1 00:00:27,000 --> 00:00:30,120 Our world is covered in giants. 2 00:00:34,600 --> 00:00:36,920 The largest things that ever lived on this planet 3 00:00:36,920 --> 00:00:40,600 weren't the dinosaurs. They're not even blue whales. 4 00:00:40,600 --> 00:00:42,080 They're trees. 5 00:00:42,080 --> 00:00:46,400 These are Mountain Ash, the largest flowering plant in the world. 6 00:00:46,400 --> 00:00:50,400 They grow about a metre a year and these trees are 60, 70, 7 00:00:50,400 --> 00:00:52,600 even 80 metres high. 8 00:00:52,600 --> 00:00:54,280 But to get this big, 9 00:00:54,280 --> 00:00:57,720 you need to face some very significant physical challenges. 10 00:01:06,720 --> 00:01:10,280 These giants can live to well over 300 years old. 11 00:01:11,360 --> 00:01:13,360 But they don't keep growing forever. 12 00:01:15,240 --> 00:01:19,600 There are limits to how big each tree can get. 13 00:01:19,600 --> 00:01:22,080 As with all living things, the structure, 14 00:01:22,080 --> 00:01:24,360 form and function of these trees 15 00:01:24,360 --> 00:01:28,840 has been shaped by the process of evolution through natural selection. 16 00:01:28,840 --> 00:01:32,480 But evolution doesn't have a free hand. 17 00:01:32,480 --> 00:01:36,680 It is constrained by the universal laws of physics. 18 00:01:42,560 --> 00:01:45,640 Each tree has to support its mass 19 00:01:45,640 --> 00:01:48,640 against the downward force of Earth's gravity. 20 00:01:50,160 --> 00:01:51,480 At the same time, 21 00:01:51,480 --> 00:01:54,840 the trees rely on the strength of the interactions 22 00:01:54,840 --> 00:01:58,880 between molecules to raise a column of water from the ground 23 00:01:58,880 --> 00:02:01,320 up to the leaves in the canopy. 24 00:02:06,080 --> 00:02:09,840 And it's these fundamental properties of nature 25 00:02:09,840 --> 00:02:14,520 that act together to limit the maximum height of a tree, 26 00:02:14,520 --> 00:02:19,480 which theoretically lies somewhere in the region of 130 metres. 27 00:02:29,120 --> 00:02:33,000 With its forests and mountains... 28 00:02:33,000 --> 00:02:35,800 Oceans and deserts... 29 00:02:36,960 --> 00:02:42,080 I've come to Australia to explore the scale of life's sizes. 30 00:02:45,680 --> 00:02:48,440 I want to see how the laws of physics 31 00:02:48,440 --> 00:02:52,080 govern the lives of all living things. 32 00:02:52,080 --> 00:02:55,000 From the very biggest... 33 00:02:55,000 --> 00:02:56,680 to the very smallest. 34 00:02:59,200 --> 00:03:02,880 The size of life on Earth spans from the tallest tree, 35 00:03:02,880 --> 00:03:07,440 over 100 metres tall and with a mass of over 1,000 tonnes, 36 00:03:07,440 --> 00:03:10,080 to the smallest bacterium cell, 37 00:03:10,080 --> 00:03:13,440 with a length less than a millionth of a millimetre 38 00:03:13,440 --> 00:03:17,520 and a mass less than a million millionths of a gram. 39 00:03:17,520 --> 00:03:21,960 And that spans over 22 orders of magnitude in mass. 40 00:03:24,800 --> 00:03:28,440 I want to see how size influences the natural world. 41 00:03:32,080 --> 00:03:34,600 How do the physical forces of nature 42 00:03:34,600 --> 00:03:38,160 dictate the lives of the big and the small? 43 00:03:40,440 --> 00:03:44,280 Do organisms face different challenges at different scales? 44 00:03:46,520 --> 00:03:51,080 And do we all experience the world differently, based on our size? 45 00:03:53,480 --> 00:03:54,680 The size you are 46 00:03:54,680 --> 00:03:57,480 profoundly influences the way that you live your life. 47 00:03:57,480 --> 00:04:00,560 It selects from the properties of the natural world 48 00:04:00,560 --> 00:04:02,160 that most affect you. 49 00:04:02,160 --> 00:04:06,360 So, I suppose that whilst we all live on the same planet, 50 00:04:06,360 --> 00:04:08,480 we occupy different worlds. 51 00:04:33,360 --> 00:04:35,640 I'm heading out to the Neptune Islands, 52 00:04:35,640 --> 00:04:37,840 west of Adelaide in South Australia... 53 00:04:42,560 --> 00:04:45,800 in search of one of nature's largest killing machines. 54 00:04:52,320 --> 00:04:55,000 These beasts are feared around the world, 55 00:04:55,000 --> 00:04:57,760 a fear not helped by Hollywood filmmakers. 56 00:05:01,720 --> 00:05:05,360 I'm here to swim with great white sharks. 57 00:05:07,920 --> 00:05:09,160 ENGINE STARTS UP 58 00:05:14,360 --> 00:05:17,640 How big... How wide can they open their jaw? Three foot wide. 59 00:05:17,640 --> 00:05:20,160 About three feet. They can swallow a man whole. Yes. 60 00:05:20,160 --> 00:05:22,440 So about three... 61 00:05:22,440 --> 00:05:25,000 Three foot wide, can swallow a man whole. 62 00:05:30,560 --> 00:05:35,080 The skipper has a special permit to use bait to lure the sharks in. 63 00:05:39,680 --> 00:05:41,880 The crew ready the cages. 64 00:05:58,600 --> 00:06:01,960 The last time I dived was in the marina in Brighton. 65 00:06:01,960 --> 00:06:03,840 I did see a fish. 66 00:06:03,840 --> 00:06:05,040 It was about that big. 67 00:06:07,320 --> 00:06:10,520 From that to the largest marine predator. 68 00:06:10,520 --> 00:06:11,960 CLEARS HIS THROAT 69 00:06:15,200 --> 00:06:18,520 As the sharks start to circle, it's time to get in. 70 00:06:30,000 --> 00:06:32,160 There he is. There he comes. 71 00:06:34,440 --> 00:06:38,800 Just look at that. He's just checking us out. 72 00:06:38,800 --> 00:06:41,600 Well, he's turning straight for us. 73 00:06:43,000 --> 00:06:45,080 Look at those teeth. 74 00:06:46,520 --> 00:06:51,400 Graceful, elegant thing. Shaped by natural selection. 75 00:06:51,400 --> 00:06:55,480 Brilliant at what it does, which is to eat things. 76 00:07:02,920 --> 00:07:03,960 HE LAUGHS 77 00:07:06,800 --> 00:07:10,840 Well, I never would've thought you could be that close to one of those. 78 00:07:17,480 --> 00:07:21,120 Great whites are highly evolved predators. 79 00:07:21,120 --> 00:07:25,480 Around two thirds of their brain is dedicated to their sense of smell. 80 00:07:27,560 --> 00:07:33,440 They can detect as little as one part per million blood. 81 00:07:33,440 --> 00:07:37,280 In this water, the tiniest speck of blood... 82 00:07:37,280 --> 00:07:38,680 will attract the shark. 83 00:07:41,520 --> 00:07:45,040 These fish can grow to a huge size. 84 00:07:45,040 --> 00:07:48,200 But still move with incredible speed and agility. 85 00:07:49,600 --> 00:07:51,720 They've been sculpted by evolution, 86 00:07:51,720 --> 00:07:55,800 acting within the bounds of the physical properties of water. 87 00:07:59,320 --> 00:08:02,000 Now, he's about five metres long. 88 00:08:02,000 --> 00:08:03,480 He weighs about a ton. 89 00:08:05,240 --> 00:08:08,360 And he's probably the most efficient predator on earth. 90 00:08:13,240 --> 00:08:15,240 When he's attacking, 91 00:08:15,240 --> 00:08:18,440 he can accelerate up to over 20 miles an hour. 92 00:08:18,440 --> 00:08:21,880 They can launch themselves straight out of the water. 93 00:08:21,880 --> 00:08:23,400 There he is! There he is. 94 00:08:30,720 --> 00:08:32,200 Whoa! 95 00:08:32,200 --> 00:08:33,480 Whoa! 96 00:08:35,680 --> 00:08:38,400 I felt the need to remove my hands. 97 00:08:51,440 --> 00:08:57,320 That was one of the most awe-inspiring sights I've ever seen. 98 00:08:57,320 --> 00:09:01,880 A great white, just straight in front of me with its mouth open. 99 00:09:04,800 --> 00:09:08,840 With the boat moored up, away from shark-infested waters, 100 00:09:08,840 --> 00:09:10,720 I want to explore why 101 00:09:10,720 --> 00:09:14,840 it's in our oceans that we find the biggest animals on Earth. 102 00:09:14,840 --> 00:09:17,240 From giant sharks to blue whales, 103 00:09:17,240 --> 00:09:21,080 the largest animals that have ever lived have lived in the sea. 104 00:09:22,720 --> 00:09:25,280 The reason why is down to physics. 105 00:09:26,840 --> 00:09:29,800 This is a container full of saltwater 106 00:09:29,800 --> 00:09:31,240 and I'm going to weigh it. 107 00:09:32,480 --> 00:09:36,320 You see, that says 25 kilograms there. 108 00:09:36,320 --> 00:09:38,360 That's actually its mass. 109 00:09:38,360 --> 00:09:43,920 Its weight is the force the Earth is exerting on it due to gravity, 110 00:09:43,920 --> 00:09:46,120 which is 25 times about ten, 111 00:09:46,120 --> 00:09:49,520 which is 250 kilogram metres per second squared. 112 00:09:49,520 --> 00:09:54,000 That might sound pedantic, but it's going to be important in a minute. 113 00:09:54,000 --> 00:09:59,040 See what happens if I lower this saltwater into the ocean. 114 00:10:03,320 --> 00:10:08,400 Its weight has effectively disappeared. It's effectively zero. 115 00:10:08,400 --> 00:10:11,440 Now, of course, gravity is still acting on this thing, 116 00:10:11,440 --> 00:10:14,040 so by the strictest sense of the word, 117 00:10:14,040 --> 00:10:16,480 it still has the same weight as it did up here, 118 00:10:16,480 --> 00:10:19,280 but Mr Archimedes told us 119 00:10:19,280 --> 00:10:21,600 that there's another force that's come into play. 120 00:10:21,600 --> 00:10:23,480 There's a force proportional 121 00:10:23,480 --> 00:10:27,560 to the weight of water that's been displaced by this thing 122 00:10:27,560 --> 00:10:30,960 and because this thing has essentially the same density as seawater, 123 00:10:30,960 --> 00:10:33,000 because it's made of seawater, 124 00:10:33,000 --> 00:10:37,440 then that force is equal and opposite to the force of gravity, 125 00:10:37,440 --> 00:10:39,080 and so they cancel, 126 00:10:39,080 --> 00:10:42,320 so it's effectively weightless 127 00:10:42,320 --> 00:10:45,120 and that is extremely important indeed 128 00:10:45,120 --> 00:10:47,400 for the animals that live in the ocean. 129 00:10:51,040 --> 00:10:55,520 The cells of all living things are predominantly made up of salty water 130 00:10:55,520 --> 00:10:58,760 so in the ocean, weight is essentially unimportant. 131 00:11:17,840 --> 00:11:22,160 Because of Archimedes' principle, the supportive nature of water 132 00:11:22,160 --> 00:11:25,800 releases organisms from the constraints of Earth's gravity, 133 00:11:25,800 --> 00:11:29,760 allowing the evolution of marine leviathans. 134 00:11:33,160 --> 00:11:35,560 But this comes at a cost. 135 00:11:35,560 --> 00:11:38,600 Water is 800 times denser than air 136 00:11:38,600 --> 00:11:41,240 and so whilst it provides support, 137 00:11:41,240 --> 00:11:44,680 it requires a huge amount of effort to move through it. 138 00:11:49,120 --> 00:11:52,920 Not only does the shark have to push the water out of the way, 139 00:11:52,920 --> 00:11:55,720 it also has to overcome drag forces 140 00:11:55,720 --> 00:11:59,320 created by the frictional contact with the water itself. 141 00:12:00,560 --> 00:12:03,400 The solution for the shark lies in its shape. 142 00:12:04,880 --> 00:12:07,400 If you look at him, that great white, 143 00:12:07,400 --> 00:12:09,960 he's got that distinctive streamlined shape. 144 00:12:11,520 --> 00:12:16,000 His maximum width is about a third of the way down his body, 145 00:12:16,000 --> 00:12:20,440 and that width itself should be around a quarter of the length. 146 00:12:22,160 --> 00:12:28,040 That ratio is set by the necessity for something that big 147 00:12:28,040 --> 00:12:33,960 to be able to swim effectively and quickly through this medium. 148 00:12:38,520 --> 00:12:41,800 This shape reduces drag forces to a minimum 149 00:12:41,800 --> 00:12:46,200 and optimises the way water flows around the shark's body. 150 00:12:46,200 --> 00:12:51,680 It is the result of evolution, shaped by the laws of physics. 151 00:12:55,720 --> 00:12:57,200 Whoa! 152 00:12:58,640 --> 00:13:01,080 HE LAUGHS 153 00:13:01,080 --> 00:13:04,520 That's cunning! That was straight out of Jaws! 154 00:13:12,400 --> 00:13:15,080 That streamlined shape of a shark 155 00:13:15,080 --> 00:13:18,480 is something that you see echoed throughout nature. 156 00:13:18,480 --> 00:13:21,400 I mean, think of a whale or a dolphin or a tuna, 157 00:13:21,400 --> 00:13:24,920 all that same torpedo-like shape, 158 00:13:24,920 --> 00:13:28,480 and that's because they're contending with problems that arise 159 00:13:28,480 --> 00:13:30,680 from the same laws of physics 160 00:13:30,680 --> 00:13:35,200 and convergent evolution has driven them to the same solution. 161 00:13:37,640 --> 00:13:41,480 For life in the sea, the evolution of giants is constrained 162 00:13:41,480 --> 00:13:44,120 directly by the physical properties of water. 163 00:13:47,560 --> 00:13:49,200 But out of the ocean, 164 00:13:49,200 --> 00:13:53,280 life now has to content with the full force of Earth's gravity. 165 00:13:53,280 --> 00:13:55,520 And it's this force of nature 166 00:13:55,520 --> 00:13:59,040 that dominates the lives of giants on land. 167 00:14:10,040 --> 00:14:14,920 This is the hot, dry outback north of Broken Hill in New South Wales. 168 00:14:19,280 --> 00:14:21,800 I'm here to explore how gravity, 169 00:14:21,800 --> 00:14:26,080 a force whose strength is governed by the mass of our whole planet, 170 00:14:26,080 --> 00:14:30,840 moulds, shapes and ultimately limits the size of life on land. 171 00:14:41,840 --> 00:14:45,440 I've come to track down one of Australia's most iconic animals... 172 00:14:47,440 --> 00:14:48,840 ..the red kangaroo. 173 00:14:51,560 --> 00:14:55,960 Red kangaroos are Australia's largest native land mammal, 174 00:14:55,960 --> 00:14:59,040 one of 50 species of macropods, 175 00:14:59,040 --> 00:15:01,920 so-called on account of their large feet. 176 00:15:05,960 --> 00:15:07,800 (WHISPERS) There! There. 177 00:15:07,800 --> 00:15:09,360 There's two very close there. 178 00:15:16,960 --> 00:15:19,960 The kangaroos are the most remarkable of mammals 179 00:15:19,960 --> 00:15:21,840 because they hop. 180 00:15:21,840 --> 00:15:24,880 There's no record, even in the fossil record, 181 00:15:24,880 --> 00:15:27,720 of any other large animal that does that 182 00:15:27,720 --> 00:15:30,440 but it makes them very fast and efficient. 183 00:15:30,440 --> 00:15:33,720 When Joseph Banks, who's one of my scientific heroes, 184 00:15:33,720 --> 00:15:38,040 first arrived here with Captain Cook on the Endeavour in 1770, 185 00:15:38,040 --> 00:15:40,320 he wrote that "They move so fast 186 00:15:40,320 --> 00:15:43,440 "over the rocky, rough ground where they're found, 187 00:15:43,440 --> 00:15:46,400 "even my greyhound couldn't catch them." 188 00:15:46,400 --> 00:15:49,080 I mean, what was he doing with a greyhound? 189 00:15:53,280 --> 00:15:55,440 Kangaroos are herbivorous 190 00:15:55,440 --> 00:15:58,160 and scratch out a living feeding on grasses. 191 00:16:01,040 --> 00:16:04,640 While foraging, they move in an ungainly fashion, 192 00:16:04,640 --> 00:16:07,760 using their large, muscular tail like a fifth leg. 193 00:16:11,840 --> 00:16:13,600 But when they want to, 194 00:16:13,600 --> 00:16:17,520 these large marsupials can cover ground at considerable speeds. 195 00:16:20,320 --> 00:16:21,760 To take a leap, 196 00:16:21,760 --> 00:16:26,160 kangaroos have to work against the downward pull of Earth's gravity. 197 00:16:26,160 --> 00:16:28,240 This takes a lot of energy. 198 00:16:30,080 --> 00:16:35,320 As animals go faster, they tend to use more energy. 199 00:16:35,320 --> 00:16:37,560 Not so with the kangaroos. 200 00:16:41,440 --> 00:16:46,280 As the roos go faster, their energy consumption actually decreases. 201 00:16:49,360 --> 00:16:51,200 It then stays constant, 202 00:16:51,200 --> 00:16:54,840 even at sustained speeds of up to 40 kilometres per hour. 203 00:17:01,760 --> 00:17:05,120 This incredibly efficiency for such a large animal 204 00:17:05,120 --> 00:17:07,680 comes directly from the kangaroos' anatomy. 205 00:17:11,120 --> 00:17:13,160 Kangaroos move so efficiently 206 00:17:13,160 --> 00:17:17,000 because they have an ingenious energy storage mechanism. 207 00:17:17,000 --> 00:17:20,720 See, when something hits the ground after falling from some height, 208 00:17:20,720 --> 00:17:23,280 then it has energy that it needs to dissipate. 209 00:17:23,280 --> 00:17:24,480 If you're a rock... 210 00:17:26,200 --> 00:17:29,760 ..that energy is dissipated as sound and a little bit of heat 211 00:17:29,760 --> 00:17:31,480 but if you're a tennis ball... 212 00:17:32,840 --> 00:17:36,240 ..then some of that energy is reused because a tennis ball is elastic, 213 00:17:36,240 --> 00:17:38,520 it can deform, spring back, 214 00:17:38,520 --> 00:17:41,920 and use some of that energy to throw itself back into the air again. 215 00:17:43,080 --> 00:17:45,320 Well, a kangaroo is very similar. 216 00:17:45,320 --> 00:17:48,120 It has very elastic tendons in its legs, 217 00:17:48,120 --> 00:17:52,440 particularly its Achilles tendon and also the tendons in its tail, 218 00:17:52,440 --> 00:17:56,400 and they store energy and then they release it, 219 00:17:56,400 --> 00:17:59,040 supplementing the power of the muscles 220 00:17:59,040 --> 00:18:01,920 to bounce the kangaroo through the air. 221 00:18:01,920 --> 00:18:06,880 Now, an adult kangaroo is 85, 90 kilos, 222 00:18:06,880 --> 00:18:08,960 which is heavier than me, 223 00:18:08,960 --> 00:18:14,080 and when it's going at full speed, it can jump around nine metres. 224 00:18:14,080 --> 00:18:15,880 That's the distance from me... 225 00:18:16,920 --> 00:18:18,600 ..to that car. 226 00:18:22,040 --> 00:18:24,640 The evolution of the ability to hop 227 00:18:24,640 --> 00:18:29,040 gives kangaroos a cheap and efficient way to move around. 228 00:18:29,040 --> 00:18:31,720 But not everything can move like a kangaroo. 229 00:18:33,560 --> 00:18:36,800 The red kangaroo is the largest animal in the world 230 00:18:36,800 --> 00:18:38,880 that moves in this unique way, 231 00:18:38,880 --> 00:18:42,120 hopping across the landscape at high speed, 232 00:18:42,120 --> 00:18:47,480 and there are reasons why there aren't giant hopping elephants 233 00:18:47,480 --> 00:18:51,040 or dinosaurs, and they're not really biological, 234 00:18:51,040 --> 00:18:54,080 it's not down to the details of evolution 235 00:18:54,080 --> 00:18:56,840 by natural selection or environmental pressures. 236 00:18:56,840 --> 00:18:58,960 The larger an animal gets, 237 00:18:58,960 --> 00:19:04,440 the more severe the restrictions on its body shape and its movements. 238 00:19:08,040 --> 00:19:10,560 To understand why this is the case, 239 00:19:10,560 --> 00:19:13,800 I want to explore what happens to the mass of a body 240 00:19:13,800 --> 00:19:16,080 when that body increases in size. 241 00:19:20,320 --> 00:19:22,080 Take a look at this block. 242 00:19:22,080 --> 00:19:23,920 Let's say it has width - one, 243 00:19:23,920 --> 00:19:26,160 length - one, and height - one, 244 00:19:26,160 --> 00:19:30,400 then its volume is one multiplied by one multiplied by one, 245 00:19:30,400 --> 00:19:32,880 which is one cubic... 246 00:19:32,880 --> 00:19:35,320 things, whatever the measurement is. 247 00:19:35,320 --> 00:19:38,160 Now, its mass is proportional to the volume, 248 00:19:38,160 --> 00:19:41,920 so we could say that the mass of this block is one unit as well. 249 00:19:41,920 --> 00:19:45,520 Let's say that we're going to double the size of this thing 250 00:19:45,520 --> 00:19:48,680 in the sense that we want to double its width, 251 00:19:48,680 --> 00:19:51,360 double its length, 252 00:19:51,360 --> 00:19:54,040 double its height. 253 00:19:54,040 --> 00:19:57,520 Then its volume is two multiplied by two multiplied by two, 254 00:19:57,520 --> 00:19:59,920 equals eight cubic things. 255 00:19:59,920 --> 00:20:02,760 Its volume has increased by a factor of eight, 256 00:20:02,760 --> 00:20:06,240 and so its mass has increased by a factor of eight as well. 257 00:20:08,640 --> 00:20:12,080 So although I've only doubled the size of the blocks, 258 00:20:12,080 --> 00:20:14,480 I've increased the total mass by eight. 259 00:20:15,720 --> 00:20:17,360 As things get bigger, 260 00:20:17,360 --> 00:20:21,400 the mass of a body goes up by the cube of the increase in size. 261 00:20:26,320 --> 00:20:28,720 Because of this scaling relationship, 262 00:20:28,720 --> 00:20:32,520 the larger you get, the greater the effect. 263 00:20:32,520 --> 00:20:34,320 As things get bigger, 264 00:20:34,320 --> 00:20:38,080 the huge increase in mass has a significant impact 265 00:20:38,080 --> 00:20:42,120 on the way large animals support themselves against gravity 266 00:20:42,120 --> 00:20:44,160 and how they move about. 267 00:20:47,440 --> 00:20:51,040 No matter how energy-efficient and advantageous it is 268 00:20:51,040 --> 00:20:52,720 to hop like a kangaroo, 269 00:20:52,720 --> 00:20:56,160 as you get bigger, it's just not physically possible. 270 00:20:59,400 --> 00:21:04,680 Going supersize on land comes with tremendous constraints attached. 271 00:21:07,560 --> 00:21:10,400 This is the left femur, the thigh bone 272 00:21:10,400 --> 00:21:13,040 of an extinct animal called a Diprotodon, 273 00:21:13,040 --> 00:21:16,800 which is the largest known marsupial ever to have existed. 274 00:21:16,800 --> 00:21:19,520 This would have stood as tall as me, 275 00:21:19,520 --> 00:21:21,560 it would have been four metres long, 276 00:21:21,560 --> 00:21:24,280 weighed between two and two-and-a-half tons, 277 00:21:24,280 --> 00:21:25,840 so the size of a rhino, 278 00:21:25,840 --> 00:21:28,840 and it's known that it was all over Australia, 279 00:21:28,840 --> 00:21:31,120 it was the big herbivore, 280 00:21:31,120 --> 00:21:33,360 and it got progressively bigger 281 00:21:33,360 --> 00:21:37,440 over the 25 million years that we have fossils for it, 282 00:21:37,440 --> 00:21:40,000 and then around 50,000 years ago, 283 00:21:40,000 --> 00:21:43,360 coincidentally, when humans arrived in Australia, 284 00:21:43,360 --> 00:21:45,640 the Diprotodon became extinct. 285 00:21:49,800 --> 00:21:53,960 The Diprotodon is thought to have looked like a giant wombat 286 00:21:53,960 --> 00:21:56,440 and being marsupials, the females 287 00:21:56,440 --> 00:22:00,920 would have carried their sheep-sized offspring in a huge pouch. 288 00:22:03,360 --> 00:22:05,840 To support their considerable bulk, 289 00:22:05,840 --> 00:22:10,080 the Diprotodon skeleton had to be very strong. 290 00:22:10,080 --> 00:22:15,160 This imposed significant constraints on the shape and size of its bones. 291 00:22:16,600 --> 00:22:20,960 This is the fever of the closest living relative of the Diprotodon. 292 00:22:20,960 --> 00:22:24,520 It's a wombat, which is an animal around the size of a small dog. 293 00:22:24,520 --> 00:22:27,040 And you see that superficially, 294 00:22:27,040 --> 00:22:29,280 the bones are very similar. 295 00:22:29,280 --> 00:22:32,440 But let me take a few measurements. 296 00:22:32,440 --> 00:22:35,680 The length of the Diprotodon femur 297 00:22:35,680 --> 00:22:41,120 is...what, around 75 cm. 298 00:22:41,120 --> 00:22:46,520 The length of the wombat femur is around 15 cm, 299 00:22:46,520 --> 00:22:51,480 so this is about five times the length of the wombat femur. 300 00:22:51,480 --> 00:22:54,200 But now look at the cross-sectional area. 301 00:22:54,200 --> 00:22:58,040 Assuming the bones are roughly circular in cross-section, 302 00:22:58,040 --> 00:23:04,000 we can calculate their area using pi multiplied by the radius squared. 303 00:23:04,000 --> 00:23:05,440 It turns out that 304 00:23:05,440 --> 00:23:09,400 although the Diprotodon femur is around five times longer, 305 00:23:09,400 --> 00:23:12,240 it has a cross-sectional area 306 00:23:12,240 --> 00:23:15,120 40 times that of the wombat femur. 307 00:23:19,240 --> 00:23:23,320 A bone's strength depends directly on its cross-sectional area. 308 00:23:25,080 --> 00:23:30,360 The Diprotodon needed thick leg bones, braced in a robust skeleton, 309 00:23:30,360 --> 00:23:35,120 just to provide enough strength to support the giant's colossal weight. 310 00:23:41,560 --> 00:23:43,640 As animals get more massive, 311 00:23:43,640 --> 00:23:45,280 the effect of gravity 312 00:23:45,280 --> 00:23:48,480 plays an increasingly restrictive role in their lives. 313 00:23:50,440 --> 00:23:54,680 The shape and form of their body is forced to change. 314 00:23:58,560 --> 00:24:02,240 If you look across the scale of Australian vertebrate life, 315 00:24:02,240 --> 00:24:05,280 you see a dramatic difference in bone thickness. 316 00:24:08,720 --> 00:24:12,160 This is a line of femur bones of animals of different sizes. 317 00:24:12,160 --> 00:24:13,880 We start with the smallest, 318 00:24:13,880 --> 00:24:16,800 one of the smallest marsupials in Australia, 319 00:24:16,800 --> 00:24:19,680 the marsupial mouse or the Antechinus. 320 00:24:19,680 --> 00:24:23,360 Then the next one is an animal known as the Potoroo. 321 00:24:23,360 --> 00:24:26,800 Again, it's a marsupial around about the size of a rabbit. 322 00:24:26,800 --> 00:24:29,160 Then we have the Tasmanian Devil, 323 00:24:29,160 --> 00:24:30,840 a wombat, 324 00:24:30,840 --> 00:24:32,320 a dingo, 325 00:24:32,320 --> 00:24:35,480 then the largest marsupial in Austria today, 326 00:24:35,480 --> 00:24:37,800 the red kangaroo. 327 00:24:37,800 --> 00:24:41,640 And this is the femur of the Diprotodon 328 00:24:41,640 --> 00:24:45,520 and then, here, the femur of a Rhoetosaurus, 329 00:24:45,520 --> 00:24:49,560 which was a sauropod dinosaur 17 metres long 330 00:24:49,560 --> 00:24:51,960 and weighing around 20 tons. 331 00:24:53,600 --> 00:24:55,360 And so, you see, 332 00:24:55,360 --> 00:24:57,480 as animals get larger, 333 00:24:57,480 --> 00:25:02,800 from the smallest marsupial mouse, all the way up to a dinosaur, 334 00:25:02,800 --> 00:25:07,160 the cross-sectional area of their bones increases enormously, 335 00:25:07,160 --> 00:25:09,280 just to support that increased mass. 336 00:25:14,160 --> 00:25:15,960 Being big and bulky, 337 00:25:15,960 --> 00:25:19,600 giants are more restricted as to the shape of their body 338 00:25:19,600 --> 00:25:21,240 and how they get about. 339 00:25:25,280 --> 00:25:26,920 That's why red kangaroos 340 00:25:26,920 --> 00:25:30,400 are the largest animals that can move in the way that they do. 341 00:25:32,440 --> 00:25:36,040 At a much greater size, their bones would be very heavy, 342 00:25:36,040 --> 00:25:38,160 have a greater risk of fracture, 343 00:25:38,160 --> 00:25:42,000 and they'd require far too much energy to move at high speeds. 344 00:25:45,840 --> 00:25:49,080 It's ultimately the strength of Earth's gravity 345 00:25:49,080 --> 00:25:50,520 that limits the size 346 00:25:50,520 --> 00:25:53,640 and the manoeuvrability of land-based giants. 347 00:25:55,720 --> 00:25:58,160 But for the bulk of life on land, 348 00:25:58,160 --> 00:26:02,600 gravity is not the defining force of nature. 349 00:26:14,880 --> 00:26:20,320 At small scales, living things seem to bend the laws of physics, 350 00:26:20,320 --> 00:26:22,200 which is, of course, not possible. 351 00:26:23,440 --> 00:26:27,080 The world of the small is often hidden from our view, 352 00:26:27,080 --> 00:26:30,280 but there are ways to draw out these tiny creatures. 353 00:26:35,400 --> 00:26:37,600 This is the domain of the insects. 354 00:26:41,280 --> 00:26:44,720 These animals can clearly do things I can't do 355 00:26:44,720 --> 00:26:47,360 and appear to have superpowers. 356 00:26:48,520 --> 00:26:50,560 They can walk up walls, 357 00:26:50,560 --> 00:26:53,280 jump many times their own height, 358 00:26:53,280 --> 00:26:56,080 and can lift many times their own weight. 359 00:26:58,320 --> 00:27:02,600 There are over 900,000 known species of insects on the planet. 360 00:27:02,600 --> 00:27:06,840 That's over 75% of all animal species. 361 00:27:06,840 --> 00:27:08,520 Some biologists think that 362 00:27:08,520 --> 00:27:12,000 there may be an order of magnitude more yet to be discovered. 363 00:27:12,000 --> 00:27:15,440 That would be ten million species, 364 00:27:15,440 --> 00:27:17,040 and they're very small, 365 00:27:17,040 --> 00:27:20,440 so you can fit a lot of them on Planet Earth at any one time. 366 00:27:20,440 --> 00:27:23,120 In fact, it's estimated there are 367 00:27:23,120 --> 00:27:28,160 over ten billion billion individual insects alive today. 368 00:27:34,480 --> 00:27:36,520 Of all the insect groups, 369 00:27:36,520 --> 00:27:39,160 it's the beetles, or coleoptera, 370 00:27:39,160 --> 00:27:41,600 that have the greatest number of species. 371 00:27:46,280 --> 00:27:48,800 The biologist JBS Haldane said that 372 00:27:48,800 --> 00:27:52,120 if one could conclude as to the nature of the Creator 373 00:27:52,120 --> 00:27:53,800 from a study of creation, 374 00:27:53,800 --> 00:27:57,440 then it would appear that God has an inordinate fondness 375 00:27:57,440 --> 00:27:59,480 for stars and beetles. 376 00:28:08,000 --> 00:28:11,960 With so much variation in colour, form and function, 377 00:28:11,960 --> 00:28:15,280 beetles have fascinated naturalists for centuries. 378 00:28:17,760 --> 00:28:22,400 Each species is wonderfully adapted to their own unique niche. 379 00:28:37,440 --> 00:28:42,000 This is the beginnings of biology as a science that you see here, 380 00:28:42,000 --> 00:28:44,960 it's this desire to collect and classify, 381 00:28:44,960 --> 00:28:49,040 which then, over time, becomes the desire to explain and understand. 382 00:28:53,560 --> 00:28:55,320 I'm going to take a picture. 383 00:29:02,240 --> 00:29:04,920 Here in the suburbs of Brisbane, 384 00:29:04,920 --> 00:29:08,120 every February, there's an invasion of beetles. 385 00:29:09,760 --> 00:29:14,000 The rules governing their lives play out very differently to ours. 386 00:29:17,560 --> 00:29:22,120 This is the Rhinoceros Beetle, named for obvious reasons. 387 00:29:22,120 --> 00:29:23,760 But actually, it's only the males 388 00:29:23,760 --> 00:29:26,560 that have the distinctive horns on their heads. 389 00:29:28,440 --> 00:29:32,000 These beetles spend much of their lives underground as larvae, 390 00:29:32,000 --> 00:29:36,520 but then emerge en masse as adults to find a mate and breed. 391 00:29:37,680 --> 00:29:41,520 Much of this time, the males spend fighting over females. 392 00:29:50,160 --> 00:29:54,000 See that distinctive posture 393 00:29:54,000 --> 00:29:55,440 that he's adopting there? 394 00:29:55,440 --> 00:29:56,600 That's because I think 395 00:29:56,600 --> 00:29:59,920 he's seeing his reflection in the camera lens, and so he rears up. 396 00:30:00,720 --> 00:30:03,760 Look at that! He's trying to scare himself off. 397 00:30:05,360 --> 00:30:06,440 Ha-ha-ha! 398 00:30:08,040 --> 00:30:09,040 INSECT BRISTLES 399 00:30:10,480 --> 00:30:12,520 You also heard that hissing sound. 400 00:30:12,520 --> 00:30:18,560 That's him contract in his abdomen which again is a defensive 401 00:30:18,560 --> 00:30:22,480 posture that he adopts to scare other males. 402 00:30:22,480 --> 00:30:23,440 INSECT HISSES 403 00:30:25,400 --> 00:30:30,360 Gramme for gramme, these insects are among the strongest animals alive. 404 00:30:33,240 --> 00:30:36,960 I can demonstrate that I just getting hold of the top of his head. 405 00:30:40,760 --> 00:30:43,920 It doesn't hurt him at all, but watch what he is able to do. 406 00:30:49,480 --> 00:30:50,920 Look at that. 407 00:30:50,920 --> 00:30:53,000 So he is hanging on to this branch, 408 00:30:53,000 --> 00:30:55,760 which is many times his own bodyweight. 409 00:30:57,000 --> 00:30:59,520 Absolutely no distress at all. 410 00:31:02,240 --> 00:31:04,520 As things get smaller, it is 411 00:31:04,520 --> 00:31:08,160 a rule of nature that they inevitably get stronger. 412 00:31:09,280 --> 00:31:11,440 The reason is quite simple. 413 00:31:11,440 --> 00:31:14,680 Small things have relatively large muscles compared 414 00:31:14,680 --> 00:31:18,560 to their tiny body mass and this makes them very powerful. 415 00:31:26,640 --> 00:31:30,200 The beetles also appear to have a cavalier attitude to 416 00:31:30,200 --> 00:31:31,720 the effects of gravity. 417 00:31:35,400 --> 00:31:37,560 They fight almost like sumo wrestlers, 418 00:31:37,560 --> 00:31:40,840 their aim is to throw each other off the branch. 419 00:31:43,200 --> 00:31:46,160 If they should fall... 420 00:31:46,160 --> 00:31:50,000 they just bounce and walk off. 421 00:31:53,480 --> 00:31:57,720 If I fail a similar distance relative to my size, I'd break. 422 00:32:00,160 --> 00:32:03,240 So why does size make such a difference? 423 00:32:10,640 --> 00:32:13,400 Time for a bit of fundamental physics. 424 00:32:13,400 --> 00:32:17,280 All things fall at the same rate under gravity. 425 00:32:17,280 --> 00:32:19,880 That's because they they're following geodesics 426 00:32:19,880 --> 00:32:22,640 through curved space-time, but that's not important. 427 00:32:22,640 --> 00:32:25,760 The important thing for biology is that although everything falls at 428 00:32:25,760 --> 00:32:30,840 the same rate, it doesn't meet the same fate when it hits the ground. 429 00:32:36,520 --> 00:32:37,920 A grape bounces. 430 00:32:43,800 --> 00:32:46,920 A melon... 431 00:32:50,200 --> 00:32:52,160 Doesn't bounce. 432 00:32:55,800 --> 00:32:59,680 The reasons for that are quite complex actually. 433 00:32:59,680 --> 00:33:05,080 First of all, the grape has a larger surface area in relation 434 00:33:05,080 --> 00:33:09,120 to its volume and therefore its mass than the melon. 435 00:33:09,120 --> 00:33:12,280 Although, in a vacuum, if you took away the air, 436 00:33:12,280 --> 00:33:15,600 they would both fall at the same rate. Actually, in reality, 437 00:33:15,600 --> 00:33:18,600 the grape falls slower than the melon. 438 00:33:18,600 --> 00:33:22,760 Also, the melon is more massive so it has more kinetic energy 439 00:33:22,760 --> 00:33:25,600 when it hits the ground. Remember physics class. 440 00:33:25,600 --> 00:33:28,920 Kinetic energy is ½ MV squared, 441 00:33:28,920 --> 00:33:31,600 so you reduce M, you reduce the energy. 442 00:33:31,600 --> 00:33:34,400 The upshot of that is that the melon has a lot more energy 443 00:33:34,400 --> 00:33:36,240 when it hits the ground. 444 00:33:36,240 --> 00:33:39,920 It has to dissipate it in some way and it dissipates it by exploding. 445 00:33:45,640 --> 00:33:49,360 The influence of Earth's gravity in your life becomes progressively 446 00:33:49,360 --> 00:33:51,480 diminished the smaller you get. 447 00:34:00,800 --> 00:34:02,800 For life at the small scale, 448 00:34:02,800 --> 00:34:07,320 a second fundamental force of nature starts to dominate. 449 00:34:07,320 --> 00:34:11,800 And it's this that explains many of those apparent superpowers. 450 00:34:14,360 --> 00:34:20,080 For me, the force of gravity is a thing that defines my existence. 451 00:34:20,080 --> 00:34:23,160 It's the force that I really feel the effects of. 452 00:34:24,160 --> 00:34:26,040 But there are other forces at work. 453 00:34:26,040 --> 00:34:30,720 For example if I lick my finger and wet it, I can pick up a piece 454 00:34:30,720 --> 00:34:35,120 of paper and can hold up against the downward pull of gravity. 455 00:34:35,120 --> 00:34:39,640 That's because the force of electromagnetism is important. 456 00:34:39,640 --> 00:34:42,920 In fact, it is the cohesive forces between water molecules 457 00:34:42,920 --> 00:34:45,840 and the molecules that make up my finger 458 00:34:45,840 --> 00:34:47,920 and the molecules that make up the paper, 459 00:34:47,920 --> 00:34:52,040 that are dominating this particular situation. 460 00:34:52,040 --> 00:34:55,640 That's why this piece of paper doesn't fall to the floor. 461 00:34:55,640 --> 00:34:58,040 Many insects can use a similar effect. 462 00:34:59,720 --> 00:35:01,360 Take a common fly for example. 463 00:35:07,480 --> 00:35:10,360 Their feet have especially enlarged pads onto which 464 00:35:10,360 --> 00:35:11,960 they secrete a sticky fluid. 465 00:35:14,520 --> 00:35:18,040 And that allows them to adhere to rather slippery 466 00:35:18,040 --> 00:35:21,240 surfaces like the glass of this jam jar. 467 00:35:21,240 --> 00:35:25,120 It allows them to do things that for me would be absolutely impossible. 468 00:35:25,120 --> 00:35:28,960 It's all down to the relative influence of the different 469 00:35:28,960 --> 00:35:31,000 forces of nature on the animal. 470 00:35:35,280 --> 00:35:40,480 So the capacity to walk up walls and fall from a great height without 471 00:35:40,480 --> 00:35:45,640 breaking, plus super strength, are not super powers at all. 472 00:35:47,080 --> 00:35:50,320 They're just abilities gained naturally by animals 473 00:35:50,320 --> 00:35:52,920 that are small and lightweight. 474 00:35:56,040 --> 00:36:00,080 But this is just the beginning of my journey into the world of the small. 475 00:36:03,440 --> 00:36:06,640 Down at the very small scale, it becomes possible to live 476 00:36:06,640 --> 00:36:10,200 within the lives of other individuals, worlds within worlds. 477 00:36:14,280 --> 00:36:16,280 But just how small can animals get? 478 00:36:29,640 --> 00:36:33,400 This macadamia nut plantation, an hour outside of Brisbane, 479 00:36:33,400 --> 00:36:37,760 is home to one of the very smallest members of the animal kingdom. 480 00:36:46,600 --> 00:36:48,360 These are a species of micro-hymenoptera 481 00:36:48,360 --> 00:36:49,680 known as Trichogramma. 482 00:36:51,480 --> 00:36:57,320 They're basically very small wasps and when I say small, 483 00:36:57,320 --> 00:36:58,880 I mean small. 484 00:36:58,880 --> 00:37:04,400 Can you see that? They're like specks of dust. 485 00:37:04,400 --> 00:37:07,240 They're less than half a millimetre long, 486 00:37:07,240 --> 00:37:08,880 but each one of those is a wasp. 487 00:37:10,520 --> 00:37:14,400 It's got compound eyes, six legs and wings. 488 00:37:14,400 --> 00:37:18,840 They've even got a little stripe on their abdomen. 489 00:37:20,120 --> 00:37:24,360 And they're very precisely adapted to a specific evolutionary niche. 490 00:37:26,800 --> 00:37:30,200 The Trichogramma wasps may be small, but they're very useful. 491 00:37:31,280 --> 00:37:34,920 They're natural parasites of an insect pest species 492 00:37:34,920 --> 00:37:38,880 called the nut borer moth which attacks the macadamia nuts. 493 00:37:44,680 --> 00:37:49,320 The micro-wasps lay their eggs inside the eggs of the moths, 494 00:37:49,320 --> 00:37:51,160 killing the developing moth larvae. 495 00:37:54,240 --> 00:37:57,480 What you're seeing here is the surface of the macadamia nut 496 00:37:57,480 --> 00:38:02,560 and here's a small cluster of moth eggs and there, 497 00:38:02,560 --> 00:38:05,520 you see the wasp is walking over the eggs. 498 00:38:05,520 --> 00:38:08,440 They're almost pacing out the size to see 499 00:38:08,440 --> 00:38:13,120 whether the eggs are suitable for their eggs to be laid inside. 500 00:38:13,120 --> 00:38:18,400 And if we're lucky, there you go, you see that... 501 00:38:18,400 --> 00:38:20,440 That... 502 00:38:20,440 --> 00:38:22,200 There we go. 503 00:38:24,800 --> 00:38:29,360 The wasps emerge just nine days later as full-grown adults. 504 00:38:30,840 --> 00:38:34,680 At this scale, they live a very sticky world, 505 00:38:34,680 --> 00:38:37,840 dominated by strong intermolecular forces. 506 00:38:39,200 --> 00:38:42,960 To them, even the air is a thick fluid through which 507 00:38:42,960 --> 00:38:46,600 they essentially swim, using paddle-like wings. 508 00:38:49,200 --> 00:38:54,160 Incredibly, these tiny animals can move about across several trees, 509 00:38:54,160 --> 00:38:55,720 seeking out the moth eggs. 510 00:38:58,400 --> 00:39:00,480 But what I find more remarkable 511 00:39:00,480 --> 00:39:04,920 is that they do all this operating with very restricted brain power. 512 00:39:06,760 --> 00:39:09,960 One of the limiting factors that determines the minimum 513 00:39:09,960 --> 00:39:14,200 size of insects is the volume of their central nervous system. 514 00:39:14,200 --> 00:39:18,080 In other words, the processing power you can fit inside their bodies 515 00:39:18,080 --> 00:39:21,240 and these little wasps are pretty much at their limit. 516 00:39:21,240 --> 00:39:25,880 They've less than 10,000 neurons in their whole nervous system. 517 00:39:25,880 --> 00:39:27,480 To put it into perspective, 518 00:39:27,480 --> 00:39:31,760 most tiny insects have 100 times that many, but that's still 519 00:39:31,760 --> 00:39:35,360 enough to allow them to exhibit quite complex behaviour. 520 00:39:37,000 --> 00:39:40,720 These micro-wasps exist at almost the minimum possible size 521 00:39:40,720 --> 00:39:43,600 for multicellular animals. 522 00:39:43,600 --> 00:39:49,240 But the scale of life on our planet gets much, much smaller. 523 00:39:49,240 --> 00:39:51,240 The wasps are giants 524 00:39:51,240 --> 00:39:56,040 compared to life at the very limit of size on earth. 525 00:40:08,640 --> 00:40:12,720 The smallest organisms on our planet are also our oldest 526 00:40:12,720 --> 00:40:14,800 and most abundant type of lifeforms. 527 00:40:19,080 --> 00:40:22,720 These weird, rocky blobs in the shallows of Lake Clifton, 528 00:40:22,720 --> 00:40:25,840 just south of Perth, are made by bacteria. 529 00:40:31,640 --> 00:40:34,280 These mounds are called thrombolites, 530 00:40:34,280 --> 00:40:36,480 on account of their clotted structure, 531 00:40:36,480 --> 00:40:38,760 and they're built up over centuries 532 00:40:38,760 --> 00:40:42,320 by colonies of microscopic bacterial cells. 533 00:40:44,080 --> 00:40:47,520 Although these colonies are rare, by most definitions, 534 00:40:47,520 --> 00:40:51,760 bacteria are THE dominant form of life on our planet. 535 00:40:51,760 --> 00:40:56,000 On every surface across every landscape, you find bacteria. 536 00:40:56,000 --> 00:40:59,680 In fact, numerically speaking, then there are more bacteria 537 00:40:59,680 --> 00:41:03,840 living on and inside my body than there are human cells. 538 00:41:05,760 --> 00:41:08,800 Bacteria come in many shapes and forms 539 00:41:08,800 --> 00:41:12,080 and are not actually animals or plants, 540 00:41:12,080 --> 00:41:15,280 instead sitting in their own unique taxonomic kingdom. 541 00:41:17,560 --> 00:41:19,640 Compared to the cells we're made of, 542 00:41:19,640 --> 00:41:25,880 bacteria are structurally much simpler and far, far smaller. 543 00:41:25,880 --> 00:41:29,760 Bacteria are typically around two microns in size. 544 00:41:29,760 --> 00:41:34,000 That's two millionths of a metre, which is very hard to picture 545 00:41:34,000 --> 00:41:36,880 but it means that you could fit around half a million of them 546 00:41:36,880 --> 00:41:40,440 on the head of a pin or, to look at it another way, 547 00:41:40,440 --> 00:41:43,560 if I took a single bacterium and scaled it up to 548 00:41:43,560 --> 00:41:49,840 the size of this coin, then I would be 25 kilometres high. 549 00:41:49,840 --> 00:41:50,800 SPLASH 550 00:41:52,080 --> 00:41:55,320 Bacterial-type organisms were the first life on Earth 551 00:41:55,320 --> 00:41:58,160 and they've dominated our planet ever since. 552 00:41:59,360 --> 00:42:03,040 Excluding viruses, which by most definitions are not alive, 553 00:42:03,040 --> 00:42:07,080 bacteria are the smallest free-living lifeforms we know of. 554 00:42:08,680 --> 00:42:12,760 But what ultimately puts the limit on the smallest size of life? 555 00:42:14,840 --> 00:42:18,320 Single-cell life needs to be big enough to accommodate all 556 00:42:18,320 --> 00:42:21,400 the molecular machinery of life 557 00:42:21,400 --> 00:42:25,800 and that size ultimately depends on the basic laws of physics. 558 00:42:25,800 --> 00:42:28,160 It depends on the size of molecules which 559 00:42:28,160 --> 00:42:30,080 depends on the size of atoms 560 00:42:30,080 --> 00:42:33,520 which depends on fundamental properties of the universe 561 00:42:33,520 --> 00:42:36,480 like the strength of the force of electromagnetism 562 00:42:36,480 --> 00:42:39,000 and the mass of an electron. 563 00:42:39,000 --> 00:42:43,880 And when you do those calculations, you find out that the minimum size 564 00:42:43,880 --> 00:42:47,560 of a free-living organism should be around 200 nanometres 565 00:42:47,560 --> 00:42:52,000 which is around 200 billionths of a metre. 566 00:42:52,000 --> 00:42:53,760 And that should be universal, 567 00:42:53,760 --> 00:42:56,280 it shouldn't only apply to life on Earth 568 00:42:56,280 --> 00:42:59,680 but it should apply to any carbon-based life 569 00:42:59,680 --> 00:43:01,480 anywhere in the universe 570 00:43:01,480 --> 00:43:06,080 because it depends on fundamental properties of the universe. 571 00:43:15,160 --> 00:43:19,880 From the smallest bacterium to the largest tree, 572 00:43:19,880 --> 00:43:23,920 it's your size that determines how the laws of physics 573 00:43:23,920 --> 00:43:28,720 govern your life. Gravity imposes itself on the large, 574 00:43:28,720 --> 00:43:33,400 and the electromagnetic force rules the world of the small. 575 00:43:37,120 --> 00:43:40,560 But the consequences of scale for life on Earth 576 00:43:40,560 --> 00:43:43,040 extend beyond dictating the relationship 577 00:43:43,040 --> 00:43:45,200 you have with the world around you. 578 00:43:47,760 --> 00:43:52,920 Your size also influences how energy itself flows through your body. 579 00:44:00,200 --> 00:44:04,480 BATS SQUEAK FAINTLY 580 00:44:09,320 --> 00:44:12,240 These are southern bent-wing bats... 581 00:44:13,640 --> 00:44:16,680 ..one of the rarest bat species in Australia. 582 00:44:19,560 --> 00:44:22,320 Every evening, they emerge in their thousands 583 00:44:22,320 --> 00:44:24,800 from this cave, in order to feed. 584 00:44:27,080 --> 00:44:31,720 When fully grown, these bats are just 5.5cm long, 585 00:44:31,720 --> 00:44:34,880 and weigh around 18 grams. 586 00:44:34,880 --> 00:44:40,280 Because of their size, they face a constant struggle to stay alive. 587 00:44:43,480 --> 00:44:46,560 BATS SQUEAK, CRICKETS CHIRP 588 00:44:48,280 --> 00:44:51,640 We're using a thermal camera here to look at the bats, 589 00:44:51,640 --> 00:44:54,680 and you can see that they appear as streaks across the sky. 590 00:44:54,680 --> 00:44:56,280 They appear as brightly as me - 591 00:44:56,280 --> 00:44:59,360 that's because they're roughly the same temperature as me. 592 00:44:59,360 --> 00:45:01,600 They're known as endotherms - 593 00:45:01,600 --> 00:45:04,800 animals that maintain their body temperature. 594 00:45:04,800 --> 00:45:07,720 And that takes a lot of effort. 595 00:45:07,720 --> 00:45:09,480 These bats have to eat something like 596 00:45:09,480 --> 00:45:12,160 three-quarters of their own body weight every night, 597 00:45:12,160 --> 00:45:16,800 and a lot of that energy goes into maintaining their temperature. 598 00:45:18,800 --> 00:45:20,720 As with all living things, 599 00:45:20,720 --> 00:45:24,880 the bats eat to provide energy to power their metabolism. 600 00:45:24,880 --> 00:45:26,160 Although, like us, 601 00:45:26,160 --> 00:45:29,080 they have a high body temperature when they're active, 602 00:45:29,080 --> 00:45:34,080 keeping warm is a considerable challenge, on account of their size. 603 00:45:37,800 --> 00:45:41,840 The bats lose heat mostly through the surface of their bodies. 604 00:45:43,160 --> 00:45:46,080 But because of simple laws governing the relationship 605 00:45:46,080 --> 00:45:49,440 between the surface area of a body and its volume, 606 00:45:49,440 --> 00:45:52,480 being small creates a problem. 607 00:45:52,480 --> 00:45:54,720 BATS SQUEAK 608 00:45:54,720 --> 00:45:56,960 So, let's look at our blocks again, 609 00:45:56,960 --> 00:45:59,680 but this time for surface area to volume. 610 00:45:59,680 --> 00:46:00,840 Here's a big thing - 611 00:46:00,840 --> 00:46:03,560 it's made of eight blocks so its volume is eight units, 612 00:46:03,560 --> 00:46:08,200 and its surface area is two by two on each side, so that's four, 613 00:46:08,200 --> 00:46:11,280 multiplied by the six faces is 24. 614 00:46:11,280 --> 00:46:15,320 so, the surface area to volume ratio is 24 to eight, 615 00:46:15,320 --> 00:46:17,880 which is 3:1. 616 00:46:17,880 --> 00:46:21,200 Now, look at a smaller thing. This is one block, 617 00:46:21,200 --> 00:46:23,240 so its volume is one unit. 618 00:46:23,240 --> 00:46:27,720 Its surface area is one by one by one, six times, so it's six. 619 00:46:27,720 --> 00:46:33,040 So, this has a surface area to volume ratio of 6:1. 620 00:46:33,040 --> 00:46:36,600 So, as you go from big to small, 621 00:46:36,600 --> 00:46:40,520 your surface area to volume ratio increases. 622 00:46:41,840 --> 00:46:44,200 Small animals, like bats, 623 00:46:44,200 --> 00:46:47,440 have a huge surface area compared to their volume. 624 00:46:47,440 --> 00:46:51,680 As a result, they naturally lose heat at a very high rate. 625 00:46:53,240 --> 00:46:57,360 To help offset the cost of losing so much energy in the form of heat, 626 00:46:57,360 --> 00:47:01,920 the bats are forced to maintain a high rate of metabolism. 627 00:47:01,920 --> 00:47:05,320 They breathe rapidly, their little heart races, 628 00:47:05,320 --> 00:47:08,320 and they have to eat a huge amount. 629 00:47:08,320 --> 00:47:11,200 So, a bat's size clearly affects 630 00:47:11,200 --> 00:47:14,160 the speed at which it lives its life. 631 00:47:21,960 --> 00:47:24,400 Right across the natural world, 632 00:47:24,400 --> 00:47:28,680 the size you are has a profound effect on your metabolic rate - 633 00:47:28,680 --> 00:47:31,080 or your "speed of life". 634 00:47:33,360 --> 00:47:37,400 EXTREMELY FAST HEARTBEAT For Australia's small marsupial mouse, 635 00:47:37,400 --> 00:47:40,600 even at rest, his heart is racing away. 636 00:47:41,960 --> 00:47:45,080 SLOWER HEARTBEAT For the fox-sized Tasmanian devil, 637 00:47:45,080 --> 00:47:47,000 he ticks along at a much slower rate. 638 00:47:48,360 --> 00:47:52,600 And then there's me, living life at a languid 60 beats a minute. 639 00:47:55,640 --> 00:47:57,920 Looking beyond heart rate, 640 00:47:57,920 --> 00:48:01,920 your size influences the amount of energy you need to consume, 641 00:48:01,920 --> 00:48:05,080 and the rate at which you need to consume it. 642 00:48:07,440 --> 00:48:09,920 Bigger bodies have more cells to feed. 643 00:48:09,920 --> 00:48:13,720 So, you might expect that the total amount of energy needed 644 00:48:13,720 --> 00:48:17,400 goes up at the same rate as any increase in size. 645 00:48:19,200 --> 00:48:21,600 But that's not what happens. 646 00:48:25,600 --> 00:48:29,800 If you plot the amount of energy an animal uses against its mass, 647 00:48:29,800 --> 00:48:35,480 for a huge range of sizes, from animals as small as flies, 648 00:48:35,480 --> 00:48:38,520 and even smaller, all the way up to whales, 649 00:48:38,520 --> 00:48:41,840 then you DO get a straight line, but the slope 650 00:48:41,840 --> 00:48:46,240 is less than one. So, that implies that gramme for gramme, 651 00:48:46,240 --> 00:48:50,600 large animals use less energy than small animals. 652 00:48:53,360 --> 00:48:56,640 This relationship between metabolism and size 653 00:48:56,640 --> 00:48:59,560 significantly affects the amount of food 654 00:48:59,560 --> 00:49:02,800 larger animals have to consume to stay alive. 655 00:49:06,760 --> 00:49:11,440 Now, if my metabolic rate scaled one-to-one with that of a mouse, 656 00:49:11,440 --> 00:49:15,520 then I would need to eat about four kilograms of food a day. 657 00:49:15,520 --> 00:49:19,960 In my language, that's around 67,000 kilojoules of energy, 658 00:49:19,960 --> 00:49:23,000 which more colloquially is 16,000 calories. 659 00:49:23,000 --> 00:49:26,440 That is eight times the amount that I take in 660 00:49:26,440 --> 00:49:29,680 on average on a daily basis. 661 00:49:31,760 --> 00:49:34,800 Each of the cells in my body requires less energy 662 00:49:34,800 --> 00:49:38,040 than the equivalent cells in a smaller-sized mammal. 663 00:49:40,880 --> 00:49:45,160 The reason why this should be so is not fully understood. 664 00:49:45,160 --> 00:49:48,760 It's also not clear whether this rule of nature 665 00:49:48,760 --> 00:49:51,480 gives an advantage to big things, 666 00:49:51,480 --> 00:49:55,160 or is actually a constraint placed on larger animals. 667 00:49:57,480 --> 00:49:59,480 Take the relationship between 668 00:49:59,480 --> 00:50:02,000 an animal's surface area and its volume. 669 00:50:03,440 --> 00:50:07,320 Big animals have a much smaller surface area to volume ratio 670 00:50:07,320 --> 00:50:11,120 than small animals, and that means that their rate of heat loss 671 00:50:11,120 --> 00:50:12,800 is much smaller. 672 00:50:12,800 --> 00:50:16,280 And that means that there's an opportunity there for large animals. 673 00:50:16,280 --> 00:50:19,560 They don't have to eat as much food to stay warm, 674 00:50:19,560 --> 00:50:23,120 and therefore they can afford a lower metabolic rate. 675 00:50:26,000 --> 00:50:28,760 Now this helps explain the lives of large, 676 00:50:28,760 --> 00:50:32,920 warm-blooded endotherms, like birds and mammals, 677 00:50:32,920 --> 00:50:36,760 but doesn't hold so well for large ectotherms, 678 00:50:36,760 --> 00:50:39,600 life's cold-blooded giants. 679 00:50:42,440 --> 00:50:45,080 Now, there's another theory that says that it wasn't really 680 00:50:45,080 --> 00:50:47,240 an evolutionary opportunity 681 00:50:47,240 --> 00:50:50,560 that large animals took to lower their metabolic rate. 682 00:50:50,560 --> 00:50:53,800 It was forced on them. It was a constraint, if you like. 683 00:50:53,800 --> 00:50:57,440 The capillaries, the supply network to cells, 684 00:50:57,440 --> 00:51:01,360 branches in such a way that it gets more and more difficult 685 00:51:01,360 --> 00:51:04,520 to get oxygen and nutrients to cells in a big animal 686 00:51:04,520 --> 00:51:06,200 than in a small animal. 687 00:51:06,200 --> 00:51:11,400 Therefore, those cells must run at a lower rate. 688 00:51:11,400 --> 00:51:14,360 They must have a lower metabolic rate. 689 00:51:18,240 --> 00:51:20,680 Or it could just be that as you get bigger, 690 00:51:20,680 --> 00:51:24,440 then more of your mass is taken up by the stuff that supports you, 691 00:51:24,440 --> 00:51:28,080 and support structures, like bones, are relatively inert. 692 00:51:28,080 --> 00:51:30,120 They don't use much energy. 693 00:51:33,440 --> 00:51:36,480 But whatever the reason, it's certainly true to say 694 00:51:36,480 --> 00:51:40,680 that the only way that large animals can exist on planet Earth 695 00:51:40,680 --> 00:51:43,960 is to operate at a reduced metabolic rate. 696 00:51:46,040 --> 00:51:47,880 If this wasn't the case, 697 00:51:47,880 --> 00:51:52,480 the maximum size of a warm-blooded endotherm like me or you 698 00:51:52,480 --> 00:51:55,160 would be around that of a goat. 699 00:51:56,320 --> 00:52:00,040 And cold-blooded animals, or ectotherms like dinosaurs, 700 00:52:00,040 --> 00:52:02,200 could only get as big as a pony. 701 00:52:03,440 --> 00:52:06,520 Any bigger, and giants would simply overheat. 702 00:52:09,800 --> 00:52:13,720 Now, there's one last consequence of all these scaling laws 703 00:52:13,720 --> 00:52:17,520 that I suspect you'll care about more than anything else, 704 00:52:17,520 --> 00:52:20,880 and it's this - there's a strong correlation 705 00:52:20,880 --> 00:52:24,800 between the effective cellular metabolic rate of an animal 706 00:52:24,800 --> 00:52:27,840 and its lifespan. In other words, 707 00:52:27,840 --> 00:52:31,200 as things get bigger, they tend to live longer. 708 00:52:46,480 --> 00:52:50,360 To explore this connection between size and longevity, 709 00:52:50,360 --> 00:52:53,240 I've left the mainland behind. 710 00:52:53,240 --> 00:52:55,040 For my final destination, 711 00:52:55,040 --> 00:52:58,520 I've come to one of Australia's remotest outposts. 712 00:53:03,200 --> 00:53:08,680 Named Christmas Island when it was spotted on Christmas Day in 1643, 713 00:53:08,680 --> 00:53:14,560 this isolated lump of rock in the Indian Ocean is a land of crabs. 714 00:53:27,440 --> 00:53:32,240 And in their midst lurks a giant wonder of the natural world. 715 00:53:36,920 --> 00:53:39,000 This is a Christmas Island robber crab, 716 00:53:39,000 --> 00:53:41,840 the largest land crab anywhere on the planet. 717 00:53:41,840 --> 00:53:45,440 These things can grow to around 50 centimetres in length, 718 00:53:45,440 --> 00:53:47,920 they can weigh over four kilograms, 719 00:53:47,920 --> 00:53:53,040 and they are supremely adapted as an adult to life on land. 720 00:53:54,160 --> 00:53:55,800 They can even climb trees. 721 00:53:59,120 --> 00:54:01,480 Over the years, the crabs have become 722 00:54:01,480 --> 00:54:04,520 well adapted to human co-habitation. 723 00:54:06,680 --> 00:54:08,800 These things are called robber crabs 724 00:54:08,800 --> 00:54:13,880 because they have a reputation for curiosity and for stealing things, 725 00:54:13,880 --> 00:54:15,720 anything that isn't bolted down. 726 00:54:15,720 --> 00:54:21,400 They'll steal food and cameras if they can get half a chance. 727 00:54:31,840 --> 00:54:35,080 These giants live on a diet of seeds and fruit, 728 00:54:35,080 --> 00:54:38,040 and occasionally other small crabs. 729 00:54:39,240 --> 00:54:41,520 Their large, powerful claws mean 730 00:54:41,520 --> 00:54:44,480 they can also rip open fallen coconuts. 731 00:54:46,800 --> 00:54:50,040 They're really quite a menacing animal, actually, for a crab! 732 00:54:53,320 --> 00:54:55,480 What's wonderful about these crabs 733 00:54:55,480 --> 00:54:58,640 is that they live through a range of scales. 734 00:54:58,640 --> 00:55:00,800 At different times of their lives, 735 00:55:00,800 --> 00:55:03,680 they have a completely different relationship 736 00:55:03,680 --> 00:55:07,080 with the world around them, simply down to their size. 737 00:55:08,360 --> 00:55:11,680 Throughout their lives, robber crabs take on many different forms. 738 00:55:11,680 --> 00:55:14,640 They begin their lives as small larvae, 739 00:55:14,640 --> 00:55:18,440 swept around by the ocean currents, and as they grow, 740 00:55:18,440 --> 00:55:21,520 some of them get swept up onto the beaches of Christmas Island, 741 00:55:21,520 --> 00:55:26,000 where they find a shell, because they are, in fact, hermit crabs. 742 00:55:26,000 --> 00:55:28,640 They live inside their shell for a while, 743 00:55:28,640 --> 00:55:31,400 they continue to grow, and eventually, as adults, 744 00:55:31,400 --> 00:55:34,480 they roam the forests like this chap here. 745 00:55:34,480 --> 00:55:40,600 So these crabs, over that lifespan, inhabit many different worlds. 746 00:55:43,600 --> 00:55:46,040 On land, the adults continue to grow 747 00:55:46,040 --> 00:55:49,920 and now have to support their weight against gravity. 748 00:55:51,520 --> 00:55:54,440 Compared to the smaller crabs whizzing around, 749 00:55:54,440 --> 00:55:57,720 these giants move about much more slowly, 750 00:55:57,720 --> 00:56:00,280 but they also live far longer. 751 00:56:03,160 --> 00:56:06,160 Of all the species of land crab here on Christmas Island, 752 00:56:06,160 --> 00:56:08,640 robber crabs are not only the biggest, 753 00:56:08,640 --> 00:56:10,720 they're also the longest-living. 754 00:56:10,720 --> 00:56:14,040 So this chap here is probably about as old as me, 755 00:56:14,040 --> 00:56:19,600 and he might live to 60, 70, even 80 years old. 756 00:56:21,680 --> 00:56:24,680 Because of the robber crab's overall body size, 757 00:56:24,680 --> 00:56:28,160 its individual cells use less energy 758 00:56:28,160 --> 00:56:30,680 and they run at a slower rate 759 00:56:30,680 --> 00:56:35,600 than the cells of their much smaller, shorter-lived cousins. 760 00:56:38,080 --> 00:56:41,880 The pace of life is slower for robber crabs, 761 00:56:41,880 --> 00:56:44,320 and it's this that's thought to allow them 762 00:56:44,320 --> 00:56:46,840 to live to a ripe old age. 763 00:56:54,760 --> 00:56:58,600 Your size influences every aspect of your life... 764 00:57:01,240 --> 00:57:03,080 ..from the way you were built... 765 00:57:05,680 --> 00:57:07,880 ..to the way you move... 766 00:57:09,600 --> 00:57:12,080 ..and even how long you live. 767 00:57:13,240 --> 00:57:18,680 Your size dictates how you interact with the universal laws of nature. 768 00:57:21,720 --> 00:57:23,640 So there's a minimum size, 769 00:57:23,640 --> 00:57:27,080 which is set ultimately by the size of atoms and molecules, 770 00:57:27,080 --> 00:57:30,440 the fundamental building blocks of the universe. 771 00:57:31,960 --> 00:57:35,440 And there's a maximum size which, certainly on land, 772 00:57:35,440 --> 00:57:38,640 is set by the size and the mass of our planet, 773 00:57:38,640 --> 00:57:43,640 because it's gravity that restricts the emergence of giants. 774 00:57:45,560 --> 00:57:48,800 But within those constraints, evolution has conspired to produce 775 00:57:48,800 --> 00:57:52,760 a huge range in size of animals and plants, 776 00:57:52,760 --> 00:57:57,600 each beautifully adapted to exploit the niches available to them. 777 00:58:00,920 --> 00:58:04,160 Your size influences your form and constriction. 778 00:58:04,160 --> 00:58:07,280 It determines how you experience the world, 779 00:58:07,280 --> 00:58:10,920 and ultimately, how long you have to enjoy it. 780 00:58:35,640 --> 00:58:38,600 Subtitles by Red Bee Media Ltd