1 00:00:02,000 --> 00:00:05,480 The natural world is full of extraordinary animals 2 00:00:05,480 --> 00:00:07,760 with amazing life histories. 3 00:00:08,760 --> 00:00:12,120 Yet certain stories are more intriguing than others. 4 00:00:14,760 --> 00:00:17,760 The mysteries of a butterfly's life cycle 5 00:00:17,760 --> 00:00:21,040 or the strange biology of the Emperor Penguin. 6 00:00:22,040 --> 00:00:26,040 Some of these creatures were surrounded by fantastic myths 7 00:00:26,040 --> 00:00:27,760 and misunderstandings. 8 00:00:29,040 --> 00:00:32,760 Others have only recently revealed their secrets. 9 00:00:33,760 --> 00:00:37,760 These are the creatures that stand out from the crowd - 10 00:00:37,760 --> 00:00:42,040 the curiosities that I find particularly fascinating. 11 00:00:51,040 --> 00:00:55,280 In this programme, we investigate two types of shell. 12 00:00:55,280 --> 00:01:00,040 The shells of tortoises and turtles, that act as body armour, 13 00:01:00,040 --> 00:01:04,040 and the shells of birds that protect their growing chicks 14 00:01:04,040 --> 00:01:05,720 until they hatch. 15 00:01:05,720 --> 00:01:08,760 How did these different shells evolve 16 00:01:08,760 --> 00:01:11,760 and do they offer more than just protection? 17 00:01:20,280 --> 00:01:24,520 Tortoises and turtles have shells that enclose their bodies almost 18 00:01:24,520 --> 00:01:28,280 completely, leaving only the legs and the head sticking out. 19 00:01:31,040 --> 00:01:36,040 It's a unique body plan that has been around for a very long time. 20 00:01:41,040 --> 00:01:44,520 Tortoises and turtles are very ancient creatures. 21 00:01:44,520 --> 00:01:48,760 They have an ancestry which stretches back 200 million years 22 00:01:48,760 --> 00:01:52,040 and individuals like this one from the island of Aldabra 23 00:01:52,040 --> 00:01:56,280 in the Indian Ocean can live for 150 years. 24 00:01:57,760 --> 00:02:01,120 Their shell is obviously very good protection, 25 00:02:01,120 --> 00:02:04,040 but it does bring disadvantages. 26 00:02:04,040 --> 00:02:09,520 It's awkward to get about, hence the legendary speed of the tortoise, 27 00:02:09,520 --> 00:02:14,040 and also, of course, it can make mating quite tricky. 28 00:02:14,040 --> 00:02:18,520 But this is much more than a lifeless, boney box. 29 00:02:18,520 --> 00:02:22,760 This is part of the living skeleton of the animal. 30 00:02:25,040 --> 00:02:28,520 There are several hundred species of tortoise and turtle 31 00:02:28,520 --> 00:02:31,760 with a great variety of beautiful shells. 32 00:02:31,760 --> 00:02:37,040 Tortoises live on land and turtles are almost exclusively aquatic, 33 00:02:37,040 --> 00:02:39,720 but the two groups have much in common. 34 00:02:40,720 --> 00:02:44,040 The shells of both are covered with horny plates 35 00:02:44,040 --> 00:02:47,280 that get bigger as the bone beneath them grows. 36 00:02:47,280 --> 00:02:50,960 In doing so, the plates develop rings of ridges 37 00:02:50,960 --> 00:02:53,880 rather like the growth rings in a tree. 38 00:02:55,520 --> 00:02:57,280 But, for a long time, 39 00:02:57,280 --> 00:03:00,520 no-one really understood how the boney shell itself 40 00:03:00,520 --> 00:03:02,280 came into existence. 41 00:03:03,280 --> 00:03:06,760 The very first fossil tortoises to be discovered 42 00:03:06,760 --> 00:03:12,040 dated from around 215 million years ago. 43 00:03:12,040 --> 00:03:15,280 The strange thing about them was that their shell 44 00:03:15,280 --> 00:03:17,280 was pretty well complete. 45 00:03:17,280 --> 00:03:20,280 In fact, they looked very much like this fossil, 46 00:03:20,280 --> 00:03:23,080 which is 60 million years old. 47 00:03:23,080 --> 00:03:27,040 And its shell, as you can see, is, in fact, fully formed. 48 00:03:27,040 --> 00:03:29,720 But nobody had discovered fossils 49 00:03:29,720 --> 00:03:34,040 which give us any idea as to how this sort of shell 50 00:03:34,040 --> 00:03:35,760 might have developed. 51 00:03:37,760 --> 00:03:42,040 Some suggested that the earliest ancestors of the group 52 00:03:42,040 --> 00:03:47,520 grew thick plates in their skin, much like those of a crocodile, 53 00:03:47,520 --> 00:03:51,280 and that these gradually expanded until they joined up 54 00:03:51,280 --> 00:03:53,520 and formed a complete shell. 55 00:03:57,840 --> 00:04:02,120 But that didn't explain how the shell became a sort of box 56 00:04:02,120 --> 00:04:05,760 into which the animal could retract its legs and neck 57 00:04:05,760 --> 00:04:07,760 when danger threatened. 58 00:04:19,520 --> 00:04:24,040 But if you look closely at the structure of a tortoise's shell 59 00:04:24,040 --> 00:04:27,520 and examine the way it forms as the animal grows, 60 00:04:27,520 --> 00:04:31,520 you can get clues that suggest a very different origin. 61 00:04:36,040 --> 00:04:40,080 This beautiful little spider tortoise from Madagascar 62 00:04:40,080 --> 00:04:43,760 has a shell in two parts, like all tortoises. 63 00:04:43,760 --> 00:04:46,520 The top part is called the carapace 64 00:04:46,520 --> 00:04:50,040 and the lower part is called the plastron. 65 00:04:50,040 --> 00:04:52,440 And the two are fused together. 66 00:04:53,440 --> 00:04:58,520 The surface of the shell is covered with these scutes, 67 00:04:58,520 --> 00:05:02,280 which are made of keratin, the same substance as our own fingernails, 68 00:05:02,280 --> 00:05:05,760 but beneath the scutes, it's solid bone, 69 00:05:05,760 --> 00:05:08,800 and it's formed in the most extraordinary way. 70 00:05:08,800 --> 00:05:12,280 The ribs grow backwards and upwards 71 00:05:12,280 --> 00:05:16,760 in order to cover the pelvis and the shoulders. 72 00:05:16,760 --> 00:05:19,760 So, in a sense, you could say a tortoise's skeleton 73 00:05:19,760 --> 00:05:21,760 has become turned inside out 74 00:05:21,760 --> 00:05:25,200 with the shoulders and the pelvis within the ribs. 75 00:05:28,760 --> 00:05:33,960 A tortoise is therefore, in effect, trapped inside a boney box, 76 00:05:33,960 --> 00:05:36,520 and that, as you might imagine, 77 00:05:36,520 --> 00:05:40,040 makes some things you want to do rather difficult. 78 00:05:40,040 --> 00:05:43,280 Mating, for example, will not be easy. 79 00:05:45,040 --> 00:05:48,040 One thing that helps is the fact that the lower half 80 00:05:48,040 --> 00:05:52,040 of the male shell, the plastron, has become somewhat concave 81 00:05:52,040 --> 00:05:55,520 so that it fits over the female's rounded back. 82 00:05:55,520 --> 00:05:59,280 But, even then, it's obviously not easy. 83 00:06:00,280 --> 00:06:03,520 Imagine making love in a suit of armour. 84 00:06:03,520 --> 00:06:05,760 Not elegant at all. 85 00:06:10,160 --> 00:06:12,520 So, for a very long time, 86 00:06:12,520 --> 00:06:15,520 the origin of the tortoise's shell was a puzzle, 87 00:06:15,520 --> 00:06:19,040 and it remained so even to that greatest of all naturalists, 88 00:06:19,040 --> 00:06:20,840 Charles Darwin. 89 00:06:22,280 --> 00:06:26,520 In the 1830s, Darwin visited the Galapagos Islands 90 00:06:26,520 --> 00:06:30,040 and was very struck by the giant tortoises that lived there. 91 00:06:31,040 --> 00:06:34,040 He was particularly fascinated to see that tortoises 92 00:06:34,040 --> 00:06:37,040 on different islands had differently-shaped shells. 93 00:06:37,040 --> 00:06:41,520 Those from Floreana and Espanola islands were like this - 94 00:06:41,520 --> 00:06:44,520 they had a prominent arch that curved up at the front 95 00:06:44,520 --> 00:06:47,280 so that the neck could reach upwards. 96 00:06:47,280 --> 00:06:52,040 And those from Santiago Island were more dome-shaped at the front. 97 00:06:52,040 --> 00:06:55,280 He watched carefully and noticed that the shell shapes 98 00:06:55,280 --> 00:06:59,040 related to the way that each type of tortoise fed. 99 00:07:01,280 --> 00:07:04,760 Here was evidence that differing environments could, 100 00:07:04,760 --> 00:07:07,040 over a long period of time, 101 00:07:07,040 --> 00:07:09,880 influence the shape of an animal's body. 102 00:07:09,880 --> 00:07:13,520 An observation that was going to be an important clue 103 00:07:13,520 --> 00:07:16,440 in unravelling the mysteries of evolution. 104 00:07:17,520 --> 00:07:22,520 On the islands with humid highlands, tortoises had short necks. 105 00:07:22,520 --> 00:07:25,760 Here, vegetation is low and close to the ground 106 00:07:25,760 --> 00:07:29,400 so the tortoises have no need to reach up to feed 107 00:07:29,400 --> 00:07:31,760 and their shells are dome-shaped. 108 00:07:32,760 --> 00:07:36,040 The neighbouring islands, however, are much drier, 109 00:07:36,040 --> 00:07:38,280 with harder-to-reach leaves. 110 00:07:38,280 --> 00:07:42,040 Darwin noticed that here the tortoises had very long necks 111 00:07:42,040 --> 00:07:44,280 and a peak in the front of the shell 112 00:07:44,280 --> 00:07:48,400 that allowed them to crane their necks upwards to reach leaves. 113 00:07:48,400 --> 00:07:51,040 If all of these tortoises were related, 114 00:07:51,040 --> 00:07:54,040 then they must have changed over time. 115 00:07:54,040 --> 00:07:57,880 The variations in shape enabled different groups of tortoises 116 00:07:57,880 --> 00:08:01,040 to survive in different island environments. 117 00:08:03,040 --> 00:08:06,040 But, sadly, their shells couldn't protect them 118 00:08:06,040 --> 00:08:08,280 from one great predator. 119 00:08:08,280 --> 00:08:12,040 While Darwin was studying Galapagos tortoises, 120 00:08:12,040 --> 00:08:15,280 the seamen had a very different interest in them. 121 00:08:15,280 --> 00:08:20,040 The animals were a convenient source of live food onboard ship 122 00:08:20,040 --> 00:08:23,040 and Darwin watched as they were carried away. 123 00:08:24,040 --> 00:08:27,760 In his journals, he wrote that single vessels had taken away 124 00:08:27,760 --> 00:08:29,520 as many as 700 125 00:08:29,520 --> 00:08:33,280 and that the ship's company of a frigate, some years since, 126 00:08:33,280 --> 00:08:37,280 brought down in one day 200 tortoises to the beach. 127 00:08:38,400 --> 00:08:42,040 Tortoises could survive for a very long time on just water, 128 00:08:42,040 --> 00:08:44,040 with little or no food, 129 00:08:44,040 --> 00:08:46,760 and so could be slaughtered after weeks at sea 130 00:08:46,760 --> 00:08:49,200 to provide welcome fresh meat. 131 00:08:49,200 --> 00:08:52,280 Between 1831 and 1868, 132 00:08:52,280 --> 00:08:58,040 79 whaling vessels collected over 13,000 Galapagos tortoises. 133 00:09:02,680 --> 00:09:06,280 Although the Galapagos tortoise had given clues to Darwin 134 00:09:06,280 --> 00:09:08,760 about the processes of evolution, 135 00:09:08,760 --> 00:09:12,520 the origin of tortoises in general was still a mystery 136 00:09:12,520 --> 00:09:16,520 and it remained that way for a very long time. 137 00:09:17,800 --> 00:09:22,760 Then, in 2008, a new clue emerged. 138 00:09:23,760 --> 00:09:27,880 A 220 million-year-old fossil tortoise, 139 00:09:27,880 --> 00:09:30,760 that predated those with fully formed shells, 140 00:09:30,760 --> 00:09:33,040 was discovered in China. 141 00:09:33,040 --> 00:09:35,760 It was named Odontochelys, 142 00:09:35,760 --> 00:09:40,040 and although it was adult, its shell was still incomplete. 143 00:09:40,040 --> 00:09:43,760 It had the beginnings of a plastron and, on its upper side, 144 00:09:43,760 --> 00:09:47,280 broad ribs that were starting to form the upper shell. 145 00:09:49,120 --> 00:09:54,040 So here was an important stepping stone in the biological journey 146 00:09:54,040 --> 00:09:58,040 that would, in time, lead to tortoises with complete shells. 147 00:09:59,520 --> 00:10:02,040 But this change came at a cost. 148 00:10:06,040 --> 00:10:10,040 When you look inside the shell of a tortoise or a turtle, 149 00:10:10,040 --> 00:10:13,040 it seems remarkably empty. 150 00:10:13,040 --> 00:10:17,720 Here are the leg bones, but where are the ribs? 151 00:10:17,720 --> 00:10:22,040 And the answer is that they have become amalgamated 152 00:10:22,040 --> 00:10:24,520 with the top of the shell 153 00:10:24,520 --> 00:10:27,040 to give it great strength. 154 00:10:28,040 --> 00:10:30,520 But that brings a problem. 155 00:10:30,520 --> 00:10:36,280 We use our ribs to help us make the lungs work like bellows. 156 00:10:37,760 --> 00:10:39,760 Breathing in and out. 157 00:10:39,760 --> 00:10:43,520 But a tortoise and a turtle can't do that. 158 00:10:43,520 --> 00:10:47,520 Instead, they have a kind of muscular sling 159 00:10:47,520 --> 00:10:50,520 which gives a kind of internal diaphragm, 160 00:10:50,520 --> 00:10:53,520 which moves the lungs in and out. 161 00:10:53,520 --> 00:10:56,040 Rather awkward, you might think, 162 00:10:56,040 --> 00:10:59,040 but a small price to pay for the great strength 163 00:10:59,040 --> 00:11:02,280 that the ribs now give to the top of the shell. 164 00:11:05,200 --> 00:11:10,240 The tortoise's shell is a wonderful example of a multipurpose structure. 165 00:11:11,240 --> 00:11:14,440 It acts as an external protection... 166 00:11:15,440 --> 00:11:19,760 ..but also as a solar panel collecting heat from the sun. 167 00:11:22,040 --> 00:11:24,120 Tortoises are cold-blooded, 168 00:11:24,120 --> 00:11:28,040 so that ability is very valuable and important to them. 169 00:11:30,280 --> 00:11:34,040 A turtle's shell obviously gives it very good protection, 170 00:11:34,040 --> 00:11:38,280 but it's not just inert bone, it's more than that. 171 00:11:38,280 --> 00:11:42,520 On the underside of this, there are great areas 172 00:11:42,520 --> 00:11:46,040 which act as reservoirs for important minerals. 173 00:11:46,040 --> 00:11:49,520 Sodium, potassium, magnesium and phosphate. 174 00:11:49,520 --> 00:11:53,760 And when those minerals are needed for the workings of the body, 175 00:11:53,760 --> 00:11:56,520 they can be withdrawn from those reservoirs 176 00:11:56,520 --> 00:11:59,280 with the help of blood and the body fluids. 177 00:12:01,280 --> 00:12:04,040 The shell's natural chemical reservoir 178 00:12:04,040 --> 00:12:07,320 is particularly valuable for sea turtles. 179 00:12:07,320 --> 00:12:11,040 When they dive, they hold their breath for long periods 180 00:12:11,040 --> 00:12:15,280 without suffering from the dangerous build-up of toxic chemicals. 181 00:12:15,280 --> 00:12:19,320 This is partly because their shell can store and release 182 00:12:19,320 --> 00:12:23,560 the minerals that prevent damage to the body tissues. 183 00:12:24,560 --> 00:12:27,520 The shell is also a survival tool. 184 00:12:28,520 --> 00:12:33,040 he North American freshwater painted turtle is forced, in winter, 185 00:12:33,040 --> 00:12:37,520 to spend months underwater because their pools are covered by ice. 186 00:12:38,520 --> 00:12:42,040 The oxygen in their body then falls to very low levels 187 00:12:42,040 --> 00:12:46,760 and that leads to lactic acid accumulating in their tissues. 188 00:12:46,760 --> 00:12:50,280 But their shell then releases carbonates 189 00:12:50,280 --> 00:12:54,760 which neutralise the acid and store it out of harm's way. 190 00:12:54,760 --> 00:13:00,280 As a result, turtles can survive submerged in oxygen-poor water 191 00:13:00,280 --> 00:13:03,040 for up to three years. 192 00:13:05,040 --> 00:13:10,040 So, the shell, in fact, provides much more than physical protection. 193 00:13:11,040 --> 00:13:16,040 Understanding its complex evolutionary history has taken time 194 00:13:16,040 --> 00:13:19,040 but, just recently, a chance discovery 195 00:13:19,040 --> 00:13:21,280 has helped complete the picture. 196 00:13:22,760 --> 00:13:25,280 In the year 2013, 197 00:13:25,280 --> 00:13:29,520 scientists working in the Denver Museum of Nature and Science 198 00:13:29,520 --> 00:13:34,520 re-examined a fossil, a 260 million-year-old fossil, 199 00:13:34,520 --> 00:13:37,520 that had been dug up in the previous century, 200 00:13:37,520 --> 00:13:39,760 given the name Eunotosaurus, 201 00:13:39,760 --> 00:13:43,520 and then put in the storage vaults and forgotten. 202 00:13:43,520 --> 00:13:46,520 But when they came to look at it, and this is it, 203 00:13:46,520 --> 00:13:49,800 it had nine flattened ribs 204 00:13:49,800 --> 00:13:52,760 and elongated vertebrae, 205 00:13:52,760 --> 00:13:55,520 just like the first tortoises. 206 00:13:55,520 --> 00:13:58,320 A missing link had been found. 207 00:13:58,320 --> 00:14:02,520 A creature that linked the early, land-living reptiles 208 00:14:02,520 --> 00:14:05,520 with the first shelled tortoises. 209 00:14:08,280 --> 00:14:11,040 This is an artist's impression of it, 210 00:14:11,040 --> 00:14:14,280 with the very beginnings of a shell on its back. 211 00:14:16,040 --> 00:14:20,040 So now the story of how the tortoise's shell developed 212 00:14:20,040 --> 00:14:21,760 can be told. 213 00:14:27,520 --> 00:14:30,040 The ribs expanded outwards, 214 00:14:30,040 --> 00:14:33,160 enclosing the hip and the shoulder joints. 215 00:14:35,760 --> 00:14:40,520 They enlarged and fused with other bones beneath the shell. 216 00:14:45,040 --> 00:14:48,040 Eventually, they widened and came together... 217 00:14:50,520 --> 00:14:53,760 ..and the scales enlarged to form a coat of horn 218 00:14:53,760 --> 00:14:56,040 on the surface of the box. 219 00:15:00,520 --> 00:15:05,640 Recent scientific research has suggested that the ancestors 220 00:15:05,640 --> 00:15:08,520 of the tortoises first developed a shell 221 00:15:08,520 --> 00:15:11,280 as a way of strengthening their bodies 222 00:15:11,280 --> 00:15:14,280 to help them dig into the ground in search of food. 223 00:15:14,280 --> 00:15:18,520 But since then, of course, it's become primarily protective 224 00:15:18,520 --> 00:15:21,760 and very big and heavy in some instances. 225 00:15:21,760 --> 00:15:24,520 That may have had another effect. 226 00:15:24,520 --> 00:15:28,040 That may have imposed upon tortoises 227 00:15:28,040 --> 00:15:31,040 a slow-moving, low-energy life 228 00:15:31,040 --> 00:15:36,040 which, in the end, has enabled them to live to such great old age, 229 00:15:36,040 --> 00:15:38,040 as indeed they do. 230 00:15:44,560 --> 00:15:49,520 Next, we look at the shells that birds provide for their eggs, 231 00:15:49,520 --> 00:15:54,040 which vary so extraordinarily in size and appearance. 232 00:15:55,960 --> 00:16:00,760 Eggs have been considered by people and cultures all over the world 233 00:16:00,760 --> 00:16:03,280 to be wondrous, magical objects. 234 00:16:04,280 --> 00:16:09,520 Myths tell of how the world was created from a cosmic egg 235 00:16:09,520 --> 00:16:13,040 which contained all the material needed for life. 236 00:16:13,040 --> 00:16:17,760 And the egg's rounded shape is fundamental to that idea. 237 00:16:21,760 --> 00:16:24,520 The shell has a complex structure. 238 00:16:27,040 --> 00:16:31,280 On the one hand, it must be strong enough to protect a growing chick. 239 00:16:32,280 --> 00:16:36,760 And on the other, fragile enough to allow the chick to break out 240 00:16:36,760 --> 00:16:38,520 when the time comes. 241 00:16:40,040 --> 00:16:42,600 That's true of all bird eggs, 242 00:16:42,600 --> 00:16:45,760 including those belonging to the largest. 243 00:16:48,520 --> 00:16:52,040 The ostrich is a truly extraordinary bird. 244 00:16:52,040 --> 00:16:55,760 It has wings and feathers but it can't fly. 245 00:16:55,760 --> 00:17:00,040 It has massive feet, somewhat like those of a camel. 246 00:17:00,040 --> 00:17:02,040 It has a digestion 247 00:17:02,040 --> 00:17:05,920 that some people used to think was strong enough to dissolve iron. 248 00:17:05,920 --> 00:17:11,040 And it lays the biggest egg of any living bird. 249 00:17:12,040 --> 00:17:17,040 It weighs 1.5 kilos - the equivalent of two dozen chicken eggs. 250 00:17:17,040 --> 00:17:20,040 The shell is three millimetres thick 251 00:17:20,040 --> 00:17:24,040 and so strong that it's said I could stand on this 252 00:17:24,040 --> 00:17:27,040 without it breaking, but I'm not going to try. 253 00:17:27,040 --> 00:17:29,760 And it takes 40 days 254 00:17:29,760 --> 00:17:33,520 for a chick developing inside this fortress 255 00:17:33,520 --> 00:17:35,960 to break out. 256 00:17:39,040 --> 00:17:43,480 So how does a chick grow while encased by shell, 257 00:17:43,480 --> 00:17:47,760 since it needs to breathe, feed and stay warm? 258 00:17:50,520 --> 00:17:54,160 Amazingly, it manages to do all these things 259 00:17:54,160 --> 00:17:56,280 right from the very start. 260 00:17:59,040 --> 00:18:03,040 A bird embryo begins as a microscopic egg cell, 261 00:18:03,040 --> 00:18:06,600 released on the surface of a globule of yolk. 262 00:18:08,600 --> 00:18:13,280 In the oviduct, it gathers the moist albumen, or egg white. 263 00:18:15,040 --> 00:18:19,040 And it's plumped up, ready to receive the shell. 264 00:18:20,640 --> 00:18:24,520 Layers of calcium carbonate are sprayed onto the egg membrane 265 00:18:24,520 --> 00:18:27,160 and they form calcite crystals. 266 00:18:27,160 --> 00:18:33,040 A final layer of cuticle is added and the egg is complete. 267 00:18:34,760 --> 00:18:36,760 Eventually, it emerges. 268 00:18:38,040 --> 00:18:41,280 And what an astonishing and beautiful thing it is. 269 00:18:46,880 --> 00:18:50,040 Birds of different species produce characteristically 270 00:18:50,040 --> 00:18:51,760 different shaped eggs. 271 00:18:51,760 --> 00:18:55,040 They range from the almost spherical eggs laid by owls, 272 00:18:55,040 --> 00:18:59,040 to eggs pointed at one end of razorbills and guillemots. 273 00:18:59,040 --> 00:19:03,280 A spherical egg has the smallest surface-to-volume ratio 274 00:19:03,280 --> 00:19:06,200 so will lose the least heat for its size. 275 00:19:06,200 --> 00:19:10,760 However, such an egg only presents a small area of shell to be warmed 276 00:19:10,760 --> 00:19:12,600 by an incubating bird. 277 00:19:12,600 --> 00:19:15,720 A longer, more egg-shaped egg, 278 00:19:15,720 --> 00:19:20,080 may be a good compromise between losing heat and absorbing it. 279 00:19:22,880 --> 00:19:25,880 It used to be thought that a guillemot's egg 280 00:19:25,880 --> 00:19:28,680 was pointed at one end because it prevented it 281 00:19:28,680 --> 00:19:31,320 from rolling off a narrow ledge. 282 00:19:31,320 --> 00:19:34,760 But such a shape makes it easier to incubate. 283 00:19:35,760 --> 00:19:38,760 The egg fits snugly beneath the bird's body, 284 00:19:38,760 --> 00:19:41,520 with its sharper end pointing towards the tail 285 00:19:41,520 --> 00:19:45,200 and the fatter end pressed against the bird's warm stomach. 286 00:19:46,200 --> 00:19:48,760 If a nest holds several eggs, 287 00:19:48,760 --> 00:19:51,520 a pointed shape doesn't work as well. 288 00:19:53,040 --> 00:19:56,760 A more rounded one enables them to be packed closely together 289 00:19:56,760 --> 00:20:00,280 and presents a large incubating surface overall. 290 00:20:02,520 --> 00:20:07,440 So the shapes of eggs vary to allow effective incubation, 291 00:20:07,440 --> 00:20:10,280 no matter where the nest is located. 292 00:20:15,760 --> 00:20:21,280 But eggs can get too hot and need protection from the heat of the sun. 293 00:20:21,280 --> 00:20:25,040 Once again, the nature of the shell can help. 294 00:20:25,040 --> 00:20:27,640 Its colour can serve as a sun block 295 00:20:27,640 --> 00:20:30,760 and prevent too much damaging ultraviolet light 296 00:20:30,760 --> 00:20:33,640 from reaching the chick inside. 297 00:20:35,920 --> 00:20:39,520 Emus' eggs are a striking dark green colour. 298 00:20:40,520 --> 00:20:45,520 This, doubtless, camouflages them as they lie on the ground among leaves. 299 00:20:47,520 --> 00:20:52,040 But it also prevents too much sunlight from penetrating the shell. 300 00:20:58,040 --> 00:21:02,040 The great range of colours and patterns laid by different birds 301 00:21:02,040 --> 00:21:05,520 are all derived from just two pigments. 302 00:21:07,040 --> 00:21:10,520 They may help to camouflage and protect the chick inside, 303 00:21:10,520 --> 00:21:13,760 but sometimes that variety has had the opposite effect 304 00:21:13,760 --> 00:21:16,760 and made them much sought after by collectors. 305 00:21:17,760 --> 00:21:22,040 In the 19th century, an amateur naturalist and egg collector, 306 00:21:22,040 --> 00:21:26,520 Major Charles Emil Bendire, serving in the United States Army, 307 00:21:26,520 --> 00:21:29,280 went to extreme lengths to collect them. 308 00:21:30,280 --> 00:21:34,040 In 1872, while on Army patrol in Arizona, 309 00:21:34,040 --> 00:21:37,040 he spotted a zone-tailed hawk's nest, 310 00:21:37,040 --> 00:21:39,520 whose eggs he didn't have. 311 00:21:39,520 --> 00:21:42,280 Leaving his troops, he climbed the tree 312 00:21:42,280 --> 00:21:45,040 while keeping an eye out for hostile Indians, 313 00:21:45,040 --> 00:21:48,280 but just as he took an egg from the nest, he was attacked. 314 00:21:48,280 --> 00:21:50,520 Quickly, he jumped onto his horse, 315 00:21:50,520 --> 00:21:53,280 putting the egg into his mouth for safekeeping. 316 00:21:53,280 --> 00:21:56,760 He reached camp alive, but the effort and stress 317 00:21:56,760 --> 00:22:01,520 of not biting the egg caused his jaw to lock and swell. 318 00:22:01,520 --> 00:22:05,280 It took several men and a broken tooth 319 00:22:05,280 --> 00:22:09,760 to extract it from his mouth, but luckily it was still intact. 320 00:22:10,760 --> 00:22:14,280 Eventually, Charles Bendire assembled an immense collection 321 00:22:14,280 --> 00:22:17,520 of over 8,000 eggs and donated them all, 322 00:22:17,520 --> 00:22:21,520 including the hawk's egg, to the Smithsonian Institution. 323 00:22:24,280 --> 00:22:27,760 Egg collectors were obsessed with the patterns and colours 324 00:22:27,760 --> 00:22:31,520 on the surface of eggshells and thought them very beautiful, 325 00:22:31,520 --> 00:22:33,440 and so they are. 326 00:22:34,760 --> 00:22:38,760 But these characteristics have an important function. 327 00:22:41,040 --> 00:22:46,280 Ostriches use communal nests which may contain as many as 40 eggs. 328 00:22:46,280 --> 00:22:49,040 They're incubated by the dominant female 329 00:22:49,040 --> 00:22:51,760 who is able to identify her own eggs 330 00:22:51,760 --> 00:22:54,520 because she sees them in extraordinary detail. 331 00:22:56,280 --> 00:22:59,760 Eggshells are covered in tiny pores. 332 00:22:59,760 --> 00:23:03,400 A small wren-sized egg, like this one, 333 00:23:03,400 --> 00:23:05,280 may have about 200. 334 00:23:05,280 --> 00:23:08,240 A chicken's egg, about 2,000. 335 00:23:08,240 --> 00:23:12,960 And ostrich eggs, like these two, have more than 30,000. 336 00:23:12,960 --> 00:23:17,040 Each tiny pore is open to let oxygen in 337 00:23:17,040 --> 00:23:20,040 and carbon dioxide and waste product out. 338 00:23:20,040 --> 00:23:23,360 And that enables the growing chick to breathe. 339 00:23:23,360 --> 00:23:26,960 These are the chick's lifelines to the outside world. 340 00:23:27,960 --> 00:23:30,520 This particular empty ostrich shell 341 00:23:30,520 --> 00:23:33,520 has been filled with water that contained a blue dye, 342 00:23:33,520 --> 00:23:35,920 and that enables you to see the pores 343 00:23:35,920 --> 00:23:38,040 that cover the shell's surface. 344 00:23:38,040 --> 00:23:41,280 It's thought that a female ostrich can recognise 345 00:23:41,280 --> 00:23:45,520 her own individual pore pattern and, in a communal nest, 346 00:23:45,520 --> 00:23:48,760 give preference to her own eggs during incubation. 347 00:23:54,040 --> 00:23:58,040 The dominant female lays her eggs towards the centre of the nest 348 00:23:58,040 --> 00:24:01,760 and other females add theirs, so creating a large clutch. 349 00:24:04,040 --> 00:24:08,520 But only about 20 eggs can be successfully incubated at one time. 350 00:24:10,040 --> 00:24:14,040 So the dominant female ensures that her own are in the centre 351 00:24:14,040 --> 00:24:16,760 and in no danger of being pushed out. 352 00:24:20,040 --> 00:24:23,040 And it's been shown that it's the pore pattern 353 00:24:23,040 --> 00:24:25,040 that enables her to do that. 354 00:24:33,160 --> 00:24:37,400 It takes about 40 days for the eggs to hatch, 355 00:24:37,400 --> 00:24:41,280 and, of course, they have to be strong enough to carry the weight 356 00:24:41,280 --> 00:24:45,520 of the incubating birds, which take it in turn to sit on the nest. 357 00:24:45,520 --> 00:24:49,760 But then, when the time comes, they also have to be fragile enough 358 00:24:49,760 --> 00:24:53,320 to allow the chick inside to break out. 359 00:24:57,040 --> 00:25:01,040 An egg shell appears to need conflicting characteristics. 360 00:25:01,040 --> 00:25:04,040 It must be robust enough to protect the chick 361 00:25:04,040 --> 00:25:08,040 but fragile enough to allow it to escape when the time comes. 362 00:25:08,040 --> 00:25:10,760 And, indeed, it's enormously strong, 363 00:25:10,760 --> 00:25:13,720 so that even predators find it hard to break. 364 00:25:16,280 --> 00:25:19,920 The Egyptian vulture, however, has learned how to do it. 365 00:25:20,920 --> 00:25:24,160 You have to give it a sharp blow to puncture the surface. 366 00:25:27,520 --> 00:25:31,040 And once you've done that, its strength is largely lost. 367 00:25:40,520 --> 00:25:44,000 So how does a chick eventually escape? 368 00:25:45,000 --> 00:25:48,280 Well, just at the right time, the shell weakens. 369 00:25:49,280 --> 00:25:52,760 When the egg is first laid, the shell is as hard as china. 370 00:25:54,040 --> 00:25:58,280 But as the chick develops, the shell releases some of its calcium, 371 00:25:58,280 --> 00:26:01,760 which strengthens the chick's growing bones. 372 00:26:03,280 --> 00:26:08,040 So the shell becomes thinner as the chick grows stronger, 373 00:26:08,040 --> 00:26:10,520 until it finally becomes brittle 374 00:26:10,520 --> 00:26:14,880 and the chick can at last peck or kick its way out. 375 00:26:18,440 --> 00:26:23,040 So the egg shell has many complex ways of first protecting 376 00:26:23,040 --> 00:26:27,040 and then, finally, releasing its precious inmate. 377 00:26:35,520 --> 00:26:39,040 The shell of an egg gives it great protection 378 00:26:39,040 --> 00:26:43,520 but its shape and beauty has also led to its destruction. 379 00:26:43,520 --> 00:26:47,280 In the 19th century, when amateur naturalist Charles Bendire 380 00:26:47,280 --> 00:26:49,280 was avidly collecting eggs, 381 00:26:49,280 --> 00:26:52,280 there was a huge craze for eggs that were coloured. 382 00:26:52,280 --> 00:26:55,760 Complete clutches were often taken from nests. 383 00:26:55,760 --> 00:27:00,760 Today, the law prohibits this, and collecting is severely punished. 384 00:27:00,760 --> 00:27:03,760 However, some of these old collections 385 00:27:03,760 --> 00:27:07,040 are now helping conserve living birds. 386 00:27:07,040 --> 00:27:10,760 Eggs of several species from as far back as 1850 387 00:27:10,760 --> 00:27:14,040 reveal that the shells have become thinner 388 00:27:14,040 --> 00:27:17,040 since the start of the Industrial Revolution. 389 00:27:17,040 --> 00:27:20,520 This was hard evidence that helped to ban the use 390 00:27:20,520 --> 00:27:25,040 of certain agricultural chemicals that were clearly responsible. 391 00:27:25,040 --> 00:27:28,640 Since then, the populations of these birds have recovered. 392 00:27:29,640 --> 00:27:32,720 There's still much to learn about eggs and their shells, 393 00:27:32,720 --> 00:27:36,520 so the next time you take a chicken's egg for your breakfast, 394 00:27:36,520 --> 00:27:38,760 marvel at its shell. 395 00:27:38,760 --> 00:27:42,040 Here is an extraordinary natural structure 396 00:27:42,040 --> 00:27:45,040 that's very much more than just packaging. 397 00:27:48,040 --> 00:27:51,280 The shells of tortoises and eggs offer great protection, 398 00:27:51,280 --> 00:27:55,040 but being able to adapt to different lifestyles and situations 399 00:27:55,040 --> 00:27:58,040 has been the real secret to their success.