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July 10th

July 10th

Birds also have several adaptations which help them fly by reducing their weight. Bird’s bones are hollow, unlike the solid bones of other vertebrates, and are supported by lightweight interior struts that help make them strong enough to support the bird’s body during flight and landing. (This is also why you are cautioned to not feed your dogs chicken bones — their hollow structure makes them splinter into sharp fragments, unlike mammal bones which dogs can gnaw down.) Birds also have a beak instead of the heavy bones and teeth of a normal vertebrate jaw.

July 9th

July 9th

Amphibians and most reptiles have a three-chambered heart, in which blood returning to the heart from the body and from the lungs mixes together. Birds, on the other hand, have a four-chambered heart in which the blood carrying oxygen from the lungs and the deoxygenated blood returning from the body are kept separate. In addition, birds have a respiratory system which includes air sacs in addition to lungs. The birds respiratory pattern allows air to flow from these sacs through the lungs, giving the bird the ability to absorb oxygen almost continuously. Both of these adaptations allow birds to maintain the high levels of energy use necessary to be endotherms and to fly.

July 8th

July 8th

You’ve seen the old videos of early attempts at flight, such as the clumsy bicycles attached to flapping wings which bounce around before finally crashing, never leaving the ground. Successful flight takes a lot of energy. Knowing this fact is the key to understanding the features of the birds. As we noted before, they are endothermic. Maintaining a high body heats supports the birds high metabolism, which is necessary to produce the energy for flight. To support these high energy needs, birds have adaptations in their circulatory and respiratory systems that support their metabolic needs. We’ll take a look at those next.

July 7th

July 7th

The amniote egg is a trait that reptiles share with our next group of animals, the birds. Birds have traits reminiscent of reptiles in many ways — in addition to laying eggs, they have scales on their legs, and their feathers resemble highly modified scales. In most other ways, however, birds are of course unique. Birds are endotherms (warm-blooded), have forelimbs modified as wings, and have developed beaks rather than toothed jaws.

July 6th

July 6th

Reptiles typically lay these eggs in buried nests — one of the most endearing examples is the thousand-mile journeys of sea turtles to lay their eggs on the same beaches, generation after generation. In some species, such as the sea turtles, these eggs are left on their own once lain; the mother does not care for them. Some reptiles, however, care for their nests and their young once hatched. Crocodiles are rightly regarded as fearsome, but are actually one the most doting examples of reptiles that care for their young.

Interestingly, many reptiles are highly dependent on the temperature of the nest during incubation for sex development. In some species, incubating the eggs at a lower temperature causes the embryos to develop as males and higher temperatures generate females, and in others the temperatures are reversed.

July 5th

July 5th

The reptilian egg is surrounded by a waterproof shell, which retains moisture inside the egg but allows for some gas exchange with the surrounding environment. Unlike the rigid shell of bird eggs, reptilian eggshells are more leathery and flexible. Inside the egg is a yolk sac attached to the embryo, which provides nourishment as the embryonic reptile grows. The embryo itself is surrounded by an amniotic membrane containing fluid that protects and cushions the developing animal. Two additional structures, the chorion and the allantois, help to carry out gas exchange and to store wastes.

July 4th

July 4th

Many of the reptilian features we have seen have given the reptiles the ability to be more independent of water than their amphibian cousins. There is one remaining reptilian feature that especially helps with this terrestrial lifestyle: the reptilian egg.

Amphibians, you will recall, used thin-skinned, jelly-like eggs — just imagine the mass of frog eggs attached to a stick in the water from which tadpoles will emerge. If these eggs were removed from their moist environment, they would dry up. Reptiles, however, lay eggs on land. The reptilian egg is surrounded by a thick, waterproof shell that retains the egg’s moisture. Inside the egg are several functional structures to support the developing embryo. One of these, the amnion, gives the egg its name: the amniotic egg.

 

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