why do some animals lay eggs and some have live births?

Question

Why can't all animals be born one way? I always wondered why some animals give birth to a live baby while others lay eggs then they hatch?

Answer 1

In extremely simplified terms, the birds, turtles, crocodilians, amphibians and mammals are all derived from a common ancestor that was an egg layer. That ancestor can be traced back to the fishes, also egg layers. The sexually-reproducing invertebrates (from which the vertebrates descended) also produced eggs.

External fertilization can be considered the ancestral characteristic and results in the formation of an egg that develops outside the body. From there, variations on the common theme have arisen, including animals that fertilise internally but still develop their eggs externally. Only in the mammals (and some of the sharks) has the foetus being retained within the mother's body and nurtured there until birth.

When the first aquatic tetrapod (a salamander-like animal) ventured onto land to become our terrestrial ancestor, a number of terrestrial adaptations were required for it to survive and reproduce successfully. One, as mentioned, was that to internal fertilisation (because there is obviously much less water on land to act as a medium for sperm to meet egg than in the ocean). Another was that the egg laid had to be protected in some way from drying out (in the form of a leathery membrane, or shell).

The benefits of laying an egg and having it developed outside the body include, as was mentioned, not having to carry around the extra weight. But it also comes at the energitic cost of having to lay the egg with a good supply of yolk and a protective shell. Contrarily, mammals have opted to carry the extra load (which is handy if you need to move around a bit to forage, rather than constantly guarding your clutch eggs)...probably finding this trait sufficiently benefial to their survival to outweigh the inconvenience of gestation.

Answer 2

Animals That Come From Eggs

Answer 3

All embryoes must be nourished prenatally, whether inside an eggshell or via a placenta. The eggs of warm-blooded ainmals (mainly birds) must be kept warm, but those of cold-blooded animals can simply be left where they're laid. Any embryo that develops inside an egg must emerge when the supply of nutrition is consumed, but a placenta enables the embryo to develop more fully before birth, because it (the placenta) provides a steady supply of nutrition as long as it's needed. Only mammals, and not even all of THEM, have placentas. (The platypus, which has a bill and lays eggs, is classed as a mammal rather than a bird because it has four legs and suckles its young.) Marsupials (e. g. kangaroos) bear live young but have no placentas. The young develop in eggs inside the mother's uterus and are born when that source of nutrition is exhausted, but they'e still little more than fetuses at that point--hence the need for the pouch. The newborns of placental mammals, from sloths on up, may be too helpless to survive on their own, but they are, essentially, fully developed animals.

Answer 4

Animals that have live births raise their young in "eggs" of sorts. The amniotic sac serves the function of an egg inside the mother's body.

The primary differences are that animals that have live young tend to have much longer gestation periods and give birth to fewer young than those who lay eggs. Live young are better able to survive on their own, but an egg-layer can gestate and produce many young at the same time.

Answer 5

Well it shouldn't even be possible for a bird to impregnate a dog because they are to completely different species and it would be like that with most egg layers and animals that have live babies

Answer 6

Obviously, God designed birds to lay eggs because that would be difficult for birds to fly with such added weight.

Answer 7

That gets heavily into evolution and natural selection. Each method has different advantages and disadvantages.

Fish, insects, amphibians, reptiles, and birds lay eggs. These generally evolved before mammals. Certainly, fish and insects do not have a lot of brain power or ability to care for young -- build shelters, collect food, guard a territory, etc. Most typically do not live that long. Thus, being able to lay eggs and leave the young is fairly important. It also allows a lot of young. Quantify over quality is the typical fish and insect survival method. The same is true of amphibians and most reptiles.

Now birds usually care for their young, but they also need to collect food and carrying large unborn young would make this difficult - especially since most fly. Thus, it is an advantage to lay eggs quickly (i.e. less time not being able to feed yourself) and to be able to return to feeding while your young develop outside the body. Pair bonding amongst many specifies allows the male and female to take turns in caring for the eggs. This requires more brain power, and explains why most birds hatch with extremely large heads and small bodies. The parents remain to feed and protect them while the rest of the body develops.

Since the brain is the most complex (and fragile) of the organs, it is more difficult to get it to develop outside of the body. It is more suseptable to injury, temperature, etc. so it does not develop much after birth. It may get larger, but it is hard for complex structures to grow after birth/hatching. Thus, most species try developing as large a brain as possible -- allowing the rest of the body to catch up after birth. However, if you aren't going to care for your young, you still need the rest of the body to develop or else it can't fend for itself.

Mammals can live birth. This has a lot of advanges, but requires a significant shift in social behaviors and physical capabilities of the animal. The longer gestation period means a more fully developed immune system, brain, and body of the young. However, it means that the female has to carry around the young longer. This typically means fewer young than egg layers, social groups to feed/protect the females while pregnant, making burrows or other shelter to rest, saving food/kills or gaining extreme weight, etc. This typically mean more brain power, but live birthing allows development of larger brains.

In short, earlier species did not life birth because they didn't need it. A fish does not need a lot of brain power since it does not care for its young or have complex social behaviors, so its young do not need to develop large brains. Mammals evolved with more powerful brains which allowed the more complex social networks and other survival methods necessary to carry and care for young for longer periods of time -- which allowed the young to develop with larger brains, which allowed for even more complex social networks and survival methods, etc.