Egg laying is the most common form of embryonic development for animal reproduction. Why not mammals?

Question

I am curious as to why mammals (apart from 2 exceptions) aren't born in eggs. Is this evolutionary? What are the advantages/disadvantages of developing in an egg or being born live?

Thanks a bunch.

Answer 1

the main answer to this is protection of the next generation. it probably started when some lizards started caring for their young. after this they evolved glands in the skin which exuded a milky substance which lengthened the time they were kept underground and protected. eventually the eggs would have started remaining in the animal until it hatched.

then this will have evolved into the present day form of pregnancy in mammals where two cells combine and never develop the outer structure that would be the egg. the membranes still develop however showing how the womb has taken on the role of the shell of an egg.

Answer 2

Eggs are considered a more "primitive" form of development, You need to spend time incubating them, you don't know if they are actually fertile, and they are prone to predation. Eggs also need a certain level of humidity and a lot of problems can happen with hatching. There is also a set amount of nutrients than a egg can have. Just because an egg is in your nest, doesn't mean you laid it- look at brown headed cowbirds for an example.
The next stage is animals that have live young but they are in eggs with little nutritional support from the mother. There are actually eggs inside of the female that hatch inside the female. The young are born and some absorb the eggs, others pass the egg shells as well.
Then you have live young with parental support and various types of placenta (from basically none to humans- one of the most advanced forms). Some reptiles even have live young.
The basic components are still the same. Both have an amnion, with the fetus growing inside.
Live birth pros- more nutritional support, mother can stay active, you know you are pregnant, you know that they are your young.

Answer 3

Let's look at mesozoic mammals.

1. Arboreal insectivore. Can't fly. Having to come back to a nest all the time to keep it warm would limit its ability to gather food... while it was gathering food, the eggs would get cold.
Birds get around that by having both parents sit on the eggs, and by being able to fly.

2. Aquatic predator (sort of like a platypus!). Does a platypus need to dry itself off before sitting on its eggs? Probably. Again, time spent not keeping the eggs warm while not hunting up food.

The real mystery is why birds haven't become placental -- especially those that live in adverse climates, such as penguins.

Answer 4

As you noted, the earliest mammals, and some of the living species, still do lay eggs as part of their reproduction.

The other mammals have evolved adaptations that allow for embryonic development inside the female.

As with most traits, there are both advantages and disadvantages to the system.

One of the major advantages is that the embryo is protected inside the female. There is no need to build a nest, or hide eggs, or find a suitable laying habitat. As long as the mother is alive, the baby is safe and nourished.

This can also be a disadvantage. It invests a lot of metabolic energy into that set of offspring, and requires a host of metabolic and anatomical investments from the female towards the development of those embryos. There's also the fact that the mother has to carry the young around until they're developed enough to be born. One of the extreme examples of this is the human female, who is nearly incapacitated near the end of term by the weight and metabolic costs of her offspring.

Also, if the female is killed or injured, the offspring is also in immediate danger, while egg layers can potentially still successfully reproduce after death (fish do this a lot).

One of the huge advantages of bearing live young is the stable growth environment for the developing embryo. With their endothermic bodies, mammalian mothers can provide a steady, reliable internal temperature and composition.

People tend to think of mammals (including humans) as more 'advanced' animals, and assume that we must have the most DNA to encode all of our awesome advancedness. However, the opposite is true. Critters like frogs and fish can have up to hundreds of times more DNA than we do. This is because they need more complex instructions to develop from an embryo.

The mammalian DNA can be much simpler, because of the known environment. The instructions can simply be the equivalent of 'cook in the standard fashion for nine months', while a frog's DNA has to code in different proteins and enzymes that are used in different situations, so that their instructions look more like 'if the temperature is between 15 C and 20 C for the first three days of development, use enzyme A for encoding limb development. if temperature is between 20 C and 25 C for the first three days of development, use enzyme B for encoding limb development. if temperature is between 25 C and 30 C for the first three days of development, use enzyme B to substitute for enzyme C in the development of the digestive system, and use enzyme C for encoding limb development...' and so on.

This simplified instruction system means there are fewer errors in development - fewer chances for mutations or other mistakes to occur, and the most efficient method can be used in each stage of development. There's no need to switch the less efficient enzyme B for enzyme C because the environmental temperature is too high. The stable environment of a mommy mammal makes for a higher degree of success in making healthy, non-deformed babies.

This is something that tends to be overlooked even by many geneticists - especially those working on cloning. Our DNA makes a lot of assumptions about the environment the developing embryo is going to be in. Changes or errors in that environment are going to have a major effect on the development of the embryo. This is not a fault in the DNA, or their replication. The DNA instructions simply assumed that the embryo was going to be in a nice amniotic fluid bath at 37 C, and didn't bother to code those instructions into the recipe, just like most cooking recipes don't bother to include that you should be cooking in a predominantly nitrogen, oxygen rich atmosphere at about 1 barr pressure. It's just assumed.

Answer 5

Both the temperature and evolutionary arguements are good. It might also have to do with the gestation period...9 months would seem to be a very long time to sit on an egg, and expect to be fed and watered! What is the maximum period for an egg to hatch? Of course length of time to develop is also related to evolutionary complexity...I think....

Answer 6

Main disadvantages to eggs is that (certainly in birds) the adult is prone to attack whilst brooding, or else has to fly off and leave the eggs open to attack.
Also, the eggs have to be kept at the right temperature artificially, at least live young are constantly protected and at the correct temperature.

Answer 7

The mammal is able to carry and care for the foetus throughout the gestation period. This care is then carried on when suckling(also unique to mammals)

Answer 8

I thought that everything laid/had an egg. But, because of Easter, women like to hide their eggs in the womb until they hatch.

Answer 9

Because if you layed an egg, you wouldn't be a mammel.. Given a couple of exceptions..