Why Are Bird/Reptile Eggs So Large?

When compared to mammalian eggs, that is. You’d think that the larger the animal, the larger the egg, and that’s true in comparing a chicken egg to an ostrich egg, but not when comparing a chicken egg to a human egg. I can see why it would be a benefit for birds and reptiles to have large eggs, but I can’t see why it’d be detrimental to mammalian survival for their eggs to be smaller.

So, just how large are platypus and echidna eggs in relation to body size? I always thought they were comparable to the egg/body size ratio in your average bird.

This is a yolk thread, right?

The human ovum, as most people will hasten to explain to you, is internally fertilized and then remains within the mother’s body, growing a placenta which interfaces with the uterine wall to absorb nourishment for the growing embryo/fetus from the maternal circulatory system. Hence it needs to be only large enough to survive up to the point of embedding in the utrine wall. This process is, of course, also true for most mammals (noting that Australia and the Americas house a number of exceptions to that rule).

Monotremes, most reptiles, and birds, on the other hand, lay eggs which contain the ovum, the chorion, the allantois, the amnion, and the yolk sac. These various organs work together to keep the embryo which develops from the ovum alive and nourished until it is ready to hatch.

The use of “egg” for the two entities – the ovum, and the entire amniote-egg complex available for $1.09 a dozen at the supermarket – confuses the issue. But if you think it through, an ostrich of about the same height and weight as a human has a much smaller ovum-and-support-setup-complex than a human; the ostrich egg is perhaps ten inches on the long axis and weighs five or ten pounds, on a guess; what the human egg requires in terms of a support system usually is five to six feet in long axis, weighs 150-200 pounds, and is generally self-propelled and quite interesting to observe, in my experience.

Think about it for a moment. A baby chick weighs a few ounces and has a gestation of maybe 21 days. A human baby weighs 9lbs and has a gestation period of nine months. If a human had to develop for nine months totally within the egg without any outside source of nutrition, then the egg would probably have to be the size of a volkswagon. But instead, the developing embryo gets its nutrition via the umbilical cord, so the egg doesn’t have to hold much more than the dna to get the whole process started.

That would hurt on the way out, despite it’s rounded shape.

Couldn’t you reverse out slowly? :eek:

Possibly, but I’m not sure even a locking differential could overcome all the lube that would be needed.