Final answer:
Mammalian eggs have less yolk than bird and reptile eggs because they rely on the placenta for nourishment after implantation, not on yolk.
Step-by-step explanation:
Why Do Mammalian Eggs Contain Less Yolk?
Mammalian eggs contain a small concentration of yolk compared to bird and reptile eggs primarily due to differences in reproductive strategies and developmental processes. In mammals, especially placental mammals, the developing embryo is nourished through the placenta after implantation in the uterus. This direct supply of nutrients from the mother negates the need for a large yolk sac within the egg. On the contrary, bird and reptile eggs have a large concentration of yolk because they must sustain the developing embryo for a longer period outside the mother’s body, relying entirely on the yolk as a source of nourishment enclosed within either a hard calcium shell or a leathery exterior for protection.
Moreover, mammalian development is significant post-fertilization, with the mother investing heavily in the growth and care of the offspring after birth, providing milk from mammary glands. In contrast, the extensive yolk in bird and reptile eggs provides a continuous and self-sufficient food source until hatching. Notably, there are egg-laying mammals, known as monotremes, such as the platypus and echidna, which do have yolky eggs, but these are exceptions rather than the rule among mammals.