1. Different modes of development:
* Birds and reptiles: They develop outside the mother's body (oviparous), relying entirely on the yolk sac for nourishment and protection. This requires a large yolk supply to provide energy, nutrients, and essential molecules for growth and development.
* Mammals: Most mammals develop inside the mother's body (viviparous), receiving direct nourishment and oxygen from the placenta. This means they don't need as much yolk since the mother provides most of the essentials.
2. Larger size at hatching/birth:
* Birds and reptiles: Many birds and reptiles hatch at a relatively large size and can move around independently shortly after birth. This requires a substantial amount of yolk for growth and energy reserves.
* Mammals: While some mammals are born relatively large, most are born relatively small and dependent on the mother for survival. This allows for more compact eggs/births and less dependence on yolk.
3. Yolk as an energy source:
* Birds and reptiles: The yolk serves as a significant energy source for the developing embryo. This is especially true for birds that migrate long distances or reptiles that need to survive harsh environments.
* Mammals: The placenta allows for direct energy transfer from the mother, reducing the need for a large yolk reserve.
4. Developmental processes:
* Birds and reptiles: The yolk provides essential building blocks for the development of organs, tissues, and bones. This requires a significant amount of nutrients and energy.
* Mammals: While some yolk is present in mammalian eggs, it plays a less significant role in development due to the placenta's support.
In conclusion:
The larger yolk in birds and reptiles is a key adaptation for their unique mode of reproduction and development. It provides essential nutrients and energy for growth, survival, and independent life after hatching/birth. This contrasts with mammals, which rely on the mother's body for support and have less dependence on yolk reserves.