External vs internal fertilisation
Vertebrates fertilise their eggs in one of two ways:
- External fertilisation — sperm and eggs are released into the environment (usually water), where fertilisation happens outside the body. This requires huge numbers of gametes, since many are lost to currents, predation, or simply failing to meet. Typical of most fish and amphibians.
- Internal fertilisation — sperm is deposited inside the female's body, where fertilisation occurs. This is more reliable, so far fewer gametes are needed. Typical of reptiles, birds, and mammals (and some fish).
Oviparity, ovoviviparity, and viviparity
How the fertilised egg develops also varies:
- Oviparity (egg-laying) — the fertilised egg is laid and develops outside the mother's body, nourished by yolk within the egg. Examples: most fish, most reptiles, all birds.
- Ovoviviparity — the fertilised egg develops and hatches inside the mother's body, but the embryo is still nourished by the egg's yolk, not the mother directly. Examples: some sharks, some reptiles.
- Viviparity (live birth) — the embryo develops inside the mother and is nourished directly by her, typically via a placenta. Examples: almost all mammals.
Ovoviviparity is often confused with viviparity — the key difference is the source of nourishment: yolk (ovoviviparity) vs the mother's body directly (viviparity).
The trend across vertebrate groups
Moving from fish → amphibians → reptiles → birds → mammals, there is a general evolutionary trend: offspring number decreases while parental care increases. This is a survival trade-off:
- Fish and amphibians produce huge numbers of eggs with little or no parental care, relying on sheer numbers ("many eggs, low survival chance each") for some to survive.
- Reptiles produce fewer eggs, often with some nest protection.
- Birds produce even fewer eggs but incubate them and feed the chicks after hatching.
- Mammals typically produce the fewest offspring, but invest heavily — through internal gestation, a placenta, and feeding milk after birth — giving each offspring the highest chance of survival.
This trade-off links directly to environment: aquatic environments allow external fertilisation and huge egg numbers, while a terrestrial (land) environment favours protecting eggs/embryos from drying out and predation — hence internal fertilisation and greater parental investment.
Altricial vs precocial development
Among birds and mammals, hatchlings/newborns fall into two broad categories based on how developed they are at birth, which links directly to how much energy-rich yolk was available during development:
| Precocial development | Altricial development |
|---|
| Yolk % in the egg | High percentage of yolk — more energy/nutrients available to the embryo | Low percentage of yolk — less energy available for development before hatching |
| Condition at hatching/birth | Well-developed: eyes open, downy feathers or fur, able to move/feed itself almost immediately | Poorly developed: born naked (no feathers/fur), eyes closed, helpless, unable to move or feed itself |
| Parental care needed | Low — the offspring is largely independent soon after birth | High — parents must keep the offspring warm, feed it, and protect it for an extended period |
| Examples | Ducks, geese, chickens (birds); most hoofed mammals | Eagles, vultures, pigeons (birds, relative to ducks/geese); many rodents, cats |
A higher percentage of yolk in the egg → more energy for embryonic development → a more well-developed ("precocial") hatchling that needs less parental care. A lower yolk percentage → an under-developed ("altricial") hatchling that depends heavily on parental care to survive.