Q.Could the number of eggs or young ones produced by an oviparous and viviparous mother be equal? Why?

No, the numbers would generally not be equal, and the reason lies in how the young are protected.

• An oviparous mother lays eggs, so the developing young are left outside the mother's body, exposed to predators, weather and other dangers.
• A viviparous mother gives birth to live young that have developed inside her body, where they are far better sheltered and cared for during development.
• Because eggs laid outside face a much higher chance of being lost, an oviparous animal tends to produce many more eggs so that at least some survive. …

This question turns on the difference between laying eggs and bearing live young, and on how much protection the developing offspring receive.

An oviparous animal lays eggs, and development happens outside the mother's body. Once laid, the eggs and the young inside them are on their own — open to predators, to harsh conditions, and to many other hazards. A large fraction of such eggs and young never survive to grow up.

A viviparous animal, by contrast, gives birth to live young. Development takes place inside the mother's body, where the offspring are sheltered and supported until they are born. Being protected in this way, a much greater proportion of the young survive.

Method: The reproductive-investment trade-off framework

Model reproduction as a fixed budget of energy that a mother can spend either on making MANY offspring, or on PROTECTING FEWER offspring, and derive the answer from this trade-off rather than by directly comparing survival odds.

Think of a mother's total reproductive effort as a fixed budget that can be spent in one of two ways: spread thin across a large NUMBER of offspring, or concentrated as PROTECTION around a small number of offspring. These two strategies sit at opposite ends of a single spectrum, and no mother can maximise both at once.

An oviparous mother, laying eggs outside her body, cannot spend budget protecting each egg individually after laying — the eggs are exposed to the environment and to predators the moment they are released. Because each individual egg's survival chance is therefore low, the only way to ensure enough offspring survive is to spend the reproductive budget on producing a LARGE NUMBER of eggs, accepting that many will be lost.

A viviparous mother, by contrast, spends her budget on carrying and protecting the young inside her own body until birth. This investment in protection raises each individual offspring's survival chance dramatically, so she does not need to produce nearly as many to ensure some survive to adulthood — her budget is better spent on FEWER, safer offspring. …