Average age and optimal age of first reproduction?

Arse Bomb

Had a though a while ago about the optimal age for females in a population to start reproducing and how it relates to the average age that they actually do start.

Suppose there is some optimal age X to start reproducing that will allow the females to leave behind the greatest number of offspring. Females that start reproducing later than this age will leave behind fewer offspring on account of having a shorter breeding peroid until death or menopause. Those that start reproducing earlier than age X will also, on average, leave behind fewer offspring due to the greater risks of death during labour or other complications.

Females that start reproducing at age X will, by definition, be the most reproductively successful and it must then follow that the average age of first reproduction in a population will stabilise around that optimal age X.

I can't be the only person to have realised this so does this principle have a name in zoology or evolutionary biology?

aidoh

It's called age-specific reproductive investment.

Basically, an individuals fitness should decrease as it gets older because there are fewer opportunities to mate. Herring gulls for example will leave their young unattended and thus vulnerable to predation if they have previously produced a number of successful clutches. Herring gulls caring for their first clutch are far more vigilant and the mother will only leave her nest to forage if the father is around to guard them in her absence (and vice versa).

Lots and lots of other examples in nature. There is also state-specific reproductive investment, where (typically) females will invest less energy in parental/reproductive behaviour if they are injured or carrying parasites etc.

Funnily, animals which breed frequently or seasonally make such trade-offs (in general), whereas animals which only breed once in their lifetime or which depend upon an unpredictable resource for mating (e.g. a putrefying carcass for carrion-feeding insects or something) don't make such a trade-off and will invest all their energy into reproductive behaviour, even if it ends up killing them!