Average age and optimal age of first reproduction?

Discussion in 'Biology & Genetics' started by Blue Banana, Jun 12, 2013.

  1. Blue Banana Registered Member

    Had a thought a while ago about the optimal age for females in a species 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 period until death or menopause. Those that start reproducing earlier than age X will also, on average, leave behind fewer offspring due to the greater risk 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 species 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?
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  3. Tiassa Let us not launch the boat ... Staff Member

    A Difficult Question to Pin Down

    I'm not certain what the term actually is, but the thing is that the optimal reproductive age fluctuates by species and circumstance. This gets even more complicated when applied to humans.

    The question of quantity is its own conundrum. In many species, volume seems to be a key. In others, though, quality trumps quantity.

    "Optimal" in this case is an elusive variable. That is, against what standard is the optimum measured?

    If we leave that definition at mere quantity, there isn't much for a strong and defining answer to the question. That is, you will at least as many answers as there are identified species.
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  5. iceaura Valued Senior Member

    Some researchers have developed sets of equations relating growth rate, adult size, age at first reproduction, lifespan, etc, (often log transformed, as with so much in biology). They have discovered that for a given kind of animal (mammal, reptile, fish, bird, etc) the same ratios tend to show up across most species (age of first reproduction and age of expected death tend to a given ratio for mammals, a different one for birds, etc).

    Ex: One tradeoff is that size abets reproduction in females (the only sex of much interest here), but resources devoted to reproduction must be diverted from growth. Clearly this will affect animals (and plants) that continually grow differently than those with a fixed adult size. And so forth.

    The math is not always elementary stuff, actually - can be a challenge. Have fun - the field is still open.

    Edit in: I forgot to provide a hint link - in this one, you might start by following the author "Charnov" elsewhere: http://www.pnas.org/content/98/16/9460.full.pdf
    Last edited: Jun 13, 2013
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  7. Fraggle Rocker Staff Member

    Menopause is not a common phenomenon among mammals. We are one of only a handful of species whose females live long past reproductive age. This was probably selected by evolution, because we are the mammal with the most incredibly long maturation period. Baby whales become adults in only two years, elephants in only five, but human children need intensive parenting for about fifteen years, in order to maximize both their survivability and their contribution to the welfare of the community.

    In order to avoid needing at least one parent to devote nearly all of his/her attention to the children until the last one goes off to college, human communities have elders who step in. These include the grandparents within the family and also other people of that age throughout the community. Many teachers, coaches, priests and police are "retired" from parenting and can devote their nurturing, mentoring and protecting abilities to the population as a whole.

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