On David Letterman tonight, Jack Hanna was on again. He had some leopards, and said that sometime male leopards eat their young. When Dave asked why, Jack didn’t give a straight answer, so I’m not sure if he didn’t know or if he was joking. In any case, I think this is a good time for some science education. Then again, every time is a good time for science…
Anyway, the reason some males eat the young is because it causes the female to go into heat again. If the young die, she no longer has any young to care for, so from an evolutionary standpoint, it is beneficial for her to go into heat and “try again”.
However, from the perspective of the male, killing the young is also genetically beneficial, because if the young aren’t his, killing them gives him the opportunity to get the female pregnant with his own young, thus passing on his own genes.
When a male kills the young, the evolutionary interests of the male and the female are influencing each other.
It is interesting to note that as you look at various organisms, this behavior varies depending on how the organisms care for their young. The tendency for males to eat young is more typical of more complex animals, like leopards. Alligators exhibit this behavior as well.
This all changes as you move down to simpler organisms. When you look at insects and spiders, it is actually the female who frequently eats the male. Such organisms usually provide very little care to their young, and instead utilize a reproductive strategy based on sheer numbers. In other words, have as many babies as possible, and even if most die, a few will go on to continue the gene line.
With this kind of reproductive strategy, the female doesn’t really bother to care for the young anyway, so there is no evolutionary incentive for the male to eat the offspring. There is, however, an incentive for the female to eat the male. Once the male has done his deed, he is pretty much useless. Meanwhile, the female is going to need quite a bit of energy to produce hundreds of little eggs. As a result, she gets the after-sex munchies and eats the male.
The tendency for some animals to eat their young or each other is the result of the male and the female doing what is in their best interest from a genetic point of view. In species that spend a lot of time caring for young, the male will sometimes eat the young. In species that don’t really bother caring for young, the female sometimes eats her partner.
As a male, I’m just glad I’m not a spider.