When male lions take over a pride of female lions, they kill or attempt to kill the cubs. They don't indiscriminately kill baby lions, however. They leave their own offspring alone but kill cubs fathered by the previous dominant male. Killing the cubs sired by the previous dominant male increases the new male's fitness, because it gives him a better shot of producing his own offspring before he gets dethroned by some other male lion.
The violent male-male competition to control a pride of females results in frequent changes in the dominant male. When a new male takes over, he must reproduce as rapidly as possible, as the risk exists that he too will soon be displaced. Because female lions with cubs are not reproductively receptive while they're caring for existing young, the fastest way for the male to gain reproductive access to females is to kill their offspring, which he does as soon as he takes over the pride. The females are then soon ready to mate with him and have his offspring, passing along his genes.
Should I stay or should I go?
In monogamous species, males and females have either a short or a long-term interest in the fitness of their partners. Non-monogamous species don't have the same concerns. But how would they evolve if they did? Brett Holland and William Rice designed an experiment with fruit flies to investigate this question. They separated the flies into two groups:
I For one group of flies, they continued to grow the flies together in large groups in which multiple non-monogamous matings would occur. In this case, an individual fly's fitness is maximized by producing as many offspring as it can, regardless of the consequences for its partners.
I In the other group, they raised the flies in monogamous pairs. In this treatment, the fitness of the two flies is interconnected. Flies whose behaviors maximize their partner's fitness also maximize their own fitness.
The researchers raised flies in these two treatments for 47 generations; at the end of the experiment, they measured female fitness characteristics such as life span and reproductive output. Here's what they found:
I The male flies in the monogamous pairs had evolved to be less harmful to the female flies. The original male flies were rough; the evolved monogamous ones more gentlemanly.
I The female flies had evolved to be less resistant to the male flies. The original female flies watched their backs; the monogamous evolved ones relax and let their hair down.
Their experiment shows that changing the mating system such that the interests of the sexes were no longer in conflict led to rapid evolution that decreased the strength of traits associated with male-female conflict.
In this scenario, the male's fitness increases at the expense of the female's fitness. She's invested time and energy in her litter, which is now lost. Because the new dominant male is so much larger and stronger than the female, she isn't able to protect her young after the new dominant male takes over.
Before then, the females aren't completely powerless, however. Groups of female lions often attempt to chase away solitary male lions that approach the pride. How much — or whether — these actions decrease the probability of the dominant male's being replaced isn't known, but the actions are certainly consistent with the view that the females are trying to prevent the deaths of their cubs.
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