Direct matemate contests

The most obvious example of male-male competition for females is the case in which males actually fight one another, and the winners are the ones that get to mate. The stakes are high, because the losers won't be passing their genes on to the next generation (at least during this particular mating season), and sometimes the contests are quite violent. Common examples of animals that engage in male-male contests are elephant seals and lions. In both cases, selection has led to increased size in males because larger males are more successful fighters.

Run away! Run away!

Individuals typically don't engage in contests that they don't stand some chance of winning. For that reason, many contests in the animal kingdom are preceded by a period of posturing in which the two combatants check each other out to judge relative size and strength. After this initial period, a weaker individual chooses to withdraw rather than risk death in a contest he probably can't win. The same logic applies when, part of the way through a fight, one individual realizes that he's beaten and withdraws. This situation is also quite common in nature.

This phenomenon is responsible for male elephant seals being the largest members of the carnivore family. Although female elephant seals are generally between 1,000 and 2,000 pounds, males routinely weigh between 3,000 and 5,000 pounds, with the record weight being more than 7,000 pounds.

Studies have shown that the larger males do indeed father most of the young in elephant seals and some other species. But living the life of a giant warrior-lover isn't all peaches and cream.

Natural selection favors genes that increase size and fighting ability, but not genes for increased longevity. Although enormous size is necessary to fight for a mate, the violent contests and huge energy costs associated with it seem to take their toll. Male elephant-seals live much shorter lives than females — about 14 years as opposed to about 20. Because of how mating is structured in this species, being an older male has no advantage; being a bigger, stronger male does. Death is definitely a bad idea and certainly something to be avoided. But if the only way to get access to females is at the cost of a reduced lifespan, natural selection will favor the bigger-and-stronger genes over the lifespan genes.

From an evolutionary point of view, not reproducing — that is, not passing your genes on to the next generation — is the same as dying. Either way, your genes aren't in the next generation, and your fitness is effectively zero.

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