9.13: Tactics for keeping a mate: mate guarding can protect a male’s reproductive investment.

If a male simply abandons a female after mating and searches for other mating opportunities, rather than making any investment in the potential offspring, he has no risk of investing further energy in offspring that are not his. This is one way to minimize the potential costs associated with paternity uncertainty. If you don’t play, you can’t lose.

This strategy, however, is not necessarily the most effective way for a male to maximize his reproductive success. If a male has a hundred or even a thousand matings, but no offspring survive, that behavior is not evolutionarily successful. Consequently, in species for which offspring’s survival can be enhanced with greater parental investment—including those species not well-developed at birth—there is an incentive for a male to provide some additional investment in offspring, even though such behavior makes him vulnerable to paternity uncertainty.

In situations where males provide parental care, it is common for the male to reduce his vulnerability through some form of mate guarding. In contrast to a female, who can be certain that any offspring emerging from her body is hers, a male inhabits a “danger zone” that lasts as long as the female is fertile. If she mates with any other males during this time, the offspring she produces may not be his. If he is going to make any investments of time or energy that will benefit the offspring, he benefits by minimizing his risk in the danger zone. It is during this period that mate guarding is particularly common.

Mate-guarding methods range from the simple to the macabre. If a male wants to ensure that a female does not mate with another male, why bother to stop mating at all? In many species, males take this approach to reduce their risk in the danger zone. Among house flies, even though the male has completed the transfer of sperm to the female in 10 minutes of copulation, he does not separate from her for a full hour. Moths go even further and continue to mate for a full 24 hours. And in the extreme case of this strategy, certain frog species continue individual bouts of mating for several months (FIGURE 9-22). In humans, this would be equivalent to almost 10 years for a single round of intercourse.

In a slightly subtler form of mate guarding that occurs in reptiles, insects, and many mammalian species, after copulation, males block the passage of additional sperm into the female by producing a copulatory plug. Formed in the female reproductive tract from coagulated sperm and mucus, copulatory plugs can be very effective. Male garter snakes that encounter a female snake with a copulatory plug, for example, do not attempt to court or mate with her, treating her instead as if she were not available.

A much more extreme form of mate guarding occurs in the black widow spider: the male breaks off his sexual organ inside the female, preventing her from ever mating again. Interestingly, when the act is completed, the female usually kills and eats the male (FIGURE 9-23). In sealing his mate’s reproductive tract, the male assures himself of fathering the offspring, and in consuming her mate’s nutrient-filled body, the female gets resources that help her produce the offspring.