The mating system of vampire bats most closely resembles resource defense polygyny (McFarlane and Denault 1994). The harem male defends his territory, which is close to the females, so that he may increase his chances of copulating. The alpha male receives the most copulations, but he is not the only male copulating with the females. The beta male engages in significant amounts of copulations and the gamma male has been known to sneak copulations, as well. The harem male enjoys only a two fold increase in copulations over the beta male (Park 1991, Wilkinson 1984).

Polygyny is seen as a compromise between monogamy, which is the most beneficial for the females, because they gain the help of the father, and promiscuity, which is the most beneficial for males, because it enables them to spread their genes the most. Polygyny is most likely to occur “when mates or resources are patchily distributed in space” (Case 2004). In the case of the vampire bat, the females are the resource that is distributed patchily. The dense, all female groups, make it beneficial for the male to mate with more than one female. They also allow mothers to solicit help from the other females in raising their young, so the father is not needed for anything after copulation.

The male dominance hierarchy prevents promiscuity in vampire bats. In guarding his territory, the harem male prevents other males from gaining access to the females. Though he is not 100% successful, there is enough of a difference between the number of copulations that the harem male receives and the other males, to call this polygyny and not promiscuity.

The females do gain some advantages from polygyny, however. Because the harem male had to fight for his position, he is probably the best mate with the best genes. This is a benefit to the females because the mate with the best genes is already chosen for them without requiring the females to spend energy finding out for themselves.

The vampire bat exhibits asynchronous polyestry, meaning that it can breed at all times of the year, and individual females breed multiple times a year. The factors that lead to what time of the year an animal can mate and how many times a year it can mate are the dynamics of necessary resources and the gestation period. Breeding requires a large amount of energy so there must be sufficient food and other necessary resources so that the survivorship of the parent or the offspring is not affected. The Vampire bats’ food source is relatively constant throughout the year, since livestock are nonmigratory and they are around all year long. The only thing that may make feeding a little more difficult in cooler seasons is the clumping of livestock to keep warm. But, according to Willig (1985) “there is no time of the year when reproducing would hinder survivorship of the parent or the offspring”. Other sources somewhat disagree with Willig, however. Nunez and Vianna ( ) found that there was “a marked seasonality” to vampire bat reproduction. Reproduction does occur throughout the year, but there are certain times of the year where more reproduction occurs. Rather than food availability being the cause, as it is with other bats, Nunez et al attributes the seasonality to temperature and humidity. The seasons where they live are fairly constant throughout the year, but the somewhat cooler and drier seasons are different enough to make it advantageous for vampire bats to reproduce most often in the warmer, wetter months.

As long as there are sufficient resources, it is beneficial for an animal to breed as many times a year as possible to maximize its reproductive fitness. The gestation period of the vampire bat is 6-8 months, so they may breed up to two times a year. One offspring is born at a time. Newborns are born blind and are carried by the mother for a few days. Their eyes open after a week, and they can fly after three weeks. During this time, the mother teaches the young how to hunt. After nine months, the young are sexually mature, and the males leave the female group (Willig 1985).

 

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