Cooperative male alliances in bats

A paper just published on gelada baboons entitled “Concessions of an alpha male? Cooperative defence and shared reproduction in multi-male primate groups” claims to be among the first demonstrations of cooperative male defense of a female group in a mammal. But the authors seemed to have forgotten that bats are also mammals.

A little background: most mammals live in female groups that are “defended” by a dominant male that attempts to monopolize female matings in the group. To do this, males might protect a territory where the females live or keep other males away from the group. In some cases, it is thought that dominant males may “concede” matings to subordinate males because the subordinates are often related and they can help dominants fend off other males. This is called a “concession model”. The authors of this study state that “to date, only three studies have provided some support for concession models in male mammals.” They cite 3 studies on lions, chimpanzees, and baboons.

Hey! What about bats?

Jorge Ortega showed that, in the Jamaican fruit-eating bat Artibeus jamaicensis, dominant males cooperate with subordinate males to defend territories. Dominant males sometimes tolerate subordinates and in return they can defend larger groups of females. Subordinates that help dominants have a better chance of later acquiring dominant status. Both dominants and subordinates also gain indirect fitness benefits because they are often genetically related. These male alliances between dominants and subordinates in neotropical fruit bats can last more than two years. And such alliances may be pretty common since the majority of bats have similar social structures of female groups with single or multiple males.

Male alliance behavior was also found by Martina Nagy and others in the insectivorous sac-winged bat, Saccopteryx bilineata, where dominant males cooperate with subordinate males that queue for dominant status. In this species, dominant male tenure duration is not correlated with number of females or forearm size. But the tenure and reproductive success of a dominant male does increase with the number of male subordinates on their territory. Such a correlation suggests that dominant and subordinate males are cooperatively defending females from other males (and this is the same evidence that the authors give in the study on geladas).
The sac of Saccopteryx bilineata. Photo by Brock Fenton.
Sac-winged bats are also interesting because the males have an intense courtship display. Not only do they sing complex songs like songbirds, but they also concoct a special “perfume” by mixing urine and other bodily secretions in special pockets in their wing membranes. After a bit of fermentation, the perfume is sprayed in the air when the bats flap their wings and do a special aerial display to impress females (see video below).
Genetic analyses have shown that even the most “dominant” male sac-winged bats only father about a third of the pups born on their territory. Hence, dominant male sac-winged bats might be able to keep other males away, but they can’t control the sexual behavior of females very much. Genetic analyses revolutionized the study of mating behavior of animals when they first appeared on the scene, showing a deep rift between the easily observable “social monogamy” and the actually rare “genetic monogamy” of songbirds. (The behaviors that songbirds want to hide from their partners is also easy to hide from human observers.)

Likewise, it’s hard to know in bats how well observable mating behavior correlates to actual reproductive success. In the last captive group of vampire bats I worked with, my genetic analyses showed that the supposed “dominant” male fathered 0 of the 3 young born recently.

My labmate Danielle is also interested in using genetic analyses to study the relative roles of female choice and male-male competition in the greater spear-nosed bat, Phyllostomus hastatus, where the dominant male is thought to have almost complete monopolization over female reproduction in the group.

In the past, ethologists have often referred to female groups as “harems” that “belong” to dominant males (a metaphor that certainly sounds like it came from culturally-biased male scientists). This hypothesis assumes that competition between males is the primary factor determining which males are chosen by females and produce offspring. An alternative possibility is that females living in groups might mainly use “dominant” males as tools to prevent harassment by other males, while still maintaining their ability to mate with whichever males they choose. This hypothesis assumes that female choice is more important than male-male competition. Something closer to this scenario is especially possible for flying animals like bats, where females can easily leave a male’s territory whenever they want.

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