In Bat Harems, a Female’s Location Determines if She is Well- or Badly-Treated
By John Barrat
For female fruit bats clinging upside down in harem clusters to the ceilings of caves in Yucatán, Mexico, life on the harem’s edge means being attacked more, getting groomed less and being frequently expelled. This was one of the findings in a recently published study by researchers Jorge Ortega of the Ecological Institute of the National Autonomous University of Mexico and Jesus Maldonado of the Smithsonian's Genetics Program, a joint project of the Smithsonian's National Zoological Park and the Smithsonian's National Museum of Natural History.
Their study focused on the interactions of female Jamaican fruit bats (Artibeus jamaicensis) living in harems averaging 14 females. Harem bats were classified by the scientists according to their position in the cluster—those on the edge, those in the central core and those in between.
Females on the edge of the harem did not interact with females in the harem's central core and were often greeted with grunts and wing flicks (signs of aggression) when they approached the middle-position bats. Edge bats shifted their positions more often, trying to maneuver to a better spot. This behavior frequently earned them ejection from the harem. Accordingly, edge bats were more often new arrivals from other areas of the cave.
Blood and tissue samples taken from the bats showed no significant genetic subdivision, as well as a low degree of family relationship among the harem members. Bats at the harem's central core had less turnover of individuals and received mainly positive interactions—in the form of licking and grooming—from middle-position bats. Females temporarily removed from the harem by the researchers quickly regained their original positions when they returned to the harem.
During breeding season, females became more tolerant and during rearing time, the bats clustered in compact groups without moving. This suggests that one possible benefit of roosting together is that it provides a more suitable place to nurse newborns and keep them warm.
"Studying the degree of association among individuals and their influences in the group can help us address questions about the importance of membership in groups, the social structure of the groups and the type of relationships between different groups," Ortega and Maldonado say. "It can also help us understand why such groups are formed, be it protection against predators, the availability of resources or the structure of a mating system."
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