Social organization and Spacing
Social Organization and SpacingThe social organization of
vampire bats consists of colonies within the roost sites. The colonies vary
in size based on the size of the roost. They often contain about 100 bats,
but they have been known to contain up to 2,000 individuals (Holzman 1999).
The males and females commonly share the same roost, though they are in different
groups within the roost. The social system is complex, made up of a main, predominately
female, group and temporary male bachelor groups (Park 1991). New born males
and females are part of the main group, but males either leave or are forced
out when they reach sexual maturity between 12 and 18 months of age (Wilkinson
1984).
Shi Ryong Park (1991) observed the development of social structure in a captive
colony of vampire bats. He concluded that the main group was very cohesive and
consisted of adult females, infants, and one harem male. The females in the main
group were integrated firmly into the group during the entire thirty months of
observation. [It must be noted that this was a captive colony where the bats
were not allowed to move to other roost sites and join other groups. Such movement
has been observed in the wild by Trajano (1996)]. But, even in the wild, the
females groups show cohesiveness, and strong social interactions while together.
Wilkinson (1984) provides an explanation for the cohesive
female groups through a test he conducted that examined eight possible
selective advantages
of
living together. He examined whether “females roost together to (1) share a suitable
microclimate, (2) avoid predators, (3) avoid ectoparasite infestations, (4) minimize
travel to mobile prey animals, (5) respond to coercive males, (6) feed simultaneously
from a bite, (7) remove ectoparasites by allogrooming, (8) share food by regurgitating
blood to other bats within roosts.” Wilkinson concluded that the only
plausible selective advantage to the female groups is for food sharing. The
data did not
indicate any advantage gained by the other hypotheses. It is possible that
avoidance of predators and ectoparasites has an influence on the aggregations,
but they
would only account for short-term aggregations and not the long term associations
that were observed.
The harem male (synonymous
with alpha male) lives within the same space as the female group, but he
rarely comes into physical contact with the females, except
for mating. The other males form much less cohesive groups, which Park labels
as temporary bachelor groups. These bachelor groups are organized by a dominance
hierarchy. Park suggests that the hierarchy is rather dynamic, because he
observed the harem male position change twice during the course of his
observations. The
order of the male hierarchy is shown by their location with respect to the
females. The alpha male resides virtually within the female group. The
beta male is the
next closest to the females, then the gamma male, and so on (see image below).
The alpha through the gamma male show territorial behavior in protecting
their
roost
sites.
Their territories are only used for roosting and mating, so they are considered
mating
only territories. The males below the gamma position do not show significant
territorial defense, and so are considered to be home range. So, it can be
said that the top three males are the only bats that are territorial and
all others,
including males and females, are home range. The selective advantage for
the male hierarchy and territory system is related to the mating system
of vampire
bats, which is discussed in another section.
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