The common vampire bat is a social animal, meaning that
it lives in a social group (Park 1991). The common vampire bat’s nesting
site is called a roost (Park 1991). The dispersion of common vampire bats
depends on prey size and roost availability (Wohlgenant 1994; Trajano 1996).
When it is cold the bats prefer the caves, and when it warm they prefer to
live
in trees (Trajano 1996). Vampire bats may even migrate to warmer
climates,
because the energy requirements are too strenuous when it is cold (Trajano
1996). When roosts are limited there is physical confrontations for roosts
(Wohlgenant 1995). The main roosting location is the place where the majority
of the bats lives and interacts with each other (Park 1991). When the roost
is a cave it contains the harem, which is also called the principal colony,
as well as a bachelor herd (Delpietro & Russo 2002). There can be up
to one hundred bats in a harem (Nowak 1991). The harem can also contain
more than one male (Delpietro & Russo 2002). These males, called resident
males, are involved in mating and territorial defense (Delpietro & Russo
2002; Park 1991).
The common vampire bat has a harem as its social structure (Delpietro & Russo 2002; Wilkinson 1988). Generally the composition of the harem is one dominant male, multiple females, and the females’ offspring (Wilkinson 2001; Park 1991). The bats in the harem are separate from the bachelor males (Lord 1993). The dominant male, known as the harem male, protects the vampire bat harem (Wilkinson 1985a). If the males in the bachelor herd, called nonresident males, try to intrude into the harem, the dominant male is very aggressive in removing them. Fighting is not uncommon between the harem male and a bachelor male (Wilkinson 1985a). The bachelor males try to invade the harems when the harem male goes out to forage (Lord 1993). When the harem male returns he has to evict the bachelor male from the harem (Lord 1993). The dominant male tries to stay close to the females in the harem at all times (Wilkinson 1985a). If the bats live in a tree, the dominant male is found at the very top of the tree where he can see and engage any intruding bachelor males (Wilkinson 1985a). The bachelor males are constantly trying to gain access to the females (Lord 1993). The harem male usually reigns at the top until a bachelor male displaces him through fighting. The average tenure for a harem male is two years (Wilkinson 2001).
photo taken from www.batcrew.com
The
females in a harem are related and unrelated, because mature female young
stay in their natal roost (Park 1991, Wilkinson 2001). The females
in the harem
are deeply rooted in the harem (Delpietro & Russo 2002). They may live
together for years, however females do switch roosts occasionally (Wilkinson
1988; Wilkinson
2001). They do not switch colonies often, because of the long-term bonds they
have among each other (Wilkinson 1985a). The females make active choices about
which females they want to have relationships with (Wilkinson 1985a). Females
actively choose which females to have relationships with; because the females
they have relationships with are the females they will share food with (Wilkinson
1985a). Therefore, the females want to pick only females that do not try to
cheat the sharing system (Wilkinson 1985a). The females spend most of their
time participating
in social activities that strengthen bonds among them (Wilkinson 1986). Activities
like grooming and food sharing require strong bonds, which keep females from
emigrating too often (Park 1991). Also, females are not guaranteed acceptance
into a new colony (Wilkinson 1985b).
Harems that contain several males are slightly different than a typical
harem. These males are not related in any way (Wilkinson 2001). The males
within these
harems have a dominance hierarchy (Park 1991). The position of each male
in the roost correlates to his rank (Park 1991). The alpha bat is found at
the
top of
the tree, the beta male below him, and the gamma male below the beta male
(Park 1991). There is conflicting evidence as to whether the dominance hierarchy
is determined by aggression. In some studies resident males including the
harem
male are not aggressive towards each other (Wilkinson 1985a). In other studies
the males are aggressive toward each other, and it is believed that this
aggression solidifies each ones rank (Park 1991). The resident males will
help fight off
invasions from the bachelor males (Delpietro & Russo 2002). The resident
males do not stay in one harem for a long time, but rather keep moving (Wilkinson
2001). The alpha male spends most of his time near the females (Park 1991).
The roles of the females do not change within the harem except for mating.
(See Mating
system)
The young within the harem disperse depending on gender (Lord 1992). Female
young stay in the principal colony they were born in where as male young disperse
out
and join bachelor herds once they are eighteen to 27 months old (Park 1991).
The dominant male expels the young males through aggression (Park 1991). The
young females only switch colonies when their mother switches.
The bachelor herds that the young male joins are found on the periphery around
the principal colony or harem (Lord 1993). There is intense fighting within
the bachelor herds (Lord 1993; Lord 1992). The males in this herd have one
goal and
that is to gain access to females so that they can mate (Wilkinson 2001). To
gain access to the females in the harem, the bachelor males enter the harem
while the alpha male is foraging, and try to copulate (Lord 1993). The females
also
go visit the bachelor males while the harem male is gone foraging (Lord 1993).
The females in the harem actively resist against mating with bachelor males
(Lord 1993). Also, the bachelor males try to invade the harem to mate with
females
while the alpha male is present (Lord 1993). This is probably why the alpha
male stays near the top of the tree and near the females. Once the alpha male
sees
the intruder agonistic behavior ensues (Wilkinson 2001, Wilkinson 1985a). The
bachelor males are successful in replacing alpha males sometimes (Wilkinson
1985a; Wilkinson 2001). Alpha males reign over the harem for about two years
on average
(Wilkinson 2001).
The common vampire bat is a territorial animal, as evidenced by its territorial
defense behavior (Park 1991). The harem male actively defends the territory
(Park 1991). The resident males also help defend the territory (Delpietro & Russo
2002). The territory can be the corner of the cave where the harem is, or if
the bats are in a tree roost it is usually the entire tree. The defense of the
tree leads to fighting between the resident males and the nonresident males (Delpietro & Russo
2002). The fights consist of gesturing, screeching, biting and beating with
the wings (Turner 1975). The fights usually end with one bat bleeding (Wilkinson
1985a). The territory is a mating-nesting territory. The bats live and mate
in
their roost, but have to leave to forage in nearby pastures.
Common vampire bats have several predators including snakes, lizards, owls,
dogs, cats, opossums, and humans (Wilkinson 1985a). When confronted by humans
the common
vampire bat will run very quickly into crevices in its roost (Turner 1975).
Bats prefer to hunt when it is very little moonlight so that predators will
not see
them (Lord 1993). The common vampire bats dark color, coupled with the darkness
of the night provides ample protection for feeding bats (Altringham 1998).
There is no evidence that social living in bats aids in detection, avoiding,
or hindering
of predators (Wilkinson 1985a). Predation does not seem to be a great concern
in vampire bats.
Common Vampire Bat Homepage--General Overview--Mating System--
Feeding Behavior--Cooperative Behavior--References