Conspecific threat,predation avoidance,and resource defense: implications for grouping in langurs |
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Authors: | Adrian Treves C A Chapman |
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Institution: | (1) Dept. of Anthropology, Harvard University, Cambridge, MA 02138, USA, US;(2) Department of Zoology, 223 Bartram Hall, University of Florida, Gainesville, FL 32611, USA, US |
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Abstract: | Data on langur (Presbytis entellus) populations were gathered from the literature to test the importance of three selective pressures in determining group size
and composition: predation pressure, intergroup resource defense, and conspecific threat. There were no detectable difference
in the size of groups in populations facing nearly intact predator communities compared to those populations where predators
were severely reduced in number or absent, although there was a trend for the number of adult males per group to increase
in areas with nearly intact predator communities. Using population density as an indirect measure of the frequency of intrusions
into a group’s home range and thereby as an index of the demographic pressure favoring resource defense, we predicted that
higher densities would result in larger defensive coalitions and higher numbers of females per group. This prediction was
not upheld. Our third selective pressure, conspecific threat, encompasses those selective forces resulting from physical attack
on females, infants, and juveniles. Our index of conspecific threat uses the number of non-group males divided by the number
of bisexual groups, because in langurs, the major source of conspecific threat derives from non-group males who, following
group take-over, kill infants, wound females, and expel juveniles from groups. This index of conspecific threat was strongly
related to the mean number of resident females, was weakly related to the mean group size, but was not related to the number
of males in the group. In addition, as predicted, populations with a high index of conspecific threat had higher levels of
juvenile expulsion. These analyses were corroborated by a simulation model which used a computer-generated series of null
populations to calculate expected slopes of immatures regressed on adult females. These randomly generated populations, matched
to means and ranges of real populations, allowed us to determine if deviations of the observed slopes from the expected null
slopes could be explained by variation in predation pressure, population density, or conspecific threat. We found no evidence
that predation pressure was associated with decreases in immature survival in smaller groups, as would be predicted by the
predation-avoidance hypothesis. We found no evidence that immature survival was compromised by small group size in high-density
populations, as would be predicted by the resource-defense hypothesis. However, as the index of conspecific threat increased,
groups with larger numbers of females were more successful than groups with fewer females in reducing mortality or expulsion
of immatures. Overall, conspecific threat received the strongest support as a selective pressure influencing langur group
size and composition, suggesting that this selective pressure should be evaluated more widely as a factor influencing composition
of animal groups.
Received: 23 January 1995/Accepted after revision: 18 February 1996 |
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Keywords: | Presbytis entellus Group living Group size Infanticide Social organization |
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