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Evolutionary determinants of modular societies in colobines


Grueter, C C; van Schaik, C P (2010). Evolutionary determinants of modular societies in colobines. Behavioral Ecology, 21(1):63-71.

Abstract

Modular societies are structurally characterized by nuclear one-male units (OMUs, or harems) embedded within larger relatively coherent social bands. Within the order Primates, modular societies are uncommon, found in only a few species, including humans. Asian colobines (Presbytini) principally form either unimale groups that forage independently and are often territorial, or modular associations, which range from tight bands composed of
OMUs to loose neighborhoods of OMUs. A phylogenetic reconstruction of modularity in the Presbytini revealed that the single OMU pattern is probably the ancestral state while the modular pattern is derived. The selective forces favoring the evolution of modular societies have thus far been virtually unexplored. Although some ecological explanations cannot be ruled out at the moment due to lack of comparative and quantitative data, preliminary circumstantial evidence does not seem to support them. Instead, a social factor, bachelor threat, is consistent
with many observations. This hypothesis argues that where the pressure from nonreproductive bachelor males is unusually high, OMUs aggregate as a means of decreasing the amount of harassment and the risk of takeovers and infanticide. A comparative test found an association between modular societies and bachelor threat, as proxied by sex ratio within social units. The concentration
of modular systems in colobines may be due to their unusual ecology, which leads to unusually low intensity of scramble
competition. Modular colobines rely more on nonlimiting ubiquitous resources than nonmodular ones and thus can afford to gather in bands. Moreover, by comparing the slopes of regressions between group size and daily travel distance for several groups of one modular and one nonmodular colobine, we found slopes in the nonmodular to be steeper by a factor 30, indicating that ecological constraints associated with scramble competition prevent higher level groupings in nonmodulars. Thus, modular
sociality in Asian colobines may have arisen because both social benefits are substantial and ecological costs are relatively low.

Modular societies are structurally characterized by nuclear one-male units (OMUs, or harems) embedded within larger relatively coherent social bands. Within the order Primates, modular societies are uncommon, found in only a few species, including humans. Asian colobines (Presbytini) principally form either unimale groups that forage independently and are often territorial, or modular associations, which range from tight bands composed of
OMUs to loose neighborhoods of OMUs. A phylogenetic reconstruction of modularity in the Presbytini revealed that the single OMU pattern is probably the ancestral state while the modular pattern is derived. The selective forces favoring the evolution of modular societies have thus far been virtually unexplored. Although some ecological explanations cannot be ruled out at the moment due to lack of comparative and quantitative data, preliminary circumstantial evidence does not seem to support them. Instead, a social factor, bachelor threat, is consistent
with many observations. This hypothesis argues that where the pressure from nonreproductive bachelor males is unusually high, OMUs aggregate as a means of decreasing the amount of harassment and the risk of takeovers and infanticide. A comparative test found an association between modular societies and bachelor threat, as proxied by sex ratio within social units. The concentration
of modular systems in colobines may be due to their unusual ecology, which leads to unusually low intensity of scramble
competition. Modular colobines rely more on nonlimiting ubiquitous resources than nonmodular ones and thus can afford to gather in bands. Moreover, by comparing the slopes of regressions between group size and daily travel distance for several groups of one modular and one nonmodular colobine, we found slopes in the nonmodular to be steeper by a factor 30, indicating that ecological constraints associated with scramble competition prevent higher level groupings in nonmodulars. Thus, modular
sociality in Asian colobines may have arisen because both social benefits are substantial and ecological costs are relatively low.

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Additional indexing

Item Type:Journal Article, refereed, original work
Communities & Collections:07 Faculty of Science > Department of Anthropology
Dewey Decimal Classification:300 Social sciences, sociology & anthropology
Language:English
Date:2010
Deposited On:20 Mar 2010 16:05
Last Modified:05 Apr 2016 13:35
Publisher:Oxford University Press
ISSN:1045-2249
Additional Information:This is a pre-copy-editing, author-produced PDF of an article accepted for publication in Behavioral Ecology following peer review. The definitive publisher-authenticated version "Evolutionary determinants of modular societies in colobines /Cyril C. Gruetera and Carel P. van Schaik. Behavioral Ecology 2010 21(1):63-71" is available online at: http://beheco.oxfordjournals.org/cgi/content/abstract/21/1/63
Publisher DOI:https://doi.org/10.1093/beheco/arp149
Permanent URL: https://doi.org/10.5167/uzh-24578

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