Facing the crowd: intruder pressure, within‐group competition, and the resolution of conflicts over group‐membership

Abstract

Recent theory in social evolution has been mainly concerned with competition and cooperation within social groups of animals and their impact on the stability of those groups. Much less attention has been paid to conflicts arising as a result of solitary floaters (outsiders) attempting to join groups of established residents (insiders). We model such conflicts over group-membership using a demographically explicit approach in which the rates of births and deaths in a population determine the availability of group-vacancies and the number of floaters competing over these vacancies. We find that the outcome of within-group competition, reflected in the partitioning of reproduction among group members, exerts surprisingly little influence on the resolution of insider-outsider conflict. The outcome of such conflict is also largely unaffected by differences in resource holding potential between insiders and outsiders. By contrast, whether or not groups form is mainly determined by demographic factors (variation in vital rates such as fecundity and mortality) and the resulting population dynamics. In particular, at high floater densities territory defense becomes too costly, and groups form because insiders give in to the intruder pressure imposed on them by outsiders. We emphasize the importance of insider-outsider conflicts in social evolution theory and highlight avenues for future research.

Patterns of male dispersal in Verreaux's sifaka (Propithecus verreauxi) at Kirindy Mitea National Park

“…Population density has been predicted to be a critical factor in determining the ability of resident males to resist immigrants [Port & Johnstone, 2013], and studies of multiple Verreaux's sifaka populations in different habitats can highlight how such factors can influence dispersal patterns.…”

“…For instance, predation risk, population density, and home range size and overlap may determine whether becoming a solitary male (floater) is a viable strategy because extragroup males may not only be vulnerable to predation [Struhsaker, 1969;Steenbeek et al, 2000] but also cannot afford the travel costs needed to monitor highly spread out potential destination groups [Moore, 1999]. High intruder pressure due to high population density [Moore, 1999;Borries et al, 2011;Shonfield et al, 2012], floater density [Port & Johnstone, 2013], many extra-group males relative to the number of female groups [Cords, 2002], and/or mean number of males per group [Moore, 1999;Borries et al, 2011] can reduce the ability of resident males to resist immigration attempts [Moore, 1999;Port & Johnstone, 2013] and thus lower immigration costs for dispersing males. Males with high competitive ability are more likely to acquire the dominant position in a group by ousting resident males with aggressive takeovers [van Noordwijk & van Schaik, 2004], whereas less competitive males may enter new groups unobtrusively as subordinates [van Noordwijk & van Schaik, 2001], reside in all-male bands [e.g., Hanuman langurs: Sommer & Rajpurohit, 1989], or become floaters.…”

A Saki Saga: Dynamic and Disruptive Relationships among Pithecia aequatorialis in Ecuador

“…Joint mate and/or territorial defense may be especially important if floaters (i.e. solitary individuals without territories of their own) are prevalent in the area [Port and Johnstone, 2013;Thompson, 2015]. Indeed, a study of Azara's owl monkeys (Aotus azarai) revealed that encounters with floaters can lead to high turnover rates among pair-mates [Fernandez-Duque and Huck, 2013].…”

Alpha male replacements in nonhuman primates: Variability in processes, outcomes, and terminology

“…Finally, the rate at which alpha male replacements occur is variable, both within and between species. As noted above, the likelihood of challenges to the alpha position has been linked to the density of non group males (Butynski, , ; Hrdy, ; Port & Johnstone, ), which may be greater in species with uni‐male, multi‐female social units (Janson & van Schaik, ). Larger female group sizes also may increase the rates at which alpha male replacements occur because these groups are more attractive to non group males (e.g., Beehner & Bergman, ; A. seniculus , Crockett & Janson, ; T. gelada , Dunbar, ; C. capucinus , Fedigan & Jack, ).…”