Female “dispersal” in hamadryas baboons: Transfer among social units in a multilevel society

Abstract

Unlike most cercopithecines, hamadryas baboons (Papio hamadryas hamadryas) are characterized by female-biased dispersal. To clarify this pattern within the context of their hierarchical social system (comprising one-male units, clans, bands, and troops), we report here 7 years of data on female transfers among social units in wild hamadryas baboons in Ethiopia. Female tenure in one-male units (OMUs) ranged from 1 to 2,556 days (N = 208) and survival analysis revealed a median tenure length of 1,217 days (40 months). Changes in OMU membership consisted almost exclusively of takeovers by males, not voluntary transfer. Of 130 takeovers, 67% occurred within the band and 33% across bands, and, of the 22 takeovers for which we have clan membership data, 77% occurred within, not between, clans. These results reinforce the notion that hamadryas female dispersal is not analogous to sex-biased dispersal in other taxa, because (1) at least in Ethiopian populations, females do not disperse voluntarily but are transferred, often forcibly, by males; (2) only dispersal between bands will promote gene flow, whereas females are most often rearranged within bands; (3) hamadryas females undergo social dispersal but not usually locational dispersal; and (4) while male hamadryas are far more philopatric than females, they have been observed to disperse. It thus appears that the ancestral baboon pattern of female philopatry and male dispersal has evolved into a system in which neither sex is motivated to disperse, but females are forcibly transferred by males, leading to female-mediated gene flow, and males more rarely disperse to find females.

Alternative routes to the leader male role in a multi‐level society: follower vs. solitary male strategies and outcomes in hamadryas baboons

“…It appears that hamadryas leader males do maintain near exclusive access to their females for their tenure as leader and are limited primarily by the length of time that they can continue to physically defend those females from rival males [Swedell & Saunders, 2006;Swedell et al, 2011]. Although many of these potential rivals focus predominantly on juvenile females and some become followers and ''inherit'' females, there will always be opportunistic solitary males that are willing and able to take over a leader's females should he show signs of weakness.…”

Male dispersal in a provisioned multilevel group ofRhinopithecus roxellanain Shennongjia Nature Reserve, China

“…Only dispersal between bands can change genetic structure. Although male hamadryas baboons are more philopatric than females, they do disperse between OMUs, clans, even between bands [Phillips- Conroy & Jolly, 2004;Phillips-Conroy et al, 1992;Swedell et al, 2011].…”

Not without a fair fight: failed abductions of females in wild hamadryas baboons

“…In contrast to the prevailing pattern of male dispersal found in most papionin monkeys, hamadryas baboons are characterized by a pattern of dispersal mainly by females (Kummer 1968;Sigg et al 1982;Abegglen 1984;Moore 1984;Swedell 2011;Swedell et al 2011). Also unusual is their complex multilevel society consisting of nested onemale units (OMUs), clans, bands, and troops (Kummer 1968;Abegglen 1984;Swedell 2006;Schreier and Swedell 2009).…”

“…The vast majority of such takeovers appears to be long lasting (Sigg et al 1982;Abegglen 1984;Swedell 2000;Swedell et al 2011;Pines et al 2011);however, Abegglen (1984) and Swedell (2000), while not observing the actual event, each described an outcome in which the previous leader male regained his female: the former in the space of a day, and the latter, in which the female was pregnant, after 17 days.…”

“…Sigg et al (1982) suggested that such aggressive encounters between groups may provide opportunities for foreign males to herd these separated females into their own unit; i.e., a possible mechanism for female-biased dispersal. Because hamadryas females do not usually disperse voluntarily (Abegglen 1984;Swedell et al 2011), abductions during aggressive intergroup encounters are still generally thought to be the main mechanism for female transfer between bands (Kummer 1968;Sigg et al 1982;Abegglen 1984). To date, however, no successful abduction of females during intergroup conflict has been observed in wild hamadryas.…”