Like humans engaged in risky activities, group members of some animal societies take turns acting as sentinels. Explanations of the evolution of sentinel behavior have frequently relied on kin selection or reciprocal altruism, but recent models suggest that guarding may be an individual's optimal activity once its stomach is full if no other animal is on guard. This paper provides support for this last explanation by showing that, in groups of meerkats (Suricata suricatta), animals guard from safe sites, and solitary individuals as well as group members spend part of their time on guard. Though individuals seldom take successive guarding bouts, there is no regular rota, and the provision of food increases contributions to guarding and reduces the latency between bouts by the same individual.
Summary 0[ In social mammals where group members cooperate to detect predators and raise young\ members of small groups commonly show higher mortality or lower breeding success than members of large ones[ It is generally assumed that this is because large group size allows individuals to detect or repel predators more e}ectively but other bene_ts of group size may also be involved\ including reduced costs of raising young and more e}ective competition for resources with neighbouring groups[ 1[ To investigate the extent to which predation rate a}ects survival\ we compared mortality rates in two populations of suricates "Suricata suricatta#\ one living in an area of high predator density "Kalahari Gemsbok Park# and one living in an area of relatively low predator density "neighbouring ranchland#[ Most aspects of feeding ecology and growth "including time spent feeding\ daily weight gain\ growth\ adult body weight\ breeding frequency and neonatal mortality# were similar in the two populations[ In contrast\ mortality of animals over 2 months old was 0=6 times higher in the Park than on ranchland[ 2[ Mortality of juveniles between emergence from the natal burrow and 5 months of age was higher in small groups than large ones in the Park but signi_cantly lower in small groups than large ones on ranchland[ Adult mortality declined in larger groups in both areas[ 3[ The tendency for survival to be low in small groups had far!reaching consequences for the risk of group extinction[ During a year of low rainfall in the Park\ all groups of less than nine animals became extinct and population density declined to around a third of its initial level[ We argue that high group extinction rates are to be expected in species where survival declines in small groups and mortality rates are high[ Key!words] cooperative breeding\ demography\ mammals\ mortality[ Journal of Animal Ecology "0888# 57\ 561Ð572
"Limited control" models of reproductive skew in cooperative societies suggest that the frequency of breeding by subordinates is determined by the outcome of power struggles with dominants. In contrast, "optimal skew" models suggest that dominants have full control of subordinate reproduction and allow subordinates to breed only when this serves to retain subordinates' assistance with rearing dominants' own litters. The results of our 7-year field study of cooperative meerkats, Suricata suricatta, support the predictions of limited control models and provide no indication that dominant females grant reproductive concessions to subordinates to retain their assistance with future breeding attempts.
In cooperative groups of suricates (Suricata suricatta), helpers of both sexes assist breeding adults in defending and feeding pups, and survival rises in larger groups. Despite this, dominant breeding females expel subordinate females from the group in the latter half of their (own) pregnancy, apparently because adult females sometimes kill their pups. Some of the females that have been expelled are allowed to rejoin the group soon after the dominant female's pups are born and subsequently assist in rearing the pups. Female helpers initially resist expulsion and repeatedly attempt to return to their natal group, indicating that it is unlikely that dominant females need to grant them reproductive concessions to retain them in the group.
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