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Population regulation: the effects of severe environmental changes on the demography and behavior of wild toque macaques

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Humans have lived in tropical Asia for at least 500,000 years, judging from fossil sites in Java. For most of human history, they were hunters and gatherers, but even at this early stage they had some impact on the environment (Rambo, 1979). Once fire became an important tool (probably several hundred thousand years ago), human impact on the environment began to grow, particularly as settlements increased along coasts and rivers. The alluvial flatlands of Asia, once clothed in various kinds of freshwater swamp forest, have long ago been almost completely replaced by rice fields. The development of shifting cultivation (see chapter 4) accelerated the impact, at least in the hilly areas. Irrigation, cultivation of wet rice, and the spread of industrial export-driven agriculture and logging during the past few decades now gives human beings the potential to overwhelm virtually all forested habitats.
During a 16-month study of semifree-ranging Barbary macaques (Macaca sylvanus) the group under observation divided into two groups. Observations were carried out in 1987–1988, at «La Montagne des Singes,” Kintzheim, France. A subgroup of monkeys, which was already cohesive at the beginning of the study, became progressively autonomous in relation to the rest of the main group, during the mating season. Overt aggression between the males of the two groups during this period brought about the fission. Only low-ranking genealogies left their group of origin. Dominance relations between females remained identical in both groups except for one lineage. The alpha male and the alpha female of the subgroup had a close relationship before the fission occurred. The sequence of agonistic intergroup relations is described and analyzed in relation to male sexual competition and female alliance power. The results suggest that: (1) the males of the subgroup instigated the fission because it was the best strategy for them to counter sexual competition; and (2) the females followed the males in order to maintain their alliance network, necessary to insure their dominance status over subordinate females.
A group of toque macaques took-over the home range of one of its subordinate neighboring groups and fused with it to form a larger cohesive group. In the 7 years before the take-over, the dominant group had consistently won all contests at common feeding sites, yet the fitnesses of the females of these two groups did not differ significantly (Fig. 2A). After the take-over the females of the subjugated group occupied the lowest ranks in the combined dominance hierarchy of the merged groups (Fig. 1) and thereby lost the advantages of an own home range, such as priority of access to food. Consequently, in the merged group, survivorship and reproductive success among the subjugated females were significantly less than among the females of the dominant subgroup (Table 2, 4). The dominant matrilines grew numerically and replaced all of the subjugated females, and all but one of their offspring, within 8 years after the take-over (Fig. 2B). These data support the hypothesis that cooperation among female kin in defending resources against strange females is important in the evolution of female-bonded groups. Before the merger all 5 natal males of the subordinate group had transferred to the dominant group, where they occupied high and mid-level dominance ranks (Fig. 1). These males survived at a significantly greater rate than their subordinate female kin. Thus, the cost of group transfer seems to be greater for females than for males, and this may be one reason that females generally do not emigrate or that groups do not fuse. The data suggested three hypotheses. First, since large body size and other adaptations for fighting, giving males an advantage in male-male competition for mates, are also of advantage in resource competition with males and females, such male characters may also be favored by non-sexual selection, especially where male reproductive strategy involves group transfer. Second, female bonded groups evolved as female defensive coalitions against not only female but also male resource competitors, there having been a mutual influence in the coevolution of large-sized males and female gregariousness. Third, female defensive coalitions against large-sized aggressive males are also advantageous out-side the context of food competition, or, independent of foraging strategy.
At Polonnaruwa, Sri Lanka, four out of 29, groups of toque macaques, Macaca sinica, divided in a period of 16 years. Temporary peripheral subgroups of varying sizes and compositions preceded fission by 9–40 months. Fission crystallized within a month through an increase and stabilization of subgroup membership and permanent division. All members in the newly seceded groups had been frequent participants in pre-fission subgroups, and belonged to subordinate matrilineages. Subgroups, and hence group divisions, were initiated by cores of mutually loyal females and occurred mostly along kinship lines. In the year of fissions, the rate of change in female dominance relations was significantly greater among groups that divided than among those that did not. It is hypothesized that low-ranking females secede to form new groups when the costs, especially of intragroup competition for food resources, outweigh the benefits of group membership. Such seceding females were easily available and familiar mates for group males that had recently lost rank. Final division, therefore, resulted from a coalition of subordinate females and males acting according to their respective interests. It was triggered in this population by rapid growth of some groups to large size and by environmental stress (the reduction and fragmentation of food resources caused by drought and a cyclone), which accentuated the costs of resource competition. Male aggression, such as infanticide, which negatively affects female fitness, might also have contributed to one group fission.
Exploited forests have frequently been regarded as being of little use in the conservation of animal species. While it is true that many species are adversely affected by selective logging and some are entirely unable to survive in disturbed forest, a large number of species are able to maintain viable populations. Logged forests may have an important rôle in the conservation of rain-forest animals as they will continue to occupy large land areas, whereas primary forest reserves will be increasingly restricted in size. Primary forests are of vital importance in supporting a full range of rain-forest species, but logged forests are able to support many animals, including some that cannot survive in small, isolated primary forest reserves.
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