Effects of group size on birth rate, infant mortality and social interactions in Formosan macaques at Mt Longevity, Taiwan

Abstract

In social animals, the influence of life-history traits on group structure and size is potentially important in the context of the evolution of social systems. Birth rate and infant survival have been differently predicted as a function of group size by models of the evolution of group living in primates. A wild population of the Formosan macaque, Macaca cyclopis that inhabits Mt Longevity, Taiwan has been monitored since July 1993 to collect demographic, reproductive and social behaviour data. Little is known about the effects of troop size on population growth in Formosan macaques. In this paper, we have presented for the first time 5-years of data (1997–2001) on the troop dynamics of 14 social troops. We also examined the effects of troop size on birth rate, infant mortality and inter-troop social interactions. The annual average troop size increment fluctuated from 1.94% to 10.97%. The annual average troop size of 14 groups was 43.57 ± 15.88 individuals in 1997, and increased to 55.97 ± 16.86 individuals in 2001. The per-capita rates of troop size increase was negatively correlated with troop size without infants (F1,12 = 5.57, P < 0.05). The average birth rate (birth per adult female) was 0.78 (± 0.07, n = 14) and it was not linearly related to troop size, nor the number of adult females (P > 0.33). The average infant mortality was 0.16 (± 0.09, n = 14), which was relatively constant regardless of the troop size category (small, medium or large). Intertroop dominance was closely dependent on the number of adult males and females rather than group size. Thus larger group size imposed an advantage on habitat utilization without the appearance of low birth rate or high infant mortality. On the other hand, infant mortality increased and infant survival per adult female decreased when the percentage of adult females increased in troops. Therefore, competition among adult females plays an important role partially supporting the predation — intra-group feeding competition hypothesis.