Zoological Studies

Troop Size and Structure in Free-ranging Formosan Macaques (Macaca cyclopis) at Mt. Longevity, Taiwan

Minna J. Hsu and Jin-Fu Lin (2001) Among the 19 extant species of the genus Macaca that are found in southern and eastern Asia as well as northwestern Africa, the Formosan macaque is one of the least known. A long-term field study to investigate the population dynamics and social behavior of 7- 16 troops of free-ranging Formosan macaques at Mt. Longevity, Taiwan has been conducted since July 1993. Between Dec. 1994 and Dec. 1997, a systematic census was conducted on a biweekly basis to record data on the demography of Formosan macaques. We used focal animal sampling and ad libitum sampling twice per week in respective troops to record data on social behavior including male replacement and fission processes. The maximum density of macaques has been estimated as 26 individuals per km² in Oct. 1997.  The average troop size was 26.1 ± 9.7 (n = 7) in Jan. 1995 and it reached the highest level of 47.0 ± 21.2 (n = 13) in Aug. 1997. Two cases of fission were observed. The branch troops, la and Aa, that were formed as a result of fission had the smallest size with 9 individuals in the beginning, while troop l had the largest size of 86 individuals. Births were recorded mainly between Apr. and June (97%) with a peak in mid-Apr. to mid-May.  The annual average overall sex ratio was 1.06 ± 0.28, while the adult sex ratio (adult males to adult females) was 0.53 ± 0.12. The average tenure length of alpha males was 16.8 ± 18.9 mo (n = 34) and ranged from 1 wk to a maximum of 6 yr. The average alpha male tenure in newly formed troops was significantly shorter (p < 0.01) than that in the remaining troops. About 88% of alpha male changes occurred between Oct. and Feb., which paralleled the peak and the end of the mating season, respectively.