Folia Primatologica 71 3 00 May-june 2000
Folia Primatol 2000;71:154--156 Received:september 2,1999
Accepted after revision October 8,1999
Occurrence of Twins In Wild
Formosan Macaques , Macaca cyclopis ,
at Mt. Longevity , Taiwan
Minna J. Hsu Jin-Fu Lin Govindasamy Agoramoorthy
Department of Biological Sciences, National Sun Yat-Sen University , and
Shi-Pu Junior High School , Kaohsiung , and Department of Wildlife Conservation ,
NPUST , Taiwan ,ROC
Key Words
Formosan Macaques . Macaca cyclopis . Mt. Longevity . Taiwan Twins
Introduction
The Formosan macaque （Macaca cyclopis）is a threatened species , endemic to Taiwan . Little is known about its reproduction in the wild , and there has been one study of the species . We report here , for the first time , the occurrence of twins in a wild population of Formosan macaques that inhabits Mt. Longevity , southern Taiwan.
Material and Methods
Mt. Longevity lies west of Kaohsiung City , Taiwan . It is an area of tropical lowland rainforest , 1,116 ha in size . A total of 209 plant species occur there , The predominant tree species are Ficus septica , F. caulocarpa . The wet season , with a monthly average rainfall of 180mm , lasts from May to September , and the dry season from October to April . The monthly averagetemperature was lowest in January （18.4°c） and highest in July （28.7°c）.
Since January 1995 , demographic data have been collected twice a month from 16 troops of habituated macaques . All troops have been monitored for few new infants three times a weekduring the birth season （February to August ）. Subjects mentioned in this paper are individually known . Kin relations and ages were verified from genealogical records compiled by the authors .
Result
A total of 479 infants were born between January 1996 and August 1999 , of which 5 were twins .
Cases 1 and 2
Two adult females in troop I were seen with newborn twins on May 11 , 1996 . both infants of female No.8 were female , which No.17 had a bisexual pair . These 4 individuals , now over 2 years old , are still alive at the time of writing （September 1999）. Female No.8 delivered a single infant a year later . Female No.17 delivered a single infant 2 years later （on May 6,1998）.
Case 3
The adult female No.6 in troop D gave birth to a pair of female twins on April 4,1998, but one of them died on May 3. Prior to the infant’s death , a subadult aunt was seen carrying it for 3 days in the proximity of the infant’s mother . The infant had nipple contact with the aunt and produced intermittent screams , but the mother neither reacted nor attempted to retrieve it . Stress and starvation might have caused its death .
Case 4
On May 4 , 1998 , the adult female No.10 in troop K was seen with a pair of male twins . One of these died after 19 days , while the other alive at the time of writing .
Case 5
On May 15 , 1999 , the adult female No.2 in troop B gave birth to an infant female at dawn , but it was dead within a few hours . On May 24 ,1999, the same female delivered a second infant , a male, which died after 7 days . On the third and sixth days , a subadult female aunt was seen carrying the infant during the day for 2.5 and 3.5h. respectively , but the infant was with the mother for the rest of time . Although the infant was seen suckling from its mother , there was nosigh of lactation.
Discussion
The occurrence of twins is rare among macaques and only a few cases have been reported in the well-studied M. mulatta【5-7】 and M. fuscata【8,9】 . Among 1,171 births recorded at Cayo Santiago（M. mulatta）,only one（0.1%）was of twins 【8】. The twinning rate for M. cyclopis at Mt. Longevity was comparatively high（1.1%）.
Two sets of twins have survived for more than 2 years . One of the mothers delivered her subsequent infant a year later , while the other took 2 years . This rises the question : is the maternalinvestment needed to raise 2 infant costly for non-human primates？ Only two previous studies have addressed this question . In one , captive M. mulatta mothers who reared twins produced significantly fewer offspring the following birth season compared to mothers who reared single infants【10】 . In the second , a lactating female howler monkey（Alouatta seniculus）caring for her own infant adopted a kin-related unfant after its mother had disappeared . She was able to raise the two until weaning , but her subsequent birth interval was twice as long as normal【11】 .
The behavior of ‘aunting ‘ described in case 3 and 5 has been reported in seceral non-human primate species【12】. A possible explanation for the lack of maternal intervention to retrieve the infant when it was under the aunt’s care（case3）might have been due to the fact that the mother was preoccupied caring for the other twin. However , though the subadult female may have gained mothering experience from handling the infant, its mother’s negligence appears to be a maladaptive response that needs further study .
Acknowledgements
Partial funding for the on-going field research on Formosan macaques at Mt. Longevity has been provided by the Republic of China’s National Council （NSC 88-2313-B-020-023）through a research grant awarded to G.A. and M.J.H.
Reference
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