Cheyne and Chivers: Gibbons Sexual Swelling - PDF document

Cheyne and Chivers: Gibbons Sexual Swelling ABSTRACT Female gibbons were observed for labial swellings from March 2002 – August 2003. Data were collected on length of time each female was swollen, changes in swelling period as females matured and cycle length (number of days between onsets of viour of the males when the females were swollen. There was a significant relationship between age of the female gibbon and number of days she had labial swagilis. albibarbis) and number of days swollen. Copulation was more common when females were swollen. Swelling in female gibbons, may be related to to provide the male with some certainty of paternity INTRODUCTION d in several primate species (Short, to be attractive to males (Bielertsexual skin swellings live in multi-male social systems. These exaggerated anogenital swellings are found only in Old World monkeys and apes and have been seen in many et al., 1979);(Dixon, 1983); Nunn, 1999). ‘best-male’ (Clutton-Brock and Harvey, 1976); ‘many male’ (Hrdy, 1981) and the hypotheses (Domb and Pagel, 2001; (Domb & et al., 2004) and disproved for several species (Emery & Whitten, 2003); (Setchell & Wickings, 2004). l swellings in a wide array of species if we are to understand the evolution of exaggerated swellings, as well as other sexual traits, in primates. In that regard, we also need more quantcomparisons. Detailed 2 Cheyne and Chivers: Gibbons Sexual Swelling measurements of successive swelling cycles are necessary to investigmales in relation to sexual swelling. (Dahl & Nadler, 1989; Dahl & Nadler, female gibbons also have conspicuous swelling of the outer labia du1999) noted that some features of exawith features that may confuse or bias paternity among males. Nunn does not consider labia is involved. In some e much larger than in others and that they sometimes involve areas of skin beyond the immediate genital (l tissues. While gibbons live in small monogamous family groups, they are not always exclusively monogamous in their mating behavior, as extra-pair copulations have been recorded in some populations (monogamous (vigorously defend a territory monogamous as they have befragmented/isolated populations (Bartlett, 2001; Palombit, 1994; Reichard, 1995). Dahl swelling may assist in maintaining the pair female to retain and/or change mates. When gibbons are socially monogamous, the swelling may serve to increase certainty of paternity, while still allowing the female to attract other mates. Other primates thatpromiscuous, and frequently have multiple sexual partners e.g. chimpanzees and macaques. While gibbons have been seen to predominantly socially monogamous yet present sexual skin swellings smaller than those of more promiscuous species of primates. Thus the mating hypothesis could explain the presence of anogenital swellings in gibbons as a female adaptation to ensure the male remains to help raise the offspr From March 2002 – September 2003, a study was conducted on the behaviour of bilitation Project, Central Kalimantan, SMC noted that female gibbons exhibited cyclical swelling of the genital area. This paper is based on data collected systematically 3 Cheyne and Chivers: Gibbons Sexual Swelling was to determine if there was any pattern to the anogenital swellings of female gibbons, if there was any relationship between age of female and number of days anogenital swelling was present, and if the presence of swellifemale and her mate. Long-term observatiovulation period of the female gibbon is imsystem as it relates to the Old World monkeys and apes. METHODS Hylobates agilis albibarbisH. muelleriKalimantan, Indonesia. The gibbons were all wildwere housed in pairs with a male. Age estimates made from canine size taken during the medical given to all gibbons when they arribout 3yrs and being fully emerged by 4yrs. Wear and tear can be used to estimate age of adults). Table 1 Study females: age, number of days swelling was present and average cycle length. Values presented are averaged across all females of the same age. Estimated age (years) Number of females Average number of days swelling was observed Average cycle length 2 1 4 23 3 1 5 23 4 2 5.5 18 5 5 5 24 7 1 7 27 8 3 7 24 9 2 7 25 12 1 8 27 13 1 7 24 Total = 17 Overall average = 6.3 days Overall average = 23.8 days 4 Cheyne and Chivers: Gibbons Sexual Swelling All females were observed from March 2002 to August 2003. Data were collected on each individual’s activity and behaviour for six days each month (during which all 10-min scan sampling: copulation data were collected during this time). Data on anogenital swelKalaweit staff). All females were observed for the presence of swelling and a total of 17 females showed regular swellings and 17 separate swelling cycles were observed for each female (17 months data collection and 1 cycle/female/month). The total number of observation hours on all 17 females over 18 approach to within 1m of all study animals)each female was observed for equal periods of time while swollen and while not swollen al swelling. The presence or absence of level of swelling scale was developed as follows: 0) No swelling (Photo 1) Small swelling (only a small area of pink a partially swollen, Large swelling (large pink-red area visible and whole labia swollen, Photo 3) 5 Cheyne and Chivers: Gibbons Sexual Swelling Photo 1 Subadult female H.muelleri (left) showing no swelling. Photo 2 Adult female H.agilis albibarbis with small swelling. 6 Cheyne and Chivers: Gibbons Sexual Swelling Photo 3 Adult female H. muelleri with large swelling. ficance difference was vations were not in agreement, Table 2). Table 2 Inter-observer bias for scoring swelling size. Observer Number of paired Number of observations not in agreement 1 30 1 2 30 2 3 30 1 4 30 3 Total 120 7 Swelling period is described as the number of days from onset of swelling (whether small or large) to complete absence of swelling (detumescence). Menstrual cycle length was described as the number of days between onset RESULTS It was impossible to measure accurately the degree of swelling of all the female e the development of the scale, by which all data were 7 Cheyne and Chivers: Gibbons Sexual Swelling for longer (in days) than younger females. Juvenile females were seen to have swellings. Despite many gibbons living in captivity in zothere appears to be very little information available regarding their the accompanying swelling of the vulvae. (Geissmann, 1991) suggests that the onset of menstruation may not be an accurate predictor of the onset of sexual maturity. Gibbons may undergo a period in which they are sexuathe observed delays between the onset of menarche and the time ofThus the focus was on the observable cycle le The most interesting result is the reswelling and solicitation rates. If the purpose of the conspicuous swelling is to retain the presence of the male, we would expect that the female would allow him more mating rights or the male would attempt more mountings during the swelling period, as he associates the swelling with sexual activity. From the data, this does appear to be the case, though there was insufficient information to determine whether the increased number of copulations was due to an increase in presenting from the female or from an increase in mounting attempts by the male due to the visual stimulus of the female swelling. This swelling has a predictable cycle length, which increases as the female ages, increasing her sexual ‘attractiveness’ to the male, thereby ensuring that he remains with the female to defend joint territory atheory is that the swelling functions as fertilisation insurance, i.e. indicates when the male should mate to ensure maximum likelihood of conception. One suggestion is that the swelling serves to attract multiple males during the initial pair-formation (Dahl and Nadler, 1992a) swelling (possibly indicating female availability) and inter-male competition. The sexual swelling may also mediate pair mainteovulation in the female gibbon would not give the male any certaintythere would be no incentive for the male to stay and invest time and energy in the female, territory. Thus, to ensure that the male has some certainly remains to defend the female, the female has a small, but obvious indicator 10 Cheyne and Chivers: Gibbons Sexual Swelling difference between the numbers of days the females showed swelling. Additionally, males copulated with females more and most frequently in the 1-2 days preceding detumescence. Most females were observed to cycle, as identified by the presence of a pronounced sexual swelling of the outer labia, which waxed and waned each month within the period of their menstrual cycle.was not seen when the females were swollen. Data on the duration over which ovulation occurred matched that noted by Nadler et al. females over a total of 6 cycles. The sexual skin swellings of gibbons cannot be explained by any of the previously proposed hypotheses: Best male – gibbons are monogamous and there is no competition among males for access to females Many males – while gibbons do appear tois not a common social strategy and EPC’this hypothesis is the main reas swellings of the gibbons in this study. More information is needed to accurately measure the size of swellings, but this hypothesis has been proposed for multi-male species and has received criticism. Following on from the work of Dahl and between sexual swelling and mate-retentiof paternity’ where the female uses the swelling to advertise to the pair-male when she is most receptive, thus ensuring his paternity and ensuring he will remain with the female to assist with territorial defence. The sexual swellings may have a as females could use these to non-pair males when they are reon the relationship between sexual swellings and EPC’s in the wild. 11 Cheyne and Chivers: Gibbons Sexual Swelling ACKNOWLEDGEMENTS with data collection: Pak Didi, Amud, Setia, Nanto, Sinar, and Penyang. Also thanks to LIPI, the Indonesian Institute of Science for permission to work in Kalimantan. Many thanks to Raffaella Commitante for comments on earlier versions of this manuscript. Funding was provided by The Gibbon Foundation, the C.K. Marr Educational Trust and Downing College, Cambridge. REFERENCES ult gibbons: implications for Bielert, C., L. Grirolama, and S. Jowell. (1989). An experimental examination of the colour component in visually mediated sexual arousal of the mala chacma baboon Ethology. Cambridge University Press, Cambridge. genitalia of female gibbons. Journal of Primatology of female gibbons er. (1992b). 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