the ovulatory week (OW), with females out side of their OW and not pregnant (NOP), ... in one female; one OW was interrupted at 4 days because the observation ...
PRIMATES, 34(4): 503-511, October 1993
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Male Dominance Rank and Reproductive Success in an Enclosed Group of Japanese Macaques: with Special Reference to Post-conception Mating MIHO INOUE, FUSAKO MITSUNAGA, MASUMI NOZAKI, HIDEYUKI OHSAWA, AKIKO TAKENAKA, YUKIMARU SUGIYAMA, KEIKO SHIMIZU, and OSAMU TAKENAKA
Kyoto University
ABSTRACT. The mating behaviour and reproductive success of male Japanese macaques (Macaca fuscata) were studied in relation to the female sexual cycles, which were monitored from the plasma profiles of gonadotropins and ovarian hormones. Based on observations of the mating behaviour during four successive mating seasons and paternity identification by DNA fingerprinting in 35 out of 37 offspring born in the subsequent birth seasons, the correlations between (1) male dominance rank and timing of mating, and (2) male dominance rank and reproductive success were examined. The results may be summarized as follows. (1) The number of copulations with ejaculation by any male was positively correlated with the male dominance rank, but not with the identified numbers of offspring fathered by each male. (2) Males could not choose ovulatory females as mating partners: the number of copulations with ejaculation with females during ovulatory weeks was not related to the male's rank. Monopolized copulations in consortship were mostly observed between high-ranking males and non-lactating parous females after conception. (3) Paternity testing showed that the male copulating most frequently with a female was not the identified father in 11 out of 15 cases. Prediction of the fathers of offspring was difficult even from the number of copulations occurring at around the estimated time of ovulation. An adaptive explanation of these correlations is discussed. Key Words: Japanese macaque; Paternity test; Mating behaviour; Hormonal status; Estimated ovulation; DNA fingerprinting.
INTRODUCTION The multimale group structure and promiscuous mating patterns o f Japanese macaques have led to difficulties in paternity discrimination. Employing D N A analysis, we previously determined the paternity of 70 offspring born in two captive groups of Japanese macaques kept at the Primate Research Institute (INOUE et al., 1990, 1991, 1992). A m o n g highranking adult males, the number of offspring did not reflect the males' social rank. Inbreeding was avoided among maternal kin, but not between paternal half-siblings or within father-daughter relations. The mating behaviour in a captive Japanese macaque group was observed in order to evaluate the relationship between male dominance rank and mating success. Although the number of copulations with ejaculation was correlated with male dominance rank, it was not correlated with number of offspring (INOUE et al., 1991). By contrast, in Macaca fascicularis (DE RUITER et al., 1992), Macaca mulatta (SMITH, 1981), and Macaca sylvanus (PAUL & KUESTER, in press; PAUL et al., 1993), male rank is reportedly related to reproductive success. The key to assessing the relation between mating success and reproductive
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success appears to be the easiness of detection of ovulatory females by partner males, such as from the degree of swelling and/or colouration of the female sex skin during the ovulatory cycles, and whether or not these characteristics represent a signal predicting the timing of ovulation. In chimpanzees, for example, the incidence of ovulation may be easy to estimate from the degree of swelling of the sex skin, and the first ranking male tends to copulate during consort relationships with ovulating females (HASEGAWA & H1RAIWAHASEGAWA, 1983; TAKASAKI, 1985). In Japanese macaques, evaluation of ovulation is difficult from visual cues such as the colouration of the sex skin or female behavioural changes. Accordingly, we monitored the females' reproductive conditions by means of plasma profiles of reproductive hormones, and examined the correlation between male dominance rank and the timing o f mating.
METHODS OBSERVATIONOF MATING BEHAVIOUR Wild Japanese macaques were introduced into Primate Research Institute, Kyoto University, from Wakasa (134~ 35~ Tottori Prefecture, western Japan, in 1974, and named as the Wakasa group. Two males and eight females were selected to make up the group which was kept in a walled outdoor enclosure of 500 m 2 in area. In 1987, the group consisted of 57 individuals of different ages including 7 of the original members (2 males and 5 females). In this group, males have been confirmed to capable of siring offspring after the age of 5 yrs, although they were observed to ejaculate at 4 yrs old (INOUE et al., 1990). Females can be impregnated at =>4 yrs. We therefore analyzed behavioural data for females aged _ 4 yrs and males aged =>5 yrs. Table 1 shows the numbers of subject animals in each year of 1987-1990. Each observation period was designated as mating season I to IV, respectively. Over the four years of mating seasons I to IV, a total of 12 high-, 13 middle-, and 13 low-ranking males were present. The average age in years of each rank of males was 15.5 (high), 7.3 (middle), and 6.2 (low). During mating season I, the mating behaviour of all individuals in the group was observed throughout the daytime, starting one hour before sunrise and finishing one hour after sunset by seven researchers in turn daily (INOUE et al., 1991). The total observation hours was about 1500. We regarded any mounting of a female by a male as 'mating,' and recorded its duration, the identity of the individuals involved, and the presence/absence of ejaculation. In the subsequent three mating seasons, we continued observations of the mating behaviour of the group for two hours daily, begin-
Table 1. Constitution of the Wakasa group.
No. of males No. of females No. of Observation period (>5 yrs) (>4 y r s ) impregnated females No. of stillbirths I: Oct. 1987-Feb. 1988 9 18 8 0 1~: Oct. 1988-Feb. 1989 9 22 10 0 IlI: Oct. 1989-Jan. 1990 10 22 12 1 IV: Oct. 1990-Jan. 1991 10 24 8# 1" #The mother of one of the offspring is presumed to have become pregnant again after she had experienced a stillbirth; *we were unable to obtain a DNA sample from the baby's dead body.
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ning from one hour after sunrise. About a half of all matings had occurred during these two hours in mating season I. The frequency of mating of the males was recorded and analyzed by counting all "pairing-days" with individual females. The number of copulations for each male was counted as the total number of days on which copulation between the male and each and any individual female was observed. For example, if a male mated with three different females in a day, the number o f copulations added to his total was three. The sum o f the copulations across all males in mating season I was 1355. In mating season I, 32% of the copulations terminated in ejaculation, and the remaining 68% of the copulations were discontinued as a result of disturbance by higher-ranking males, or for other reasons. Since only the number of copulations with ejaculation was related to male rank (INOUE et al., 1991), the data for "copulation" given below refer only to the number of days in which copulation with ejaculation was observed. METHODS FOR ESTIMATION OF THE FEMALE OVULATORY PERIODS
Blood samples were collected weekly from all females aged > 4 yrs in mating seasons III and IV, and the plasma concentrations of estradiol-17t3, progesterone, inhibin, and luteinizing hormone were measured by appropriate radioimmunoassays (NOZAKI et al., 1990, 1991). Ovulations were inferred on the basis of increases in the plasma estradiol level coinciding with peak levels of L H secretion, followed by sustained elevations of progesterone ( > 1 ng/ml). When an L H surge or an estradiol surge was not detected due to infrequent sampling, the day of the L H surge was estimated from the profiles for estradiol, progesterone, and inhibin during the menstrual cycle (NOZAKI et al., 1991; MITSUNAGAet al., 1992). The ovulation period is expressed as the "ovulatory week" (OW), which means the period of one week containing + 3 days either side of the estimated day of ovulation. Among the 12 females who gave birth in 1990, 9 were parous and 3 were primiparous. Two of the nine parous females were lactating and the remaining seven were not. All eight females who gave birth in 1991 were parous females, and two of them were lactating. The estimated period of pregnancy for one of the offspring born alive was apparently 221 days, which was extraordinarily longer than the average (173 + 3.4 days). We presume therefore that the mother may have experienced stillbirth and then become pregnant again in the late mating season after hormonal monitoring had been discontinued. Two offspring were stillborn, although a DNA sample from one of their dead bodies was available. We omitted these three abnormal cases from behavioural analysis, although we did combined two of these three cases with the subjects of paternity discrimination. In all, ten non-lactating parous mothers, four lactating mothers, and three primiparous mothers formed the subjects for behavioural analysis. The subjects of paternity discrimination comprised a total of 37 offspring born in 1988-1991. PATERNITY DISCRIMINATION
Blood samples were collected from each of the offspring, its mother, and potential fathers. To discriminate paternity, detection of polymorphisms o f minisatellite DNA (JEFFREYS et al., 1985; INOUE et al., 1990) and microsatellite DNA, and dinucleotide repeat
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Fig. 1. An example of paternity discrimination by DNA fingerprinting. The band patterns of a mother (M), her offspring (O), and two candidates for paternity are shown. Four bands indicated by black triangles are present in the offspring but not in its mother. One of the two candidates for the father, male 1 has all these bands, but male 2 does not. This set of data indicates that these four bands of the child were inherited from its father, male 1. polymorphism detected by the P C R (polymerase chain reaction), customized for Japanese macaques (INOUE & TAKENAKA, 1993) were combined. Figure 1 illustrates an example of paternity exclusion based on polymorphism of minisatellite DNA.
RESULTS All 17 females who gave birth normally in the birth seasons following mating seasons III and IV became pregnant after their first ovulation of the mating season, as MITSUNAGA et al. (1992) reported in the same group of Japanese macaques. We were able to identify the fathers of 15 of.the above-mentioned 17 offspring. Behavioural analysis of the females who gave birth in these birth seasons was carried out only on these 15 females. Figure 2 shows the percentage of expected and observed ejaculations with females during the ovulatory week (OW), with females out side of their OW and not pregnant (NOP), and with females during early pregnancy (P) in mating seasons III and IV. The expected number of copulations was calculated from the total numbers of days on which females were found in each reproductive state. A m o n g the total number of 5658 day-females (123 days times 46 females present in mating seasons III and IV), 326 (7 days times 46 females, plus 4 days in one female; one OW was interrupted at 4 days because the observation period ended)
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t~
5O. O
o
100 50 0 NOP
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Fig. 2. The number of copulation to females in each reproductive stage. The observed (solid bars) and expected (dotted bars) number of copulations with females in each reproductive state. Females (n=46) were divided into three reproductive states; females in their ovulatory week (OW), females outside the OW and not pregnant (NOP), and pregnant females (P). Expected number of copulations was calculated from total number of day-females in each reproductive stage.
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t .L1
2
8
0,
7-~ LP ~N
-5 N-1 0 +1 ~+7 Weeks from estimated ovulation
Fig. 3. The average number of copulation of females in each reproductive stage. Ovulatory week indicated by "0" means one week containing estimated ovulatory day from LH surge. " - w e e k s " mean average number of copulation per week during - 5 to - 1 weeks before ovulation, " + weeks" mean average number of copulation per week during + 1 to + 7 weeks after ovulation when the female became pregnant. Females who gave normal birth were divided into three classes, nine non-lactating parous females (NLP), three lactating parous females (LP) who got pregnant continuous two years, and three primiparous females (N).
were estimated to correspond to an ovulatory condition (5.8%). Therefore, among the total 330 copulations observed during mating seasons III and IV, the expected number o f copulations with females during the OW was 19 (5.8% o f 330). However, the observed number of copulations with females at the OW was in fact 58. In both the OW and P, the numbers of observed copulations were more than double those expected. These findings suggest that the females were sexually more active at both the OW and P than at NOP. The males may have some means of distinguishing the ovulatory period of females. Figure 3 shows the average number of copulations per week for each female who gave birth during the subsequent birth season (n=15) in each reproductive state. Nine non-lactating parous females copulated more frequently after fertilization than during the OW. On the other hand, the numbers of copulations of three nulliparous females were lower than those of other females at all reproductive stages. Three lactating females copulated more frequently at the OW than at other weeks. Thus, copulations during post-conception estrus (OKAYASU, 1992; WILSON et al., 1982) were observed with nonlactating parous females but not with lactating or nulliparous females. Figure 4 shows the average number of copulations of non-lactating parous females with their respective partner males, in each reproductive state. The number of mating partners ranged between 2 and 10 (average, 5.5), whereas the number of ejaculating partners was
3
NFC [-'-q NFC1 mF
o
O
(.3 0
-5N-1
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Weeks from estimated ovulation
Fig. 4. Copulation partners of non-lactating parous females. The male partners were divided into three classes; identified father of offspring (F), the non-father male partner most frequently copulated with (NFC-1), and all other male partners (NFC).
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NOP ,-
20
IOW
0
15 o
ii~::i~iNiN!i~ii
10 5 i/////////l
High
Middle
Low
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rank
Fig. 5. The average number of copulation of each rank male with female at each reproductive stage. The total 20 males of five and more years in mating season III and IV were divided into three classes, 6 high-, 7 middle-, and 7 low-ranking. The ovulatory states of female partners who gave birth in the next spring (n= 15) were allocated into three classes; those before estimated ovulatory weeks (NOP), those at estimated ovulatory weeks (OW), and those after impregnation (P).
1 to 6 (average, 3.1). The male partners could be divided into three classes: (1) those identified as the resulting offspring's father (F); (2) the most frequently copulating nonfather-copulator (NFC-1); and (3) all other non-father-copulators (NFC) ( n = 0 - 4 ) . As demonstrated in Figure 4, the number of copulations with NFC-1 increased conspicuously at the P stage. The large number of copulations of non-lactating parous females observed at the P stage reflected such increased numbers of copulations with NFC-1. Monopolized copulations in consort relationships were mostly noted between highranking males and non-lactating parous females during the P stage. In 11 out of 15 cases the male which most frequently copulated was not the identified father of the offspring. The male which copulated most frequently with a female during the estimated OW was the father of the offspring in only two cases. Figure 5 shows the average number of copulations of males of differing ranks with the females in each reproductive state. The total number of copulations was high for highranking males, but most of these copulations were irrelevant in terms of reproductive outcome, including ejaculations with pregnant females (P) or with females outside of their OW and not pregnant (NOP). The total number of offspring born in subsequent birth seasons was 35. The average number of offspring born to each rank class of males per year was 0.7 (high), 1.8 (middle), and 0.2 (low). The number of offspring fathered was not related to male rank. The larger number of offspring fathered by the middle-ranking males as compared to those fathered by high- and low-ranking males may reflect the slightly large number of "effective" copulations with females during the ovulatory periods (OW) by middle-ranking males. During 96 days out of the 123-day observation period in mating season IV, at least one OW female was present in the group. On average, there were 2.5 such females at any one time during the 96 days, with a m a x i m u m of 4 females. There were 21.5 nonovulatory or pregnant females per day on average. Observations made during mating season I indicated that adult males could ejaculate at most 5 - 6 times in one day. Thus, a high-ranking male should have been able to monopolize all OW females if he wished, and if he could identify the time of ovulation. However, the observations made during mating seasons III and IV revealed that the first ranking male only copulated four times with OW females. High-ranking males did not show a tendency to choose OW females as copulatory partners.
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DISCUSSIONS COPULATIONS DURING THE FERTILE PERIOD
High-ranking males often formed consort relationships and so achieved periods of undisturbed multiple copulations with individual females. Such mating behaviour would appear to be an effective strategy for monopolizing females, if the timing o f these consortships were to be optimized around the predicted time of ovulation. However, this was not the case in the present study, since most of the consortships of high-ranking males were formed with pregnant females. Furthermore, it also proved difficult to predict the father of any individual offspring on the basis of the number of copulations with a given female during her ovulatory period. This may be due to sperm competition. This finding may partially explain why previous studies have failed to demonstrate a relationship between the number of infants sired by a given male and his rank in an enclosed group o f Japanese macaques (INOUE et al., 1990, 1991), since large numbers of copulations of high-ranking males are carried out with pregnant females. Consortships have been evolved as a potentially effective method for males to sire offspring. However, such behaviour proved not to be effective in the subject Japanese macaque group, despite some costs in monopolizing females. There must have been some other benefits. Males who formed intimate relations with many females could be able to stay longer in their troop. Long stay within the group or certain other benefits arising from good relations with females could bring long-term reproductive success for such males, not in the immediately following birth season, but in seasons two or more years later. Although the males' rank appears to be a weak factor in their reproductive success, we must compare the life-time reproductive success o f individuals in order to reach a definitive conclusion. FEMALE CHOICE
Female choice may be an important factor in deciding copulatory strategy, although there is no evidence that females positively choose high-ranking males as potential sires of their offspring. Male rank changes throughout a Japanese macaque's lifespan, and cannot be passed on to the offspring, so that this offers little incentive to a female to choose a high-ranking male partner. However, as a mating strategy, it may be advantageous for a female to copulate more frequently with a high-ranking male, who could offer greater protection and security for any resulting offspring. The number o f copulations after insemination of lactating females and nulliparous females is smaller than that of nonlactating parous females. It might perhaps be better for lactating females to take care of their infants rather than to copulate even after insemination. The nulliparous females were 4 or 5 yrs of age and their male partners also tended to be young. Due to the low ejaculation rate of these young male partners, the number of copulations with ejaculation with nulliparous females was small. There is evidence in the Japanese macaque to suggest that proximity during consortships between females and high-ranking males is maintained by the male, whereas consortships between females and middle-ranking males have been reported to be maintained by females (PERLOE, 1992). Although such approach and invitation behaviour is not as obvious as
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that of the male, some females actively approached mid- and low-ranking males, especially during the ovulatory period. Even given such apparent choice, the frequency of copulation with these preferred males did not correlate with the paternity of the females' offspring. Females did not reproduce with the same male in successive mating seasons (INOUE et al., 1990). Hence, female choice is only one element of the reproductive strategy. SPERM COMPETITION
From the standpoint of sperm competition, females do not need to choose individual males. Sperm competition may explain the observation that the frequent copulator cannot always sire many offspring. In the subject group, the males' rank and age are correlated. Therefore, some high-ranking males are more than 20 yrs old. Although changes in sperm activity have not yet been studied, the ability for insemination may decrease in old males. This could be one reason why high-ranking old males cannot sire offspring in spite of their large number of ejaculations. Including copulation without ejaculation, all females copulated with more than one partner. It is better for the females to copulate with as many partners as they can during their ovulatory period. In this way, the best sperm, which is active and healthy, will inseminate them. Sperm competition is one advantageous factor which can explain copulations with many partners.
Acknowledgments. We thank the staff of the Laboratory Primate Center, Primate Research Institute, for their assistance in collecting blood samples and for providing us with demographic data concerning the present macaque population. Thanks are extended to Dr. ALY GASPARDSOUMAHwho contributed to the observations made during mating season I. We would also like to express our gratitude to Dr. JEANWICKINGSand Dr. JAN R. DE RUITERwho kindly commented on our paper.
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J. A. R. A. M. VAr~HOOFF, 1992. Male social rank and reproductive success in wild long-tailed macaques. In: Paternity in Primates: Genetic Tests and Theories, R. D. MARTIN,A. F. DIXSON, & E. J. WICKINGS(eds.), Karger, Basel, pp. 173-191. HASEGAWA,T. & M. HIRAIWA-HASEGAWA,1983. Opportunistic and restrictive matings among wild chimpanzees in the Mahale mountains, Tanzania. J.. EthoL, l: 75-85. INOUE, M., F. M1TSUNAGA,H. OHSAWA,A. TAKENAKA,Y. SUGIYAMA,A. G. SOUMAH,(~ O. TAKENAKA, 1991. Male mating behavior and paternity discrimination by DNA fingerprinting in a Japanese macaque group. Folia PrimatoL, 56: 202-210. - - , & - - , 1992. Paternity dis-
crimination by DNA fingerprinting and male mating behavior in an enclosed Japanese macaque group, ln: Topics in Primatology, N. ITOIGAWA,Y. SUGIYAMA,G. P. SACKETT,& K. R. THOMPSON (eds.), Univ. of Tokyo Press, Tokyo, pp. 35-45. - - , A. TAKENAKA,S. TANAKA,R. KOMINAMI,& O. TAKENAKA,1990. Paternity discrimination in a Japanese macaque troop by DNA fingerprinting. Primates, 31: 563- 570. & O. TAKENAKA,1993. Japanese macaque microsatellite PCR primers for paternity testing. Primates, 34: 3 7 - 45. JEFFREYS, A. J., V. WILSON, & S. L. THEIN, 1985. Hypervariable 'minisatellite' regions in human DNA. Nature, 314: 6 7 - 73. -
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MITSUNAGA, E, M. NOZAKI, M. INOUE, A. TAKENAKA,O. TAKENAKA,O. SAKURA,Y. SUGIYAMA,8~ H. OHSAWA,1992. Steroid hormones and sexual behavior of female Japanese monkeys in an enclosed group. In: Topics in Primatology, N. ITOIGAWA,Y. SUGIYAMA,G. P. SACKETT,& K. R. THOMPSON (eds.), Univ. of Tokyo Press, Tokyo, pp. 2 3 - 34. NOZAKI, M., G. WATANABE,K. TAYA, Y. KATAKAI,Y. WADA, I. SASAMOTO,8*: K. OHSHIMA, 1990. Changes in circulating inhibin levels during pregnancy and early lactation in the Japanese monkey. Biol. Reprod., 43: 444-449. , , , - - , 8s S. SASAMO"fO, 1991. Changes in circulating inhibin levels during normal menstrual cycle in the Japanese monkey. Jpn. J. Anita. Reprod., 37: 97 - 103. OKAYASU,N., 1992. Prolonged estrus in female Japanese macaques (Macacafuscata yakul) and the social influence on estrus: with special reference to male intertroop movement. In: Topics in Primatology, N. ITOIGAWA,Y. SUGIYAMA,G. P. SACKETT, & K. R. THOMPSON (eds.), Univ. of Tokyo Press, Tokyo, pp. 163-178. PAUL, A. & J. KUESTER, in press. Differential reproduction in male and female Barbary macaques (Macaca sylvanus). In: Evolutionary Ecology and Behaviour o f the Macaques. J. E. FA & D. G. LINDBURG (eds.), Cambridge Univ. Press, Cambridge. - - , A. TIMME, & J. ARNEMANN,1993. The association between rank, mating effort and reproductive success in male Barbary macaques (Macaca sylvanus). Primates, 34: 0 0 0 - 000. PERLOE, S. I., 1992. Male mating competition, female choice and dominance in a free ranging group of Japanese macaques. Primates, 33: 289-304. SMITH, D. G., 1981. The association between rank and reproductive success of male rhesus monkeys. Amer. J. Primatol., 1: 8 3 - 9 0 . SPRAGUE, D. S., 1992. Life history and male inter-troop mobility among Japanese macaques (Macaca fuscata). Int. J. Primatol., 13: 4 3 7 - 454. TAKASAKI,H., 1985. Female life history and mating patterns among the M group chimpanzees of the Mahale National Park, Tanzania. Primates, 26: 121-129. WILSON, M. E., T. P. GORDON, & D. C. COLLINS, 1982. Serum 17/3-estradiol and progesterone associated with mating behavior during early pregnancy in female rhesus monkeys. Hormones Behav., 16: 94-106.
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Received: January 5, 1993; Accepted: March 15, 1993
Authors' Names and Present Addresses: MIHo ]NOUE, Shirakawa Institute of Animal Genetics, Odakura, Nishigo, Nishishirakawa, Fukushima 961, Japan; FUSAKOMITSUNAGA,MASUMINOZAK1,and HIDEYUKIOHSAWA, Primate Research Institute, Kyoto University, Inuyama, Aichi 484, Japan; AKIKOTAKENAKA,Department of Nutrition Science, Nagoya Bunri College, 2-L Sasazuka, Nishi-ku, Nagoya, Aichi 451, Japan; YUKtMARU SUGIYAMA,KEIKOSHIMIZU,and OSAMOTAKENAKA,Primate Research Institute, Kyoto University, Inuyama, Aichi 484, Japan.
