Demography and reproductive parameters of a free-ranging group of Japanese macaques (Macaca fuscata) at Katsuyama

Decline in the Conception Rate of Wild Japanese Monkeys after the Fukushima Daiichi Nuclear Power Plant Accident

We examined the conception rate of wild Japanese monkeys (Macaca fuscata) in Fukushima City that were exposed to radiation as a result of the Fukushima Daiichi Nuclear Power Plant accident in March 2011. The conception rate in the year of delivery from 2009 to 2022 was estimated by dissecting individuals that were euthanized by the government for population control as a countermeasure against crop damage. To evaluate the effects of exposure, the cumulative exposure dose for each individual was calculated using the concentration of radiocesium deposited in the soil at the capture site and the concentration of radiocesium in muscle estimated from the aggregated transfer factor. There were no significant differences in conception rates across all age classes over time. In terms of conception rates by age class, there was a significant decrease post-exposure compared with pre-exposure in the age class ≥ 8 years, but no significant differences in the age class 5-7 years. The non-ovulation rate did not significantly differ between the pre- and post-exposure periods for any age class. Body fat index, which can affect fertility, was compared between the pre- and post-exposure periods, and no significant differences were found in either age class. In contrast, the median total cumulative exposure (cumulative internal exposure + cumulative external exposure) was significantly higher in the age class ≥ 8 years compared with the age class 5-7 years. These results suggest that the total cumulative exposure dose may be one of the reasons for the lower conception rate in the post-exposure period among the age class ≥ 8 years.

Timing of conceptions in Phayre's leaf monkeys: Energy and phytochemical intake

OBJECTIVES

Raising offspring imposes energetic costs, especially for female mammals. Consequently, seasons favoring high energy intake and sustained positive energy balance often result in a conception peak. Factors that may weaken this coordinated effect include premature offspring loss and adolescent subfertility. Furthermore, seasonal ingestion of phytochemicals may facilitate conception peaks. We examined these factors and potential benefits of a conception peak (infant survival and interbirth interval) in Phayre's leaf monkeys (Trachypithecus phayrei crepusculus).

MATERIALS AND METHODS

Data were collected at Phu Khieo Wildlife Sanctuary, Thailand (78 conceptions). We estimated periods of high energy intake based on fruit and young leaf feeding and via monthly energy intake rates. Phytochemical intake was based on fecal progestin. We examined seasonality (circular statistics and cox proportional hazard models) and compared consequences of timing (infant survival and interbirth intervals, t-test, and Fisher exact test).

RESULTS

Conceptions occurred in all months but peaked from May to August. This peak coincided with high fecal progestin rather than presumed positive energy balance. Primipara conceived significantly later than multipara. Neither infant survival nor interbirth intervals were related to the timing of conception.

DISCUSSION

Periods of high energy intake may not exist and would not explain the conception peak in this population. However, the presumed high intake of phytochemicals was tightly linked to the conception peak. Timing conceptions to the peak season did not provide benefits, suggesting that the clustering of conceptions may be a mere by-product of phytochemical intake. To confirm this conclusion, seasonal changes in phytochemical intake and hormone levels need to be studied more directly.

My studies of primates: Sex, affinity, and competition

In this essay, I summarize my research career, with reference to the early days of the Laboratory of Physical Anthropology (LPA) at Kyoto University led by Kinji Imanishi and Junichiro Itani. When I started conducting research on the sexual behavior of Japanese macaques in 1975, I made some unexpected observations. High-ranking males did not obtain high mating success. Estrous females often rejected the courtships of high-ranking males and chose to mate with lower-ranking males. Some male-female dyads exhibited long-lasting affinitive relations, but they avoided mating. Females frequently showed 'excessive' sexuality. Clear explanations for some of these observations do not exist. After that, I changed my study subjects several times from chimpanzees, Yakushima macaques (a subspecies of Japanese macaque), and ringtailed lemurs. It is difficult to summarize my findings into a consistent story. Instead, I review my research and experiences. Throughout my career, I kept two things in mind. The first was established by Imanishi at the Laboratory of Physical Anthropology at Kyoto University: to explore the evolution of human society. Second, I tried to understand seemingly incomprehensible phenomena using evolutionary theory. Despite adhering to these foundational concepts, things did not always work out as planned.

Male care in a free-ranging group of Japanese macaques (Macaca fuscata)

Among the Macaca species, adult male Japanese macaques are the least likely to perform male care (i.e., affiliative interactions between adult males and immatures, including holding, carrying, and grooming); however, they perform male care for infants, albeit infrequently. We examined 17 cases of male care observed for the first time when the immature was younger than 1 year of age. Eleven of the 31 adult males who remained as central males during the 30-year observation period performed male care. Their age and dominance rank did not influence the occurrence of male care. Most cases were first recorded between the last part of the mating season and the first part of the birth season (January-March), whereas male care was rarely observed during the mating season (October-December). In 12 of the 17 cases, male care ceased within 6 months after the first observation, whereas in the remaining cases, it continued for at least 1 year. In 15 of the 17 cases, males tended to perform male care for matrilineally unrelated female infants of low-ranking mothers. In some cases, the male and infant mother showed grooming interactions for 6 months both before and after the start of male care, whereas such grooming interactions were never recorded either before or after the start of male care in other cases. We also examined some hypotheses on male-immature associations and the probable benefits that males and infants might acquire through male care.

Twenty-three-year demographic history of the Affenberg Japanese macaques (Macaca fuscata), a translocated semi-free-ranging group in southern Austria

Demographic studies on translocated primate groups provide a unique opportunity to study population dynamics, social strategies, and reproductive parameters of a species adapting to new environments. In 1996, 38 Japanese macaques (Macaca fuscata) of the Minoo-H group (Osaka Prefecture, Japan) were translocated to Affenberg Landskron, a four-hectare naturally forested park in southern Austria. By January 2020, the population had increased to 160 individuals, and a total of 223 births were recorded. Births peaked in late April to late May, and the timing was influenced by neither offspring sex nor parity status of the mother. Infant mortality was low (8.97%), mostly involving primiparous females, and the average interbirth intervals were shorter following the death of an infant (1.10 years) than a surviving infant (1.77 years). Females rarely had offspring with the same males repeatedly, and the reproductive success among males declined with increasing years of presence in the group. The main aspects of reproduction, mortality, and mate choice are consistent with published data on natural and provisioned populations in Japan and those translocated to other countries. The life expectancy for females, however, was relatively high (11.72% chance of reaching the age of 20), whereas birth control prevented them from using their lifetime reproductive potential. By January 2020, the number of old individuals (> 18 years; 17.5%) was close to that of juveniles (< 4 years; 22.5%). The specific group composition, along with the inability of males to emigrate out of their natal group, may affect the social dynamics of the population, which merits further attention in future studies.

Reproductive characteristics of Japanese monkeys on Shimokita Peninsula, Japan, the northernmost habitat of wild primates in the world

The Shimokita Peninsula in Aomori Prefecture, Japan, which is inhabited by Japanese monkeys (Macaca fuscata), is the northernmost habitat for wild primates in the world. This study was the first to determine the conception dates of specific individuals and estimate the pregnancy rate of wild populations in this region. The pregnancy rate of animals aged 5 years or more at delivery was estimated to be 40.9% (27/66). Conception dates of each fetus were also estimated using a regression line of Pig-tail monkeys (Macaca nemestrina), which are taxonomically related to Japanese monkeys and have a similar physique. The conception dates were distributed across 90 days between September 24th and December 23rd, with a mean conception date of November 4th (SD=22.3 days, n=53). Using these findings, the mean birth date was estimated as April 25th, more than two weeks earlier than the mean birth date in previous research determined using direct observations carried out over the past 20 years ago. Global warming due to climate change is thought to be one of the main causes of this difference.

Philopatry at the frontier: A demographically driven scenario for the evolution of multilevel societies in baboons (Papio)

The baboons (Papio sp.) exhibit marked interspecies variation in social behavior. The thesis presented here argues, first, that male philopatry is a crucial factor, arguably the crucial factor, underlying the other distinctive features (one-male units, multilevel society) shared by hamadryas and Guinea baboons, but not other species of Papio. The second suggestion is that male philopatry as a population norm was not an adaptation to a particular habitat or set of ecological circumstances but evolved in the common ancestor of hamadryas and Guinea baboons as a response to natural selection in the demographic context peculiar to the frontier of a rapidly expanding population. Other derived features of social structure (male-male tolerance, some facultative female dispersal) subsequently evolved to accommodate male philopatry. The mitochondrial genetic population structure of extant baboons preserves a footprint of the initial expansion of 'modern' Papio. Immediately after the expansion, male-philopatric, multilevel populations with a general physical and behavioral resemblance to Guinea baboons occupied the whole northern hemisphere range of the genus. Behavioral and physical autapomorphies of hamadryas baboons evolved in a subpopulation of this ancestral northern base, in response to a less productive habitat of the Horn of Africa. Subsequently, ancestral olive baboons 'reinvented' male dispersal. They and yellow baboons, another male-dispersing species, then replaced most of the male-philopatric northern populations, by male-driven introgression and nuclear swamping.

Triadic grooming among adult females in a free-ranging group of Japanese macaques

This study aimed to quantitatively describe triadic grooming bouts in a free-ranging group of Japanese macaques (Macaca fuscata). Two types of triadic grooming bouts occurred less frequently and for a shorter duration than dyadic grooming bouts: straight-line type (SL), where individual A grooms B, who then grooms C; and two-to-one type (TO), where individuals X1 and X2 groom Y. TO-type grooming was recorded more often than SL-type grooming. As in the dyadic grooming bouts, two females who had direct grooming interactions in both SL-type (i.e., between A and B or between B and C) and TO-type groomings (i.e., between X1 and Y and between X2 and Y) and two groomers who did not have direct grooming interactions in TO-type grooming were closely related to each other in more than the half of the pairs recorded. Groomers were more likely to be subordinate to groomees in triadic grooming. Almost all of the observed pairs in SL- and TO-type groomings were also recorded in dyadic grooming bouts. These findings indicate that like dyadic grooming, triadic grooming bouts are largely influenced by blood relatedness through maternal lines and dominance relationships between participants. Based on affiliative relationships maintained through dyadic grooming, triadic grooming could be influenced by such affiliative relationships and then function to strengthen bonds between participants. Possible relationships between triadic grooming and the level of social tolerance among individuals are discussed from the viewpoints of interspecies differences among macaque species and regional differences in Japanese macaques.

Sex roles and adult sex ratios: insights from mammalian biology and consequences for primate behaviour

Theoretical models and empirical studies in various taxa have identified important links between variation in sex roles and the number of adult males and females (adult sex ratio (ASR)) in a population. In this review, I examine these relationships in non-human primates. Because most existing theoretical models of the evolution of sex roles focus on the evolutionary origins of sex-biased behaviour, they offer only a general scaffold for predicting variation in sex roles among and within species. I argue that studies examining sex role variation at these more specific levels need to take social organization into account to identify meaningful levels for the measurement of ASR and to account for the fact that ASR and sex roles mutually influence each other. Moreover, taxon-specific life-history traits can constrain sex role flexibility and impact the operational sex ratio (OSR) by specifying the minimum length of female time outs from reproduction. Using examples from the primate literature, I highlight practical problems in estimating ASR and OSR. I then argue that interspecific variation in the occurrence of indirect forms of paternal care might indeed be linked to variation in ASR. Some studies also indicate that female aggression and bonding, as well as components of inter-sexual relationships, are sensitive to variation in ASR. Thus, links between primate sex roles and sex ratios merit further study, and such studies could prompt the development of more specific theoretical models that make realistic assumptions about taxon-specific life history and social organization.This article is part of the themed issue 'Adult sex ratios and reproductive decisions: a critical re-examination of sex differences in human and animal societies'.

The role of individuals in the history of a free-ranging group of Japanese macaques: Some thoughts on the relevance for human behavioural research

Taking the stance that the study of nonhuman primates can be useful in understanding the mechanism of long-term changes in social organisation, this short paper seeks to shed light on the role the individual might play in changes in future human society. This is done by examining the causes of social order change in a group of free-ranging macaque monkeys. The paper suggests that it may be useful to examine the role of genetically transmitted characteristics in human “success” stories, particularly concerning adaptability in times of change, and that the examination of individual life-history data across generations, will be essential for clarifying the mechanism of changes in human social organisation.

The effect of solicitations on grooming exchanges among female Japanese macaques at Katsuyama

In group-living primates, individuals often exchange grooming with not only kin but also non-kin. We investigated the effect of soliciting behaviors on grooming exchanges in a free-ranging Japanese macaque (Macaca fuscata) group at Katsuyama. In this study, we used a focal animal sampling method, targeting 14 females. Data were collected for 15.75 ± 2.67 (mean ± SD) hours per focal female. We classified female-female pairs into three pair types: kin pairs, affiliated non-kin pairs, and unaffiliated non-kin pairs. Females received grooming more frequently when they solicited after grooming their partners than when they did not solicit in all pair types. In addition, females received grooming less frequently when they did not groom their unaffiliated non-kin partners before soliciting; prior grooming was not needed to receive grooming from kin or affiliated non-kin partners. The degree of grooming reciprocity did not differ according to the frequency with which females in kin or affiliated non-kin pairs solicited after grooming. On the other hand, grooming reciprocity between unaffiliated non-kin females was more balanced when they solicited frequently after grooming, as compared with when they did not. In conclusion, our study suggests that soliciting behaviors promote grooming exchanges in female Japanese macaques.

Pregnancy rate and conception date in a wild population of Japanese monkeys

This study is the first to determine the conception dates of specific individuals and estimate the pregnancy rate of a wild population in Japanese monkeys. The conception dates estimated from the embryonic ages of 37 fetuses were distributed over 128 days between September 17 and January 23, with a mean conception date of November 19 (SD=29.2 days). A comparison of the mean conception dates among age groups showed a significantly delayed conception in the subadult animals (aged 4 to 5 years; December 5, n=8) than in the adult animals (aged 6 or more years; November 15, n=29; P<0.03). The mean pregnancy rate among animals aged 4 years or more in this local population was estimated to be 48.5% (32/66).

Stochastic variation in sex ratios in infant mortality rates due to small samples in provisioned Japanese macaque (Macaca fuscata) populations

Sex differences in infant mortality in provisioned Japanese macaque populations were examined using 10 data sets from five populations. The results indicate that there was no available data set in which a sex difference in infant mortality was statistically significant. To examine whether the observed sex ratios in infant mortality rates could be the product of stochastic variation in small samples, a correlation between sample size and the magnitude of sex ratios in infant mortality rates was also examined. Notably, the magnitude of sex ratios in infant mortality rates declined significantly as sample sizes increased. These results suggest that previously reported marked sex ratios in infant mortality could be the product of stochastic variation in small samples.

Menopause in nonhuman primates?

A gradual alteration in the mechanisms underlying reproduction and fertility characterizes the aging process in human females. These changes culminate in menopause, conventionally defined as a cessation of menstrual cycles that marks the end of reproductive capacity. In fact, a central and defining event in menopause is the discontinuation of ovulation, which is correlated with a number of structural and functional changes in the reproductive axis. Despite several decades of research, a degree of uncertainty remains as to whether nonhuman primates undergo menopause, and whether they are suitable models of human reproductive senescence. We review some of the controversies that have clouded our understanding of reproductive aging in nonhuman primates, including issues of definition, timing, comparability of data from wild versus captive populations, and cross-species comparisons. The existing data support the view that menopause occurs in a number of primate species and is not unique to humans.

Long-term grooming partnerships between unrelated adult females in a free-ranging group of Japanese monkeys (Macaca fuscata)

In order to examine the presence of long-term grooming relationships among unrelated females, grooming interactions of 18 adult females (range: 16-32 years) in a free-ranging group of Japanese monkeys (Macaca fuscata) were recorded in 2003 and compared with those recorded 10 years earlier, i.e., in 1993. In 2003, on average, each female who had survived the 10 years had grooming interactions with 2.2 surviving old partners with whom she was recorded to have grooming interactions in 1993, 3.5 females related to the surviving old partners, and 4.5 unrelated females who were other than the surviving old partners or their related females. By calculating the ratio of actual grooming partners to available females in 2003, we concluded that females had a greater possibility of selecting surviving old partners as their grooming partners than other unrelated partners, and that they also had a greater possibility of selecting females related to surviving old partners than females other than surviving old partners and their related females. These findings indicate that with regard to grooming relationships, female Japanese monkeys are basically conservative, showing a tendency to concentrate their grooming interactions on closely related females and certain familiar unrelated females such as surviving old partners and some females closely related to these partners. At the same time, however, female Japanese monkeys also showed a progressive trait for grooming since they formed grooming relationships with new partners. The necessity of long-term psychological bonding for long-term grooming relationships between unrelated females is discussed in this work.

A fatal attack on an unweaned infant by a non-resident male in a free-ranging group of Japanese macaques (Macaca fuscata) at Katsuyama

A non-resident male attacked a 4-month-old unweaned infant in a free-ranging group of Japanese macaques (Macaca fuscata) and it died 2 days later from a severe wound. When the infant was alone at the feeding site, the non-resident male rushed at it. The infant ran away as soon as it became aware of the male, but was captured. The male bit the infant on its hand, foot, and arm, while continuously scanning his surroundings. He did not kill the infant immediately and, after the infant escaped, he did not chase or attack it at all. Although the infant's right arm was bleeding heavily, it survived until the following day. The infanticide occurred a few weeks before the mating season began, so the victim's mother soon resumed estrus and the subsequent inter-birth interval was shortened. The first-, second-, and fourth-ranking adult males had died or disappeared a few months before this infanticide, and there were no other group members near the infant when the infanticidal male appeared. The infanticidal male had not been observed before this incident. Compared to one-male groups, the occurrence of infanticide in multi-male groups of Japanese macaques is extremely infrequent. However, in other reports of infanticide in Japanese macaques as well as in this case, it has been noted that infanticide is likely to occur in the pre- or early-mating season, when there are no resident males to defend the infant against attacks, and when a threatening male is least likely to be the infant's father.

Playback experiment to test maternal responses of Japanese macaques (Macaca fuscata) to their own infant's call when the infants were four to six months old

Calls emitted by infants when the infant loses sight of its parents are useful to estimate the infant's requirement for parental care. When an Old World monkey infant loses sight of its mother it emits whistles. Therefore, it would be interesting to determine whether mothers could distinguish their own infant's whistles from the whistles of other infants. The response of each of seven Japanese macaque (Macaca fuscata) mothers to her own infant's whistle was compared with their responses to another infant's whistle. Matched control playback experiments were performed when the infants were four to six months old. The results showed that each mother could distinguish her own infant's whistle from that of another infant when the infants were four to six months old. Although a stricter experimental plan is required to further examine the issue, we found that the dominance rank of the infant's mother was another important factor in the female response to the infant's call. The finding suggests that females can associate a call emitted by an infant with its mother's rank, even before the infant begins to wander far from its mother.

Hormone profiles and reproductive characteristics in wild female Japanese macaques (Macaca fuscata)

In this study we investigated the reproductive characteristics of wild female Japanese macaques (Macaca fuscata fuscata) in 2 nonconsecutive years using noninvasive methods to monitor physiological events. We detected ovulation dates and ascertained conceptions from fecal hormone profiles. First ovulations occurred from middle October to early November in 1997, and from middle to late November in 1999. Most females conceived during their first ovarian cycle. On average, postconception bleeding occurred 18.4 days after ovulation, and menstruation occurred 13.7 days after ovulation. The average gestation length was 176.3 days. The average degree of facial redness and the percentage of females that copulated synchronously changed across the ovarian cycle and peaked in the periovulatory period. Although prolonged periods of postconception copulation have been reported in previous studies, they did not occur in this study, which suggests that such behavior may not be a species-typical characteristic. Female fertility varied between the 2 years. The copulation rates of females with no infant <1 year of age were 100% (14/14) in 1997 and 45.5% (5/11) in 1999. The ovulation rates of the female subjects that we chose for hormonal assays were 100% (9/9) in 1997 and 50.0% (3/6) in 1999. Th conception rates of these selected females were 100% (9/9) in 1997 and 16.7% (1/6) in 1999. The birth rates (the number of females that delivered divided by the total number of adult females in the troop) were 73.3% (11/15) in 1998 and 6.7% (1/15) in 2000. The modified birth rates (the number of females that delivered /the number of adult females with no infant <1 year of age) x 100 were 78.6% (11/14) in 1998 and 9.1% (1/11) in 2000.

Birth-season variation in Japanese macaques, Macaca fuscata

Japanese macaques, Macaca fuscata, exhibit an annual reproductive cycle that apparently is maintained intrinsically. Translocation of nine troops to new latitudes within the northern hemisphere has had minimal effect on the timing of birth seasonality in these troops; translocation of one troop to the southern hemisphere has resulted in a 6-month forward displacement of birth seasonality in this troop. Limited available evidence indicates that, in the latitudinal zone between Toimisaki (31 degrees 22'N) and Kinkazan (38 degrees 17'N), mean birth date in in-situ troops becomes earlier as latitude of troop localities increases; the same relationship between mean birth date and latitude apparently does not apply to in-situ troops south and north of the Toimisaki-Kinkazan latitudinal zone. Within the Toimisaki-Kinkazan latitudinal zone, earlier mean birth dates at higher latitudes may permit infants to achieve an adequate level of development before the earlier onset of poor winter food conditions. South of the Toimisaki-Kinkazan latitudinal zone, winters are relatively mild and may be less of a factor in infant survival; north of this zone, poor winter food conditions persist so long that earlier infant births may be maladaptive.

Demographic analysis of the washington regional primate research center pigtailed macaque colony, 1967-1996

This work presents the results of a demographic analysis of 30 years of breeding records from the University of Washington's recently closed Primate Field Station at Medical Lake, Washington. Summaries of population growth, age-specific fertility and mortality rates, first-year survival, and seasonality of reproduction are presented, as well as an analysis of survival by decade. In addition, we present data on interbirth intervals in this population. In general, pigtailed macaques represent a typical Old World monkey pattern of age-specific fertility and mortality, with a few minor exceptions. We suggest that pigtailed macaques are most similar to rhesus and Barbary macaques, and that Japanese and bonnet macaques differ somewhat in their demographics.

Life history and demography of wild moor macaques (Macaca maurus): summary of ten years of observations

Data on the life history and demography of individual species are indispensable when we discuss social behavior from an evolutionary perspective, and when we attempt to make adequate conservation plans. This is the first report on the life history and demography of moor macaques in their natural habitat. Moor macaques (Macaca maurus) in the Karaenta Nature Reserve, South Sulawesi, Indonesia, have been observed since 1981. Individual identification of group members began in 1988. The size of the study group increased continuously, from 20 to 43, over this 10-year period of observation (1988-1998). The average population growth rate was 8.0%, and 45 births were confirmed during this period. They were categorized as moderately seasonal breeders. Mortality rate within one year after birth was 17.1%. Average inter-birth interval following surviving infants was 24.1 months, while that following early infant death was 15.0 months. As is the case in other species of macaques, males moved between groups while females stayed in their natal groups. Females seemed to exhibit their first perineal swelling at 4-6 years of age, and to have their first infant at 6-7 years. Males left their natal group at 7-9 years. Solitary males were seldom observed around the study group. The late dispersal of males from their natal groups and their infrequent movement between groups contrast with patterns in well-known macaque species such as Japanese macaques. Recently, differences in social characteristics among macaque species have attracted the attention of researchers. Our findings would be useful to further understanding of such social differences.

Birth rate and mortality rate of infants with congenital malformations of the limbs in the Awajishima free-ranging group of Japanese monkeys (Macaca fuscata)

The birth rate and mortality rate of infants with congenital malformations of the limbs were examined in the Awajishima free-ranging group of Japanese macaques (Macaca fuscata). Of the 606 infants born between 1978 and 1995, 86 (14.2%) were malformed. The male-female ratio did not differ between malformed and normal infants. Most kin-groups included females who gave birth to malformed infants at least once. The mortality rate within the first year after birth for malformed infants (28.2%) was significantly higher than that for normal infants (10.0%). However, this indicates that more than 70% of malformed infants were able to survive for the first year of life, even though they were unable to cling to their mother's ventrum due to their limb deformities. This finding indicates that maternal care-taking is sufficient to enable malformed infants to survive during the early stages of development and that clinging by the infant is not necessary for the display of maternal care.

Continuity of affiliative relationships among infants and juveniles in a free-ranging group of Japanese monkeys (Macaca fuscata)

5 infants were observed during the first 13 mo. after birth in a free-ranging group of Japanese monkeys (Macaca fuscata) to examine whether they maintained relatively frequent affiliative relationships with particular immature individuals. The number of immature affiliative partners rapidly increased from the first 3-mo. period after birth to the second 4-mo. period after birth, and the infants thereafter maintained affiliative relationships with particular individuals, mostly same-aged infants, at least in the second half of their first year. These findings show that infants were likely to interact with a larger number of different animals in the early stage of development, and thereafter selected particular individuals with whom they maintained affiliative relationships. The effects of sex on the selection of these long-term partners are also discussed.