Male residence and the patterning of serum testosterone in vervet monkeys (Cercopithecus aethiops)

A natural history of Chlorocebus dryas from camera traps in Lomami National Park and its buffer zone, Democratic Republic of the Congo, with notes on the species status of Cercopithecus salongo

The natural history and taxonomic status of two central African primates, Cercopithecus dryas and Cercopithecus salongo have long been in question. Recent studies confirmed that C. dryas is a basal member of the savanna monkey clade, and that it prefers dense undergrowth in lowland rainforest. While these studies advanced our knowledge of this enigmatic species, key aspects of its natural history remain poorly documented. Furthermore, the lack of a field study that documents pelage patterns of both sexes and different age classes of C. dryas has led to a disagreement over the validity of C. salongo as a sister taxon to C. dryas. Using the results of two multi-strata camera trap surveys in Lomami National Park (Democratic Republic of the Congo) and its buffer zone, we conducted a third survey in the understory of degraded forest to accumulate videos of C. dryas/salongo. We used these videos to test the hypothesis that C. dryas and C. salongo are synonymous, and to assess the species' group composition, density, behavior and vocalizations. Camera traps revealed an ontogenetic change in pelage pattern that supports the view that C. salongo is the adult of C. dryas. Videos revealed that adult males develop a blue perineum and scrotum, and a red subcaudal patch, similar to other savanna monkeys. We provide a preliminary assessment of C. dryas' group composition, density, behavior, and vocalizations. This long-overlooked monkey is an exceptional member of the Chlorocebus clade, and all aspects of its biology require further investigation.

Social and demographic correlates of male androgen levels in wild white-faced capuchin monkeys (Cebus capucinus)

The Challenge Hypothesis, designed originally to explain the patterning of competitive behavior and androgen levels in seasonally breeding birds, predicts that males will increase their androgen levels in order to become more competitive in reproductive contexts. Here we test predictions derived from the Challenge Hypothesis in white-faced capuchin monkeys (Cebus capucinus), a species that has somewhat seasonal reproduction. We analyzed demographic and hormonal data collected over a 5.25-year period, from 18 males in nine social groups living in or near Lomas Barbudal Biological Reserve, Costa Rica. Alpha males had higher androgen levels than subordinates. Contrary to our predictions, neither the number of breeding-age males nor the number of potentially fertile females was obviously associated with androgen levels. Furthermore, male androgen levels were not significantly linked to social stability, as measured by stability of male group membership or recency of change in the alpha male position. Androgen levels changed seasonally, but not in a manner that had an obvious relationship to predictions from the Challenge Hypothesis: levels were generally at their lowest near the beginning of the conception season, but instead of peaking when reproductive opportunities were greatest, they were at their highest near the end of the conception season or shortly thereafter. This lack of correspondence to the timing of conceptions suggests that there may be ecological factors not yet identified that influence ifA levels. We expected that the presence of offspring who were young enough to be vulnerable to infanticide during an alpha male takeover might influence androgen levels, at least in the alpha male, but this variable did not significantly impact results.

Hormonal correlates of male life history stages in wild white-faced capuchin monkeys (Cebus capucinus)

Much attention has been paid to hormonal variation in relation to male dominance status and reproductive seasonality, but we know relatively little about how hormones vary across life history stages. Here we examine fecal testosterone (fT), dihydrotestosterone (fDHT), and glucocorticoid (fGC) profiles across male life history stages in wild white-faced capuchins (Cebus capucinus). Study subjects included 37 males residing in three habituated social groups in the Área de Conservacíon Guanacaste, Costa Rica. Male life history stages included infant (0 to <12months; N=3), early juvenile (1 to <3years; N=10), late juvenile (3 to <6years; N=9), subadult (6 to <10years; N=8), subordinate adult (⩾10years; N=3), and alpha adult (⩾10years; N=4, including one recently deposed alpha). Life history stage was a significant predictor of fT; levels were low throughout the infant and juvenile phases, doubled in subadult and subordinate adults, and were highest for alpha males. Life history stage was not a significant predictor of fDHT, fDHT:fT, or fGC levels. Puberty in white-faced capuchins appears to begin in earnest during the subadult male phase, indicated by the first significant rise in fT. Given their high fT levels and exaggerated secondary sexual characteristics, we argue that alpha adult males represent a distinctive life history stage not experienced by all male capuchins. This study is the first to physiologically validate observable male life history stages using patterns of hormone excretion in wild Neotropical primates, with evidence for a strong association between fT levels and life history stage.

The association of intergroup encounters, dominance status, and fecal androgen and glucocorticoid profiles in wild male white-faced capuchins (Cebus capucinus)

Androgens play a role in male reproductive competition, frequently via aggression, while glucocorticoids are associated with the stress response. However, the relationships of these hormones with different sources of competition (intra- vs. intergroup) and dominance status are highly variable. Here, we consider the fecal androgen (fA) and glucocorticoid (fGC) profiles of alpha and subordinate male Cebus capucinus in the context of intergroup competition during a rare period of low intragroup competition (i.e. all females were either pregnant or lactating). Intergroup encounters (IGEs) are a long-term reproductive strategy in male white-faced capuchins, enabling them to assess the composition of neighboring groups. IGEs pose a threat to resident males as these can result in injury or death, loss of dominance rank, group eviction, and group takeovers that are frequently associated with infanticide. From February to July 2007, fecal samples were collected from eight males in three groups of white-faced capuchins in the Santa Rosa Sector of the Área de Conservación Guanacaste, Costa Rica. IGE rate was positively associated with both fA and fGC levels, indicating that IGEs are perceived as reproductive challenges by resident males, and may be associated with elevated metabolic costs. Alpha males sire the majority of group offspring and, accordingly, the threat of IGEs to both future (via rank loss or eviction) and current (via infanticide) reproductive success is greater than for subordinate males. Consistent with this observation, alpha males had higher fA and fGC levels than subordinate males. Given that all females were either pregnant or lactating and pronounced overt intragroup competition was absent, we interpret the difference in hormone profiles of alpha and subordinate males as being primarily associated with variation in the perceived threats of IGEs according to dominance status. Future studies should focus on the interaction of intra- and intergroup competition by examining hormone levels in the presence of periovulatory females.

Social modulation of testosterone levels in male black howlers (Alouatta pigra)

The influence of social factors on the modulation of male testosterone levels has been demonstrated among several vertebrate species. In addition to sexual activity, parental care and reproductive competition affect testosterone secretion. We examined variations in testosterone levels among male black howlers (Alouatta pigra) in various social contexts. Fecal samples were collected from nine males living in five different groups in the Mexican state of Campeche. The potential for intragroup and extragroup competition varied among the groups. The number of resident males living in the groups was the only variable that significantly explained variations in testosterone levels. Males living in unimale groups had higher testosterone levels; the highest testosterone levels were recorded for males that had experienced a shift from multimale to unimale group compositions. In this species, the probability of being challenged by extragroup males and evicted from the group during immigration events increases when males live in unimale groups. Therefore, our results suggest that male black howlers respond to competition for group membership by increasing their testosterone levels. In this context, testosterone secretion represents an anticipatory response to reproductive conflicts. Therefore, although males living in unimale groups have exclusive access to females, they face higher physiological costs associated with sustaining high testosterone levels for extended time periods.

Social correlates of fecal testosterone in male ursine colobus monkeys (Colobus vellerosus): the effect of male reproductive competition in aseasonal breeders

Male testosterone (T) levels are thought to be linked with the mating system, degree of parental care, and male-male aggression in reproductive contexts (The 'challenge hypothesis'; Wingfield et al., 1990). In many species though, T increases associated with mating behavior cannot be separated from those associated with male-male aggression. We tested the challenge hypothesis on aseasonally breeding ursine colobus (Colobus vellerosus), where male-male competition is intense outside of mating contexts. Fecal samples (N=109) were collected from >27 subadult and adult males in seven groups during 13-months of research in Ghana in 2004-2005. Fecal T (fT) levels were determined by enzyme immunosorbant assays. Behavioral data was collected using focal-animal and ad libitum sampling. The number of receptive females in each group did not positively correlate with male fT. There was a trend for adult males to have higher fT than subadult males; however there was no effect of rank on fT. The level of male-male aggression experienced was positively correlated with fT and individual males showed higher mean fT during 'challenge' than during 'non-challenge' periods. The number of male incursions experienced positively correlated with fT whereas the number of between-group encounters did not. Males attempt to gain reproductive opportunities during incursions, thus these results support the 'challenge hypothesis' in C. vellerosus. Outside of mating contexts, higher male fT levels are associated with increased aggression. Male parental investment in the form of infant defense was associated with increased fT, rather than the decline expected from other forms of paternal care.

Age, rank, and personality effects on the cortisol sedation stress response in young chimpanzees

Primates and other mammals exhibit a glucocorticoid response to somatic and psychosocial stressors. The pattern and degree of response varies both within and between species, but the factors affecting within-species variability have rarely been considered. Here we describe the 90-min serum cortisol response of 14 juvenile and adolescent chimpanzees to the stress of sedation with ketamine hydrochloride. We show age differences in both baseline cortisol and time of peak cortisol, with younger individuals (ages 4-5 years) showing higher baseline levels and an earlier peak response than older individuals (ages 8-9 years). However, we found no sex or rank-related differences in any of the sedation measures: baseline cortisol, cortisol peak time, peak cortisol level, or cortisol change from baseline. We also examined the relationship between these sedation stress measures and behavioral style score, and found that individuals scoring high in the "mellow" behavioral style component showed a greater stress response than individuals scoring lower in this component. Future studies should consider the role of individual differences in age and personality in the cortisol response to stress.

Testosterone in juvenile and adolescent male chimpanzees (Pan troglodytes): Effects of dominance rank, aggression, and behavioral style

Testosterone is a steroid hormone with diverse effects on male reproductive function and behavior. The relationship between testosterone and social behavior such as mating and aggression has been investigated in a variety of primate species, but few such studies have been conducted on chimpanzees, and even fewer on primates during the juvenile and adolescent periods. This study explores the relationship between baseline urinary testosterone and behavioral variables including dominance rank, rates of aggression toward peers, and behavioral style in 16 juvenile and adolescent male chimpanzees (Pan troglodytes) living at the New Iberia Research Center in Louisiana. Behavioral observations and urine collection occurred during four research periods, each a year apart. After correcting for the positive association between testosterone and age, testosterone was positively associated with both dominance rank and rates of aggression directed at others. It was negatively associated with rates of aggression received. Individuals scoring highest in the "mellow" behavioral style component showed higher levels of testosterone than individuals scoring lowest in this component, an effect that may be partially due to the confounding effect of rank. The results of this study suggest that hormonal changes during the period preceding adulthood are not simply programmed physiological processes tied primarily to age-related change, but that important age-independent relationships with behavior also exist.