► Unknown relationships affect dominance hierarchy indices and rank order. ► We used a series of reduced datasets based on observations of real animals. ► Hierarchy linearity and steepness decreased ...as % unknown relationships increased. ► I&SI rank order became inconsistent at 26–38% unknown relationships. ► Group size and specific input data influence effects of unknown relationships.
The presence of unknown dyadic relationships is a common problem in constructing dominance hierarchies for groups of social animals. Although previously acknowledged, the influence of unknown relationships on hierarchy measures like linearity and steepness has not been studied in detail. Using real data-sets from four groups of wild monkeys, we illustrate how unknown relationships affect linearity and steepness of hierarchies and the consistency of rank ordering based on de Vries’ I&SI method. Monte Carlo simulations revealed significant negative linear relationships between the proportion of unknown relationships and both linearity and steepness. These simulations over-estimated steepness and linearity indices relative to additional real-data input matrices. Rank orders became inconsistent at 26–38% unknown relationships, depending on the group. Group size and the specific input matrix substantially affected how much unknown relationships influenced steepness and linearity, the values of these indices and the point at which rank order became inconsistent. We recommend caution in characterizing the dominance structure of a group with many unknown relationships, and in drawing conclusions about hierarchy linearity and steepness based on few input matrices, especially if they contain many unknown relationships. Quantitative characterizations of hierarchies are perhaps best viewed as a somewhat fluid range rather than fixed values.
Women rarely give birth after ∼45 y of age, and they experience the cessation of reproductive cycles, menopause, at ∼50 y of age after a fertility decline lasting almost two decades. Such ...reproductive senescence in mid-lifespan is an evolutionary puzzle of enduring interest because it should be inherently disadvantageous. Furthermore, comparative data on reproductive senescence from other primates, or indeed other mammals, remains relatively rare. Here we carried out a unique detailed comparative study of reproductive senescence in seven species of nonhuman primates in natural populations, using long-term, individual-based data, and compared them to a population of humans experiencing natural fertility and mortality. In four of seven primate species we found that reproductive senescence occurred before death only in a small minority of individuals. In three primate species we found evidence of reproductive senescence that accelerated throughout adulthood; however, its initial rate was much lower than mortality, so that relatively few individuals experienced reproductive senescence before death. In contrast, the human population showed the predicted and well-known pattern in which reproductive senescence occurred before death for many women and its rate accelerated throughout adulthood. These results provide strong support for the hypothesis that reproductive senescence in midlife, although apparent in natural-fertility, natural-mortality populations of humans, is generally absent in other primates living in such populations.
Female mate choice may drive sexual selection, but discerning whether female behaviors reflect free expression of choice or responses to constraints can be difficult. We investigated the efficacy of ...female choice in wild blue monkeys using 10 years of behavior and paternity data (N = 178 male–female dyads). Although blue monkeys live modally in one-male polygynous groups, where male-biased intersexual power is expected, females can access multiple potential mates during seasonal male influxes and occasional intergroup encounters. Additionally, extra-group males sire offspring. We examined female resistance rates to male-initiated sexual interactions, and unsolicited proceptive behavior that females directed to males (corrected for male availability). Females seldom resisted male solicitation, but initiated sexual interactions more than males. Females generally preferred residents. Those who preferred non-residents tended to have residents with longer tenures, but neither female parity nor rank influenced the tendency to prefer non-residents vs. residents. The male most solicited by a particular female fathered that female’s infant 82% of the time; odds of siring were 26 times higher for most vs. nonpreferred males. Female preference predicted paternity even more strongly among non-resident males, with odds of siring 33 times higher for most vs. nonpreferred non-residents. Neither female rank nor parity influenced her likelihood of having her preferred partner as sire. Paternity by preferred males did not affect infant survival. While we cannot fully discount the effect of male–male competition on paternity, these results suggest that blue monkey females can exercise choice successfully, even in a polygynous mating system.
Human senescence patterns--late onset of mortality increase, slow mortality acceleration, and exceptional longevity--are often described as unique in the animal world. Using an individual-based data ...set from longitudinal studies of wild populations of seven primate species, we show that contrary to assumptions of human uniqueness, human senescence falls within the primate continuum of aging; the tendency for males to have shorter life spans and higher age-specific mortality than females throughout much of adulthood is a common feature in many, but not all, primates; and the aging profiles of primate species do not reflect phylogenetic position. These findings suggest that mortality patterns in primates are shaped by local selective forces rather than phylogenetic history.
Earth's rapidly changing climate creates a growing need to understand how demographic processes in natural populations are affected by climate variability, particularly among organisms threatened by ...extinction. Long‐term, large‐scale, and cross‐taxon studies of vital rate variation in relation to climate variability can be particularly valuable because they can reveal environmental drivers that affect multiple species over extensive regions. Few such data exist for animals with slow life histories, particularly in the tropics, where climate variation over large‐scale space is asynchronous. As our closest relatives, nonhuman primates are especially valuable as a resource to understand the roles of climate variability and climate change in human evolutionary history. Here, we provide the first comprehensive investigation of vital rate variation in relation to climate variability among wild primates. We ask whether primates are sensitive to global changes that are universal (e.g., higher temperature, large‐scale climate oscillations) or whether they are more sensitive to global change effects that are local (e.g., more rain in some places), which would complicate predictions of how primates in general will respond to climate change. To address these questions, we use a database of long‐term life‐history data for natural populations of seven primate species that have been studied for 29–52 years to investigate associations between vital rate variation, local climate variability, and global climate oscillations. Associations between vital rates and climate variability varied among species and depended on the time windows considered, highlighting the importance of temporal scale in detection of such effects. We found strong climate signals in the fertility rates of three species. However, survival, which has a greater impact on population growth, was little affected by climate variability. Thus, we found evidence for demographic buffering of life histories, but also evidence of mechanisms by which climate change could affect the fates of wild primates.
We use long‐term life‐history data for natural populations of seven primate species representing the four major radiations of primates to investigate associations between vital rate variation, local climate variability, and global climate oscillations. We ask whether primates are sensitive to global changes that are universal (e.g., higher temperature, large‐scale climate oscillations) or whether they are more sensitive to global change effects that are local (e.g., more rain in some places), which would complicate predictions of how primates in general will respond to climate change. We found strong climate signals in the fertility rates of three species, but most survival rates were little affected by climate variability. These findings indicate demographic buffering of life histories and provide new insights into the implications of climate change for the fates of wild primates.
Animals facing seasonal variation in food availability experience selective pressures that favor behavioral adjustments such as migration, changes in activity, or shifts in diet. Eclectic omnivores ...such as many primates can process low-quality fallback food when preferred food is unavailable. Such dietary flexibility, however, may be insufficient to eliminate constraints on reproduction even for species that live in relatively permissive environments, such as moist tropical forests. Focusing on a forest-dwelling primate with a flexible diet (Cercopithecus mitis) we investigated whether females experience seasonal energetic stress and how it may relate to reproductive seasonality. We used fecal glucocorticoids (fGCs) as an indicator of energetic stress, controlling for the potentially confounding effects of social interactions and reproductive state. We modeled within-female fGC variation with General Linear Mixed Models, evaluating changes in feeding behavior and food availability as main effects. Regardless of reproductive state, fGCs increased when females shifted their diet towards fallback foods (mature leaves and other non-preferred items) and when they spent more time feeding, while fGCs decreased with feeding time on preferred items (insects, fruits, young leaves) and with the availability of young leaves. Changes in fruit availability had no general effects on fGCs, likely because fruits were sought out regardless of availability. As predicted, females in the energetically demanding stages of late pregnancy and early lactation showed greater increases in fGCs between periods of low versus high availability of fruits and young leaves than females in other reproductive states. Potential social stressors had no measurable effects on fGCs. Preliminary evidence suggests that seasonal energetic stress may affect the timing of infant independence from mothers and contribute to unusually long inter-birth intervals compared to closely related species of similar body size. Our findings highlight how the study of stress responses can provide insights into the proximate control of reproductive strategies.
In group-living animals, between-group relations are often revealed during intergroup encounters, which may be intolerant (competitive), affiliative (cooperative) or neutral. We investigated factors ...that predict different encounter types in blue monkeys, Cercopithecus mitis stuhlmanni, where intolerant encounters predominate but neutral encounters also occur. Using data from nine groups monitored over 24 months in equatorial forest, we first examined whether availability of fruit, a preferred and important food, predicted encounter rates. Fruit availability related positively only to the rate of intolerant encounters, suggesting that energetic costs of aggression and attractiveness of large fruiting trees influence contest rates. We then examined how fruit availability and time of day influenced the odds of intolerant versus neutral encounters, finding no evidence for an effect. We examined additional social factors for a subset of encounters between groups with well-known histories. The odds of intolerant versus neutral encounters declined when group sizes were more disparate and when groups had split from each other longer ago. We found no evidence that the odds of intolerant intergroup encounters were related to the proportion of females with infants or the encounter's location on edge–centre axes for both groups. Finally, we examined how ecological and social factors predicted group behaviour (aggressive, fleeing, neutral) during encounters. Encounter location, group size, group size differential and the time since two interacting groups split from each other all influenced group behaviour. Unexpectedly, aggressive behaviour was less likely at the start and end of the day. There was no evidence for an effect of fruit availability. Overall, while some evidence implicated ecological factors affecting rates of intolerant encounters, social factors seemed more important in predicting intolerant versus neutral encounters and aggressive or fleeing versus neutral group behaviour. More detailed measures of contested resources may improve evaluation of ecological effects on types of intergroup encounters.
•Blue monkeys exhibit both intolerant and neutral intergroup encounters.•Monthly rates of intolerant encounters increased with fruit availability.•Odds of intolerant encounters reflected group size differences and shared histories.•Group behaviour reflected location, time of day, group size and group history.•Intergroup relations are affected by both ecological and social factors.
In wild animals, injuries often arise from intraspecific contests and, therefore, may reveal challenges and selection pressures related to social conflict. We evaluated whether known aggressive ...patterns predicted injury patterns in wild blue monkeys, in which most injuries of known cause resulted from conspecifics. After describing the injuries and their causes in 12 groups monitored over 13 years, we compared injury distributions across age–sex class and season. Adult males were much more likely, adult females about equally likely, and immatures much less likely to sustain injuries relative to random expectation. Among adult males, injuries were most common in the mating season. Using multivariate models, we examined how individual, social, and demographic factors predicted injury rates in adult females and immatures. Rates were higher for adult females versus all immatures, for individuals in smaller groups, and for those living in groups frequently exposed to novel adult males and with few immature members. Among older juvenile and adult females, these trends persisted; additionally, pregnant and lactating females were injured less often than others, but there was no evidence that dominance rank influenced injury rates. For juveniles, injury rates increased similarly with age in both sexes, providing only limited evidence that heightened aggression motivates natal dispersal by males. Most findings supported the hypothesis that aggression increases injury risk. The costs of within-group aggressive competition may manifest in ways other than injury, however, especially for females. We highlight the importance in comparative analyses of considering both within- and between-group aggression and demographic variables as influencing injury risk.
Significance statement
Injuries in wild animals often result from intraspecific aggression, so their distribution may reveal both causes and consequences of social conflict. We studied the distribution of injuries in wild monkeys to examine how closely injuries matched known patterns of intra-specific aggression and, thus, to better understand where such conflict is costly. After verifying that most injuries were indeed caused by conspecifics, we examined how injury rates varied with individual attributes (age, sex, female reproductive state, and dominance rank), season, and socio-demographic context (group size and composition). Most of these variables predicted injury rates as expected from previously known patterns of aggression. Unexpectedly, however, injury rates were higher in smaller groups and unrelated to female dominance rank, suggesting the importance of inter-group competition as a source of injury and the possibility that within-group competition imposes costs other than enhanced injury risk.
Postings on social media on Twitter (now X), BioAnthropology News (Facebook), and other venues, as well as recent publications in prominent journals, show that primatologists, ecologists, and other ...researchers are questioning the terms “Old World” and “New World” due to their colonial implications and history. The terms are offensive if they result in erasing Indigenous voices and history, ignoring the fact that Indigenous peoples were in the Americas long before European colonization. Language use is not without context, but alternative terminology is not always obvious and available. In this perspective, we share opinions expressed by an international group of primatologists who considered questions about the use of these terms, whether primatologists should adjust language use, and how to move forward. The diversity of opinions provides insight into how conventional terms used in primatological research and conservation may impact our effectiveness in these domains.
Infanticide by males is widespread across mammals and especially prevalent among primates. Considerable research has examined how fitness benefits can explain the occurrence of this behavior; less is ...known, however, about intrapopulation variation in its occurrence. We evaluated 10 infanticides by males in wild blue monkeys according to the sexual selection hypothesis. To explore intrapopulation variation in occurrence of infanticide, we compared these cases to 38 cases that were contextually similar but in which infanticide did not occur. We examined male reproductive benefit, infant age, maternal parity, postconception estrus, group defense, available mating partners, and context of takeover. We based comparisons on daily or near daily records of male presence in the study groups, infant birth dates, and male-female sexual interactions. Infanticides followed predictions of the sexual selection hypothesis: males were unlikely to kill their own offspring, the period for the mother's return to conception was reduced by half, and males increased their chance of siring her next offspring. Difference in male reproductive benefit, costs, and motivation did not fully explain the observed variation in infanticide occurrence. Infants were more likely to be spared if they were older when a male first arrived, or if their mother had mated with the male in the second month after conception. The most important determinant of infant fate, however, was male identity, a finding consistent with 2 scenarios: 1) an infanticidal tendency may be influenced by a genetic polymorphism that is not fixed in this population or 2) infanticidal behavior may be a conditional male strategy. Further research on intrapopulation variation in infanticidal behavior should focus especially on characteristics of males.
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