“Monogamy” and pair bonding have long been of interest to anthropologists and primatologists. Their study contributes to our knowledge of human evolutionary biology and social evolution without the ...cultural trappings associated with studying human societies directly. Here, we first provide an overview of theoretical considerations, followed by an evaluation of recent comparative studies of the evolution of “social monogamy”; we are left with serious doubts about the conclusions of these studies that stem from the often poor quality of the data used and an overreliance on secondary sources without vetting the data therein. We then describe our field research program on four “monogamous” platyrrhines (owl monkeys, titis, sakis, and tamarins), evaluate how well our data support various hypotheses proposed to explain “monogamy,” and compare our data to those reported on the same genera in comparative studies. Overall, we found a distressing lack of agreement between the data used in comparative studies and data from the literature for the taxa that we work with. In the final section, we propose areas of research that deserve more attention. We stress the need for more high‐quality natural history data, and we urge researchers to be cautious about the uncritical use of variables of uncertain internal validity. Overall, it is imperative that biological anthropologists establish and follow clear criteria for comparing and combining results from published studies and that researchers, reviewers, and editors alike comply with these standards to improve the transparency, reproducibility, and interpretability of causal inferences made in comparative studies.
The function of intergroup encounters (IGEs) may differ substantially among species of different group sizes and social organizations. Research in group‐living primates has shown that the behavioral ...responses during IGEs can vary widely from affiliative to neutral or aggressive interactions; still, little is known about IGEs in pair‐living taxa. We conducted a systematic literature review to find relevant studies on the functions of IGEs in pair‐living nonhuman primates that could inform analyses of IGE data (n = 242 IGEs, 21 groups and 10 solitary individuals, 1997−2020) from wild owl monkeys, a pair‐living, monogamous primate with extensive biparental care. We identified 1315 studies published between 1965 and 2021; only 13 of them (n = 10 species) contained raw data on the number of IGEs. Our review of those studies showed that IGEs are common, but highly variable in their nature and characteristics in pair‐living primates. To examine the non‐mutually exclusive hypotheses of resource‐, and mate defense, and infanticide avoidance we analyzed data from the Owl Monkey Project 27‐year long database to build first an a priori model set. To incorporate prior knowledge from the literature review, we conducted our analyses as a consecutive series of binomial logistic regressions. All IGEs including all biologically relevant parameters (N = 156) were codified into three different behavioral categories (Reaction, Agonism, and Physical Aggression). The analysis showed that owl monkeys regularly engaged in IGEs, most of which were agonistic. They showed more reaction when infants were present, but reactions were less physically aggressive when infants and pregnant females were involved. Overall, our results lend more support for the infant and mate defense hypotheses than they do for the resource defense one.
Intergroup encounters in pair‐living owl monkeys seem to be more related to infanticide avoidance and mate defense than to resource defense.
Research Highlights
Our systematic review of the literature showed that intergroup encounters (IGEs) in pair‐living primates are common, but highly variable in their characteristics and possible function.
In Azara's owl monkeys IGEs are most frequently agonistic and primarily related to group size and the presence of females and/or infants—IGEs involving bigger groups were more agonistic and less physically aggressive when infants and pregnant females were involved.
The lack of standardized terminology and methodologies across studies hampers comparative analyses. Multi‐year data on IGEs is important for understanding the role of territoriality, reproductive strategies, dispersal, and intergroup spacing patterns on the evolution of pair‐living in primates.
Patterns of ranging behavior and space use are key for evaluating current ideas about the evolution and maintenance of pair‐living and sexual monogamy as they provide insights into the dispersion of ...females, the potential for territoriality, and whether males are limited to defending an area that can support only one female and her offspring. We examined ranging behavior and space use to evaluate the potential for territoriality in five groups of red titi monkeys (Plecturocebus discolor) during a 10‐year study in Ecuadorian Amazonia. Mean home range size, calculated using a time‐sensitive local convex hull estimation procedure, was 4.0 ± 1.4 ha. Annual home ranges of neighboring groups overlapped, on average, 0%–7%. Mean daily path length was 670 ± 194 m, resulting in defendability indices of 2.2–3.6 across groups. Groups visited, on average, 4 of 12 sections of their home range border area per day, but that was not more often than would be expected by chance, and intergroup encounters were infrequent. We did not find evidence of active monitoring for intruders in border areas, in that groups did not travel either faster or slower when at the border than when in central areas of their range. The absence of overt monitoring might be compensated for by engaging in loud calls, which the study groups did throughout their home ranges; these calls may serve as an advertisement of occupancy and a deterrent to intruding conspecifics. Our finding that red titis have a high potential for territoriality is consistent with several of the main hypotheses proposed to explain pair‐living in mammals.
Graphical
Small, stable home ranges of five red titi monkey groups during the study period at the Tiputinu Biodiversity Station, Amazonian Ecuador.
Research Highlights
Red titis live in small, stable home ranges with low overlap with neighbors and that are theoretically defendable. Groups visit home range borders daily, but not more often than expected by chance, nor do they adjust travel speed at their borders.
Thus, red titis have the potential to be territorial, but evidence of active monitoring is lacking. Loud calling throughout the range may help discourage intrusions as intergroup encounters are infrequent.
These findings are consistent with several of the main hypotheses proposed to explain pair‐living and sexual monogamy in mammals: the female distribution, male mate‐guarding, and resource defense hypotheses.
Objectives
We quantified variation in fecal cortisol across reproductive periods in Azara's owl monkeys (Aotus azarae) to examine physiological mechanisms that may facilitate biparental care. ...Specifically, we evaluated evidence for the explanation that owl monkeys have hormonal mechanisms to mobilize energy during periods when each sex is investing heavily in reproduction, that is, the gestation period for females and the infant care period for males.
Materials and methods
Between 2011 and 2015, we monitored 10 groups of Azara's owl monkeys from a wild population in Formosa, Argentina and collected fecal samples from 26 adults (13 males, 13 females). Using enzyme‐linked immunosorbent assays, we quantified fecal cortisol as a proxy for evaluating stress responses, including energetic demands, on both sexes during periods of reproduction and parental care.
Results
Male cortisol was lowest during periods when they were caring for young infants (<3 months) compared with periods with older infants or no infant. Female cortisol was elevated during gestation compared with other periods. Mean fecal cortisol in both males and females was lower when an infant was present compared with when females were gestating.
Discussion
Our results do not support the hypothesis that owl monkey males have elevated fecal cortisol during periods when they need to mobilize energy to provide intensive infant care. Our findings are also inconsistent with the Maternal Relief hypothesis. However, results from studies measuring fecal cortisol must be interpreted with care and alternative explanations, such as seasonal fluctuations in diet and thermoenergic demands, should be considered when drawing conclusions.
The current extinction and climate change crises pressure us to predict population dynamics with ever‐greater accuracy. Although predictions rest on the well‐advanced theory of age‐structured ...populations, two key issues remain poorly explored. Specifically, how the age‐dependency in demographic rates and the year‐to‐year interactions between survival and fecundity affect stochastic population growth rates. We use inference, simulations and mathematical derivations to explore how environmental perturbations determine population growth rates for populations with different age‐specific demographic rates and when ages are reduced to stages. We find that stage‐ vs. age‐based models can produce markedly divergent stochastic population growth rates. The differences are most pronounced when there are survival‐fecundity‐trade‐offs, which reduce the variance in the population growth rate. Finally, the expected value and variance of the stochastic growth rates of populations with different age‐specific demographic rates can diverge to the extent that, while some populations may thrive, others will inevitably go extinct.
Objectives
Territoriality refers to the consistent defense of an area within the home range (HR) against intrusions of conspecifics. It implies exclusive space use with low degree of overlap among ...neighboring groups, high site fidelity, specific ranging behavior such as high mobility relative to HR size and frequent visits of territory borders, and monitoring behavior. We examined ranging behavior and use of space to evaluate territoriality in Pithecia aequatorialis in Ecuador.
Materials and methods
Between 2005 and 2015, we monitored one main study group continuously and five additional groups for shorter periods (5 months to 2.5 years) at the Tiputini Biodiversity Station, in eastern Ecuador. We scored the location of the study groups at 20 min intervals during, on average, 5 days per month. We estimated saki HRs and core areas (CAs) using the fixed kernel density method (95 and 50%, respectively).
Results
The average HR size was 57 ha and the average CA 14 ha. The degree of overlap between HRs of neighboring groups was low (2–9%). For the main study group, the average overlap between annual HRs was 82%. Mean daily path length across groups was 1,151 m; the defensibility index varied between 1.1 and 2.3 (values >1 are suggestive of territoriality), and the fractional monitoring rate varied between 0.06 and 0.15 (values >0.08 are suggestive of territoriality). Groups did not visit their HR borders (100 m inner buffer) more often than would be expected by chance. Travel speed and directness were comparable between the borders and the centers of groups' HRs.
Discussion
Our multiyear study suggests that equatorial sakis show low degree of range overlap and high site fidelity and have the potential to be territorial, given their high mobility relative to HR size that allows for frequent border monitoring. Nevertheless, their movement patterns in border areas did not reveal evidence for monitoring behavior.
Loud calls play an important function in regulating the use of space and structuring social groups and mating systems in a wide range of taxa. In pair‐living territorial animals, where encounters ...with neighbors and solitary conspecifics are common, these calls are mainly associated with resource defense or mate guarding behaviors. Owl monkeys (Aotus azarae) live in groups of one pair of reproducing adults and 1–4 younger, non‐reproducing individuals. Both sexes disperse when they are around 3 years of age; they become solitary floaters who compete to replace same‐sex adults from other groups. Here, we examined the behavioral responses of Azara's owl monkey pairs toward calls of unpaired and unfamiliar males and females to better understand if the competition between floaters and groups is in relationship to the defense of their territory, their mates, or both. We collected behavioral data from six groups, before, during, and after the playing back of unfamiliar male and female loud calls and of a control stimulus at the center and border of their home ranges. Overall, our results showed that the playback location did not elicit differential responses in the monkeys and that both sexes were more reactive to male than female unfamiliar calls, as evidenced by higher rates of sociosexual and vocal responses, movement toward the playback, and intergroup encounters during and after the experiments. Our study indicates that paired male and female owl monkeys mainly defend their partners toward intruders and emphasizes the need of including the role of sexual competition on both sexes in models about the evolution of pair‐living social organizations and sexual monogamy mating systems.
Objectives
Sexual selection has seemingly influenced chemical communication in numerous non‐human primates, although it is unclear whether it has influenced strictly pair‐living and pair‐bonded taxa. ...The physical similarities between male and female owl monkeys suggest that disruptive selection has not played a role in this taxon. However, given their nocturnality, olfactory traits may show differing patterns of sexual selection than visual traits. If sexual selection has influenced chemical communication in owl monkeys, we expect larger scent glands and greater scent‐marking in females given the high degree of paternal care, as has been proposed for callitrichines.
Materials and Methods
We evaluated sex differences in the qualitative and quantitative descriptions of the subcaudal and perianal glandular regions of captive male (n = 39) and female (n = 36) owl monkeys (A. nancymaae), and in the olfactory behaviors performed within breeding pairs (n = 16).
Results
Males had larger areas of secretion retained in the hairs covering the subcaudal gland, and females had more and darker secretion than males covering the perianal region. Males inspected the genital region of their partners more frequently than females did, but the sexes did not differ much in other investigative and marking behaviors.
Discussion
The observed sex differences and variation in olfactory traits are consistent with the hypothesis that sexual selection has influenced chemical communication in owl monkeys, with males having larger subcaudal glands and spending more time investigating odors. Still, sex differences in monogamous owl monkeys were less extreme than those in other, non‐monogamous, taxa.
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