Ranging behavior and the potential for territoriality in pair-living titi monkeys (Plecturocebus discolor)

Who you live with and what you duet for: a review of the function of primate duets in relation to their social organization

Duets are one of the most fascinating displays in animal vocal communication, where two animals fine-tune the timing of their emissions to create a coordinated signal. Duetting behavior is widespread in the animal kingdom and is present in insects, birds, and mammals. Duets are essential to regulate activities within and between social units. Few studies assessed the functions of these vocal emissions experimentally, and for many species, there is still no consensus on what duets are used for. Here, we reviewed the literature on the function of duets in non-human primates, investigating a possible link between the social organization of the species and the function of its duetting behavior. In primates and birds, social conditions characterized by higher promiscuity might relate to the emergence of duetting behavior. We considered both quantitative and qualitative studies, which led us to hypothesize that the shift in the social organization from pair living to a mixed social organization might have led to the emergence of mate defense and mate guarding as critical functions of duetting behavior. Territory/resource ownership and defense functions are more critical in obligate pair-living species. Finally, we encourage future experimental research on this topic to allow the formulation of empirically testable predictions.

Intergroup encounters in pair-living primates: Comparative analysis and a case study of pair-living and monogamous owl monkeys (Aotus azarae) of Argentina

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.

Linking glucocorticoid variations to monthly and daily behavior in a wild endangered neotropical primate

Identifying the factors swaying physiological stress levels in wild animals can help depict how they cope with environmental and social stressors, shedding light on their feeding ecology, behavioral plasticity, and adaptability. Here, we used noninvasive methods to explore the link between glucocorticoid levels and behavior in an endangered neotropical primate facing habitat fragmentation pressure, the black lion tamarin (Leontopithecus chrysopygus). We investigated monthly and day-to-day glucocorticoid variations independently to attempt to disentangle the complex nature of the adrenocortical activity. Between May 2019 to March 2020, we followed two groups of black lion tamarins in two different areas, a continuous forest and a small fragment, and gathered behavioral data (over 95 days in total; 8.6 ± 3.9 days/month) and fecal samples (N_samples  = 468; 4.93 ± 3.5 samples/day) simultaneously. Preliminary analyses enabled us to identify circadian variations linked to the biological rhythm, which were taken into account in subsequent models. Monthly analyses revealed that black lion tamarin fecal glucocorticoid metabolite levels vary according to changes in activity budget associated with the fruit consumption, movement, and resting time of the groups. At a day-to-day level, while intergroup encounters led to increases in fecal glucocorticoid metabolite concentrations, we found that changes in food intake or activity level did not trigger physiological stress responses. These findings suggest that diet and ranging patterns, driven by food availability and distribution, influence physiological stress at a seasonal scale, while acute stressors such as interspecific competition trigger short-term stress responses. Exploring fecal glucocorticoid metabolite variations over different timescales can help uncover the predictive and reactive facets of physiological stress in wild species. Moreover, having a comprehensive understanding of the physiological state of species is a valuable conservation tool for evaluating how they cope in changing environments.

Coordinated Singing in Coppery Titi Monkeys (Plecturocebus cupreus): Resource or Mate Defense?

Coordinated singing, performed as duets by mated pairs and often joined by offspring to form choruses, is a distinctive behavioral attribute of the social system of pair-living and pair-bonded Neotropical titi monkeys. Duets and choruses are presumed to be associated with mate or territorial defense, but no consensus has yet been reached regarding their function. Here, we examined temporal and spatial patterns of coordinated singing in eight wild groups of coppery titi monkeys, Plecturocebus cupreus, in Peruvian Amazonia to test predictions of the joint resource and mate defense. We investigated singing rates in relation to female reproductive state, fruit consumption and demographic context using a dataset based on 227 observation days and analyzed temporal and spatial distribution of songs using a dataset based on 150 songs, collected between June 2017 and September 2021. Titi monkeys sang least frequently when females were likely to be sexually receptive and most frequently when females were likely to be pregnant. Groups also sang slightly more often when fruits were consumed more intensively, although this association did not reach statistical significance. The duration of songs was not associated with female reproductive state or fruit consumption, but songs were longer during inter-group encounters compared to non-encounter contexts. Songs were not concentrated in the core areas of home ranges; rather, they were distributed throughout the home ranges in concordance with its use. Finally, songs were concentrated around dawn. Our results provide support for a function in joint resource defense and inter-group communication of coordinated songs in coppery titi monkeys. The function of coordinated songs for mate defense in the form of paternity guarding, on the other hand, was not supported by our findings.

Social evolution in mammals

[Figure: see text].