Humans are thought to be unique in their ability to help others voluntarily even though it may sometimes incur substantial costs. However, there are a growing number of studies showing that prosocial behaviors can be observed, not only in humans, but also among nonhuman primates that live in complex social groups. Prosociality has often been described as a major factor that facilitates group living. Nonetheless, it has seldom been explored whether solitary living primates, such as orangutans, share this propensity. In the present study, we tested four captive orangutans (Pongo abelii × pigmaeus, Pongo pigmaeus) in a simple food-delivering task. They had a choice, incurring the same cost, between getting a food reward for themselves and providing an additional food reward to a conspecific recipient passively sitting in an adjacent booth. Two orangutans played the actor's role, and two orangutans participated as recipients. The results showed that the actors did not choose to deliver food to the recipients more often than expected by chance (51.3 % on average). The control condition demonstrated that this tendency was independent of the actor's understanding of the task. These findings suggest that orangutans do not spontaneously share benefits with other conspecifics, even when the prosocial choice does not disadvantage them. This study gives the first experimental evidence that socially housed captive orangutans do not behave prosocially in a choice paradigm experiment. Further studies using a different experimental paradigm should be conducted to examine whether this tendency is consistent with previous findings hypothesizing that the enhanced prosocial propensity shown in humans and other group living primates is an evolutionary outcome of living in complex social environments.

The adaptive functions of food transfer from parents to their offspring have been explained mainly by two mutually non-exclusive hypotheses: the nutritional and informational hypotheses. In this study, we examined the functions of food transfer in wild Javan gibbons (Hylobates moloch) by testing these hypotheses from both infants' and mothers' perspectives. We observed 83 cases of food solicitations that resulted in 54 occasions of food transfers in three groups over a 19-month period in Gunung Halimun-Salak National Park, Indonesia. Infants initiated all solicitations directed at their mothers with one solicitation towards a father. Food solicitation rate decreased as infant age increased and ceased before weaning. As predicted by the informational hypothesis, infants solicited more food items difficult to obtain and preferred by their parents. On the contrary to the nutritional hypothesis, infants solicited low-quality items more often than high-quality items. Mothers did not change probability of food transfer according to the food characteristics or infant age. Hence, our results suggest that the primary function of food transfer from mother to infant Javan gibbons seems to be information transfer rather than nutritional aids, similarly to great apes.Although food sharing among unrelated adults is somewhat rare, parent-offspring food transfer is reported in majority of primate species 1,2 . Food transfer from parents to offspring has been principally explained as a result of kin selection. Transferred food provides the offspring with a direct benefit, which in return provides parents with an indirect fitness benefit of increased offspring survival 3 . The adaptive functions of food transfer to offspring have been mainly explained by two mutually non-exclusive hypotheses: the nutritional and informational hypotheses 4-6 . The nutritional hypothesis states that food transfer enables offspring to grow and wean faster through the provision of extra nutrients 7 . Thus, the main predictions of the nutritional hypothesis are that food transfer will be most frequent during the weaning period, and that nutritionally rich food items will be transferred more than nutritionally poor items. The informational hypothesis claims that food transfer allows the immature to acquire food processing skills or information about food items which they cannot access themselves 7 . Therefore, following the informational hypothesis, as infants become older and more skilled, food solicitation rates are predicted to decrease 8 . Moreover, according to the informational hypothesis, novel or difficult-to-access/process food items are more likely to be transferred, regardless of their nutritional value.Empirical findings in support of the nutritional hypothesis have been mostly found in cooperatively breeding primate species, such as the callitrichids, including tamarins and marmosets 9-11 . In these species, females typically produce 2-3 litters and become pregnant while lactating, which poses a greater energetic burden for the females [12]...

Rhinopithecus roxellana are generally seasonal breeders, although copulation can occur throughout the year. Previous studies suggest that estradiol modulates female sexual behavior during the mating season. However, the effects of social context on estrogen levels and behavior have not been fully explored. We studied the relationship between sexual behaviors and fecal estrogens in a group of captive R. roxellana during a period of social instability. We collected behavioral data for six months and collected fecal samples at 2-3-day intervals for four months spanning the mating and nonmating seasons, and analyzed fecal estrogen levels via RIA. Females showed clear cyclic solicitation and copulation peaks in the mating season, which corresponded with sharp peaks in fecal estrogens. During the nonmating season, solicitation rates, copulation rates, and fecal estrogens were generally low. However, one nonpregnant female displayed a sharp peak in solicitations, copulations, and estrogens during the nonmating season 10-14 days after a male replacement. Our results provide preliminary evidence that social and behavioral changes affect estrogen levels in R. roxellana.

The expression of emotions and their recognition in conspecifics are pivotal to social life. As Darwin postulated in his pioneering book The expression of the emotions in man and animals, many morphological features and functions of emotional expressions characterized in humans are homologous to those of other animals. Intriguingly, despite this early work, scientists have been skeptical about the feasibility of studying emotions in nonhumans and, therefore, the study of their emotional expressions has been limited. However, recent technological advances in neuroscience, genetics, and fine-scale behavioral analyses enable researchers to investigate human emotions in direct comparison with other animals. Throughout this chapter, the authors provide convincing evidence that nonhuman primates produce and recognize conspecific emotional expressions. Some of them, especially the bared-teeth display, are used in multiple contexts, suggesting cognitively sophisticated functions. The flexible use of emotional expressions seems to be tightly linked to species sociality, such as level of tolerance.