Two wild female bonobos adopted infants from a different social group at Wamba

Behavioral adaptation in an adoptive free-ranging female Tibetan macaque

Adoption is an important form of allomaternal care in nonhuman primates, with implications for reproductive output and infant survival. Here, we report a kidnapping that became an adoption of a 3-week-old infant by a mother with her own infant in Tibetan macaques (Macaca thibetana). The adoptive mother nursed her "new" infant (allonursing), the first observation of this behavior in the species. The case provided a natural experiment for comparing how a female copes with a heavier burden of care for both her biological infant and another female's infant, compared to mothers caring for only one infant. Our results showed that the adoptive female spent more time foraging and resting, and less time in group social activity compared to females with a single infant. The adoptive female showed more instances of social bridging. Although the duration of post-bridging grooming received from group members decreased, the frequency of such grooming increased. We discuss this adoption with reference to possible factors involved in the evolution of adoption and allonursing behavior in Tibetan macaques.

Fearful apes, happy apes: Is fearfulness associated with uniquely human cooperation?

In the fearful ape hypothesis, Grossmann argues that heightened fearfulness increases human-unique cooperation. We suggest that this conclusion, however, may be premature. In particular, we question Grossmann's singling out of fear as the affective trait that enhances cooperative care. Additionally, we problematize the extent to which heightened fearfulness in humans, and its association with human-unique cooperation, are supported empirically.

Human cooperation and evolutionary transitions in individuality

A major evolutionary transition in individuality involves the formation of a cooperative group and the transformation of that group into an evolutionary entity. Human cooperation shares principles with those of multicellular organisms that have undergone transitions in individuality: division of labour, communication, and fitness interdependence. After the split from the last common ancestor of hominoids, early hominins adapted to an increasingly terrestrial niche for several million years. We posit that new challenges in this niche set in motion a positive feedback loop in selection pressure for cooperation that ratcheted coevolutionary changes in sociality, communication, brains, cognition, kin relations and technology, eventually resulting in egalitarian societies with suppressed competition and rapid cumulative culture. The increasing pace of information innovation and transmission became a key aspect of the evolutionary niche that enabled humans to become formidable cooperators with explosive population growth, the ability to cooperate and compete in groups of millions, and emergent social norms, e.g. private property. Despite considerable fitness interdependence, the rise of private property, in concert with population explosion and socioeconomic inequality, subverts potential transition of human groups into evolutionary entities due to resurgence of latent competition and conflict. This article is part of the theme issue 'Human socio-cultural evolution in light of evolutionary transitions'.

Why help others? Insights from rodent to human early childhood research

Helping behavior are actions aiming at assisting another individual in need or to relieve their distress. The occurrence of this behavior not only depends on automated physiological mechanisms, such as imitation or emotional contagion, that is, the individual's emotion and physiological state matching with others, but also needs motivation to sustain. From a comparative and developmental perspective, we discover that the motivation for helping behavior has a deep foundation both phylogenetically and ontogenetically. For example, empathic concern for others, relieving personal distress and the desire for social contact are universal motivations across rodents, non-human primates and human early childhoods. Therefore, a circle-layered model integrating evidences for motivation for helping behavior from rodent to human early childhood research is proposed: the inner circle contains the emotional-behavioral system and the outer circle contains the affective-cognitive system. The application of this model has significance for both behavioral neuroscience research and cultivating prosocial behavior in human society.

Infant adoptions in wild bonnet macaques (Macaca radiata)

Though uncommon, adoption of orphaned infants has been observed in both wild and captive non-human primates. In two groups of wild bonnet macaques (Macaca radiata), we observed five instances of infants being cared for after they lost their mothers at a pre-weaning age (< 6 months). Orphaned infants had one or more caregivers (juvenile, subadult, and adult female or male) involved in carrying, grooming, hugging, and protecting them. Adoption did not appear to be related to the age/sex class of the infant, or directly to the mother's rank. Although the dominance rank of the mother of an orphaned infant did not have a direct effect on orphan survivorship, it determined the number of caregivers available to the orphaned infant, and infant survivorship was positively related to the number of caregivers of the orphaned infant. Thus, survivorship was likely a function of the mother's sociality. Two other infants born to high-ranking mothers were also adopted by more individuals and survived longer than the infants of low-ranking mothers.

Transition to siblinghood causes a substantial and long-lasting increase in urinary cortisol levels in wild bonobos

In animals with slow ontogeny and long-term maternal investment, immatures are likely to experience the birth of a younger sibling before reaching maturity. In these species, the birth of a sibling marks a major event in an offspring’s early life as the older siblings experience a decrease in maternal support. The transition to siblinghood (TTS) is often considered to be stressful for the older offspring, but physiological evidence is lacking. To explore the TTS in wild bonobos, we investigated physiological changes in urinary cortisol (stress response), neopterin (cell-mediated immunity), and total triiodothyronine (T3, metabolic rate), as well as changes in behaviors that reflect the mother–offspring relationship. Following a sibling’s birth, urinary cortisol levels of the older offspring increased fivefold, independent of their age, and remained elevated for 7 months. The cortisol level increase was associated with declining neopterin levels; however, T3 levels and behavioral measures did not change. Our results indicate that the TTS is accompanied by elevated cortisol levels and that this change does not coincide with nutritional weaning and attainment of physical independence. Our results suggest that bonobos and humans experience TTS in similar ways and that this developmental event may have emerged in the last common ancestor.