Not as expected: Outgroup - not ingroup - females mirror the parturient hormonal shifts in bonobos

Oxytocin (OT) and cortisol (CORT) are key to parturition and lactation and they are also implicated in maternal behaviour and social bonding; OT is often linked to affiliation, whereas CORT may be associated with anxiety and social inhibition. Around birth, both OT and CORT are known to vary not only in the parturient female, reflecting the physiological and behavioural demands of parturition, but also in other females, in response to socially salient birth-related cues. In this study, we checked if and how in bonobos - phylogenetically closest to humans together with chimpanzees - hormonal levels in the parturient female and in the non-parturient females (of the same and of a different, control group) varied around the socially salient cue change represented by birth. From October, 16 to December, 3 2019, we collected urine samples for hormonal analyses at La Vallée des Singes (France) from two bonobo groups housed in adjacent enclosures: one included a pregnant female (Omanga, who gave birth on 10 November), and one included no pregnant females. We also collected, as a control, affiliation and anxiety related behavior (grooming/sit-in contact; scratching). We defined ingroup females as those in the parturient’s group and outgroup females as controls, and compared two periods: around birth (±2 weeks around delivery) and out of birth (±16 days before/after that window). Urinary OT and CORT concentrations were quantified using liquid chromatography–mass spectrometry. Because OT undergoes transient variations around birth whereas CORT varies over larger time windows, we predicted that OT, but not CORT, would increase around birth in the parturient female and, as a response to birth as a salient cue, in ingroup females. Results supported the prediction for the parturient: OT increased around birth, whereas CORT did not vary. Contrary to expectations, OT did not rise in ingroup females but increased in outgroup females, with no CORT changes in either group. Because affiliative behaviour increased in ingroup (not outgroup) females and swelling phase did not change in the females of either group, the outgroup OT increase may be related to the changes related to birth in the parturient’s group. Visual and auditory contact with infants can enhance oxytocinergic activity in mammals and heighten sensitivity to socially salient cues in outgroups. Further work is needed to confirm possible physiological synchrony patterns and clarify the underlying mechanisms.