Costs and benefits of allomaternal care to mothers and others in wild Phayre's leaf monkeys.

Objectives: Allomaternal care (AMC) is suggested to be energetically beneficial to mothers and costly to allomothers. However, among primates, AMC is a heterogeneous phenomenon and its implications are less clear especially in female dispersal species. Here, we investigated infant care in a female dispersal species, Phayre's leaf monkeys (Trachypithecus phayrei crepusculus), to evaluate whether mothers were constrained by infant care and benefitted energetically from AMC, whether AMC was energetically costly for allomothers and how maternal experience was associated with AMC.

Materials and methods: Data were collected via instantaneous focal animal sampling between 2004 and 2005 for juvenile and adult females (N = 18) from two groups at the Phu Khieo Wildlife Sanctuary, Thailand (440 h). We used generalized linear mixed models to determine how infant care during the first month after birth affected the time mothers and allomothers spent feeding, socializing, resting, and locomoting and how AMC varied.

Results: In the first month, infants spent 26% of their time with an allomother. We found no differences in mothers' overall activity before versus after birth, although mothers fed significantly more and rested less when without their infant. Allomothers fed and rested less when with an infant. AMC varied between 0.0% and 20.5%, with immature females being most active.

Discussion: Mothers appear to benefit energetically from AMC such that their overall activity after birth remained unchanged. Costs and benefits for allomothers seem to be variable. Some very active immature females may be benefitting from learning-to-mother. The overall low cost of AMC may facilitate a reciprocal social network among unrelated females.