Preliminary isotopic assessment of weaning in bonobos shows evidence for extended nursing, sibling competition and invested first-time mothers

Although considered a hallmark in early ontogeny, weaning from breastmilk is difficult to monitor in wild primates and weaning ages remain unknown for wild bonobos (Pan Paniscus). Here, we calculated inter-birth intervals from demographic data and measured the isotopic offsets (Delta N-15 and Delta C-13) between mother (n = 17) and offspring (n = 28) fecal sample pairs (n = 131, total n = 246) in the LuiKotale bonobos to assess nutritional weaning for the first time. We tested the effects of infant age, female parity, and sibling competition on Delta N-15 and Delta C-13 values. We found bonobo inter-birth intervals ranging from 2.2 to 7.3 years ((x) over bar = 4.7 +/- 1.3 years) at LuiKotale. The Delta N-15 and Delta C-13 values suggested nutritional weaning on average by 6.6 and 7.0 years of age respectively, considerably exceeding weaning ages reported for chimpanzees (P. troglodytes) using the same approach. Our Delta C-13 data suggested that the number of offspring present affected nursing, with first-time mothers nursing more and possibly longer. The Delta N-15 and Delta C-13 values decreased with the arrival of the next sibling, suggesting sibling competition reduces milk access. Nevertheless, offspring may continue nursing 2.5-3 years after the birth of the next sibling, corresponding well with observations on low infant mortality. In conclusion, bonobo mothers provide remarkably enduring materna l support in the form of nursing concurrently to several offspring.