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 (ΔN and ΔC) 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 ΔN and ΔC values. We found bonobo inter-birth intervals ranging from 2.2 to 7.3 years (x̄ = 4.7 ± 1.3 years) at LuiKotale. The ΔN and ΔC 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 ΔC data suggested that the number of offspring present affected nursing, with first-time mothers nursing more and possibly longer. The ΔN and ΔC 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.