Ontogenetic dietary shifts in the Indo-Pacific humpback dolphin (Sousa chinensis)

Abstract

The deposition of growth layer groups (GLGs) in odontocetes teeth provides a temporal archive of dietary nitrogen (N), which is valuable in studies of foraging ecology. Although the Indo-Pacific humpback dolphins (Sousa chinensis) inhabiting Hong Kong waters have received considerable research attention since the mid-1990s, their early postnatal life history is poorly understood. The age of weaning, a period of transition from milk-dependent diet to solid prey consumption, has never been investigated; although mother-calf disassociation is thought to occur when calves reach ~3 years of age. In this study, we extracted high-resolution dentine materials from teeth of humpback dolphins by micro-milling. We analysed d15N (13.7‰ ± 1.68‰) in all extractable dentine layers (n=110) from 11 individuals (5 males, 4 females and 2 of unknown sex) that stranded in Hong Kong between 2007 to 2015. The growth layer groups count ranged from 5 to 16; and the ontogenetic pattern was compared between the GLGs, sex, total body length, calendar year and C:N ratio. Segmentation analysis showed that the point of dietary change is at the age of 3.4 years (95% CI: 2.2 - 4.5 years). The variance of the estimated complete weaning age may reflect an individual variability and, likely, the presently still relatively small sample size. This ongoing study provides novel insights into individual dietary life histories that cannot be obtained through traditional field observations. Moreover, the teeth of Indo-Pacific humpback dolphins are among the smallest cetacean teeth ever analysed with the micro-mill technique. Future work will incorporate more samples from the broader Pearl River Delta (PRD) region. Given the unusually high calf mortality reported in the past 2-3 years, a period marked by extensive coastal construction projects in Hong Kong, our results may prove valuable to dolphin conservation in both Hong Kong and the PRD.