Fostering behaviour in New Zealand sea lions Phocarctos hookeri

The fostering behaviour of female New Zealand sea lions Phocarctos hookeri was studied to investigate simultaneously nursing of two or more pups. Surveys (n=44) documenting the frequency of this fostering behaviour found that it is uncommon with a mean of 6.0% (S.E.=0.5) of nursing females at any one time seen nursing two or more pups simultaneously. Most of the multiple suckling events (n = 149) observed were females simultaneously nursing two pups (78%), although three (20% and four (2%) pups per female were also recorded. We used time spent nursing and a nursing index to investigate relative nursing times. Females seen to nurse more than one pup simultaneously (multiple nursers) spent significantly more time nursing and had a significantly higher nursing index than females seen to nurse only a single pup at any one time (single nursers). The impact of this on the female and her pup are unknown but could represent a significant increase in the cost of lactation over this period. Single nursers rejected almost all (95%) pups that attempted to suckle from them when there was already a pup suckling, while multiple nursers rejected less than half (46%) of the extra pups and appeared more tolerant of additional pups suckling. The mechanism through which this behaviour may have evolved has not been determined.