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. Surv eys (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 on e time seen nursing two or more pups simultaneously. Most of the multiple s uckling events (n = 149) observed were females simultaneously nursing two p ups (78%), although three (20% and four (2%) pups per female were also reco rded. We used time spent nursing and a nursing index to investigate relativ e nursing times. Females seen to nurse more than one pup simultaneously (mu ltiple nursers) spent significantly more time nursing and had a significant ly 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 ar e unknown but could represent a significant increase in the cost of lactati on over this period. Single nursers rejected almost all (95%) pups that att empted to suckle from them when there was already a pup suckling, while mul tiple 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.