Fostering behavior and the effect of female density in Hawaiian monk seals, Monachus schauinslandi

Frequent nursing of pups by non-filial females (fostering) has been reported in Hawaiian monk seals (Monachus schauinslandi) at French Frigate Shoals in the Northwestern-Hawaiian Islands. We present data on the occurrence of fostering at Laysan Island, NWHI, and compare spatial patterns and behavior of seals at Laysan Island and East Island, French Frigate Shoals, to investigate the importance of density in frequency of fostering. Fostering was common at Laysan Island; 53% of 17 females nursed pups other than their own for some part of their lactation period. This level of fostering was significantly less than that at East Island, where ca. 90% of females fostered pups in 2 separate years (n = 30 in 1987 and 10 in 1989). Density of females was significantly lower at Laysan Island than East Island (0.5 versus 1.5 femaIes/1,000 m(2)), and nearest female neighbors were significantly farther away (58 versus 27 m) at Laysan Island. Stage of lactation at which fostering started, the total duration of foster care and the duration of fostering episodes did not differ between colonies. However, mean number of pups fostered per female at the denser colony (2.3 pups, East Island) was greater than at the less dense colony (1.3 pups, Laysan Island). No difference occurred between islands in relative frequency of aggressive interactions between females, which are known to result in females exchanging pups. High female density does appear to increase fostering frequency but not through female-female aggression as expected. Instead, high density increases the likelihood that separated females and pups will encounter another potential partner before reuniting.