Brenda L. Becker

THE HAWAIIAN MONK SEAL ON LAYSAN ISLAND, 1988

The endangered Hawaiian monk seal, Monachus schauinslandi, was studied on TysanIslandintheNorthwesternHawaiianIslandsfrom28Februaryto20 June andfrom 24 October to 13 November 1988. Data were collected on haul-out patterns, population structure, reproductive patterns, and factors affecting survival (primarily adult male aggression toward adult female and immature seals). Census counts including pups ranged from 73 to 128 seals (3 = 105); counts excluding pups ranged from 61 to 116 seals (5 = 87). There were 33 1 individual seals identified (286 excluding pups). All weaned pups and 41 adult males were tagged, and 3 subadults were retagged. Minimum first-year survival of tagged 1987 pups was 86%. At least 45 pups were born (24 males, 19 females, and 2 of unknown sex), and 40 survived to weaning. This represents the highest number of births documented on Laysan Island since extensive research began in 1977. Average nursing period was 39-40 days. Exchanges of pups were observed 17 times between 14 nursing females. Adult female reproductive rate was at least 61%. Of the 45 parturient females observed, 31 were identifiable from previous years. Average time between births in successive years was 382 days. This season, three 5-year-olds and a 22-year-old pupped, representing the youngest and oldest ages at parturition on record. One seal was observed giving birth, and two were observed immediately following parturition. In 1988, interatoll movement was documented for 11 seals, which moved between Laysan Island and Lisianski Island or French Frigate Shoals. At least 92 injuries were sustained across a l l age groups. Seven seals were found entangled in debris and were released by observers. In all, 485 items of debris were inventoried and destroyed. At least 17 seals disappeared during the 1988 field season. Nothing unusual was noted before 12 of these disappearances, but circumstantial evidenceindicates 5were probably deaths. Inaddition, at least 14 seals died. Adult male aggression resulted in 35 injuries, 4 probable deaths, and 7 hown deaths.

Dizygotic twinning in the Hawaiian monk seal

Abstract Twinning is extremely rare in pinnipeds. Instances in which a female is observed simultaneously nursing 2 pups may represent alloparenting, or nonfilial nursing, a relatively common trait in pinnipeds. We explored the incidence of twinning in the Hawaiian monk seal (Monachus schauinslandi). We observed 7 sets of putative twins (i.e., an isolated female nursing 2 newborn pups) of 4,965 total births recorded between 1983 and 2008 (minimum twinning rate  =  0.1%). Genetic specimens from the mother and both pups were available only for 5 of the 7 sets. Microsatellite genotyping revealed all of these to be dizygotic twins, with low probability of false assignment (P < 0.001). On average, Hawaiian monk seal twins were less likely to survive to weaning than singletons born in the same year and on the same island (P  =  0.008). Those that weaned were smaller in size (i.e., axillary girths 1–4 SDs lower than singletons). In the critically endangered Hawaiian monk seal twinning appears to be a rare trait that results in overall lower survival rates but also offers a small opportunity for stock enhancement.