252 



Fishery Bulletin 103(2) 



Discussion 



Compared to other species of sea lions and fur seals, SSL 

 pups are large, although this species produces smaller 

 pups in relation to adult size than do smaller otariids 

 (Kovacs and Lavigne, 1992; McLaren, 1993). In the 

 present study, male pups averaged 22.6 kg and female 

 pups averaged 19.6 kg at 1-5 days of age, which is in 

 the range of birth masses reported in the literature. Two 

 studies conducted before the recent population decline 

 reported 17 kg for male pups at birth (Scheffer, 1945) 

 and a range of 9.1-21.8 kg for male and female pups 

 (Mathisen et al., 1962). Late in the population decline, 

 studies reported a range of 16-23 kg for pups at birth in 

 Alaska (Calkins and Pitcher, 1982) and an extrapolated 

 birth mass of 17.9 kg for five pups for which growth rates 

 were measured in California (Higgins et al., 1988). 



This is the first, large-scale (in terms of sample size 

 and geographic area) longitudinal study of growth in 

 Steller sea lion pups. Growth rates reported in our 

 study are the highest absolute growth rates reported for 

 any sea lion or fur seal. This is to be expected because 

 adult SSLs are the largest otariids (Kovacs and Lavi- 

 gne, 1992). The growth rate of 0.38 kg/day measured 

 for five SSL pups at Afio Nuevo Island in California 

 (Higgins et al., 1988) falls within the range of average 

 growth rates measured in the present study (0.23-0.48 

 kg/day). The only other measurement of pup growth in 

 SSLs was conducted on captive pups that were already 



Chmkof Island 

 1993 



Lowne Island 

 1993-94, 1997 



Marmot Island 

 1994 



15 20 25 



Age (days) 



35 



Figure 3 



Summary of Steller sea lion lEumetopias jubatus) pup growth (body 

 mass) during the first six weeks after birth for all five rookeries. 

 The length of each line indicates the length of the study period at 

 that location. Pups from Seguam, Yunaska, and Chirikof Islands, 

 in the declining population, grew significantly faster than pups 

 from Lowrie Island, in the stable population. Pups from Seguam 

 and Yunaska Islands also grew significantly faster than pups from 

 Marmot Island. 



several months old. In terms of growth rate in relation 

 to size at birth, SSL pups gained 1-2.3% of their birth 

 weight per day (Lowrie Island and Seguam and Yu- 

 naska Islands, respectively, based on an average birth 

 mass of 21.1 kg), which was faster than the relative 

 growth rates reported for other otariid species (Kovacs 

 and Lavigne, 1992, calculated from Table 1), except for 

 northern fur seals. In contrast, seals (order Carnivora, 

 family Phocidae) exhibit faster growth rates (1.3-5.6 

 kg/day or 8-26% birth weight per day) (Stewart and 

 Lavigne, 1980; Bowen et al., 1985; Kovacs and Lavigne, 

 1985; Bowen et al., 1987; Bowen et al., 1992; Campagna 

 et al., 1992). Although adult SSLs are larger than many 

 species of phocid seals, phocids have much shorter lacta- 

 tion periods and their pups grow at a more accelerated 

 rate than do otariids. 



Male-female differences 



Male pups weighed 159c more than females at birth, indi- 

 cating a difference in maternal investment during gesta- 

 tion, which has been found in other otariids including 

 Antarctic fur seals (Doidge et al., 1984; Lunn and Boyd, 

 1993; Goldsworthy, 1995; Boyd, 1996), South American 

 fur seals (Arctocephalus australis) (Lima and Paez. 1995), 

 California sea lions (Ono and Boness, 1996), and southern 

 sea lions (Otaria byronia) (Cappozzo et al., 1991). These 

 results are consistent with the predictions of Maynard- 

 Smith's (1980) theory on sexual investment. Steller sea 

 lion adults are extremely sexually dimorphic; 

 females weigh 263 kg on average (maximum 

 of approximately 350 kg); males weigh more 

 than twice as much (average of 566 kg, maxi- 

 mum of approximately 1120 kg) (Calkins and 

 Pitcher, 1982). In view of this dimorphism 

 and the fact that size is more important to 

 male fitness than to female fitness in a polyg- 

 ynous species (McCann, 1981) such as the 

 SSL, theory predicts that males would be 

 heavier than females at birth. Northern fur 

 seal females with male fetuses are in poorer 

 condition than mothers with female fetuses 

 (Trites, 1992), and male fetuses grow at a 

 faster rate than female fetuses (Trites, 1991), 

 indicating that mothers invest more in male 

 offspring during gestation. 



However, there were no male-female dif- 

 ferences in neonatal growth (BM, SL, and 

 AG) rate in SSL during the first six weeks 

 after birth. In a species as sexually dimor- 

 phic as SSL, one would expect males to grow 

 at a faster rate than females during devel- 

 opment. However, this difference may not 

 occur until the animals are older. There is 

 some evidence that male otariids undergo a 

 sharp increase in growth rate near sexual 

 maturity (McLaren, 1993; Bester and Van 

 Jaarsveld, 1994), after females have already 

 reached sexual maturity and their growth 

 has slowed. 



"H 

 40 



