Brandon et al.: Neonatal growth of Eumetopias jubatus 



253 



Conflicting results have been reported in other 

 growth studies of otariids. Several studies reported 

 that male pups grew faster than female pups (Antarctic 

 fur seals: Payne, 1979; Doidge et al.. 1984; Antarctic 

 and Subantarctic fur seals: Kerley, 1985; New Zealand 

 fur seals: Mattlin 1981). However, cross-sectional data 

 on growth rate were used in these studies. Conversely, 

 longitudinal data, considered to be more accurate, dem- 

 onstrate no differences in neonatal growth rate between 

 male and female Antarctic fur seal pups (Doidge and 

 Croxall, 1989; Lunn et al., 1993; Lunn and Arnould, 

 1997); Goldsworthy (1995), however, is the exception. 

 Ono and Boness (1996) collected longitudinal growth 

 data on California sea lion pups and found that males 

 grew faster than females, but they found no other evi- 

 dence of differential maternal investment. In phocids, 

 most studies have found no difference in neonatal male 

 and female growth rates, regardless of whether the 

 data were longitudinal or cross sectional (Stewart and 

 Lavigne, 1980; Innes et al., 1981; Bowen et al., 1992). 

 This is true for species with extreme sexual dimor- 

 phism such as elephant seals (McCann et al., 1989; 

 Campagna et al., 1992). The only other study where 

 growth rates for SSL pups were measured did not have 

 a large enough sample size for a comparison between 

 males and females (Higgins et al., 1988). No differences 

 between male and female pups were found for suckling 

 behavior or maternal attendance behavior (Higgins et 

 al., 1988). 



Total body lipid 



Average %TBL of neonatal pups was low (5.6% BM). 

 Steller sea pups are born with small energy stores and 

 normally fast for short periods (about one day) while 

 their mothers make foraging trips to sea. There have 

 been few measurements of lipid content in otariid neo- 

 nates. Jonker and Trites (2000) found a blubber content 

 of 9.7% BM in five SSL pups in the first month after 

 birth. However, this measurement does not correspond 

 directly to body fat content because they measured blub- 

 ber content by weighing the sculp (skin plus blubber) and 

 then calculating the fraction of sculp that was blubber by 

 measuring skin and blubber thicknesses. Using the same 

 labeled water method as in the present study, Arnould et 

 al. (1996b) found a %TBL of 9.4% BM in four Antarctic 

 fur seal pups in the first month after birth. In a similar 

 study of one-day-old Antarctic fur seal pups, Arnould et 

 al. (1996a) found a %TBL of 7.0% BM for female pups 

 and 4.9% BM for male pups. Also using labeled water, 

 Oftedal et al. (1987a) found an average %TBL of 5% BM 

 for neonatal California sea lion pups. 



Arnould et al. (1996b) suggested two explanations 

 for the higher lipid content that they found in Antarc- 

 tic fur seal pups in comparison to California sea lion 

 pups (Oftedal et al. 1987b). First, in colder habitats, a 

 larger subcutaneous lipid store may be necessary for 

 thermoregulation. The data here do not support that 

 explanation. SSL live in a colder habitat than Cali- 

 fornia sea lions, but have a similar %TBL. The more 



likely explanation is that larger lipid stores are found 

 in species in which pups normally fast longer while 

 their mothers are foraging. Steller sea lion pups have 

 the smallest lipid stores and shortest fasting periods 

 (Brandon, 2000) of the three species. 



Differences in pup size among rookeries 



Although male and female pups differed significantly 

 in size, there were no significant differences in pup size 

 at birth among the rookeries studied. Rookery location 

 should have less influence on pup size at birth than on 

 neonatal growth because maternal foraging range is 

 much greater during gestation than during lactation 

 (Merrick and Loughlin, 1997). This greater maternal 

 foraging range during gestation reduces, among rook- 

 eries, variation in maternal size and feeding conditions 

 (quantity and quality of prey available) during gestation, 

 both of which have been shown to influence pup birth 

 mass in pinnipeds (Calambokidis and Gentry, 1985; 

 Kovacs and Lavigne, 1986; Trites, 1991; Trites 1992). 

 The lack of a difference in pup BM at birth among rook- 

 eries could also be explained by the fact that females 

 that are "successful" (i.e.. carry their fetuses to term) 

 have a significantly better body condition than females 

 that do not carry their fetuses to term (Pitcher et al., 

 1998). As a consequence of our study design, only those 

 females that were successful were used, and therefore 

 our sample was biased toward females in the population 

 with better body condition. In addition, gestation is less 

 energetically expensive than early lactation; therefore 

 differences in food availability would have less of an 

 effect during gestation (Robbins and Robbins, 1979; 

 Albon et al., 1983; Oftedal, 1984). 



Although most pup morphometries at first capture did 

 not differ among rookeries, growth parameters differed 

 significantly (Table 3). Growth rates of pups on Seguam 

 and Yunaska Islands (0.48 kg/day) and on Lowrie Is- 

 land (0.23 kg/day) represented the extremes, whereas 

 growth rates of pups on Chirikof, Marmot, and Fish 

 Islands fell between these two extremes. In general, 

 faster growth rates occurred in the west and slower 

 growth rates in the east. In terms of mass, Seguam and 

 Yunaska Islands and Chirikof Island pups grew twice 

 as fast as Lowrie Island pups. A concurrent study of 

 the attendance patterns of lactating females (Brandon, 

 2000) showed that foraging trip duration decreased 

 from east (25.6 hours on Lowrie Island) to west (an 

 average of9.4 hours on Chirikof and Seguam Islands). 

 Therefore, it is possible that the higher growth rates in 

 SSL pups in the western Gulf of Alaska and Aleutian 

 Islands resulted from shorter periods of fasting while 

 females were foraging at sea (Arnould et al., 1996a; 

 Goldsworthy, 1995). 



Is food limiting growth in Steller sea lion pups 

 in the area of population decline? 



If the cause of the population decline were decreased 

 food availability, which is one of the leading hypotheses 



