Ontogeny of diving behaviour in the Australian sea lion: trials of adolescence in a late bloomer

Abstract

Summary1. Foraging behaviours of the Australian sea lion ( Neophoca cinerea ) reflect an animal working hard to exploit benthic habitats. Lactating females demonstrate almost continuous diving, maximize bottom time, exhibit elevated field metabolism and frequently exceed their calculated aerobic dive limit. Given that larger animals have disproportionately greater diving capabilities, we wanted to examine how pups and juveniles forage successfully. 2. Time/depth recorders were deployed on pups, juveniles and adult females at Seal Bay Conservation Park, Kangaroo Island, South Australia. Ten different mother/pup pairs were equipped at three stages of development (6, 15 and 23 months) to record the diving behaviours of 51 (nine instruments failed) animals. 3. Dive depth and duration increased with age. However, development was slow. At 6 months, pups demonstrated minimal diving activity and the mean depth for 23-month-old juveniles was only 44 ± 4 m, or 62% of adult mean depth. 4. Although pups and juveniles did not reach adult depths or durations, dive records for young sea lions indicate benthic diving with mean bottom times (2·0 ± 0·2 min) similar to those of females (2·1 ± 0·2 min). This was accomplished by spending higher proportions of each dive and total time at sea on or near the bottom than adults. Immature sea lions also spent a higher percentage of time at sea diving. 5. Juveniles may have to work harder because they are weaned before reaching full diving capability. For benthic foragers, reduced diving ability limits available foraging habitat. Furthermore, as juveniles appear to operate close to their physiological maximum, they would have a difficult time increasing foraging effort in response to reductions in prey. Although benthic prey are less influenced by seasonal fluctuations and oceanographic perturbations than epipelagic prey, demersal fishery trawls may impact juvenile survival by disrupting habitat and removing larger size classes of prey. These issues may be an important factor as to why the Australian sea lion population is currently at risk.

The ontogeny of aerobic and diving capacity in the skeletal muscles of Weddell seals

“…Previous studies investigating body oxygen stores in diving mammals have reported neonates with significantly lower total body oxygen stores compared with adults. This suggests that increases in oxygen stores are triggered by foraging (Burns et al, 2005;Clark et al, 2006;Clark et al, 2007;Fowler et al, 2006;Weise and Costa, 2007). Our findings that the pups had significantly lower concentrations of myoglobin and hence lower intramuscular stores of oxygen compared with either the adults or juveniles support these previous studies.…”

“…Together with the thermoregulatory benefits of lanugo, an increased aerobic capacity may provide additional thermogenesis during lactation and weaning. Juveniles possess low whole-body oxygen stores and hence relatively poor diving capacity compared with adults (Burns et al, 2005;Burns et al, 2007;Clark et al, 2006;Clark et al, 2007;Fowler et al, 2006;Weise and Costa, 2007). This is reflected in both skeletal muscle physiology and mean dive durations (Burns, 1996) that are consistent with those of short-duration, shallower divers such as the Steller sea lion (Fig.…”

Rapid maturation of the muscle biochemistry that supports diving in Pacific walruses (Odobenus rosmarus divergens)

“…Differences in diving ability at this stage are associated with differences in body size because, by weaning, the blood oxygen stores of walruses are mature (S.R.N., unpublished), and it is likely that the muscle oxygen store is mature, based on the observation that the longissimus dorsi myoglobin content was already 79% of adult levels by 3 months post-partum (Table 1). Although there are no published accounts of the diving behaviors of immature walruses for comparison, studies on other pinnipeds have demonstrated shorter dive durations, shallower dive depths and a lower percentage of time spent diving by immature seals compared with adult conspecifics (Thorson, 1993;Lydersen and Hammill, 1993;Lydersen et al, 1994;Horning and Trillmich, 1997;McCafferty et al, 1998;Bowen et al, 1999;Burns, 1999;Greaves et al, 2005;Fowler et al, 2006;Rehberg and Burns, 2008). The dive durations of immature pinnipeds increase with age as body size increases (Kooyman et al, 1983;Lydersen and Hammill, 1993;Thorson, 1993;Lydersen et al, 1994;Burns and Castellini, 1996;Burns et al, 1998;Horning and Trillmich, 1997;Burns et al, 1998;Costa et al, 1998;McCafferty et al, 1998;Burns, 1999), and these ontogenetic differences in diving capacity have been associated with inter-age class variations in diet (Bowen et al, 1999;Field et al, 2007;Jeglinski et al, 2012).…”

Stable isotope and scat analyses indicate diet and habitat partitioning in northern fur seals Callorhinus ursinus across the eastern Pacific

“…Immature animals may not have the swimming speed or the mouthparts to capture adult prey. Additionally, young animals may have different nutritional requirements, insufficient experience, or may be avoiding competition with older conspecifics (Fowler et al 2006). Shifts in diet or changes in foraging/diving behaviors with increasing age have been observed in other otariid pinnipeds including the Galápagos fur seal (Arctocephalus galapagoensis; Horning & Trillmich 1997), New Zealand fur seal (A. forsteri; Page et al 2006), Australian sea lion (Neophoca cinerea; Fowler et al 2006), Steller sea lion (Eumetopias jubatus; Raum-Suryan et al 2004, Pitcher et al 2005, and California sea lion (Zalophus californianus; NMML unpubl.…”