AWA: Academic Writing at Auckland

Female southern elephant seals and Weddell seals share certain similarities in breeding strategies, but differ mainly in lactation length. Males of the two species differ in several ways, most notably in their degrees of sexual dimorphism (which relate to the differences in breeding strategies generally). The diets of both species are not well known, but elephant seals are so dimorphic that they show ontogenetic “niche partitioning,” resulting in a broader niche than Weddell seals (Bailleul et al., 2010, p. 709).

Females of both species ‘haul out’ each year to give birth in large colonies, and both do so in the same places (that is, they are both philopatric to their own birth sites) (Boeuf & Laws, 1994, p. 19; de Bruyn et al., 2011, p. 446).  The breeding seasons for both species are also roughly the same, in the austral spring from September to November (Boeuf & Laws, 1994, p. 9). Female southern elephant seals give birth on rocky terrestrial shores of islands like Macquarie, while Weddell seals give birth on the stable fast ice attached to the continent (they are the most southerly breeding mammals in the world) (Boeuf & Laws, 1994, p. 13; McGonigal, 2008, p. 176). Females of the two species also differ in time spent lactating. Lactation is a crucially important part of a mammalian reproductive strategy and has important consequences for pup weaning and survival (Wheatley, Bradshaw, Harcourt, & Hindel, 2008, p. 12). Female southern elephant seals lactate for around 23 days while remaining in the harem (Boeuf & Laws, 1994, p. 17). Weddell seals, on the other hand, lactate for more than twice that time, around 50 days (Wheatley, et al., 2008, p. 12). This affects the feeding strategy of both species while breeding. In female southern elephant seals, fasting is a key strategy of breeding, and the survival of the pup largely depends on the nutrients that pregnant females were able to gain before giving birth (McMahon & Burton, 2005, p. 923). Female Weddell seals, however, have much higher energy requirements due to the length of lactation, and they need to feed during the breeding season (McGonigal, 2008, p. 177). In turn, whether or not females leave the rookeries to feed is an essential determinant of the male breeding strategies.

Male southern elephant seals and Weddell seals differ significantly in their breeding strategies. Male southern elephant seals base their breeding strategy and subsequent reproductive success on the ability to occupy the rookery uninterrupted for the duration of the breeding season, relying on stored blubber (Boeuf & Laws, 1994, p. 19). Southern elephant seals are extremely sexually dimorphic; fully adult males can be up to ten times larger than females (Boeuf & Laws, 1994, p. 4). This has much to do with the fact that large size confers advantages in male-male competition over females (Harcourt, Kingston, Waas, & Hindall, 2008, p. 869).  The ‘beachmaster’ is established through fighting as the dominant male, and this male can monopolise harems of up to 100 females (Boeuf & Laws, 1994, p. 15). When more females are present, a dominance hierarchy is established, whereby continued fighting is too energy consuming for the beachmaster and next in rank males are permitted to mate at the periphery (Boeuf & Laws, 1994, p. 15). Dominant males achieved over 98% of all mating is southern elephant seal rookeries (Boeuf & Laws, 1994, p. 16).

Weddell seal males, on the other hand, are smaller than females, and breed aquatically (McGonigal, 2008, p. 177). Size does not confer as much of an advantage when there is no need to fast, and when there is a greater need to manoeuvre (Harcourt, et al., 2008, p. 568; McGonigal, 2008, p. 177). This is because males defend territories around entry holes in the ice where females enter the water to feed during the breeding season; males mate with them there (McGonigal, 2008, p. 176). Males can monopolise large numbers of females with this strategy, making them similarly polygynous to southern elephant seals, but different in that mating success is not related to size, strength, and fasting so much as the learning of where foraging sites are for females (Harcourt, et al., 2008, p. 575; McGonigal, 2008, p. 575). It would be easy to view male Weddell seal strategies as more flexible and favourable to weaker males than the apparent ‘winner-take-all’ strategy male southern elephant seals, but this is not necessarily the case. Nonpolygynous opportunistic mating by southern elephant seal males does take place at sea during the breeding season, as up to 75% of males are at sea during the breeding season taking advantage of foraging opportunities by not hauling out to compete on land (de Bruyn, et al., 2011, pp. 445, 449). 

Breeding and feeding strategies are inseparable from each other given that energy requirements for successful mating dictate feeding strategies as well (Bailleul, et al., 2010, p. 717). The diet of southern elephant seals is not well known, due to the fact that their long migrations mean stomach contents are digested by the time they arrive back on land (Biuw et al., 2007, p. 13706). Generally, though, they are thought to eat mostly cephalopods and fish, though relative proportions remain unknown (Bailleul, et al., 2010, p. 717). During their foraging migrations, up to 90% of their pelagic life is spent doing deep dives of up to 1500 metres (Bailleul, et al., 2010, p. 710). Elephant seals have an interesting divergence in foraging ecologies between sex and age, due to sexually dimorphic growth requirements resulting in different energy needs: while females reach maturity at 3-4 years, males have not fully developed their extensive size and secondary sexual characteristics (like the proboscis) until around 7 years of age (Bailleul, et al., 2010, p. 717; Boeuf & Laws, 1994, p. 4). Juvenile males thus invest much of their dietary requirements in growth, while females focus on fat reserves for upcoming breeding (Bailleul, et al., 2010, p. 717) Juvenile males not only shift their foraging areas with age, they also potentially prey at higher trophic levels than females (Bailleul, et al., 2010, p. 717). The ‘ecological niche’ is inseparable from both feeding and breeding strategies; forms a complex feedback loop with each other (Bailleul, et al., 2010, p. 717) The ecological niche of southern elephant seals can be said to be broad (in geography and in trophic levels) due to shifts in foraging ecologies between ontogenetic stages, even to the point where females and males can occupy different niches (Bailleul, et al., 2010, p. 717; Field, Bradshaw, Burton, Sumner, & Hindel, 2005, p. 132).

Weddell seals, on the other hand, are not known to have any significant differences in foraging between sexes. They are believed to consume a similar diet to elephant seals (fish and cephalopods), with the addition of krill and other invertebrates (McGonigal, 2008, p. 177). They are also known to eat penguins on occasion (Casaux, Carlini, Corbalán, Bertolin, & DiPrinzio, 2009, pp. 833-837). Nonetheless, the finer details of Weddell seal foraging ecologies are difficult to ascertain given the large variations in diving patterns (Plotz, Bornemann, Knust, Schroder, & Bester, 2001, p. 908). What is known is that Weddell seals make holes in their fast ice haul out zones with their incisors, and use these to breathe and enter the water. They are also loyal to their foraging sites (Harcourt, et al., 2008, p. 575).  The Weddell seal niche is not as broad as elephant seals since they do not have vast differences in age/sex in their foraging ecologies and breeding (Bailleul, et al., 2010, p. 717).  The Weddell seal niche can therefore be classified as less dimorphic than that of southern elephant seals.

Female southern elephant seals and Weddell seals share similar breeding seasons, give birth terrestrially, and in colonies. They differ in lactation and subsequent energy requirements. Males differ in degrees of sexual dimorphism, and this reflects upon their different breeding strategies. The diets of both species are not well known, but elephant seals differ in their foraging ecologies enough across age and sex to be classified as having a broader niche than Weddell seals.

References Cited

Bailleul, F., Authier, M., Ducatez, S., Roquet, F., Charrassin, J., Cherel, Y., & Guine, C. (2010). Looking at the unseen: combining animal bio-logging and stable isotopes to reveal a shift in the ecological niche of a deep diving predator. Ecography, 33(4), 709-717.

Biuw, M., Boehme, L., Guinet, C., Hindell, M., Costa, D., Charrassin, J.-B., . . . Fedak, M. A. (2007). Variations in behavior and condition of a Southern Ocean top predator in relation to in situ oceanographic conditions. Proceedings of the National Academy of Sciences, 104(34), 13705-13710.

Boeuf, B., & Laws, R. (1994). Elephant Seals: an introduction to the genus. In B. Boeuf & R. Laws (Eds.), Elephant seals: population ecology, behavior, and physiology (pp. 1-29). Berkeley; Los Angeles: University of California Press.

Casaux, R., Carlini, A., Corbalán, A., Bertolin, L., & DiPrinzio, C. (2009). The diet of the Weddell seal Leptonychotes weddellii at Laurie Island, South Orkney Island. Polar Biology, 32, 833-838.

de Bruyn, P., Tosh, C., Bester, M., Cameron, E., McIntyre, T., & Wilkinson, I. (2011). Sex at sea: alternative mating system in an extremely polygynous mammal. Animal Behavior, 82(3), 445-451.

Field, I., Bradshaw, C., Burton, H., Sumner, M., & Hindel, M. (2005). Resource partitioning through oceanic segregation of foraging juvenile southern elephant seals (Mirounga leonin. Behavioural Ecology, 142(1), 127-135.

Harcourt, R., Kingston, J., Waas, J., & Hindall, M. (2008). Foraging while breeding: alternative mating strategies by male Weddell seals? Aquatic Conservation: Marine and  Freshwater Ecosystems, 17(S68-S78).

McGonigal, D. (2008). Antarctica: Secrets of the Southern Continent. London: Publisher Pty Ltd.

McMahon, C., & Burton, H. (2005). Climate change and seal survival: evidence for environmentally mediated changes in elephant seal, Mirounga leonina, pup survival. Proceedings of the Royal Society B:Biological Sciences, 272, 923-928.

Plotz, J., Bornemann, H., Knust, R., Schroder, A., & Bester, M. (2001). The foraging behaviour of Weddell seals, and its ecological implications. Polar Biology, 24, 901-909.

Wheatley, K., Bradshaw, C., Harcourt, R., & Hindel, M. (2008). Feast or famine: evidence for mixed capital–income breeding strategies in Weddell seals. Oecologia, 155(1), 11-20.