Sex-specific foraging strategies and resource partitioning in the southern elephant seal (Mirounga leonina)

Abstract

The evolution of resource specializations is poorly understood, especially in marine systems. The southern elephant seal (Mirounga leonina) is the largest of the phocid seals, sexually dimorphic, and thought to prey predominantly on fish and squid. We collected vibrissae from male and female southern elephant seals, and assessed stable C and N isotope ratios along the length of the vibrissae. Given that whiskers grow slowly, this sampling strategy reflects any variation in feeding behaviour over a period of time. We found that isotopic variation among females was relatively small, and that the apparent prey choice and trophic level of females was different from that for males. Further, males showed a very broad range of trophic/prey choice positions, grouped into several clusters, and this included isotopic values too low to match a broad range of potential fish and cephalopod prey tested. One of these clusters overlapped with data for South American sea lions (Otaria flavescens), which were measured for comparison. Both male southern elephant seals and southern sea lions forage over the continental shelf, providing the potential for competition. We discuss the possibility that individual southern elephant seals are pursuing specialist foraging strategies to avoid competition, both with one another, and with the South American sea lions that breed nearby.

Change in the foraging strategy of female South American sea lions (Carnivora: Pinnipedia) after parturition

“…Neither prey-to-predator and female-to-offspring enrichment rates nor the tissue turnover have been determined for the South America sea lion in controlled experiments and hence extrapolation from studies conducted on phylogenetically close organisms is required (Lewis et al, 2006;Hückstädt et al, 2007). Fortunately, the prey-to-predator enrichment rate has been determined in other pinnipeds in captivity (Hobson et al, 1996;Kurle, 2002), the female-to-offspring enrichment rate has been assessed in other carnivores, including pinnipeds (Jenkins et al, 2001;Ducatez et al, 2008), and the turnover rate of blood components has been established for a number of birds and mammals (Hobson and Clark, 1993;Hilderbrand et al, 1996).…”

Development of foraging strategies with age in a long-lived marine predator

“…In sexually size-dimorphic species, size is thought to drive many observed behavioural differences (LeBoeuf et al 2000, Ruckstuhl & Neuhaus 2005, Lewis et al 2006. In most known examples, behavioural differences are minimal or nonexistent at birth and manifest later as one sex grows larger than the other (Shine 1989, Ruckstuhl & Neuhaus 2005.…”

Vibrissal growth parameters of southern elephant seals Mirounga leonina: obtaining fine-scale, time‑based stable isotope data

“…To ensure that the SI signature is represented similarly across different vibrissae grown at the same time, we compared the SI profile of two vibrissae collected simultaneously from a third individual (hereafter, Seal 3, 0.8 yo) and eight vibrissae collected from a deceased adult female SES (hereafter, Seal 4) (see Lewis et al (2006) and Newland et al (2011)). We controlled for different vibrissa lengths and growth rates in Seal 4 (Greaves et al 2004;Beltran et al 2015) by applying the growth rate of adult M. angustirostris from Beltran et al (2015), using the maximum length of each of the sampled vibrissae as the asymptotic length in the equation.…”

“…To prepare and analyse samples for stable isotopes, each vibrissa was cleaned using a chloroform:ethanol rinse as described by Lewis et al (2006). All samples were then ovendried for 24 h at 70 °C (Lowther et al 2013), sequentially sub-sampled into 2 mm sections ranging from the proximal portion (base) to the distal portion (tip) following Hindell et al ± 0.8 times within a given moult season; similar to the 2.6 ± 0.7 times of those that did not shed their vibrissae.…”

“…The vibrissae (whiskers) of pinnipeds (Ling 1966) are particularly useful for dietary studies when using SI analysis (e.g., Hirons et al 2001;Greaves et al 2004;Zhao & Schell 2004;Hall-Aspland et al 2005; Lewis et al 2006;Cherel et al 2009;Hindell et al 2012;Walters et al 2014;Rea et al 2015). Vibrissae can be sampled relatively non-invasively and archive ecological information over longer temporal scales at a finer resolution than other tissues, such as blood (Tieszen et al 1983).…”