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. 2022 Jan 19;9(1):210522.
doi: 10.1098/rsos.210522. eCollection 2022 Jan.

Trade-offs between foraging reward and mortality risk drive sex-specific foraging strategies in sexually dimorphic northern elephant seals

Sarah S Kienle  1   2 , Ari S Friedlaender  3 , Daniel E Crocker  4 , Rita S Mehta  1 , Daniel P Costa  1
Affiliations

Trade-offs between foraging reward and mortality risk drive sex-specific foraging strategies in sexually dimorphic northern elephant seals

Sarah S Kienle et al. R Soc Open Sci. .

Abstract

Sex-specific phenotypic differences are widespread throughout the animal kingdom. Reproductive advantages provided by trait differences come at a cost. Here, we link sex-specific foraging strategies to trade-offs between foraging reward and mortality risk in sexually dimorphic northern elephant seals (Mirounga angustirostris). We analyse a decadal dataset on movement patterns, dive behaviour, foraging success and mortality rates. Females are deep-diving predators in open ocean habitats. Males are shallow-diving benthic predators in continental shelf habitats. Males gain six times more mass and acquire energy 4.1 times faster than females. High foraging success comes with a high mortality rate. Males are six times more likely to die than females. These foraging strategies and trade-offs are related to different energy demands and life-history strategies. Males use a foraging strategy with a high mortality risk to attain large body sizes necessary to compete for females, as only a fraction of the largest males ever mate. Females use a foraging strategy with a lower mortality risk, maximizing reproductive success by pupping annually over a long lifespan. Our results highlight how sex-specific traits can drive disparity in mortality rates and expand species' niche space. Further, trade-offs between foraging rewards and mortality risk can differentially affect each sex's ability to maximize fitness.

Keywords: feeding; fitness; marine mammal; niche divergence; spatial ecology; survival.

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Figures

Figure 1.
Figure 1.
Comparison of satellite tracks and dive behaviour of 39 male and 178 female northern elephant seals (Mirounga angustirostris). (a) Males travel (blue lines) to coastal areas and forage (blue circles) on the continental shelf (grey area). Females travel (orange lines) and forage (orange circles) throughout the North Pacific. The three mesopelagic ecosystems (boundaries defined by Sutton et al. [35] used by northern elephant seals are colour-coded and labelled, with the California Current ecoregion in aqua, the Subarctic Pacific in light blue and the North Central Pacific in blue-grey. (b) Expanded view of differential male and female habitat use on/near the continental shelf in the Subarctic Pacific. Northern elephant seal illustration by Pieter Folkens. (c) Representative dive profile of daytime benthic foraging (black line) and pelagic foraging dives (grey line) from a male and female seal, respectively. The benthic foraging dives represented here occurred on the continental shelf, and the pelagic foraging dives were adjacent to the shelf edge. (d) Boxplots comparing the proportions of the two foraging dive types used by both sexes on their foraging trips: benthic foraging dives (black) and pelagic foraging dives (grey). Horizontal bars denote the 25th, 50th (median) and 75th quartile.
Figure 2.
Figure 2.
Comparison of foraging success and mortality rate metrics between male (blue) and female (orange) northern elephant seals. Boxplots compare sex-specific differences in (a) mass gain (kg), and (b) rate of energy gain (MJ d−1) of 32 males and 128 females. Horizontal bars represent the 25th, 50th (median) and 75th quartile. (c) Probability of at-sea mortality for each sex on the foraging trip. Values are model mean ± 95% confidence intervals. (d) Satellite tracks of males (n = 17) and females (n = 22) that died at sea. Circles represent the point of last satellite transmission. The three mesopelagic ecosystems (boundaries defined by Sutton et al. [35]) used by northern elephant seals are colour-coded and labelled, with the California Current ecoregion in aqua, the Subarctic Pacific in light blue, and the North Central Pacific in blue-grey. (e) Density plot of dead males and females showing their distance to the continental shelf edge at their last satellite transmission. Vertical lines are the mean value for each sex.
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