Abstract
Sea otters (Enhydra lutris) have the highest mass-specific metabolic rate of any marine mammal, which is superimposed on the inherently high costs of reproduction and lactation in adult females. These combined energetic demands have been implicated in the poor body condition and increased mortality of female sea otters nearing the end of lactation along the central California coast. However, the cost of lactation is unknown and currently cannot be directly measured for this marine species in the wild. Here, we quantified the energetic demands of immature sea otters across five developmental stages as a means of assessing the underlying energetic challenges associated with pup rearing that may contribute to poor maternal condition. Activity-specific metabolic rates, daily activity budgets and field metabolic rates (FMR) were determined for each developmental stage. Mean FMR of pre-molt pups was 2.29 ± 0.81 MJ day(-1) and increased to 6.16 ± 2.46 and 7.41 ± 3.17 MJ day(-1) in post-molt pups and dependent immature animals, respectively. Consequently, daily energy demands of adult females increase 17% by 3 weeks postpartum and continue increasing to 96% above pre-pregnancy levels by the average age of weaning. Our results suggest that the energetics of pup rearing superimposed on small body size, marine living and limited on-board energetic reserves conspire to make female sea otters exceptionally vulnerable to energetic shortfalls. By controlling individual fitness, maternal behavior and pup provisioning strategies, this underlying metabolic challenge appears to be a major factor influencing current population trends in southern sea otters (Enhydra lutris nereis).
Highlights
Across mammalian species, the most energetically taxing lifehistory period for females is lactation, which can require as much as three to four times the energy intake needed during non-lactating periods (Millar, 1977; Williams et al, 2007)
It is likely that resting metabolic rate (RMR) reaches an asymptotic value shortly after weaning, and the lack of statistical support for a breakpoint in this case probably reflects sample size limitations
Meeting the remarkably high energetic demands of immature sea otters throughout dependency is physiologically challenging for lactating females and often results in a substantial reduction in maternal body mass by pup weaning (Monson et al, 2000)
Summary
The most energetically taxing lifehistory period for females is lactation, which can require as much as three to four times the energy intake needed during non-lactating periods (Millar, 1977; Williams et al, 2007). Received 6 November 2013; Accepted 23 February 2014 duration of lactation, metabolic demands of offspring, energetic reserves of the female, and the availability of prey resources will determine the success of the female in accomplishing this task (Boness and Bowen, 1996; Bowen et al, 2001; Burns et al, 2004; Georges and Guinet, 2000; Verrier et al, 2011) These factors will impact daily behavioral responses, the level of energy invested in reproduction, and the overall fitness of adult females and offspring (Andersen et al, 2000; Georges and Guinet, 2000; Millar, 1975; Oftedal et al, 1987; Pontier et al, 1993). Energetic costs associated with reproduction must be added onto these inherently high metabolic demands
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