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

Abstract

The development of foraging strategies as animals mature depends upon experience, stage-specific physiology, the onset of reproductive maturity and the reproductive costs incurred by each sex. To understand the ontogeny of foraging behaviour, we compared movement behaviour of 24 young-of-year (YOY) juveniles (12 male, 12 female) with 6 subadult (4 male, 2 female) and 81 adult (43 male, 38 female) grey seals Halichoerus grypus. We used a behaviour discriminating state-space model followed by a series of mixed-effects models to examine trip structure and habitat use in these age classes. In foraging trips, tortuosity and speed of outbound travel were not different in YOY, subadults and adults, suggesting that YOY navigate as well as older animals. On average, however, YOY trips lasted 1.2 to 3.5 d longer and required up to twice as much transit time to reach foraging areas that were 1.5 to 3 times farther from haul-out sites than those of subadults and adults. This suggests an overall apparent higher foraging effort for YOY. Differences, however, were highly seasonally dependent. In all groups, apparent foraging effort decreased in the summer and increased in the winter, which is consistent with seasonal changes in prey distribution and energy content. Adult foraging patterns showed complex seasonal patterns influenced by both reproductive cycles and seasonal environmental variation, whereas annual foraging patterns of YOY and subadults, neither of which invest in reproduction, were simpler and appeared more closely tied to seasonal changes in prey availability and condition. Although foraging in the same habitats, male and female YOY showed small but significant differences in movement behaviour and trip structure. These differences are unlikely to be due to size difference, which is minimal at this age, and suggest that sex-related differences in foraging develop early before sexual size dimorphism is significantly expressed.

Offspring size at weaning affects survival to recruitment and reproductive performance of primiparous gray seals

“…Young of the year gray seals do make longer foraging trips and forage farther from haul-out sites than older animals (Breed et al. 2011), indicating that greater effort associated with foraging could reflect ineffective foraging behaviors (Marchetti and Price 1989). Longer pups may also be better equipped to avoid predators (Hindell et al.…”

The Grey Seal (Halichoerus grypus) as a Predator of Harbour Porpoises (Phocoena phocoena)?

“…As the fat has the highest energy content, the seals would have had the highest calorific return in the shortest time period, a strategy suggested to be followed-if choice is possible-by grey seals and other marine mammals (Evans, 1987;Bowen et al, 2002;Beck et al, 2007;Breed et al, 2010;Spitz et al, 2010). Polar bears (Ursus maritimus) apparently digest fat more easily than protein (Best, 1985) and also do not usually eat the whole prey.…”

Navigating uncertain waters: a critical review of inferring foraging behaviour from location and dive data in pinnipeds

“…Using biological rationale, ecologists infer behaviours from the discrete movement states identified by the SSM. SSMs enable us to quantify foraging behaviour in relation to other behaviours such as resting and travelling, and thus tackle more complex questions of how these activities interact through time and space [117, 121–123]. Importantly, models can test the influence of explanatory covariates on the probability of switching into a certain movement state.…”

“…This is most notable in the otariid literature. Nevertheless, foraging strategies are known to vary seasonally, between the sexes [104, 122, 153–155], age classes [123, 156–158], and indeed between individuals in general [92, 112, 159]. Due to cost and logistics, tagging studies are often constrained by relatively small sample sizes.…”

“…In order to address this knowledge gap, researchers should attempt to track recently-weaned pups as they explore their environment, learning how to dive and find food [157]. Numerous studies have used pup movement data to address the ontogeny of diving from a physiological perspective [40, 157, 167, 168], and comparatively fewer investigate the ontogeny of foraging strategies [123, 156, 169]. For otariids, and some phocid species, pups may learn to dive (and potentially forage) alongside their mother [16, 38, 39].…”