From pup to predator; generalized hidden Markov models reveal rapid development of movement strategies in a naïve long-lived vertebrate
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Rapid development of a successful foraging strategy is critical for juvenile survival, especially for naïve animals that receive no parental guidance. However, this process is poorly understood for many species. Although observation of early-life movements is increasingly possible with miniaturisation of animal-borne telemetry devices, analytical limitations remain. Here, we tracked 29 recently-weaned grey seal Halichoerus grypus pups from colonies in two geographically distinct regions of the United Kingdom. We analysed at-sea movements of pups throughout their initial months of nutritional independence to investigate the ontogeny of behaviour-specific (foraging and travelling) movement patterns. Using generalized hidden Markov models (HMMs), we extended the conventional HMM framework to account for temporal changes in putative foraging and travelling movement characteristics, and investigate the effects of intrinsic (sex) and extrinsic (environment) factors on this process. Putative foraging behaviour became more tortuous with time, and travelling became faster and more directed, suggesting a reduction in search scale and an increase in travel efficiency as pups shifted from exploration to an adult-like repeatable foraging strategy. Sex differences in movement characteristics were evident from colony departure, but sex-specific activity budgets were only detected in one region. We show that sex-specific behavioural strategies emerge before sexual size dimorphism in grey seals, and suggest that this phenomenon may occur in other long-lived species. Our results also indicate that environmental variation may affect the emergence of sex-specific foraging behaviour, highlighting the need to consider interacting intrinsic and extrinsic factors in shaping movement strategies of long-lived vertebrates. Moreover, comparing the behavioural state estimations to those of a conventional HMM (no variation in state-specific movement parameters) revealed differences in the amount and location of foraging activity, with implications for spatial conservation management. Overlooking intrinsic and extrinsic variation in movement processes could distort our understanding of foraging ecology, population dynamics and conservation requirements.
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