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Abstract
Predator-prey relationships play a key role in the evolution and ecology of carnivores. An understanding of predator-prey relationships and how this differs across species and environments provides information on how carnivorous strategies have evolved and how they may change in response to environmental change. We aim to determine how mammals overcame the challenges of living within the marine environment; specifically, how this altered predator-prey body mass relationships relative to terrestrial mammals. Using predator and prey mass data collected from the literature, we applied phylogenetic piecewise regressions to investigate the relationship between predator and prey size across carnivorous mammals (51 terrestrial and 56 marine mammals). We demonstrate that carnivorous mammals have four broad dietary groups: small marine carnivores (<11000kg) and small terrestrial carnivores (<11kg) feed on prey less than 5kg and 2kg, respectively. On average, large marine carnivores (>11000kg) feed on prey equal to 0.01% of the carnivore's body size, compared to 45% or greater in large terrestrial carnivores (>11kg). We propose that differences in prey availability, and the relative ease of processing large prey in the terrestrial environment and small prey in marine environment, have led to the evolution of these novel foraging behaviours. Our results provide important insights into the selection pressures that may have been faced by early marine mammals and ultimately led to the evolution of a range of feeding strategies and predatory behaviours.
Highlights
There is a strong link between physiology and behaviour in carnivorous mammals
We propose that the marine environment may have: (1) enabled marine carnivorous mammals to ‘escape’ or minimise the energetic constraints apparent for carnivores on land, and (2) that above a given mass, allometry is no longer the key factor determining what carnivorous marine mammals feed upon, but rather that the distribution and type of prey available and the ability of the carnivorous marine mammals to process this prey has resulted in the evolution of different feeding categories
The daily energy expenditure (DEE) model applied to data for all carnivorous mammals that had the lowest AIC value inferred that there was a single breakpoint at 14.5kg, and an increase in energy expenditure above the breakpoint of approximately 120%
Summary
There is a strong link between physiology and behaviour in carnivorous mammals. For example, much of the variation in ranging behaviour of carnivorous mammals can be attributed to the energetic requirements of being a carnivore and the distribution of preferred prey (Carbone et al, 2005, Kelt & van Vuren, 2001). The population dynamics of predators can be impacted by prey choice and the population fluctuations of those prey species. The polar bear (Ursus maritimus) feeds on large prey (e.g. seals; Derocher et al, 2002), they process the prey carcases on the ice and they do not have fully aquatic-adapted forelimbs, or the simplified dentition typical of other marine mammals. Polar bears use their clawed forelimbs to hold and position the carcass between crushing postcanies. The polar bear is a comparably recent convert to a marine lifestyle
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