Review: The inducible defenses of large mammals to human lethality

Robert A. Montgomery, David W. Macdonald,  Matthew W. Hayward

A key area of ecological inquiry is the evolution of defense mechanisms induced by a previous encounter with a biotic agent, whether that be a consumer or a competitor. Research has now catalogued many morphological and behavioral adaptations of hosts to their parasites, of plants to their herbivores, and of prey to their predators. Within predator-prey systems however, humans as predators have largely been overlooked. Humans have demonstrated an ability to consume animals at rates many times higher than any other non-human predator. But the extent to which human predation might induce adaptive morphological or behavioral changes in prey that are subsequently heritable remains unclear.

We reviewed the literature on the defenses induced by human predation. We focused our assessment on terrestrial large mammalian herbivores and carnivores, as these species have been disproportionately exploited by humans over time. We reviewed a total of 187 studies published between 1958 and 2019. These studies recorded the behavioral plasticity of >60 species of large mammals to humans, with none focusing on morphological adaptations. Interestingly, we could not attribute these examples of behavioral plasticity to defenses induced by human predation, because a key condition associated with the evolution of inducible defenses was not commonly assessed.

For inducible defenses to evolve, the application of such defenses must be costly (i.e., present fitness tradeoffs). But among the studies that we reviewed, just 16 (i.e., ~9%) attempted to assess the costs of the observed behavioral plasticity. Synthesizing the results of our analysis, we make a number of recommendations including renewed ingenuity of field experiments that can reveal the fitness tradeoffs that large mammals may make when encountering humans.  

Read the paper in full here.

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