Pacing polar bears and stoical sheep: testing ecological and evolutionary hypotheses about animal welfare

Responses to potential threats to welfare vary greatly between species. Even closely related animals often differ in their fear of humans and/or novelty, their behavioural responses to pain; and when captive, their overall welfare and the form and frequency of their stereotypies. Such species differences stimulate hypotheses about 1) the way that responses to challenge co-vary with other biological traits, 2) the adaptive value of particular responses, and 3) the factors predicting responses to evolutionarily new scenarios, such as captivity. We illustrate how these ideas can be statistically tested with multi-species comparisons, and show how techniques such as the Comparative Analysis of Independent Contrasts can be used to control for any non-independence of data points caused by species' relatedness. For each of the three types of hypothesis, we then provide several welfare-relevant examples including one that has been fully tested (respectively, the relationships between sociality and anti-predator behaviour in antelopes, predation pressure, foraging niche and neophobia in parrots, and home range size and stereotypy in carnivores). Ultimate explanations such as these, based on species' ecology and evolutionary history, have great explanatory appeal. Species comparisons can also have great practical value, allowing the test of hypotheses that would be almost impossible to investigate experimentally, and generating principles that allow predictions about the welfare of similar unstudied species. Multi-species data, for example from the many taxa held in zoos, thus hold enormous potential for increasing the fundamental understanding of animal welfare.