The pathophysiology of survival in harsh environments.

An individual's ability to survive harsh conditions might depend on its available energy, and also on its health, which is expected to decline as conditions deteriorate. Yet, we know little about how health and energy expenditure are shaped by harsh environmental conditions in free-living vertebrates. Here, we studied how African striped mice (Rhabdomys pumilio) that survived summer droughts differed in their energy expenditure and health from non-survivors. Specifically, we tested whether: (1) survivors' and non-survivors' health and energy expenditure differed before environmental conditions declined; (2) non-survivors were in poorer health and had greater energy expenditure than survivors when conditions were harshest; (3) non-survivors' health deteriorated more than that of survivors as conditions deteriorated; and (4) survivors recovered once conditions improved. Survivors and non-survivors' health was assessed using VetsScan ABAXIS, while energy expenditure was measured as resting metabolic rate (RMR). Before conditions declined, non-survivors had lower energy stores and higher globulin levels than survivors. As conditions became harsher, survivors' and non-survivors' health deteriorated but only non-survivors showed signs of permanent pathology (increased glucose and decreased globulin). Once conditions improved, survivors' health improved but was not fully restored (increased alanine aminotransferase and decreased globulin). Furthermore, while survivors and non-survivors had similar RMR before conditions became harsh; their levels diverged considerably when conditions deteriorated, with survivors having a decreased RMR and non-survivors having an increased RMR. Our results show that an individual's health before facing an environmental challenge and the way it regulates its RMR influences its ability to maintain homeostasis when conditions become more taxing.