The metabolic response of the Bradypus sloth to temperature

Abstract

Poikilotherms and homeotherms have different, well-defined metabolic responses to ambient temperature (T _a ), but both groups have high power costs at high temperatures. Sloths (Bradypus) are critically limited by rates of energy acquisition and it has previously been suggested that their unusual departure from homeothermy mitigates the associated costs. No studies, however, have examined how sloth body temperature and metabolic rate vary with T _a . Here we measured the oxygen consumption (VO₂) of eight brown-throated sloths (B. variegatus) at variable T _a 's and found that VO₂ indeed varied in an unusual manner with what appeared to be a reversal of the standard homeotherm pattern. Sloth VO₂ increased with T _a , peaking in a metabolic plateau (nominal 'thermally-active zone' (TAZ)) before decreasing again at higher T _a values. We suggest that this pattern enables sloths to minimise energy expenditure over a wide range of conditions, which is likely to be crucial for survival in an animal that operates under severe energetic constraints. To our knowledge, this is the first evidence of a mammal provisionally invoking metabolic depression in response to increasing T _a 's, without entering into a state of torpor, aestivation or hibernation.

Keywords: Arboreal folivore; Bradypus; Energetics; Metabolic depression; Metabolic rate; Sloth; Temperature response.

Figure 1. The effect of ambient temperature (T_a) on resting metabolic rate (RMR), rectal temperature (T_b) and posture of Bradypus variegatus sloths.

Means presented (±1SE) are taken from 8 animals over a total of 10 different trials (repeated measurements for individual sloths). Number of individuals and trials at each temperature bracket listed across the top. Posture was graded visually on a scale of 1–6 (1 = tight ball, 6 = all limbs spread) and is presented as a frequency distribution with bars representing the proportion of cases. T_a significantly affected RMR, with the lowest metabolic values occurring at 22 °C (561 ml O₂/h ± 95 ml O₂/h). RMR increased with increasing T_a, before peaking and remaining constant between 26–30 °C (1,102 ml O₂/h ± 119 ml O₂/h). Above 30 °C, RMR decreased rapidly. Both sloth T_b and posture were significantly affected by changes in T_a. Photo credit: Rebecca Cliffe