Heterothermy in two mole‐rat species subjected to interacting thermoregulatory challenges

Abstract

Maintaining a high and constant body temperature (T(b) ) is often viewed as a fundamental benefit of endothermy, but variation in T(b) is likely the norm rather than an exception among endotherms. Thus, attempts to elucidate which factors cause T(b) of endotherms to deviate away from the T(b) that maximizes performance are becoming more common. One approach relies on an adaptive framework of thermoregulation, used for a long time to predict variation in T(b) of ectotherms, as a starting point to make predictions about the factors that should lead to thermoregulatory variation in endotherms. Here we test the predictions that when confronted with thermoregulatory challenges endotherms should (1) become more heterothermic, (2) lower their T(b) setpoint, and/or (3) increase behavioral thermoregulation (e.g., activity levels or social thermoregulation). We exposed two species of relatively homeothermic mole-rats to two such challenges: (a) ambient temperatures (T(a)) well below the thermoneutral zone and (b) increased heat loss caused by the removal of dorsal fur. In general, our results support the adaptive framework of endothermic thermoregulation with each species conforming to some of the predictions. For example, Mashona mole-rats (Fukomys darlingi) increased heterothermy as T(a) decreased, highveld mole-rats (Cryptomys hottentotus pretoriae) displayed lower T(b) 's after shaving, and both species increased behavioral thermoregulation as T(a) decreased. This suggests that there is some merit in extending the adaptive framework to endotherms. However, none of the three predictions we tested was supported under all experimental conditions, reiterating that attempts to determine universal factors causing variation in T(b) of endotherms may prove challenging.

Body Temperature Patterns and Rhythmicity in Free-Ranging Subterranean Damaraland Mole-Rats, Fukomys damarensis

“…This corroborates previous suggestions that subterranean rodents may be at high risk of overheating and therefore carefully regulate any increases in T b [14]. The HI values and T b characteristics recorded herein were quite similar to those recorded in two other mole-rat species in the laboratory [45].…”

Locomotor Activity and Body Temperature Patterns over a Temperature Gradient in the Highveld Mole-Rat (Cryptomys hottentotus pretoriae)

“…A reduction in activity levels when the ambient temperature approaches the thermoneutral zone (TNZ) has also been reported for other species such as laboratory mice [41]. In social mole-rat species, the TNZ’s typically range between 28–36°C [6,29,32,42,43]. The TNZ of the highveld mole-rat is around 30–32°C [44], thus 30°C falls within this zone.…”

“…It has been suggested that the highveld mole-rat is more heterothermic, however, the activity analysis of this study was not very comprehensive [29]. We anticipated that the average T b would remain relatively stable over the range of T a ’s, but that locomotor activity and the range of T b variation would increase at the lower T a ‘s.…”

“…Naked mole-rats ( Heterocephalus glaber ) are considered poikilothermic and at ambient temperatures (T a ) below 25°C their body temperature may fall more than 5°C [27]. Mashona mole-rats ( Fukomys darling ) have also been shown to show greater heterothermy at lower T a ’s, and T b can reduce significantly at T a below 25°C, while this appears not to be the case for the highveld mole-rat [28,29]. Social mole-rat species employ behavioural thermoregulation to regulate body temperatures; they select deeper, warmer areas of the burrows for resting [30] and also huddle together to conserve heat [31,32].…”

Functional histology of the skin in the subterranean African giant mole-rat: thermal windows are determined solely by pelage characteristics

“…Similar differences between ventral and dorsal body regions were found in 15 species of subterranean rodents of different phylogenetic lineages (F. Vejmělka and R. Šumbera, 2015, unpublished data). The importance of fur for heat dissipation has been demonstrated in the South African highveld mole-rat Cryptomys hottentotus pretoriae experimentally, with fur shaving decreasing body temperature, probably as a result of increased heat dissipation (Boyles et al, 2012). This finding indicates that fur characteristics are probably highly relevant for heat dissipation in subterranean species.…”