Exogenous passive heating during torpor arousal in free-ranging rock elephant shrews, Elephantulus myurus

Abstract

In the laboratory rock elephant shrews (Elephantulus myurus; mean body mass 56.6 g) displayed the lowest torpor T yet recorded (ca. 5°C) in a placental daily heterotherm. It was unknown whether these low Ts were characteristic of daily heterothermy in free-ranging animals. It was also unclear how cost effective these low T s were since considerable energy is required to arouse from low Ts on a daily basis. We continuously measured body temperature once every hour for 85 days in 13 free-ranging E. myurus from May to August 2001 (winter) in Weenen Game Reserve, KwaZulu-Natal, South Africa. We recorded a total of 412 torpor bouts. Free-ranging E. myurus had a high propensity for torpor with females displaying higher torpor frequency than males. The lowest T recorded was 7.5°C at T=2.7°C and the minimum torpor T was strongly correlated with ambient temperature. Torpor arousal was tightly coupled with ambient temperature cycles. Low torpor T at low T s was therefore cost-effective because the animals offset the high cost of arousal through exogenous passive heating. Laboratory studies under constant ambient temperatures may therefore underestimate the energetic benefits of torpor in free-ranging small mammals that inhabit regions where seasonality is moderate.

Basking and diurnal foraging in the dasyurid marsupial Pseudantechinus macdonnellensis

“…Similar behaviour has been observed in other saxicoline, basking mammals (Mzilikazi et al 2002;Brown and Downs 2005). Close proximity of refuge and basking sites reduces commuting time when animals are potentially exposed to predators while their locomotor abilities are impaired at low T b (Rodriguez-Gironés 2002), and, as with ectothermic reptiles, ensures a greater probability of escape in the event of attempted predation (Cooper 2000).…”

“…This trade-off may be particularly challenging for those endothermic mammals that bask to passively rewarm during arousal from torpor when their body temperature (T b ) and coordination are reduced. Nevertheless, recent studies have documented this previously unknown behaviour in several small (body mass <100 g) marsupial and placental mammals (Geiser et al 2002(Geiser et al , 2008Mzilikazi et al 2002). A specific challenge faced by nocturnal endothermic mammals that passively rewarm by basking is that these species undergo torpor during the latter half of the night, during what is normally their active phase, and subsequently rewarm by basking in sun-exposed microhabitats in the morning, during their normal rest phase.…”

Timing of the daily temperature cycle affects the critical arousal temperature and energy expenditure of lesser long-eared bats

“…The circadian timing of torpor entry and arousal is also well known (Willis, 1982;Körtner and Geiser, 2000), but the application of these studies to wild populations is often limited by the use of a constant T a generally employed in the laboratory. Whereas, many animals experience a daily T a cycle in their resting shelter and this appears to be an important cue for the timing of torpor and arousal (Körtner and Geiser, 2000;Mzilikazi et al, 2002;Turbill et al, 2003a). In general, small nocturnal mammals show a high propensity for torpor in the early morning, when daily T a are minimal, and arouse at around midday or in the early afternoon, seemingly in response to a rising T a in their shelter and some passive rewarming of torpid T b (Davis and Reite, 1967;Schmid, 1996;Körtner and Geiser, 2000;Geiser et al, 2004;Körtner et al, 2008).…”

Torpor in Dark Times: Patterns of Heterothermy Are Associated with the Lunar Cycle in a Nocturnal Bird

“…3 predation risk or variable environmental temperatures) might also be important determinants of heterothermy in endotherms (Bieber and Ruf 2009;Mzilikazi et al 2002;Stawski and Geiser 2010;Warnecke et al 2008). Efforts have been made to generalize all factors affecting the expression of T b , whether well studied or not, into an adaptive framework (Angilletta et al 2010;Humphries et al 2003).…”