Are Tropical Small Mammals Physiologically Vulnerable to Arrhenius Effects and Climate Change?

Abstract

There is some urgency in the necessity to incorporate physiological data into mechanistic, trait-based, demographic climate change models. Physiological responses at the individual level provide the mechanistic link between environmental changes and individual performances and hence population dynamics. Here we consider the causal relationship between ambient temperature (Ta) and metabolic rate (MR), namely, the Arrhenius effect, which is directly affected by global warming through increases in average global air temperatures and the increase in the frequency and intensity of extreme climate events. We measured and collated data for several small, free-ranging tropical arboreal mammals and evaluated their vulnerability to Arrhenius effects and putative heat stress associated with climate change. Skin temperatures (Tskin) were obtained from free-ranging tarsiers (Tarsius syrichta) on Bohol Island, Philippines. Core body temperature (Tb) was obtained from the greater hedgehog tenrec (Setifer setosus) and the gray brown mouse lemur (Microcebus ravelobensis) from Ankarafantsika, Madagascar. Tskin for another mouse lemur, Microcebus griseorufus, was obtained from the literature. All four species showed evidence of hyperthermia during the daytime rest phase in the form of either Tskin or Tb that was higher than the normothermic Tb during the nighttime active phase. Potentially, tropical arboreal mammals with the lowest MRs and Tb, such as tarsiers, are the most vulnerable to sustained heat stress because their Tb is already close to Ta. Climate change may involve increases in MRs due to Arrhenius effects, especially during the rest phase or during torpor and hibernation. The most likely outcome of increased Arrhenius effects with climate change will be an increase in energy expenditure at the expense of other critical functions such as reproduction or growth and will thus affect fitness. However, we propose that these hypothetical Arrhenius costs can be, and in some species probably are, offset by the use of hyperthermic daily torpor, that is, hypometabolism at high Ta.

Mammal survival at the Cretaceous–Palaeogene boundary: metabolic homeostasis in prolonged tropical hibernation in tenrecs

“…Cold environments appeared in the Late Cenozoic only, and certainly did not exist at the K-Pg boundary [19,20]. Indeed, until relatively recent reports of hibernation in Malagasy primates [21,22] and tenrecs [23,24], and in the echidna (a monotreme) from semi-tropical Queensland, Australia, [25,26], it was thought that hibernation was a physiological capacity limited to high latitude, temperate mammals only [17].…”

Increased homeothermy during reproduction in a basal placental mammal

“…Such a delayed deployment of heat loss mechanisms would indicate that T b is flexible until a certain point, after which it is defended against potential hyperthermia, and efforts are made by the animal to offload stored heat. The T b measurements from S. setosus presented in our previous study (Lovegrove et al, 2014) indicate that high T b can be…”

“…In addition to circadian rhythms, in which T b typically decreases during the rest phase (but see Lovegrove et al, 2014), many mammals also lower T b and metabolic rate (MR) for extended periods of time during daily torpor and hibernation (Geiser and Ruf, 1995;Lyman et al, 1982).…”

“…We sought to fully characterize the thermoregulatory patterns during reproduction in a basoendotherm by measuring RMR over a range of T a in a freeranging population of greater hedgehog tenrecs [Setifer setosus (Schreber 1778)] in the dry deciduous forest of western Madagascar. Although S. setosus is known to use torpor less than its sister species E. telfairi (Eisenberg and Gould, 1969;Eisenberg and Muckenhirn, 1968), it has been shown to enter into torpor over a wide range of T a and to express a high degree of thermolability when not torpid Lovegrove et al, 2014). In addition, S. setosus is both larger and more abundant than E. telfairi and, unlike the larger common tenrec (Tenrec ecaudatus), is not readily consumed by humans at the study site, making it an ideal candidate for a long-term study on the energetics in a free-ranging population (Randrianjafy, 2003).…”

Lowland biotic attrition revisited: body size and variation among climate change ‘winners’ and ‘losers’

“…Ongoing warming could push ambient temperatures above the thermal neutral zones of many tropical endotherms [3][4][5], suggesting that these species & 2017 The Author(s) Published by the Royal Society. All rights reserved.…”