Summit metabolism and metabolic expansibility in Wahlberg's epauletted fruit bats (Epomophorus wahlbergi): seasonal acclimatisation and effects of captivity

Abstract

Summit metabolism (M sum ), the maximum rate of resting metabolic thermogenesis, has been found to be broadly correlated with climatic variables and the use of heterothermy in some endotherms. Far less is known about M sum and metabolic expansibility [ME, the ratio of M sum to basal metabolic rate (BMR)] in bats compared with many other endotherm taxa. We measured BMR and M sum during winter and summer in captive and wild populations of a pteropodid from the southern subtropics, Wahlberg's epauletted fruit bat (Epomophorus wahlbergi) in Pretoria, South Africa. The M sum of fruit bats ranged from 5.178±0.611 W (captive, summer) to 6.006±0.890 W (captive, winter), and did not vary significantly between seasons. In contrast, BMR decreased by 17-25% in winter. The combination of seasonally stable M sum but flexible BMR resulted in ME being significantly higher in winter than in summer, ranging from 7.24±1.49 (wild, summer) to 13.11±2.14 (captive, winter). The latter value is well above the typical mammalian range. Moreover, both M sum and ME were significantly higher in captive bats than in wild individuals; we speculate this represents a phenotypic response to a reduction in exerciseassociated heat production while in captivity. Our data for E. wahlbergi, combined with those currently available for other chiropterans, reveal that M sum in bats is highly variable compared with allometrically expected values for other mammals.KEY WORDS: Acclimatisation, Cold exposure, Helox, Phenotypic flexibility, Thermogenic capacity INTRODUCTIONThe lowest environmental temperature to which an endotherm can defend normothermic body temperature (T b ) is determined primarily by its maximum capacity for metabolic thermogenesis (Scholander et al., 1950). Summit metabolism (M sum ) is the maximum rate of resting metabolic thermogenesis in the absence of exercise-associated heat production (Swanson et al., 1996) [also referred to as cold-induced peak metabolic rate (Wiersma et al., 2007)]. Among mammals, metabolic expansibility [ME, the ratio of M sum to basal metabolic rate (BMR); also referred to as factorial aerobic scope] is typically 4-8, but may be as high as 10-13 (Careau, 2013;Hinds et al., 1993). Most avian ME values are similar to those typical of mammals, with maximum reported values of 9.0-9. production capacity is correlated with climate, with M sum generally being higher in species inhabiting colder regions (Rezende et al., 2004;Swanson and Garland, 2009).One endotherm order about which remarkably little is known in terms of resting heat production capacity is the Chiroptera. The first published estimate of M sum in a bat of which we are aware was for the molossid Tadarida brasiliensis, in which mass-specific M sum was equivalent to ~21× BMR (Canals et al., 2005). The very high ME value for T. brasiliensis contrasts with more recent data for three frugivorous phyllostomids (Artibeus lituratus, Sturnira lilium and Carollia perspicillata) in which ME ranged from 3.4 to 5.2 (Almeida and Cruz-Neto, 2011).A priori, two...

Global patterns of seasonal acclimatization in avian resting metabolic rates

“…Two populations of Wahlberg's epauletted fruit bats (Epomophorus wahlbergi) from South Africa showed seasonal changes in BMR in opposite directions, with one population decreasing whole-animal BMR by 16-25 % but another increasing it by 40 % (Minnaar et al 2014;Downs et al 2012). …”

Seasonal Metabolic Acclimatization Varies in Direction and Magnitude among Populations of an Afrotropical Passerine Bird

“…An open flow-through respirometry system was used to estimate metabolic rates, by measuring O 2 consumption ( _ VO 2 ; mL min 21 ), CO 2 production ( _ VCO 2 ; mL min 21 ), and total evaporative water loss (TEWL; mg min 21 ), using an experimental setup similar to that described by Minnaar et al (2014), with modifications listed below. We used airtight respirometry chambers constructed from 4-L clear plastic containers (Lock and Lock, Anaheim, CA) and elevated birds about 10 cm above a layer of mineral oil (∼1 cm; to prevent evaporation of excreta influencing TEWL estimates), using a plastic mesh platform.…”

“…Excurrent air or helox from the chambers and from the baseline channel was alternately subsampled and pulled through an O 2 analyzer (FC-10A, Sable Systems, Las Vegas, NV) and a CO 2 /H 2 O analyzer (LI-840A, LI-COR, Lincoln, NE) at ∼200 mL min 21 by a custom-built pump with a rotameter (Omega, Stamford, CT). All mass flow controllers and analyzers were calibrated regularly, using the protocol described by Minnaar et al (2014).…”

“…The T a (7C) within each chamber and core body temperature (T b ; 7C) were measured as described by Minnaar et al (2014), except that a Cu-Cn thermocouple (IT-18, Physitemp Instruments, Clifton, NJ) and temperature recorder (RDXL12SD, Omega) was used to monitor T a during M sum measurements. Temperaturesensitive passive integrated transponder (PIT) tags (Destron Fearing, St. Paul, MN) were injected into the abdominal cavities of birds to measure T b after they were calibrated against a Cu-Cn thermocouple in a temperature-controlled circulating water bath (F34-ME, Julabo, Seelbach, Germany).…”

“…To quantify M sum in sparrow-weavers (using the same individuals as those for BMR measurements), gas exchange rates were measured individually during the daytime, using the sliding cold exposure method (Swanson et al 1996) and a similar protocol to that described by Minnaar et al (2014), with modifications listed below. M sum was measured within 12 h before BMR measurements for about half of the sparrow-weavers and during the 12 h after for the rest of the birds, in order to control for the effect of habituation during repeated metabolic measurements (Jacobs and McKechnie 2014).…”