Effect of fur removal on the thermal conductance and energy budget

INTRODUCTION Sustained energy intake (SusEI) is the maximal rate of energy intake that animals can sustain over sufficiently long periods so that energy demands are fuelled by food intake rather than by transient depletion of energy reserves (Speakman and Krol, 2005). In many situations, the SusEI may be constrained. The factors causing these constraints on SusEI are important because they establish the upper energetic limits on the ability of animals to distribute, survive and reproduce (Karasov, 1986; Peterson et al., 1990; Hammond and Diamond, 1997; Speakman, 2000; Krol and Speakman, 2003a; Krol and Speakman, 2003b; Speakman and Krol, 2005). Earlier studies suggested that the SusEI might be imposed by food availability; called the extrinsic limitation hypothesis (Hammond and Diamond, 1997; Speakman and Krol, 2005). However, the extrinsic limitation hypothesis is inconsistent with the fact that most animals do not increase food intake when given free access to unlimited food (Akbar and Gorman, 1993; Akbar and Gorman, 1996; Hammond and Diamond, 1997; Speakman and Krol, 2005). To explain such situations two further hypotheses emerged – the central and peripheral limitation hypotheses (Weiner, 1992; Peterson et al., 1990; Hammond and Diamond, 1997; Speakman and Krol, 2005; Speakman, 2007). The central limitation hypothesis suggested the constraint on SusEI might be imposed by the capacity of the gastrointestinal tract to acquire, process and absorb energy. The peripheral limitation hypothesis in contrast suggested that the limits on SusEI were imposed by the expenditure capacities of the energyconsuming organs, such as skeletal muscle during physical exercise, brown adipose tissue and muscles during cold exposure and the mammary glands during lactation (Hammond and Diamond, 1997; Speakman and Krol, 2005; Speakman, 2007; Speakman, 2008). Lactation is the most energetically demanding period encountered by small mammals (Hammond and Diamond, 1997; Johnson et al., 2001a; Johnson et al., 2001b; Thompson and Nicol, 2002; Speakman and Krol, 2005; Speakman, 2007). Hammond and Diamond (Hammond and Diamond, 1997) calculated the sustained metabolic scope (SusMS, the ratio SusEI/resting metabolic rate, RMR) of laboratory mice and found that maximal SusMS was 3.6 for physical exercise, 4.8 for heat production during cold exposure, and 6.5 for lactation. This suggests that lactation is an excellent model for the study of factors limiting SusEI (Hammond and Diamond, 1997). When litter sizes were manipulated at birth, both Swiss Webster mice and MF1 mice raising additional offspring were not able to further increase food intake to match the greater number of pups, showing that SusEI was limited, but it was not possible to determine whether the limitation was acting centrally or peripherally (Hammond and Diamond, 1992; Johnson et al., 2001a). Other studies showed that both house mice (Mus domesticus) and deer mice (Peromyscus maniculatus) were not capable of increasing food intake to meet the combined energy demands of lactation and wheelrunning activity, suggesting that limits to SusEI might be set The Journal of Experimental Biology 212, 2541-2549 Published by The Company of Biologists 2009 doi:10.1242/jeb.029603