Meeting Abstract

S10.12  Friday, Jan. 7  Heat dissipation and hyperthermia risk as limiting factors in endotherm ecology SPEAKMAN, JR*; KROL, E; University of Aberdeen, Aberdeen, Scotland, UK j.speakman@abdn.ac.uk

The maximum rate at which an animal can ingest energy is important because it defines a constraint on competing processes that use energy, such as reproduction and survival. Historically it has been widely assumed that environmental supplies of energy or the capacity of animals to collect it from the environment impose such limits. However, several lines of evidence suggest that intrinsic limitations on intake and expenditure may be more important. Potential locations of this intrinsic limit include the capacity of the alimentary tract to assimilate ingested energy ‘the central limitation theory’. Experimental evidence in lactating mice exposed to different ambient temperatures allows us to reject this idea, since they are capable of upregulating their intake at low ambient temperatures above levels that at 21 oC limit performance. Failure of the extrinsic limit and central limitation hypothesis has led to attention switching to potential limitations on the energy expenditure side of the energy balance equation. Two alternative theories have been proposed. The ‘peripheral limitation hypothesis’ suggests that the maximal sustained energy intake reflects the summed demands of individual tissues. The ‘heat dissipation limitation’ theory suggests that animals are constrained by the maximal capacity to dissipate body heat. Evidence in domesticated livestock supports the heat dissipation limitation theory. Data from several smaller species also suggests their lactation performance is also limited by heat dissipation capacity. However several studies have generated data that conflict with the heat dissipation theory and are more consistent with the peripheral limitation hypothesis. It is unclear at present which limit pertains more generally, and hence the wider ecological relevance of the two theories.