Thermal substitution and aerobic efficiency: measuring and predicting effects of heat balance on endotherm diving energetics

Abstract

For diving endotherms, modelling costs of locomotion as a function of prey dispersion requires estimates of the costs of diving to different depths. One approach is to estimate the physical costs of locomotion (P mech ) with biomechanical models and to convert those estimates to chemical energy needs by an aerobic efficiency (hZP mech /Vo 2 ) based on oxygen consumption (Vo 2 ) in captive animals. Variations in h with temperature depend partly on thermal substitution, whereby heat from the inefficiency of exercising muscles or the heat increment of feeding (HIF) can substitute for thermogenesis. However, measurements of substitution have ranged from lack of detection to nearly complete use of exercise heat or HIF. This inconsistency may reflect (i) problems in methods of calculating substitution, (ii) confounding mechanisms of thermoregulatory control, or (iii) varying conditions that affect heat balance and allow substitution to be expressed. At present, understanding of how heat generation is regulated, and how heat is transported among tissues during exercise, digestion, thermal challenge and breath holding, is inadequate for predicting substitution and aerobic efficiencies without direct measurements for conditions of interest. Confirming that work rates during exercise are generally conserved, and identifying temperatures at those work rates below which shivering begins, may allow better prediction of aerobic efficiencies for ecological models.

Modeling profitability for the smallest marine endotherms: auklets foraging within pelagic prey patches

“…data), and by 1 CAAU instrumented in British Columbia (Burger & Powell 1990), were to depths < 20 m; thus, I assumed that no dives by either CAAU or LEAU were deeper than 20 m. To estimate costs of diving, I used the modeling approach of Lovvorn et al (2004). This approach estimates mechanical work against drag, buoyancy, and inertia of the body and entrained water (added mass) during each swimming stroke at different depths, and then converts mechanical work into aerobic work (chemical substrate requirements) by means of an aerobic efficiency η (Lovvorn 2007). For a larger wing-propelled alcid (TBMU) it was previously shown that inertial work is a negligible fraction of the total mechanical work of diving compared to work against drag and buoyancy (Lovvorn et al 1999(Lovvorn et al , 2004.…”

“…Values of η depend strongly on the accuracy of the mechanical model (i.e. η is specific to a given model), and subsume thermoregulation costs (Lovvorn 2007). Depending on body size, water temperature, and exercise level, heat generated by exercising muscles may be less than heat lost, or may completely replace heat lost so that no shivering thermogenesis is needed.…”

“…Water temperature also acted mainly by varying the cost of RMR during passive ascent. Temperature effects on costs of descent and bottom foraging would be considered through changes in aerobic efficiency (Lovvorn 2007), which was also varied in this analysis.…”

“…The latter measurements do not account for major changes in the buoyancy of air volumes over a greater range of depths, or corresponding adjustments of swim speed at depths below 2 m (Lovvorn et al 2004). To account for such effects, an alternative approach is to estimate mechanical work against drag and buoyancy, and then to convert mechanical costs to aerobic costs with an aerobic efficiency factor based on experimental measurements (Lovvorn et al 2004, Lovvorn 2007. Given the shortage of studies, dive patterns and costs for small auklets must be scaled down from data and models for larger birds.…”

Environmental constraints upon locomotion and predator–prey interactions in aquatic organisms: an introduction

“…One response to that difficulty has been the development of bioenergetics modelling. The contribution by Lovvorn (2007) illustrates this type of approach in understanding thermoregulation in diving animals. However, Lovvorn also raises the point that it remains very difficult to reconcile the time-scales at which the various subcomponents involved in preserving heat balance proceed, i.e.…”