ORIGINAL PAPER The cranial arterio-venous temperature difference is related to respiratory evaporative heat loss in a panting species, the sheep (Ovis aries) Kristine Vesterdorf • Dominique Blache • Shane K. Maloney Received: 13 May 2010 / Revised: 16 August 2010 / Accepted: 7 September 2010 / Published online: 28 September 2010 Ó Springer-Verlag 2010 Abstract Panting is a mechanism that increases respira- tory evaporative heat loss (REHL) under heat load. Because REHL uses body water, it is physiologically and ecologically relevant to know under what conditions free- ranging animals use panting. We investigated whether the cranial arterio-venous temperature difference could pro- vide information about REHL. We exposed sheep to environments varying in ambient dry bulb temperatures (Env 1: *15°C, Env 2: *25°C, Env 3: *40°C, Env 4: *40°C ? infrared radiation) and measured REHL simul- taneously with carotid arterial (T car ) and jugular venous (T jug ) blood temperatures, as well as brain (T brain ) and rectal (T rec ) temperatures. REHL increased significantly with ambient temperature, from 18.4 ± 4.5 W at Env 1 to 79.5 ± 12.6 W at Env 4 (P \ 10 -6 ). While there was no effect of environment on T car (P = 0.7) or T jug (P = 0.09), the difference between them (T a-v = T car - T jug ) increased from Env 1 to Env 2 (P = 0.04) and from Env 3 to Env 4 (P = 0.008). T a-v reached a maximum of 0.7 ± 0.2°C at Env 4 and was positively correlated with REHL across environments (r 2 = 0.78, F = 34.7, P \ 10 -3 ). Calculated cranial blood flow changed only from Env 2 to Env 3 (P = 0.002). The increase in REHL maintained homeo- thermy when dry heat loss decreased. While REHL could increase without generating an increase in T a-v , any increase in T a-v was always associated with an increase in REHL. We conclude that the cranial T a-v provides useful information about REHL in panting animals. Keywords Mammal  Thermoregulation  Panting  Respiratory evaporative heat loss  Arterio-venous temperature difference  Heat stress  Selective brain cooling Introduction Many mammals have adapted to heat load by the evolution of physiological and behavioural mechanisms for heat dissipation. Panting is one such thermoregulatory mecha- nism (Findlay 1957). In panting species, respiratory fre- quency, f R , increases when ambient or body temperature increases, leading to an increase in respiratory evaporative heat loss (REHL; Bligh 1963; Hales 1973a; Kronert and Pleschka 1976; Murrish 1973; Schmidt-Nielsen et al. 1970). During inspiration, heat is removed from the nasal mucosa predominantly via the latent heat of evaporation and also sensible heat is exchanged if the air is cooler than the mucosa. When blood perfuses the nasal mucosa, it loses heat to the cooled surface (Hales 1973a; Hales and Brown 1974; Murrish 1973; Schmidt-Nielsen et al. 1970). The cooling effect of REHL is achieved when this cooled blood returns to the heart and perfuses the body. The extent to Communicated by I. D. Hume. K. Vesterdorf  S. K. Maloney Physiology M311, School of Biomedical, Biomolecular, and Chemical Science, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia D. Blache School of Animal Biology M085, Faculty of Natural and Agricultural Sciences, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia Present Address: K. Vesterdorf (&) Section for Biochemistry, Physiology and Nutrition, Department of Basic Animal and Veterinary Sciences, Faculty of Life Sciences, University of Copenhagen, Dyrlægevej 100, 1870 Frederiksberg C, Denmark e-mail: krisvester@gmail.com 123 J Comp Physiol B (2011) 181:277–288 DOI 10.1007/s00360-010-0513-7