Comparison of brain and ear temperature changes during diving in seals

	Comparison of brain and ear temperature changes during diving in seals
Research field:	Other
Authors:	Jenstad M, Folkow LP, Kvadsheim PH
Address of presenting author:	M. Jenstad Department of Arctic Biology University of Tromsø N-9037 Tromsø Norway
E-mail:	monicaj@fagmed.uit.no
Phone:	+4777644855
Fax:	+4777645770
Text of abstract	Introduction Seals are able to stay submerged for extended periods of time due to their large oxygen stores in blood and skeletal muscles, and due to dramatic cardiovascular changes that occur during diving in order to economise with these stores. One factor that may reduce oxygen consumption during diving is a remarkable drop in brain temperature, which may amount to 3.5°C during a 15 min simulated dive in a hooded seal. This presumably causes a substantial decrease in brain metabolism through a Q₁₀-effect (Odden et al. 1999). Studies of the mechanism of the brain cooling phenomenon require that brain temperature is measured, which, if done invasively, is associated with risks of losing valuable research animals. We have investigated whether tympanic temperature is a useful index of brain temperature in seals, as it has been shown to be in some other species (e.g., Baker et al. 1972). If so, this would offer a non-invasive technique to monitor brain temperature changes in these mammals. Methods Four subadult harp seals (Phoca groenlandica) were surgically instrumented under full isoflurane anaesthesia with a 30 mm long blind-ended guide tube that was inserted into the left cerebral hemisphere through a hole in the skull, 10 mm lateral to the midline. After recovery from surgery, the seals were restrained on a board and instrumented with copper-constantan thermocouples for measurement of brain, ear and rectal temperature (T_b_r, T_e, and T_r, respectively), as well as with subcutaneous ECG-electrodes for monitoring of heat rate (HR). The thermosensitive tip of the ear thermocouple was placed within 3 mm of the tympanic membrane under X-ray surveillance. The seals were then subjected to simulated diving by submerging the board with the seal in an 800 l tank filled with 5°C water, for durations of 10 or 15 minutes. Results All seals displayed instant bradycardia upon submersion, with HR decreasing from pre-dive values of 70-120, to 8-25 beats per minute during the dive. Pre-dive T_e was 2.2±0.7°C (average±SD; n=4) lower than T_b_r. T_b_r and T_r always decreased during the course of a dive (DT_b_r=-1.0±0.1°C and DT_r=-1.4±0.9°C (n=4) during 10-15 min dives), whereas T_e decreased in some experiments and increased in some (DT_e=-0.5±0.5°). The overall correlation between DT_b_r and DT_e was poor (r²=0.01, n=27 dives). Conclusions Based on the poor correlation between DT_b_r and DT_e and the differences in the pattern with which the two temperatures changed during a dive, we conclude that T_e is not a good indicator of T_b_r in diving harp seals. Further studies of the mechanism behind the brain cooling phenomenon in seals must therefore be based on direct measurements of brain temperature. References Baker, M.A., Stocking, R.A. & Meehan, J.P. 1972. J Appl Physiol 32, 739-742 Odden, Å., Folkow, L.P., Caputa, M., Hotvedt, R. & Blix, A.S. 1999. Acta Physiol Scand 166, 77-78
Keywords:	brain cooling, diving, pinnipeds, temperature regulation

Created 20000-05-24