Do reindeer aggregate on snow patches to reduce harassment by parasitic flies or to thermoregulate?

During warm, sunny days (maximum temperature 22-25°C) CO2-baited traps operated at sites on and off snow patches (SPs) in subarctic Norway caught significantly fewer mosquitoes (Aedes communis, A. nigripes, A. pionips, A. punctor), simuliids (Metacnephia saileri, Prosimulium hirtipes, Simulium corbis, S. ornatum, S. rostratum, S. vernum) and tabanids (Hybomitra auripila) on snow in both 1985 and 1987. However, for overnight catches (18.30-07.30 h), there was no significant difference in the number of culicids caught on vs off SPs. Analysis of videotapes taken in 1987 showed that defensive anti-fly behaviours of reindeer (Rangifer tarandus) on and off SPs remained low (<0.25 min) throughout the day. Based on reindeer anti-fly behaviours, harassment was greatest from 10.30 to 12.30 h (Norwegian Standard Time), but reindeer continued to aggregate on SPs while anti-fly behaviours were lowest (13.30-20.00 h). Groups of >150 animals often occupied the entire surface of a snow patch. At the fly densities and climatic conditions encountered, it seemed apparent that reindeer intermittently used SPs primarily to thermoregulate following periods of foraging. Almost all reindeer remained on SPs from 11.00 to 12.30 h, but at other times between 08.00 and 19.00 h about half the herd (ca. 800 animals) foraged for about an hour while the other half aggregated on SPs. However, by 20.00 h, during the cooler period when trap catches of mosquitoes were increasing, almost all reindeer had moved off SPs. The small decreases in anti-fly defensive behaviours observed for reindeer on SPs versus animals foraging in snow-free areas indicated that their presence on SPs may have resulted in a minor, coincidental reduction in harassment. Significantly more tabanids were caught during the morning trapping period than at other times, and significantly more mosquitoes were caught during the evening/overnight trapping period than at other times.