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

During warm, sunny days (Max. temp. 22 °C to 25 °C) C02-baited traps operated at sites on and off snow patches (SP's) in subarctic Norway caught significantly fewer culicids, simuliids and tabanids 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 versus off SP's. Analysis of videotapes taken in 1987 showed that defensive anti-fly behaviors of reindeer on and off SP's remained low (< 1/4 min) throughout the day. Based on reindeer anti-fly behaviors, harassment was greatest from 10.30 to 12.30 h (Norwegian Standard Time), but reindeer continued to aggregate on SP's while anti-fly behaviors were lowest (13.30 to 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 SP's primarily to thermoregulate following periods of foraging. Almost all reindeer remained on SP's 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 SP's. However, by 20.00 h, during the cooler period when trap catches of mosquitoes were increasing, almost all reindeer had moved off SP's. The small decreases in anti-fly defensive behaviors observed for reindeer on SP's versus animals foraging in snow-free areas indicated that their presence on SP's 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.