Between a rock and a hard place: impact of nest selection behavior on the altitudinal range of an alpine ant, Formica neorufibarbis.

Formica neorufibarbis Emery is a dominant alpine ant in North America. Above timberline, colonies nest under rocks, placing brood against the rock surface to warm during the day. We tested the hypothesis that nest rock preference represents a mechanism of behavioral thermoregulation and used a demographic model to explore its impact on the species' altitudinal range. Under sunny conditions rocks provide warm microclimates. Across a 400-m altitudinal gradient from below timberline to high alpine tundra, nest rocks warmed on average 5°C above ambient at mid-day, but cooled to near ambient before dawn. Soil was warmer at mid-day under rocks occupied by ants than under randomly chosen unoccupied rocks in the same area. Occupied nest rocks were painted white or black to manipulate temperature and test its impact on colony occupancy. In addition, black-painted and white-painted rocks of similar size were placed in the field to test for a temperature effect on nest site colonization. Likelihood of nest persistence and colonization decreased under cooler, white-painted rocks and depended strongly on rock warming capacity. Experimental results showed that nest site colonization and occupancy are favored in microsites maintaining mid-day temperature ≥22°C. Across the altitudinal gradient, 66-90% of nest microsites occupied by F. neorufibarbis met this temperature threshold. Conversely, none of the exposed microsites and only 15% of those microsites under randomly selected rocks warmed to 22°C at the high altitude site (3,900 m). Results support the conclusion that nest site preference contributes to the broad altitudinal distribution of F. neorufibarbis.