While photographing acorns yesterday in a patch of melting snow, I wondered about the fate of the small Temnothorax that live in the acorns. Most temperate ants survive the season by tunneling through the soil to hibernate below the freezing zone. But Temnothorax don’t. They stay at the soil surface, holed up in walnuts, acorns, twigs, and other small cavities that freeze solid for several months. That can’t be the easy winter survival option.
I moved on to ponder other things- namely, a warm cup of coffee- but today I was surprised to see a timely new study on just this problem:
Abstract: Most species of ants inhabiting the temperate zone overwinter underground, whereas those of the genus Temnothorax remain in nests aboveground. I studied the cost of aboveground overwintering. Workers of Temnothorax crassispinus survived in higher numbers (median = 88%) in artificial nests experimentally buried at a depth of 5 cm than those in nests on the surface (48%) of the soil. The results support the hypothesis that overwintering aboveground could be a consequence of a limited supply of nests and/or the advantage of being able to respond quickly to warm temperatures in spring.
Slawomir Mitrus buried 18 equal-sized nests of a European acorn ant under the soil and placed 18 control nests on the surface. The worker survival data between the two treatments were stark:
Most workers in the buried nests survived the winter while about half of the surface ants perished. With such a steep cost for overwintering above ground (see also Joan Herber’s earlier studies), remaining on the surface must provide some non-trivial benefits to the ants. The author suggests the surface-nesters are holding valuable nesting sites against usurpers, or perhaps the ants are able to jump-start their spring growth. These are plausible explanations, and I suspect that a deep-soil migration may also prove more costly in terms of fungal infection or exposure to predators than remaining in their nests. We still don’t know a whole lot about the natural history of even common species.
My own overwintering strategy consists of travelling to sunny Belize, of course.
source: Mitrus, S. 2013. Cost to the cavity-nest ant Temnothorax crassispinus (Hymenoptera: Formicidae) of overwintering aboveground. Eur. J. Entomol. 110: 177–179.