Nest temperatures and thermal preferences of a forest ant species: Is seasonal polydomy a thermoregulatory mechanism?

We measured nest temperatures and performed laboratory and field experiments that determined thermal preferences of the forest ant Myrmica punctiventris Roger. We tested the hypothesis that the seasonal cycle of colony splitting (polydomy) that this species undergoes allows it to warm the brood, enhancing reproductive output. This ant species nests in performed cavities such as sticks, acorns, and hickory nuts. Laboratory choice experiments indicated that ants preferred to nest at a temperature between 16 degrees C and 21 degrees C. Workers and brood had very similar temperature preferences; queens had slightly higher temperature preferences. Field measurements of nest temperatures in Vermont recorded an average summer daytime nest temperature of 22.6 +/- 0.38 degrees C s.e., n = 53 nests, with few differences between temperatures in different types of nesting materials. Field experiments demonstrated that ants colonized artificial nest sites on cooler plots preferentially to those on warmer plots (artificial nest sites on these plots differed by 1 degrees C in temperature, on average). Our results indicate that M. punctiventris preferred to nest at temperatures at the cooler end of the spectrum of available nest sites in the field. Thus, our results lend no support to the hypothesis that seasonal polydomy is a thermoregulatory mechanism that these ants employ to place their brood in the warmest available nest sites.