Date Published: January 23, 2017
Publisher: Public Library of Science
Author(s): Štěpánka Kadochová, Jan Frouz, Flavio Roces, Claudio R. Lazzari.
In early spring, red wood ants Formica polyctena are often observed clustering on the nest surface in large numbers basking in the sun. It has been hypothesized that sun-basking behaviour may contribute to nest heating because of both heat carriage into the nest by sun-basking workers, and catabolic heat production from the mobilization of the workers’ lipid reserves. We investigated sun-basking behaviour in laboratory colonies of F. polyctena exposed to an artificial heat source. Observations on identified individuals revealed that not all ants bask in the sun. Sun-basking and non-sun-basking workers did not differ in body size nor in respiration rates. The number of sun-basking ants and the number of their visits to the hot spot depended on the temperature of both the air and the hot spot. To investigate whether sun basking leads to a physiological activation linked with increased lipolysis, we measured respiration rates of individual workers as a function of temperature, and compared respiration rates of sun-basking workers before and two days after they were allowed to expose themselves to a heat source over 10 days, at self-determined intervals. As expected for ectothermic animals, respiration rates increased with increasing temperatures in the range 5 to 35°C. However, the respiration rates of sun-basking workers measured two days after a long-term exposure to the heat source were similar to those before sun basking, providing no evidence for a sustained increase of the basal metabolic rates after prolonged sun basking. Based on our measurements, we argue that self-heating of the nest mound in early spring has therefore to rely on alternative heat sources, and speculate that physical transport of heat in the ant bodies may have a significant effect.
Wood ants of the genus Formica belong to those social insect species whose colonies are able to keep relatively stable temperatures inside the nest [1–4], as for instance honey bees , fungus-growing termites , some bumble bees , stingless bees  and leaf-cutting ants . For the control of climate conditions inside the nest, large nests with good insulation properties are crucial . Metabolic heat produced by ant workers [3, 11] or by associated microorganisms [12, 13, 14] is supposed to be an important inner source of heat. The level of nest thermoregulation depends on many factors other than size, e.g., population size, moisture and thermal conductivity of the nest material .
In our laboratory experiments, we succeeded in triggering sun-basking and clustering behaviours in F. polyctena workers on an artificial hot spot provided by an infrared source, as previously described by [11, 29]. Sun-basking behaviour was similar to that described under field conditions, which is assumed to serve as one of the mechanisms underlying nest heating in early spring [2, 3, 15, 24]. Our results go beyond previous studies on sun basking in wood ants [11, 29] by describing the behaviour of individually-marked workers over several days.
There are sun-basking ants and non-sun-basking ants in colonies of wood ants; the fraction of sun-basking ants in a colony decreases over time. Among sun-basking ants, some individuals spend significantly more time sun basking and perform more visits to the hot spot than others. Sun-basking ants and non-sun-basking ants do not differ in body size nor in standardized metabolic rates. There are no differences in the respiration rate of ants before sun basking and two days after they are allowed to expose themselves to a heat source over 10 days, at self-determined intervals. Therefore, the transient increase in the workers’ body temperatures and, as a consequence, in their respiration rate during sun basking does not lead to a sustained, long-term increase in respiration rate consistent with a higher rate of lipid catabolism. Based on our measurements, we argue that self-heating of the nest mound in early spring has therefore to rely on alternative heat sources, and speculate that physical transport of heat in the ant bodies may have a significant effect.