Critical thermal minima, their spatial and temporal variation and response to hardening in Myrmica ants

In a changing environment the ability to adjust physiological functions to new conditions might be especially valuable for long-living ectothermic animals such as ants. With a simple method for estimating critical thermal minima of Myrmica ants we assessed variation of the minima and their response to lowered temperature. At a cooling rate of about 1 degree C per minute the ants first displayed knock-down (at temperatures 1.5 to -0.2 degree C) and only later immobilized completely (at -1.3 to -3.1 degree C). Pre-chilling of ants at 5 degree capital ES, Cyrillic for 1 h lowered the parameters significantly but not greatly (about 0.9 degree capital ES, Cyrillic). Constant laboratory conditions (20 degree capital ES, Cyrillic) raised the knock-down temperature and tended to lower the temperature of immobilization. In the same manner the two parameters changed in field conditions in June, but no significant change occurred through August and until the end of study season in mid-September. When populations from different geographic localities were compared, populations from the north showed lower knock-down temperatures. The magnitude of these apparently genetic differences among populations was comparable to the magnitude of plastic changes that occurred either naturally, or in experiments, or in laboratory culture. The little plasticity and low geographic variation of the critical thermal minima may indicate that the ability of the ants to withstand cold events and populate varying climates bases mainly on the protective properties of nesting substrate.