Operative temperatures of eastern garter snakes ( Thamnophis sirtalis sirtalis ) reveal a Goldilocks effect for habitat use

Garter snakes ( Thamnophis spp.) are the most widespread reptiles in North America, although evidence suggests that thermal preference has not diverged much among populations or Thamnophis species. To shed light on how thermal decisions influence local habitat use by the eastern garter snake ( Thamnophis sirtalis sirtalis ), we measured the thermal profiles of three habitats differing in canopy cover: open peat, mixed shrub, and closed forest. We installed biophysical models to record operative temperatures at a fine scale and assess habitat thermal quality. We also used coverboards to survey habitat usage. While the open canopy offered the highest thermal quality, we recorded the greatest number of snakes in the mixed shrub which had a lower thermal quality. Since environmental temperatures regularly exceeded the upper thermal limit of T. s. sirtalis in the open canopy, snakes might favour the use of habitats that minimise the odds of overheating. Therefore, open habitats potentially restrict snakes’ activity window and may not be thermally attractive. Our data show that T. s. sirtalis use habitats that vary in thermal quality, but warmer habitats are not necessarily better. Rather, snakes preferentially seek areas that offer a mix of open and closed canopies to suit their thermoregulatory needs.