Does thermoregulatory behaviour of green toads (Bufo viridis) constrain geographical range in the west? A comparison with the performance of syntopic natterjacks (Bufo calamita)

(1)Behavioural regulation of body temperature (tb) was monitored in 23 free-ranging Bufo calamita (Bc) and 17 syntopic Bufo viridis (Bv) at Urmitz (Rhineland-Palatinate, Germany) using temperature-sensitive transmitters implanted to the abdominal cavity.(2)In field tb varied between +0.5 and 37.4 °C in Bc and between +0.6 and 33.7 °C in Bv. Maximum tb of a Bc measured during an experimental trial was 38.8 °C.(3)Natterjack toads avoided environmental temperature extremes by burrowing actively into moist sandy soil (2–90 cm deep), whereas green toads hid exclusively in mammal burrows or pre-existing subterranean cavities. Shelter choice did not vary between summer and winter.(4)Average tb of Bc exceeded significantly that of Bv during summer (26.7 °C vs. 24.7 °C), while the reverse was true during winter (4.2 °C vs. 7.2 °C). Following hibernation the body condition of Bv was significantly lower than that of Bc.(5)We conclude that green toads fail to colonise regions west of the Rhine valley because of a combination of winter temperatures impeding foraging trips for prolonged periods, the choice of warm hibernacula increasing metabolic costs and/or predation risk and reduced fecundity.

► Core temperatures of 40 free-ranging radio-tracked toads were recorded at the western range limit of Bufo viridis.
► Green toads spent winter in hibernacula allowing core temperature of up to 10 °C.
► Body condition as a measure of fitness decreased significantly during winter causing high mortality rates.