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

Abstract

(1) Behavioural regulation of body temperature ( t b) 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 t b varied between +0.5 and 37.4 °C in Bc and between +0.6 and 33.7 °C in Bv. Maximum t b 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 t b 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.