Physiological responses of the frog Engystomops pustulosus to cold temperature

Frogs from temperate climates have been known to tolerate low temperatures and possess physiological mechanisms that increase their thermal tolerance to cold. Two of these mechanisms are hardening and acclimation. Hardening occurs after short exposures to extreme temperatures and involves quick (within minutes) biochemical modifications on a cellular level to increase their chances of survival. Longer exposures to less extreme temperatures can trigger slower physiological responses in a process called acclimation. Frogs from tropical climates fail to acclimate to low temperatures. They might have lost this mechanism potentially due to disuse in their benign habitat. We tested the generality of this pattern by characterizing the thermal tolerance, hardening and acclimation responses of the túngara frog (Engystomops pustulosus), which belongs to a family with unknown thermal ecology that inhabits an extremely constant neotropical climate. Tadpoles became unresponsive around 10° C and none withstood temperatures below 5°C. Premetamorphic tadpoles hardened by 1.18 +/- 1.05° C (SD; P = 0.01), while metamorphic tadpoles hardened by 0.36 +/- 0.37° C (SD; P = 0.04). Preliminary data for acclimation indicate that túngara tadpoles can acclimate to cold temperatures, since tadpoles raised in a cold environment express enhanced cold tolerance. Our results show that this species has a greatly reduced cold tolerance when compared to species from temperate climate, but it responds to cold temperatures with comparable hardening and acclimation. The previously proposed loss of response to cold by disuse cannot, therefore, be generalized to neotropical frogs