Larval amphibians seek warm temperatures and do not avoid harmful UVB radiation

Habitat use by animals often reflects the balance between conflicting demands such as foraging and avoiding predation. Environmental stressors such as temperature can also affect habitat use in many organisms, particularly in ectothermic animals. For example, warm, shallow thermal regimes in ponds can optimize growth and developmental rate of amphibian larvae but may also expose larvae to potentially harmful levels of ultraviolet B (UVB) radiation. Thus, optimally, amphibians seeking sunlight for thermoregulation must balance this behavior while limiting their exposure to harmful UVB radiation. We conducted a series of laboratory and field experiments to test the hypothesis that larval amphibians avoid UVB by selecting microhabitats with lower exposure to UVB. We then quantified habitat use of the larvae of 4 amphibian species using field transects in 3 ponds with different UVB transmission. Tadpoles did not avoid UVB radiation in laboratory or field experiments and preferred warmer temperatures in laboratory thermal gradients regardless of UVB exposure. The majority of anuran larvae were observed in water less than 10-15 cm deep in field surveys, whereas salamander larvae were most often observed in deeper, cooler water. The similarity in habitat use across different sites and the lack of evidence of UVB avoidance in choice tests suggest that larval anuran amphibians may be exposed to harmful levels of UVB radiation due to habitat choice behaviors that have been established over evolutionary time. Levels of UVB radiation at the earth's surface have recently increased due to stratospheric ozone depletion. Thus, long-term selection pressures such as thermoregulation may override the relatively recent selection pressure of increased UVB radiation.