Abstract:
Variation in geographical range size among closely related species may result from differences in physiological traits, such as desiccation tolerance, that enable these species to interact with their environment or adapt to new surroundings.
We tested the hypothesis that insect species with a broader geographical range have either a higher basal desiccation tolerance or mount a more plastic response than more narrowly distributed species by exposing four fruit fly species (Ceratitis capitata, Ceratitis rosa, Ceratitis cosyra and Ceratitis podocarpi) to one of three acclimation treatments (control: standard relative humidity (RH) and temperature; desiccation: standard temperature and low humidity; and temperature: low RH and high temperature) and measuring metabolic rate, activity, water loss rates and survival.
The targeted physiological responses differed between species and acclimation treatments. Survival of the widely distributed C. capitata improved by up to 43% after short-term exposure to high temperature and desiccation (35°C; 0% RH) treatment, while survival in the more narrowly distributed species only improved by 4%–30% after a desiccation treatment (25°C; 0% RH).
Less water was lost by broadly distributed C. capitata through excretion after both high temperature and desiccation treatments, but only activity and respiratory water loss (RWL) were reduced after the temperature treatment, and total water loss and cuticular water loss declined after the desiccation treatment. The narrowly distributed C. rosa also lost less water through excretion after both acclimation treatments but showed reduced cuticular and RWL only after desiccation. While basal tolerance in C. cosyra was high, acclimation responses in this species and C. podocarpi were insignificant in that they did not produce a measurable survival benefit.
Broadly distributed species successfully employed unique combinations of physiological strategies, with some having highly flexible responses to stressful environmental conditions, which ultimately results in beneficial acclimation to enhance survival during dry conditions. By contrast, range restricted species showed limited responses to desiccation stress. Flexible desiccation responses likely contribute to species geographical ranges in changing climate conditions.
