Physiological traits and their relationships vary along an aridity gradient within and among Fijian bee species

Abstract

AbstractTemperature and water availability are independently hypothesised to be important abiotic drivers of the evolution of metabolic rates and gas exchange patterns, respectively. Specifically, the metabolic cold adaptation hypothesis (MCA) predicts that cold environments select for faster metabolic rates to counter the thermodynamics of biochemical reactions while the hygric hypothesis predicts that dry environments select for discontinuous gas exchange to reduce water loss. Although these two hypotheses consider different physiological traits and how they vary along different abiotic gradients, metabolic rate drives gas exchange patterns in insects meaning these two traits are inherently linked. Despite this link, the MCA and hygric hypotheses are rarely considered together and the extent to which metabolic rates and respiratory patterns vary and co-vary with temperature and aridity along climatic gradients remains unclear. We tested the MCA and hygric hypotheses within a species of endemic Fijian bee, Homalictus fijiensis, across an altitudinal gradient of 1100 m, and among four Fijian bee species, including H. fijiensis, that inhabit different altitudinal bands. In Fiji, environmental temperature is ~5°C lower in the central highlands than in the coastal lowlands with the highlands receiving ~100 mm of additional precipitation than the lowlands each month. We found an MCA-like pattern within H. fijiensis and among Fijian bee species, where metabolic rate decreased with increasing temperature, but precipitation also explained variation in metabolic rate. We also found support for the hygric hypothesis within H. fijiensis and across species. Lastly, we found that the relationship between metabolic rate and ventilation rate changed depending on precipitation of the driest month within H. fijiensis and among bee species. Bees that inhabited wet regions had low ventilation rates of greater volume but bees that inhabited drier locations had rapid ventilations of smaller volume potentially as a mechanism to exchange gas without desiccating. As the correlation between metabolic rate and ventilation rate changes across the precipitation gradient, it is possible that these two traits can evolve independently of each other in response to abiotic variables despite being positively correlated.