Competing for mates is hard enough, but when you live in a sunny, hot environment, the competition can get even harder when the threat of overheating is real. Digger bees (Centris pallida) living in the Sonoran Desert have evolved some tricks that allow them to stay cool while looking for mates. Males of this species come in two sizes: large, which are pale grey in colour, and small, which are a darker brown. Emerging from their solitary burrows in April to form huge groups, the larger males spend most of their time digging and fighting over unmated females on the hot ground, while the smaller males hover near vegetation, watching for females that may have escaped the affray below. Even in the shade, desert air temperatures are hot, so how do the larger male bees tolerate the sizzling temperatures in direct sunlight? Meghan Barrett and Sean O'Donnell at Drexel University, USA, believe that lighter-coloured males can stay on the hotter ground longer without overheating in part because their pale hairs can reflect more sunlight.
After collecting large and small male bees from the desert in Arizona, USA, Barrett and O'Donnell brought them to the University of Illinois, USA, to measure just how much light reflects off the bees’ bodies. They found that the pale backs of large males were more reflective than those of the smaller, dark bees. Reflecting more light means that larger male digger bees heat up slower in direct sunlight and can withstand the scorching sun better than their darker, smaller counterparts. However, the researchers still didn't know what the light was reflecting off – was it the hairs on their body or their hard exoskeleton?
To answer this question, they measured the amount of light reflecting off the bees after shaving the hairs from the insects’ backs. They found that removing the hairs from both the large and small males decreased their ability to reflect sunlight. But, having paler hairs enables larger male digger bees to stay cooler on the hot ground, where they can get first access to unmated females emerging from their underground burrows. So why might smaller males have darker hairs, if being a paler colour would better help them avoid overheating? The darker hairs of the smaller males could allow them to warm up quicker by absorbing more sunlight, so they can fly and find females earlier in the cooler mornings than larger males.
Generally, researchers make predictions about how climatic warming will affect a given species. However, Barrett and O'Donnell suggest that we need to consider that not all members of the same species will respond in the same way. For example, as climate change threatens animals with warmer and more variable temperatures, large male digger bees may have an advantage when competing for mates on hotter days. Yet, small males may benefit on cooler days by having a colouration that allows them to fly earlier by absorbing more heat. The need to focus on understanding the behaviour and abilities of individual organisms is paramount, especially if we are to predict how they will fare in a changing world.
