Deer mice (Peromyscus maniculatus) are versatile little critters, making their homes in the mountains, prairies and forests of north America, adjusting their diet to the seasons as seeds and vegetation come and go. But how will the adaptable little rodents cope as global temperatures continue inching upward? ‘Few studies of endotherms [warm-blooded animals] have examined how performance capacities vary across a range of warm temperatures’, says Graham Scott, from McMaster University, Canada, explaining that researchers have largely focused on the effects of climate change on ectotherms (cold-blooded creatures) until now. To find out how warmer climes will impact deer mice, Matthew Eizenga, Luke Flewwelling, Tanisha Warrier (all from McMaster University) and Scott set the rodents a running challenge, to find out how long they could keep running as the mercury climbed.
‘We wanted to control for potential differences in early life conditions, so we caught wild mice and brought them into the lab to raise offspring in controlled conditions’, says Scott. When the youngsters were 6–18 months old, the team gently inserted a minute temperature sensor just beneath their skin, to record their body temperature, and began training the animals to run on a mini treadmill at 20°C. ‘The mice would initially try to avoid running by suspending themselves from the sides of the treadmill chamber – they're pretty smart little animals – but they got with the programme after a couple of training runs’, he chuckles. Once the mice had found their feet, the team increased each rodent's running speed gradually, keeping track of how much time the animal spent running until the tiny athlete could run no more; some were stronger runners than others, keeping going for an impressive 139 min, compared with the less athletic rodents, which only ran for 45 min.
Then the team set each mouse a final test – to run for as long as possible at one of six temperatures, ranging from 15 to 38°C – and this time the air temperature really affected the animals’ endurance. While the mice seemed most comfortable running at 25°C, continuing for ∼130 min, at warmer temperatures they began to struggle and when the temperature hit 38°C they only managed ∼32 min on the treadmill. Also, when the team checked the runners’ body temperature, the mice running at 38°C were 3.9°C warmer (with a body temperature of 41°C) than the mice running in milder conditions. Additionally, the body temperature of the mice in the hottest conditions continued rising until the animals were exhausted, in contrast to the mice on the 15–35°C treadmills, whose body temperature stabilised after rising initially. The team then checked how the mice dealt with warmth while relaxing, by raising the temperature in their chamber, and found that the deer mice started breathing harder and losing more water to keep cool as the air temperature rose to 36°C and above.
But what does all this mean for deer mice roaming free? ‘Mice do a lot of running in their daily lives to explore, find food and avoid predators, so harmful effects of warming temperatures on their ability to run could be a big problem that might impact their health and fitness’, says Scott. But he is optimistic that the resilient little rodents will cope because they are mainly active at night, when temperatures are generally cooler. Although, he points out that night-time temperatures remain high during heatwaves, so the animals may still be in danger, and deer mice that live in poorly insulated shallow burrows could be at risk on hot days.
