A sweaty brow, dry lips and unslakeable thirst – we all know what it's like to swelter through a hot summer. While we can ease our discomfort by switching on an air conditioner or visiting an ice cream shop, animals don't have that luxury. They can, however, change their activity times to avoid high temperatures. This can be as simple as staying hidden during the middle of a hot day or, at the other extreme, changing tack and switching to being active at night. It was this latter strategy that interested Ofir Levy from Tel Aviv University, Israel, and his collaborators, so they set about examining the phenomenon of ‘temporal niche switching’, in the context of surviving increased temperatures associated with climate change.

In general, a warming climate will reduce how much food mammals and birds need to eat. However, daytime water loss will increase under hotter conditions, making the night an attractive option for day-active animals. In fact, saving water is probably the most important factor in stopping animals from overheating. Levy and his team identified three factors with the potential to drive animal activity patterns – heat from the environment, access to shade and the availability of water – and examined how changing the time of day when they're active may help animals cope with the impacts of climate change.

The researchers built a computer simulation looking at how climate change may cause a shift in the activity patterns of golden spiny mice. They expected that a shift to a nocturnal lifestyle would make life easier for the tiny rodents by reducing their energy expenditure and water loss. Dialling their calculations 80 years into the future as the temperature increases, they found that nightly energy expenditure of the mice decreased and their daily water costs increased. Additionally, the team realised that the amount of time when it will be cool enough for the animals to be active during the day will decline, and the amount of shade provided by vegetation, as well as free water to drink, are also predicted to decrease. This could spell trouble for day-active mice, as individuals that are unable to meet their daily energy demands will need to compete over scarcer resources, find new areas to live and may even be in danger of dying out in areas where it's simply too hot to trot. Changing to a nocturnal lifestyle seems like a no-brainer.

Unfortunately, switching from a day- to night-active lifestyle will not solve all of the mice's problems. Switchers must compete with animals that have better night vision, be aware of nocturnal predators and have the ability to stay warm when active by moonlight, instead of sunlight. To top things off, climate change is probably happening too quickly for switchers to completely adapt to the nightlife, or for those that stay day-active to get used to the daytime heat.

Levy and his colleagues leave us with the thought that time – whether minutes, days or millennia – is an ecological resource similar to space. They argue that the importance of time is under-appreciated and that time use is likely to be impacted by climate change: animals will need to shift when they go about their daily business as temperatures rise. Changing activity from day to night may help species endure a warmer world but more research is still needed to understand the breadth of this potential. Fortunately, thanks to the team's work, we now know that some species could go into the night to save the day.

W. P.
Time and ecological resilience: can diurnal animals compensate for climate change by shifting to nocturnal activity?
Ecol. Monogr.