Turtles Around the World Are Basking in Moonlight

While the rest of us are sleeping, freshwater turtles from Central America to Asia are slinking out of the water and basking at night. There have been occasional anecdotal reports of this behavior before, but now a new study is the first to document the widespread occurrence. The findings suggest that nocturnal basking "may be a common and almost entirely overlooked aspect of many species’ ecology," write the authors.

The study was led by La Trobe University and published in Global Ecology and Conservation. In it, the authors document nocturnal basking in six families of freshwater turtle species, occurring in Central America, Trinidad and Tobago, Seychelles, Africa, Asia, and Australia.

Postdoctoral Researcher at La Trobe University Dr. Donald McKnight said he and a colleague, Dr. Eric Nordberg from the University of New England, first observed freshwater turtles nocturnal basking at the Ross River in Townsville, Australia.

“We think it's related to temperature. The water is staying so warm at night that it's actually warmer than the turtles like to be and they can cool down by coming out of the water,” says McKnight.

“It’s widespread across the turtle family tree, with the caveat that it is only in the tropics and the subtropics where it occurs,” he added. “They were coming up at night and sitting on logs exhibiting very much the same behavior they do during the day; when we looked into it, it wasn’t something that turtles reportedly did."

For the study, researchers from around the world did what anyone would do when wondering what turtles do at night: They put cameras on basking logs to monitor the nocturnal activity of as many freshwater turtle species as possible.

The cameras were set up in 25 locations across Australia, Belize, Germany, India, Seychelles, Senegal, Trinidad and Tobago, the United States, and South Africa. They were programmed to take a photo every two minutes; they captured data on 29 species from seven of the freshwater turtle families.

It would be lovely to think the turtles are just enjoying some leisure time when things are quiet and the diurnal world is asleep. But the researchers note that in most cases, the turtles emerged from the water when the water was too warm and the air was cooler. Given the warming planet, that's admittedly a bit depressing. However, not all of the nighttime baskers were escaping too-warm water. From the study:

"... [T]urtles in India exhibited more nocturnal basking on cooler nights, rather than warmer nights, and P. adansonii in Africa spent more time basking nocturnally in winter than in summer. The reasons for these differences are unclear. It may be that at some sites or seasons, turtles are escaping unfavourably warm water temperatures, while at others, they are taking advantage of the warm tropical air to increase their body temperature and escape unfavourably cold water."

"Alternatively," the authors add, "at some locations, nocturnal basking may not be directly for the purpose of thermoregulation, but the higher night-time temperatures in the tropics may allow turtles to emerge for other purposes, such as predator avoidance."

Regardless of the "why" behind different species' reasons for midnight basking, it's a fascinating look at behavior not previously documented by scientists. And it is at least heartening to know that turtles are figuring out important behaviors tied to thermoregulation. As the study concludes, this "could have implications for turtles’ persistence and behavioural changes under climate change."

The study "Nocturnal basking in freshwater turtles: a global assessment," was published in Global Ecology and Conservation.