Diurnal basking (sunning) is common in many ectotherms (cold-blooded animals) and is generally thought to be a behavioral mechanism for thermoregulation. Recent studies have reported the occurrence of nocturnal basking in a few distantly-related species of freshwater turtles, but the true extent of this behavior is unknown. Therefore, scientists from La Trobe University and elsewhere initiated a global, collaborative effort to systematically document and quantify diurnal and nocturnal basking activities across a wide range of freshwater turtle species and locations. They conducted camera trap or manual surveys in North America, the Caribbean, Europe, Asia, Africa, the Seychelles, and Australia.
Diurnal (A-C) and nocturnal basking (D-F) in freshwater turtles: (A) and (D) Emydura macquarii plus Crocodylus johnstoni in Australia; (B) Pelomedusa galeata in South Africa; (C) several Emydidae species in the United States; (E) Chelodina expansa in Australia; (F) Pelusios adansonii in Senegal. Image credit: Eric Nordberg / Cormac Price / Brad Glorioso / Malcolm Kennedy / Pearson McGovern.
“We first observed freshwater turtles nocturnal basking at the Ross River in Townsville, Australia,” said Dr. Donald McKnight, a postdoctoral researcher at La Trobe University.
“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.”
“We think it’s related to temperature,” he added.
“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.”
“It’s widespread across the turtle family tree, with the caveat that it is only in the tropics and the subtropics where it occurs.”
For the study, Dr. McKnight and his colleagues collected 873,111 trail camera photographs (25,273 hours of search effort) and obtained data on 29 species of freshwater turtle representing seven of the 11 living freshwater turtle families.
They collected data from Africa, Asia, Australia, Central America, Europe, North America, the Seychelles, and Trinidad and Tobago (in the Caribbean); however, most of the data came from Australia and North America.
In most cases, a minimum of three wildlife trail camera traps, set to time-lapse mode (two-minute photograph interval) with an infrared flash, were positioned at different basking structures (e.g., rocks, logs, muskrat lodges, artificial basking platforms, or riverbanks) within an area (river, series of ponds, etc.) to record basking behaviors for seven consecutive days.
Some level of nocturnal basking was documented in 13 of the 29 freshwater turtle species sampled, belonging to six (Chelidae, Emydidae, Geoemydidae, Kinosternidae, Pelomedusidae, and Trionychidae) of the seven sampled families.
Nocturnal basking was confined to tropical and a few sub-tropical sites, and nocturnally basking species were found significantly closer to the equator than species that did not bask at night, regardless of whether captive populations were or were not included.
When considering all nocturnal basking events, most events were short (around 22 min), but there were numerous long events.
“Nocturnal basking was restricted to tropical and sub-tropical locations, suggesting that environmental temperature plays a role in this behavior,” the authors said.
“However, the primary factors driving nocturnal basking are yet to be determined and may vary geographically and by species.”
“The frequency and duration of nocturnal basking varied among species and seasons, but nocturnal basking events were often substantially longer than diurnal events.”
“This study is simply the first critical step in understanding this behavior, and now that it has been widely documented, more focused research can be conducted to better understand the factors that influence it.”
“These initial results suggest that nocturnal basking is likely an overlooked aspect of the ecology of many species and given that it appears to be influenced by environmental temperatures, it may also have implications for turtles’ persistence and behavioral changes under climate change.”
The study was published in the journal Global Ecology and Conservation.
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Donald T. McKnight et al. 2023. Nocturnal basking in freshwater turtles: A global assessment. Global Ecology and Conservation 43: e02444; doi: 10.1016/j.gecco.2023.e02444
