Data - Leech removal is not the primary driver of basking behavior in a freshwater turtle

Leaving the water to bask (usually in the sun) is a common behavior for
many freshwater turtles, with some species also engaging in “nocturnal
basking.” Ectoparasite removal is an obvious hypothesis to explain
nocturnal basking and has also been proposed as a key driver of diurnal
basking. However, the efficacy of basking, day or night, to remove leeches
has not been experimentally tested. Therefore, we examined the number of
leeches that were removed from Krefft’s river turtles (Emydura macquarii
krefftii) after experimentally making turtles bask at a range of times of
day, durations, and temperatures. Turtles had high initial leech loads,
with a mean of 32.1 leeches per turtle. Diurnal basking under a heat lamp
for 3 h at ~28°C significantly reduced numbers of leeches relative to
controls. In diurnal trials, 90.9% of turtles lost leeches (mean loss of
7.1 leeches per turtle), whereas basking for 30 mins under the same
conditions was not effective (no turtles lost leeches, and all turtles
were still visibly wet). Similarly, “nocturnal basking” at ~23°C for 3 h
was not effective at removing leeches. Only 18% of turtles lost leeches
(one turtle lost one leech and another lost four leeches). Diurnal basking
outdoors under direct sunlight for 20 min (mean temp = 34.5°C) resulted in
a small reduction in leeches, with 50% of turtles losing leeches and an
average loss of 0.7 leeches per turtle. These results indicate basking can
remove leeches if temperatures are high or basking durations are long.
However, it was only effective at unusually long basking durations in this
system. Our data showed even the 20-min period was longer than 70.1% of
natural diurnal basking events, many of which took place at cooler
temperatures. Therefore, leech removal does not appear to be the purpose
of the majority of basking events.