A novel method to study the ecological role of sleep in small mammals.

Sleep, is a complex, vital, and universal behavior that strongly differs from mere inactivity. Its ecological role remains, however, largely unknown mostly owing to the lack of methodological tools to record animal sleep states in the wild. By using a small, low power consumption biologger, capable of recording brain activity, body movements, and core physiology, we were able to record and quantify key sleep parameters (circadian distribution, sleep stages and their fragmentation, …), in wild black rats (Rattus rattus) in their natural environment over multiple days. We developed a simple, rapid (<1h), surgical procedure using a custom subdermal flexible electrode that provides signal quality equivalent to the cortical electrodes classically used in lab experiments. We also validated a semi-captive procedure, where the animals could be recorded in their own environment, with ad libitum access to standardized food pellets, and contact with conspecifics without close interactions. Such a protocol allows for the direct investigation of biotic and abiotic factors, like social interactions, food availability or type, light, temperature, and stress, all of which may strongly impact sleep. By evaluating general behavior and sleep patterns in four wild rats over up to ten days after surgery and by tracking feeding behavior for over a month, we show that the animals do not display any obvious signs of pain or stress and stabilized their sleep patterns two days after manipulation. Altogether this novel method and procedure constitute a unique tool for assessing sleep variability and flexibility and provides a proof-of concept for sleep studies in small (<200 g) wild animals.