The concept of cathemerality: history and definition

During a field study in 1974 it was noticed, and further fieldwork in 1977 and 1980 confirmed, that the activity of the Mayotte lemur, Eulemur fulvus fulvus, is distributed fairly evenly throughout the daily 24-hour cycle: by a very crude approximation the daytime activity:rest ratio averages 0.271, while the night-time ratio averages 0.283. In 1978 I proposed the new word 'cathemeral', compounded from the Greek roots kappaalphataualpha (through) and etamuepsilonrhoalpha (the day), to describe this unusual activity pattern, though for reasons described in this article the term was not formally published until 1988. Since then cathemeral activity has been routinely recorded, with some variation, in species of the strepsirhine genera Eulemur and Hapalemur; and in one form or another it apparently also occurs, at least occasionally, among platyrrhines (species of Aotus and Alouatta) as well as among a variety of non-primate mammals. It may well thus be a more widespread activity pattern than earlier appreciated.