Cathemerality, sometimes called metaturnality, is an organismal activity pattern of irregular intervals during the day or night in which food is acquired, socializing with other organisms occurs, and any other activities necessary for livelihood are performed. It has been defined as follows: "The activity of an organism may be regarded as cathemeral when it is distributed approximately evenly throughout the 24 h of the daily cycle, or when significant amounts of activity, particularly feeding and/or traveling, occur within both the light and dark portions of that cycle."
Many animals do not fit the traditional definitions of being strictly nocturnal, diurnal, or crepuscular, often by factors that include the availability of food, predation pressure, and variable ambient temperature. As a result, many species, particularly among primates, may be classified as cathemeral.
Alternative patterns of cathemeral activity have been observed in specific lemurs. Seasonal cathemerality has been described for the mongoose lemur (Eulemur mongoz) as activity that shifts from being predominantly diurnal to being predominantly nocturnal over a yearly cycle. The common brown lemurs (Eulemur fulvus fulvus) have been observed as seasonally shifting from diurnal activity to cathemerality.
In the original manuscript for his paper, "Patterns of activity in the mayotte lemur, Lemur fulvus mayottensis," Ian Tattersall introduced the term "cathemerality" to describe a pattern of observed activity that was neither diurnal nor nocturnal. Though the term "cathemeral" was proposed, "a reviewer took exception to the introduction of what he regarded as unnecessary new jargon. The result was that the term 'diel' was substituted for 'cathemeral' in the published version." In 1987, Tattersall gave a formal definition of "cathemeral", turning to its Ancient Greek roots.
The word is a compound of two Greek terms: κᾰτᾰ́ (katá), meaning "through," and ἡμέρᾱ (hēmérā), meaning "day." The term "cathemeral," then, means through the day, with "day" meaning the full day from midnight to midnight. Tattersall credits his father, Mr. Arthur Tattersall, and Dr. Robert Ireland, two classicists, for considering this lexical problem and proposing its solution.
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