Latitudinal gradients of biologically useful semi-darkness

Biological rhythms and activity patterns are strongly influenced by light levels, involving both the sun and the moon. The Earth's orbit around the sun, the moon's orbit around the Earth, and the intersecting angles of planes of orbit and of rotation conspire to create temporal variability in those light levels. Organisms that are preferentially active in the semi-darkness of twilight and moonlight must cope with these patterns of variability and are presumably adapted for the constraints and opportunities they present. Using field results for activity patterns of a model species (whip-poor-will: Aves: Caprimulgidae: Caprimulgus vociferus), I define biologically useful semi-darkness by reference to the position of the sun and the percentage of moonface illuminated. For such biologically useful semi-darkness, I then calculate latitudinal gradients and the temporal patterns of variation in those gradients. The substantial variability in night-time illumination contributed by the lunar cycle is largely independent of latitude, except at high latitudes during periods when there is little or no dark night. The total annual duration of semi-darkness generated by both sun and moon increases modestly with latitude. However, the seasonal variation in the distribution of that semi-darkness increases dramatically over the same gradient. At mid and high latitudes, the availability of semi-darkness is least around the summer solstice and greatest around the winter solstice, patterns that are likely to influence the phenology and related behaviours of the organisms experiencing them.