Wild clocks: preface and glossary

Abstract

The critical significance of biological timing and timekeeping is well appreciated by both chronobiologists and ecologists, and historically the two fields were linked early on (e.g. [1–3]). Sixty years ago, a diagram appeared in a book of papers from the Fifteenth Symposium for the Study of Development and Growth (figure 1; [4]) that set the stage for future research on biological time. This schematic—with daily rhythmicity generated by a ‘clock’ composed of one or more ‘endogenous self-sustained oscillators’ (ESSOs) entrained by 24-h rhythms of light and temperature—became a blueprint for research by many chronobiologists on the mechanisms of internal timekeeping within organisms. Since then, our mechanistic understanding of daily and annual timing has blossomed, now encompassing details at the molecular, cellular, tissue and organismal levels. Figure 1 also included an input from ‘residual periodic variables’ (RPVs); these were meant to represent abiotic factors such as ‘pressure, humidity, air ionization, cosmic ray showers’ [4], but could also pertain to biotic factors such as food availability, predators, competitors and mating opportunities. Since then, ecologists have demonstrated the importance of daily and annual timing for individual fitness, with deviations from optimal timing possibly resulting in reduced foraging success, survival and reproductive output. Figure 1. Block diagram of an entrained field rhythm (FR). NP, natural period. To some extent, over the next decades the research programmes of the two fields became non-overlapping, with chronobiologists focusing on unravelling the endogenous clock machinery and ecologists addressing the functional significance of timing in nature. Both by necessity and design, mechanistic work mostly has been conducted using a limited number of model organisms, each living in isolation, housed under standard (and, except for …