Sex steroids modulate circadian behavioral rhythms in captive animals, but does this matter in the wild?

Nearly all organisms alter physiological and behavioral activities across the twenty-four-hour day. Endogenous timekeeping mechanisms, which are responsive to environmental and internal cues, allow organisms to anticipate predictable environmental changes and time their daily activities. Among-individual variation in the chronotype, or phenotypic output of these timekeeping mechanisms (i.e. timing of daily behaviors), is often observed in organisms studied under naturalistic environmental conditions. The neuroendocrine system, including sex steroids, has been implicated in the regulation and modulation of endogenous clocks and their behavioral outputs. Numerous studies have found clear evidence that sex steroids modulate circadian and daily timing of activities in captive animals under controlled conditions. However, little is known about how sex steroids influence daily behavioral rhythms in wild organisms or what, if any, implication this may have for survival and reproductive fitness. Here we review the evidence that sex steroids modulate daily timing in vertebrates under controlled conditions. We then discuss how this relationship may be relevant for the reproductive success and fitness of wild organisms and discuss the limited evidence that sex steroids modulate circadian rhythms in wild organisms.

The World Is Not Flat: Accounting for the Dynamic Nature of the Environment as We Move Beyond Static Experimental Manipulations

Although we have long understood that environmental variation affects both physiology and behavior, historically, most studies have limited or simplified environmental variation to focus more directly on traits of interest. Recently, a number of investigators have turned their focus toward attempting to incorporate such variation into studies of physiology and behavior, and not surprisingly, are finding that the results from studies that include more realistic variation, both from the environment as well as in physiological processes within individuals, can differ substantially from those of studies that attempt to hold the parameters constant. Understanding the role that this dynamic variation plays in shaping phenotypes is critical given that, under most predictions from future climate change models, increased variability in factors such as temperature and rainfall are predicted.