Circadian Rhythms and Masking

As the earth spins on is axis the environments of animals change in dramatic but predictable ways. Animals respond directly to ambient changes (e.g. in light intensity) via a process referred to as “masking,” but they can also anticipate them with the help of an internal timekeeping system that includes a network of self-sustaining “circadian” oscillators that generate rhythms of approximately 24 hours. These systems have changed over the course of evolutionary time enabling animals to move from one temporal niche to another, and they can change within an animals’ lifespan as, for example, when the intensity of light increases in the spring and decreases in the fall. Over the course of evolutionary time, as mammals first descended from their reptilian ancestors, they underwent a remarkable transformation involving the emergence of many novel adaptations. One of the key changes that made possible the radiation of mammals was the transition from a diurnal to a nocturnal pattern of adaptation to the day-night cycle. Diurnality, however, has resurfaced in a variety of independent mammalian lineages and, at each of these transitions, changes occurred in the biological timekeeping mechanisms that coordinate a far-ranging suite of behavioral and physiological processes. One main question being addressed in our group is that of how the neural mechanisms underlying these adaptations differ in nocturnal and diurnal species.

Temporal niche transitions and the evolution of a sensory brain

As animals moved from one temporal niche to another it was not just the timekeeping systems that changed. A range of adaptations supporting activity in a cold/dark world vs. a warm/bright one diverged as well. The light provided by the sun can provide information to diurnal animals that is simply not available to nocturnal ones, but only if they have the brain mechanisms able to extract that information. However, brain tissue is energetically “expensive” which raises questions about the extent to which tradeoffs in investment into the varying sensory systems were made. One set of studies being undertaken here involves examination of evolutionary patterns of change in size and shape of brain structures dedicated to different sensory modalities.

Seasonal Rhythms

Species from temperate zones experience considerable seasonal variation in their environments. Accurate prediction of seasonal changes in food availability, temperature, weather, or predator activity is crucial for survival of many species. The most reliable environmental cue that animals use to predict the flow of the seasons is the progressive changes in day-length (photoperiod). The SCN serves not only as a daily clock but also as coordinator of seasonal time, through its ability to encode day-length.We are interested in understanding the mechanisms underlying day-length encoding by the SCN, and the subsequent changes in the SCN targets in both nocturnal and diurnal species.

In humans, there is evidence that seasonality exists on a variety of traits, and that these seasonal changes are driven by photoperiod. Some of us show seasonal changes in physiology and behavior, but for a few individuals these changes are extreme enough as to be considered a form of seasonal depression or seasonal affective disorder (SAD). Knowledge derived from work with photoperiodic animal models is informing therapeutic interventions for SAD patients and is providing insights about milder forms of seasonal cycles in our species.