Biological rhythms

Endogenous rhythms allow most organisms to synchronize their behavior and physiology with physical cycles that vary on a daily, lunar or annual cycle. Populations within species often show variation in the timing of functionally identical rhythms. This variation occurs because physical cycles may differ with geography. The purpose of this study was to determine whether hatching rhythms shown by fiddler crabs (Genus Uca) on one coastline could be entrained by the different tide patterns present at another coastline. To test this I transferred breeding females (Uca thayeri) from mangroves on the west coast of Florida to mangroves on the east coast. On the west coast, females are exposed to "mixed" tides; most release their larvae during the day or night (early summer), or during the day (mid- to late summer). On the east coast, females are exposed to "semidiurnal" tides; they release their larvae between dusk and midnight. After four weeks of exposure to the East Coast tides, crabs from the West Coast showed hatching rhythms identical to the resident crabs. This change indicates that the crabs show behavioral (phenotypic) "plasticity". These observations provide further evidence for the adaptive value of behavioral plasticity.

In this thesis the transition region between two modes of behavior is explored using a novel technique, delayed feedback, and a variety of dynamical systems measures. In a previous study, Engstrom, Kelso, and Holroyd (to appear) established the existence of a transition between anticipatory and reactive behavior in a sensorimotor coordination task as a control parameter (frequency) was varied. Here, in order to explore the hypothesis that the behavioral dynamics during this transition are intermittent in character, subjects were asked to synchronize with a metronome that was actually a delayed copy of their own response pattern. The use of delayed feedback was expected to destabilize the behavioral dynamics enough to allow the observation of hypothesized intermittent phenomena. Use of delayed feedback was shown to destabilize synchronization, resulting in the emergence of a new behavioral pattern in the transition region that exhibited complex "bursting" dynamics. Analysis revealed that this bursting behavior displays many of the characteristics common to intermittency, which supports the idea that the anticipation-reaction transition is the result of a neurobehavioral dynamical system losing stability. Living in the vicinity of instabilities may be an important mechanism for biological organisms to maintain both flexibility and stability.