The role of the biological clock in the sun compass orientation of free-running individuals of Talitrus saltator

We used a 'variable-drift' paradigm to investigate the relation between the angle of solar orientation and the circadian rhythm of locomotor activity in Talitrus saltator. We recorded the circadian rhythms of individual sandhoppers in constant darkness before testing them at a set time during the day. Because of differences in the free-running period between individuals, they were thus tested at different phase points in their circadian cycle, so that the angular measurements were distributed across the subjective scotophase and photophase. To investigate the association between sun orientation angle and circadian time we used circular correlation statistics, and constructed circular-circular regression models for comparison with the biometric data. The sun orientation angles were strongly dependent on time of day, suggesting that orientation and locomotor activity are regulated by the same time-keeping system. The model fitting the experimental data best was a sinusoidal one, and the distribution of angular orientation predicted on this basis followed closely that of the nonparametrically smoothed data. During the day this followed the arc of movement of the sun, with a phase delay of about 90° and the sign of the angle of orientation reversed around 1200 and 2400 hours. We interpret the data in terms of a mean angular velocity register and discuss their ecological significance.