Sensitivity and evolution of sea-turtle magnetoreception: observations, modelling and constraints from geomagnetic secular variation

In a series of papers, Lohmann and Lohmann (1991, 1994a, 1994b, 1996) provide evidence for remarkable sensitivity of sea-turtles to the earth's magnetic field and suggest that it is used by these animals to determine global position and to navigate. In this paper, we emphasize that a consequence of these observations taken together is that sea-turtles should be able to accurately detect the full (vector) magnetic-field, and perhaps spatial gradients. In order to interpret these observations, we propose a simple model in which the turtle is considered as a small permanent magnet, on which the geomagnetic field exerts a torque. This torque varies as a function of turtle azimuth and field parameters which depend mainly on latitude. Although this simple model accounts for some of the observational evidence, discrepancies might be due to a number of other factors, such as speed of magnetic field changes during experiments or lack of field homogeneity. Also, the earth's field has varied significantly over the last few centuries and some of the magnetic features observed today and suggested by the Lohmanns for use in sea-turtle navigation were very different or even not present two or three centuries ago. This would place constraints on the rate at which genetically inherited magnetic behavioural preferences can change with time. Alternately, it may imply that the experimental results need to be re-evaluated and complemented.