Magnetic field distortions produced by protective cages around sea turtle nests: unintended consequences for orientation and navigation?

Abstract

The Earth’s magnetic field plays an important role in the orientation and navigation of sea turtles. Galvanized steel wire cages are often placed over turtle nests to protect them from predators, but the material typically used in cages has a high magnetic permeability and might therefore affect the nearby field. Here we report magnetometer measurements indicating that standard nest cages do indeed significantly alter the local magnetic field in the area where eggs develop. The mean change in total intensity was 26% at a level corresponding to the top of the egg chamber and 5% at a level corresponding to the bottom. Similarly, the mean change in field inclination was 20% for the top level and 4% for the bottom. In principle, the altered magnetic environment might affect subsequent magnetic orientation and navigation behavior in several ways, although whether turtles that develop in an unnatural magnetic field actually suffer navigational impairment has not yet been studied. Constructing protective cages out of magnetically inert materials provides a way to deter predators without risking unintended behavioral consequences of distorting the ambient field.

Introduction

The Earth’s magnetic field plays an important role in the orientation and navigation of sea turtles (reviewed by Lohmann and Lohmann, 1998, Lohmann and Lohmann, 2003). Hatchling loggerheads have a well-developed magnetic compass that enables them to maintain consistent headings in the absence of other cues (Lohmann, 1991; Light et al., 1993; Irwin and Lohmann, 2003), an ability that presumably helps guide them offshore during their initial seaward migration (Lohmann and Lohmann, 1996a). In addition, sea turtles are sensitive to small differences in field inclination (Lohmann and Lohmann, 1994) and intensity (Lohmann and Lohmann, 1996b) and can exploit the regional fields that exist in different locations along their migratory route as navigational markers (Lohmann et al., 2001). Finally, it has been hypothesized, though not yet demonstrated, that young turtles imprint on the magnetic fields that mark their natal beaches and use this information to return to the same region as adults (Lohmann et al., 1999).

Despite the prominent role that magnetism appears to play in the sensory world of sea turtles, little thought has been given to how anthropogenic influences in general, and conservation practices in particular, might disrupt their natural magnetic environment. A first step toward assessing possible risks is to identify circumstances in which turtles are exposed to unnatural fields that might interfere with the development of normal magnetosensory abilities or prevent the detection of important magnetic information.

A common conservation practice on many sea turtle nesting beaches is to cover turtle nests with galvanized wire mesh cages or screens to protect the eggs from raccoons, foxes, and other predators (Addison and Henricy, 1994; Jordan, 1994; Ratnaswamy et al., 1997; Yerli et al., 1997; Kinsella et al., 1998). Here we report, however, that wire mesh cages significantly alter the magnetic field around the eggs and developing embryos. Whether such field distortions affect the development of the turtles’ navigational system is not yet known. In principle, however, the field changes produced by cages might produce effects on subsequent orientation ability and navigational performance, particularly if turtles do indeed imprint on the magnetic features of their natal beaches. The results underscore the need to consider carefully the sensory biology of animals when designing conservation practices.