Abstract:
Vertebrates navigate over great distances in fluid environments, which are often unfamiliar or featureless. Despite the mechanisms for compass navigation and piloting being well understood, how animals determine their current position in order to plot their course is unknown. Many different models have been proposed, but to date none have been conclusively demonstrated or gained widespread acceptance as the compass models have. I investigated a model that proposes magnetic intensity isodynamics form a global map used for navigation, specifically testing the main predictions that animal tracks will have fine-scale alignments to the magnetic field and show disorientation by anomalies for both the intensity and curvature of Earth’s magnetic field. After confirming the criticism that the published Monte Carlo method to assess the alignment of tracks to the magnetic field breaks the autocorrelation inherent in the data, I developed new randomisation methods that maintain the autocorrelation. I further improved the method by using a single biologically meaningful index window to determine alignment, instead of four arbitrary alignment windows. The new methods where applied to adult Egyptian fruit bats (Rousettus aegyptiacus) and juvenile homing pigeons (Columbia livia) in Cape Town, South Africa. The new analysis methods found some tracks significantly aligned to the magnetic field, but considerably fewer than with the original method. The bats demonstrated very strong homing behaviour even in foggy conditions when no visual cues were present. Topography did not appear to affect the bats’ behaviour, but some tree lines and roads were followed when these features aligned with the direction the bat was already traveling. The pigeons’ homing performance was poorer, likely due to their young age. Increasing the sample sizes and using adult birds may produce more conclusive results. Linear models comparing track straightness as a measure of navigation difficulty with variation in magnetic anomalies and homing experience of adult pigeons in Auckland, New Zealand, revealed some weak, but significant correlations. Track straightness was positively correlated with increasing experience. As expected, increased variation in magnetic intensity was correlated with a decrease in track straightness, however, magnetic curvature had no effect. Magnetic intensity and curvature are highly correlated to each other at most sites, which may have obscured a clear pattern if magnetic intensity stronger effect. Some evidence of magnetic effects on homing performance were found, but improvements in the methods used are required for more conclusive results.