Behavioral and Geophysical Factors Influencing Success in Long Distance Navigation

Many animals can sense the earth’s magnetic field and use it to perform incredible feats of navigation; however, studying this phenomenon in the lab is difficult because behavioral responses to magnetic cues can be highly variable. My Ph.D. research attempts to fill this knowledge gap in the following ways: we first explore potential sources for this variability, including both natural and artificial sources of noise. We then examine the ways in which these natural sources of noise could be used to study magnetoreception in animals that are not feasible to study in the laboratory. Finally, we propose a possible solution for how navigating animals may overcome noise to still accomplish highly accurate migrations. Chapter 1 contains the relevant background and introduction. In Chapter 2, we conduct a synthetic review of natural and anthropogenic sources of radio frequency electromagnetic noise (RF) and its effects on magnetoreception. Anthropogenic RF has been shown to disrupt magnetic orientation behavior in some animals. Two sources of natural RF might also have the potential to disturb magnetic orientation behavior under some conditions: solar RF and atmospheric RF. In this review, we outline the frequency ranges and electric/magnetic field magnitudes of RF that have been shown to disturb magnetoreceptive behavior in laboratory studies and compare these to the ranges of solar and atmospheric RF. Frequencies shown to be disruptive in laboratory studies range from 0.1 to 10 MHz, with magnetic magnitudes as low as 1 nT reported to have effects. Based on these values, it appears unlikely that solar RF alone routinely disrupts magnetic orientation. In contrast, atmospheric RF does sometimes exceed the levels known to disrupt magnetic orientation in laboratory studies. We provide a reference for when and where atmospheric RF can be expected to reach these levels, as well as a guide for quantifying RF measurements.

In Chapter 3, we explore how these natural sources of noise may allow us to study magnetoreception in animals that are not feasible to study in the laboratory. Although it is difficult to perform behavioral experiments on baleen whales, it may be possible to use live stranding data (strandings that indicate the whale may have made a navigational error, rather than those having died at sea and washed ashore) as a tool to investigate the cues they use while navigating. Here we show that there is a 2.1-fold increase in the likelihood of a live gray whale (Eschrichtius robustus) stranding (n=186) on days with a high sunspot count than on low sunspot days (p<0.0001). Increased sunspot count is strongly correlated with solar storms – sudden releases of high-energy particles from the sun which have the potential to disrupt magnetic orientation behavior when they interact with earth’s magnetosphere. We further explore this relationship by examining portions of earth’s electromagnetic spectrum that are affected by solar storms and found a 3.7-fold increase in the likelihood of a live stranding on days with high solar radio flux (RF) as measured from earth (p<0.0001). One hypothesized mechanism for magnetoreception, the radical-pair theory, predicts that magnetoreception can be disrupted by RF radiation, and RF noise has been shown to disrupt magnetic orientation in certain species. To our knowledge, this is the first evidence that provides support for a specific magnetoreception mechanism in whales.

Finally, in Chapter 4, we propose a mechanism for how magnetoreceptive animals may overcome noise to perform incredibly accurate migrations. Many animals use the geomagnetic field to migrate long distances with high accuracy; however, research has shown that individual responses to magnetic cues in the laboratory can be highly variable. Thus, it has been hypothesized that magnetoreception alone is insufficient for accurate migrations and animals must either switch to a more accurate sensory cue or integrate their magnetic sense over time. Here we suggest that magnetoreceptive migrators could also use collective navigation strategies. Using agent-based models, we compare agents utilizing collective navigation to both the use of a secondary sensory system and time-integration. In our models, by using collective navigation alone, over 70% of the group is still able to successfully reach their goal even as their ability to navigate becomes extremely noisy. To reach the same success rates, in our models, a secondary sensory system must provide perfect navigation for over 73% of the migratory route, and time integration must integrate over 50 time-steps, indicating that magnetoreceptive animals could benefit from using collective navigation. Finally, we explore the impact of population loss on animals relying on collective navigation. We show that as population density decreases, a greater proportion of individuals fail to reach their destination and, in our models, a 50% population reduction resulted in up to a 37% decrease in the proportion of individuals completing their migration. We additionally show that this process is compounding, eventually resulting in complete population collapse.