Switchbacks and Salinity: The Effects of Environmental Degradation on Tadpole Defense Mechanisms

Over the last 25 years, climate change has progressively worsened, leading to increased salinization of freshwater habitats in North America and causing widespread degradation of aquatic environments. This degradation is often caused by increased wintertime runoff containing road salts. These environments are often home to vulnerable wildlife, such as tadpoles. Tadpoles have been found to experience higher predation rates, slower burst-speed performance, and developmental and anatomical limitations after chronic exposure to increased salinity. Tadpoles may perform switchbacks when confronted with stressful stimuli (i.e., predators), in which they enact 180° turns, potentially rushing toward a source of danger instead of away. In this study, pickerel frog tadpoles (Lithobates palustris) were collected from a New Jersey wetland and exposed to three increasing levels of salinity. Switchbacks were counted in 3-minute trials where tadpoles were forced to swim in an O-shaped apparatus after an initial, 15-minute exposure to NaCl at their test concentration. After conducting an ANOVA (Analysis of Variance) and a Tukey’s HSD (Honestly Significant Difference) test, we found that there was a statistically significant difference between some treatment groups. These results suggest that exposure to road salts and other forms of salinization may impact tadpole defense mechanisms, though further experimentation is needed.

These days, we have become perhaps too familiar with the wide-reaching effects of climate change on everything from human health to weather patterns. However, a thing often overlooked is the indirect consequences of these effects on habitats other than our own. With natural disasters and pollution on the rise, animal species across the globe are currently having to adapt and survive in some of the most extreme and unnatural conditions to date. Frog species are no exception, especially during their most vulnerable life stage as a tadpole. Tadpoles are particularly susceptible to illness and even death for a variety of reasons. To start, tadpoles emerge from jelly-like, exposed eggs that their mothers lay en masse in bodies of water, leaving them somewhat defenseless against increasing aquatic pollution.

During this next stage as tadpoles, many developmental milestones occur that ensure the survival of these future frogs, all while their skin is still quite permeable to the environment. A pollutant of concern for tadpoles in North America, what with increasing runoff, is the leaching of road salts from urban areas into local bodies of water. Road salt usage during the wintertime has increased drastically in the US in the past 50 years, leaving experts concerned about runoff in local watersheds.

In prior studies, it has been found that unnatural salinization of tadpole habitats leads to higher predation rates, slower burst-speed performance, and developmental and anatomical limitations. In this study, I aimed to analyze the impacts unnatural salinization has on a newly discovered tadpole defense mechanism: switchbacks. Switchbacks occur when tadpoles perform 180-degree turns while actively swimming, potentially turning towards a predator or other stressful stimulus.

Switchbacks have not been widely studied, so we decided to further the limited research regarding this unique aspect of tadpole defense. Prior to experimentation, Pickerel frog tadpoles, otherwise known as Lithobates palustris, were collected from Assunpink Wildlife Management Area in Monmouth County, New Jersey, from May 14th to 17th 2020. Experimentation occurred outdoors due to the constraints of the pandemic. In our study, tadpoles were exposed to a control and three different levels of salinity: 0, 0.05, 0.1, and 0.2M NaCl.

After an acute 15-minute exposure, the tadpoles were forced to swim in an O-shaped apparatus for 3-minute trials, and the number of switchbacks were counted. Tadpoles were gently prodded with a rubber spatula after each 5 second interval of inactivity during the trials to both provide a stressful stimulus and encourage continuous swimming. An ANOVA test and Tukey’s post hoc test were used to determine the statistical significance of the results. After analysis, we found that only two of the six salinity level comparisons’ switchback differences were statistically significant: 0.05-0.1M and 0-0.2M.

These results imply that salinity may have some impact on tadpole defense mechanisms, but further experimentation is needed to determine the extent. Future experimentation would benefit from performing a MANOVA analysis testing whether switchback count and lateralization are correlated and adding additional salinity levels. Special thanks to OSCAR for our funding and to Dr. Tina Bell for making this project a reality. Thanks for watching!