By: Mallory Crispens, Sarah Faley, and Lilly Leach (Stonehill College, Bio323: Evolution Spring 2024)
Overview
One of the most beneficial characteristics for survival in the natural world is the ability to adapt to changing environments. In the face of climate change and increasing urbanization of rural areas, it has become even more important for wildlife to be able to adapt and endure increasing temperatures. Urban heat islands are a significant consequence of urbanization, occurring when cities replace natural land cover with dense concentrations of pavement, buildings, and other surfaces that absorb and retain heat. These heat islands create a brand-new environment for wildlife, particularly for pond-dwelling animals such as tadpoles whose aquatic environment is sensitive to change. While most organisms living in urban environments have elevated tolerance to heat, the thermal adaptations of aquatic organisms are relatively unknown. In the article, “Tadpoles Develop Elevated Heat Tolerances in Urban Heat Islands Regardless of Sex”, Dr. Bokony analyzes tolerance to increasing temperature, as a result of urbanization, in tadpoles.
The Research
The researchers tested the Critical thermal maxima (CTmax), a thermal indicator of how much heat an individual can withstand, in tadpoles originating from two different environments, woodland and urban ponds. The tadpoles used in this experiment were selected from these two ponds, as they differ in temperature. Urban ponds are found to be relatively warmer than woodland ponds due to the effects of urbanization. The researchers collected three cohorts of Agile Frogs, Rana Dalmatina, from 6 locations: 3 woodland pond habitats and 3 urban pond habitats. Preliminary temperatures were measured in the previous year, every 30 minutes from April 6th to June 8th, 2022. For the first cohort, they collected 144 eggs from both urban and woodland ponds. In the second cohort, they collected another 144 embryos from both urban and woodland ponds. Finally, in the third cohort, they collected 102 free-living tadpoles. The smaller sample size in this cohort was due to a lack of an absence of tadpoles in one of the urban ponds, and insufficient number of tadpoles found in a woodland pond (6 individuals collected compared to 24 in the other two cohorts). The eggs and embryos collected in the first two cohorts were raised in the laboratory in modified tanks set to mimic the urban and woodland environment temperatures and day length. CTmax was measured randomly in groups of eight within each cohort. The water temperature was raised to 0.6 Celsius per minute, and after twenty-one minutes, the tadpoles were observed by prodding them every six seconds at the base of their tail and CTmax was collected. CTmax was defined as the temperature at which the tadpole failed to respond to motion after three prods to the base of its tail.
The Results:
The researchers found urban ponds to be consistently warmer than woodland ponds, which supported their original hypothesis about urban heat islands. Due to this heat island effect, the researchers found a statistically significant increase in CTmax for the tadpoles originating in urban ponds compared to tadpoles originating from woodland ponds. This study also suggests that individual phenotypic plasticity, the ability for genotypes to produce different phenotypes in an individual, is one of the main drivers of modifications of CTmax as opposed to generational adaptations. The researchers theorize that this is because of the relatively long generation time in tadpoles which doesn’t allow for quick enough evolutionary change in response to the rapid changes in the aquatic environment. The researchers also found no significant difference in CTmax between males and females, however, due to physiological differences between males and females more studies will be needed to further evaluate possible differences in response to changing environments.
Figure 1: Critical thermal maximum (CTmax) of frog tadpoles originating from urban and woodland ponds raised from the egg, embryo, or tadpole stage. https://link.springer.com/article/10.1007/s11692-024-09626-7
The Consequences:
This phenotypic response, studied by Dr. Bokony, is important for several reasons. In woodland ponds, temperatures occasionally spike high enough that they approach or exceed the thermal limits of aquatic organisms. This is even more exaggerated in urban ponds due to urban heat islands. These heat events are expected to become more frequent with gradually changing climates making this adaptation that much more critical to the survival of tadpoles in both urban and woodland habitats. The upper limit of this phenotypic adaptation is something to consider and research further as temperatures change, including sex-based differences and tolerance at different life stages.
Original Article:
Bókony, Veronika, et al. “Tadpoles develop elevated heat tolerance in urban heat islands regardless of sex.” Evolutionary Biology (2024): 1-8. https://doi.org/10.1007/s11692-024-09626-7