The Common European Toad, Bufo bufo, is a pond-breeding amphibian found throughout Europe, including in urban environments. Ecological changes (e.g., predator communities) in anthropogenic habitats present novel challenges to the common toad, but abiotic changes (e.g., pollution) may also influence phenotypes in these landscapes. Nevertheless, we know relatively little about how these urban-dwellers (and specifically their toxins) are affected by anthropogenic change. Veronika Bókony and colleagues set out to change that with their 2019 paper “Toads phenotypically adjust their chemical defences to anthropogenic habitat change” in which they investigated how the morphology and chemical defenses of the common toad varied in natural, agricultural, and urban habitats across Hungary.
A Toxic Response
The common toad and related Bufonid species synthesize potent toxins, which they store and secrete from large glands on the sides of their heads known as the parotoid glands. These toxins act as a powerful predator deterrent — would be predators are treated to a bitter tasting, nausea inducing compound, which (if they’re lucky) convinces them to quickly abandon any thought of eating the toad. Should the predator persist in its unwise venture of toad consumption, the powerful bufaldienolides — bufagenins and bufotoxins — will cause cardiac arrest (and death) of the predator. In natural habitats, these toads are known to alter the production of these toxins depending on ecological conditions, so it is reasonable to suspect that in urban habitats they may do so as well in response to differences in competitors, predators, or resources. Moreover, exposure to agricultural pollutants has been shown to experimentally induce increased bufadienolide production in B. bufo, providing a second mechanism by which the production of these toxins might be altered in urban habitats.
More Toxic Urban Toads
Bókony and team captured 166 adult toads and measured the size of the parotoid glands as a representative measure of the amount of toxins produced by the toads as well as the chemical composition of the toxic secretion. They found that adults from both urban and agricultural habitats had larger parotoid glands, suggesting they store more toxins compared to toads from natural habitats. They also found that the toxin secreted from the glands of toads from urban and agriculture habitats had higher concentrations of bufagenins — the more potent of the two toxins. Interestingly, urban toads also had lower concentrations of the less toxic bufotoxins.
To determine if these differences were attributable to heritable changes or if they were environmentally induced, the researchers also conducted a common-garden rearing experiment. They housed the 166 toads they captured in their laboratory and allowed them to spawn with members of their respective populations. They then raised the offspring in common conditions until they were 5 months post-metamorphosis. They found that the differences observed in the wild were not maintained in the common-garden populations, but rather that toadlets from agriculture habitat parents actually had smaller parotoids and lower amounts of bufotoxins! Urban toadlets on the other hand had similar sized parotoid glands and similar amounts of toxins as the natural habitat toadlets. This finding suggests that the morphological and chemical changes observed in the wild are plastic in nature.
Adaptive Toxins?
Changes in toxin production in anthropogenic habitats may be adaptive or non-adaptive responses. On one hand, altered toxin production may be an adaptive response to the novel selection pressures and challenges of city life. We’ve seen this before, for example, in white clover that have reduced hydrogen cyanide in urban habitats. In toads, these changes might arise in response to altered ecological communities in anthropogenic habitats that place shifted demands on predator defense versus competitive or reproductive success. If predation pressures are elevated in anthropogenic habitats, toads that produce more toxins and more potent toxins would have a survival advantage. Alternatively, these differences may be a non-adaptive consequence of environmental exposure to endocrine-disrupting chemicals present in polluted waterways, and/or an adaptive response to elevated physiological stress of urban life. Future research teasing apart the adaptive versus non-adaptive mechanisms producing these patterns will help us understand how and why this phenotypic change is occurring. These changes may also have additional consequences for individual toads (e.g., autotoxicity or trade-offs) as well as the ecological communities and predators in urban habitats.
Want to know more? Read the paper here:
Bókony, V., Üveges, B., Verebélyi, V. et al. Toads phenotypically adjust their chemical defences to anthropogenic habitat change. Sci Rep 9, 3163 (2019). https://doi.org/10.1038/s41598-019-39587-3
Figures from Bókony et al. (2019) are shared here under a Creative Commons Attribution 4.0 International License.
Featured image: Hans-Willi Thomas, Erdkröte (Bufo bufo), CC BY-SA 3.0
Shout out to Herpetological Highlights — this paper wasn’t on my radar until I heard it on the podcast.
- New Lit Alert: Urban environment determines population genetics in the green toad, Bufotes viridis - September 11, 2023
- Contribute to the New York Canid Project! - July 21, 2023
- Parallel Urban Adaptation from Phenotype to Genotype in Anolis Lizards - January 19, 2023
Very cool write up! I wonder if paratoid glands scale with body size such as that by ataining larger size they also increase in toxicity regardless of external pressures (predation or expsosure to pollutants). Makes me think of the mammals increases in fecundity in urban areas because of higher resources and body masses.
Good question! It looks like they included body size (SVL) as a covariate in their analysis of parotoid size and that it does scale positively. Even so, the urban animals still had relatively larger glands for their body size. They mention that females were larger than males, but I don’t think they compared body size by land use. If urban animals are also larger, then maybe they can better deal with the tradeoffs and autotoxicity of having more toxins?