Spadefoot toads are one of the best known examples of fossorial frogs. They gained their name because three of the genera have a specially adapted hind foot which enables effective burrowing. Many spadefoot toads live in arid habitats and they are uniquely adapted to harsh, dry environments. The adults are highly secretive, often spending most of the year buried underground. Individuals of some species may burrow to depths of two metres below the surface (AmphibiaWeb, 2008). North American spadefoot toads typically have a highly explosive breeding strategy, often emerging after heavy rains to breed (Figure 1). The adults are able to burrow into sand to avoid hot summer temperatures, while the larvae have an extremely rapid development, enabling some species to metamorphose into terrestrial juveniles within just 30 days (Pfenning, 1992). However, European spadefoot toads have a different life history with a prolonged breeding period of several months and having large larvae which take several months to metamorphose (Degani, 2015).
Spadefoot toads are spread widely across the whole of the northern Hemisphere from eastern Russia to western United States. Most, but not all, spadefoot toads possess an adapted hind foot (Figure 2) and it was assumed that it was only the species which lived in arid environments which possessed this feature. However, recent evolutionary research by Chen et al. (2016) has suggested that this may not be the case. Chen et al. (2016) report on a new spade-foot bearing fossil toad from eastern Mongolia. The fossil, Prospea holoserisca, is estimated to be 56 million years old and resembles modern spadefoot toads. It is the earliest definite fossil frog with an enlarged hind limb which was used for effective burrowing (Chen et al., 2016). Analysis by Chen et al. (2016) has shown that this fossil is the ancestor of both the living spadefoots which possess the specially adapted hind foot and those which do not. The anatomy of the fossil spadefoot toad suggests that it could burrow, but analysis of the environment at the time indicates that it did not live in an arid environment, like modern spadefoot toads. This has led Chen et al. (2016) to the conclusion that burrowing in an arid environment is an exaptation instead of an adaptation. In other words, burrowing behaviour did not evolve in response to an arid environment, but instead the frogs already had the hind foot morphology which enabled the frogs to burrow in sand when the environment subsequently became arid (Chen et al., 2016). Indeed, another study has suggested that the rapid larval development often observed in North American spadefoot toads did not evolve in response to an increasingly arid environment but relates instead to the size of the frogs’ genome and evolutionary history (Zeng et al. 2014). These studies show that amphibian species may utilise morphology and behaviour which they already possess when environmental conditions change.
There are six species of spadefoot toad belonging to the genus Pelobates which are distributed across Europe, Western Asia and North Africa. Due to their fossorial existence for most of the year, information on their behaviour is generally lacking. The common spadefoot toad, Pelobates fuscus, has a prolonged breeding period lasting from April to June during which time the males defend territories around the pond and, unlike many breeding toads, call underwater (AmphibiaWeb, 2008) (Figure 3). Eggert & Guyétant (2003) carried out a study of calling males in northeast France and found that males with a lower body condition arrived later than males with higher body condition. Males arriving early at the breeding pond experienced high competition for females and it is risky for males in low body condition as they are unlikely to be successful in mating. Therefore, Eggert & Guyétant (2003) propose that males optimise their breeding migration by arriving earlier or later, depending on their body condition. In addition, older (and more experienced) males tended to stay for less time at breeding ponds, presumably because they were able to obtain a mate more quickly (Eggert et al., 2003). Unlike species of toad which have an explosive breeding strategy where males have little time to secure a mate, species with a longer breeding season like the common spadefoot toad have the opportunity to utilise different mating tactics to ensure greater chances of successfully mating.
The common spadefoot toad (P. fuscus) and eastern spadefoot toad (P. syriacus) overlap in parts of their range. Both species have highly similar ecology and life history: both burrow underground, are nocturnal, reproduce at the same period of the year, use similar aquatic habitats for reproduction and forage in the same terrestrial habitats (Cogălniceanu et al., 2014). Therefore, there is a high potential for niche overlap and competition between species. In their recent research, Székely et al. (2017) found that each species had different foraging patterns which avoided competition. The eastern spadefoot toad emerged from the soil less often than the common spadefoot toad but was active for much longer and moved over twice the distance. These differences in movement patterns are likely to allow the different species to exploit different prey types which would allow coexistence and reduce competition (Székely et al., 2017). Since the eastern spadefoot toad is a widely foraging species it is likely to encounter more sedentary prey species that are clumped and unpredictable. However, this is likely to be costly in terms of energetic expenditure. The common spadefoot toad is less active, but will be able to exploit larger, faster moving and active prey. However, these are generally fewer in number (Székely et al., 2017). Therefore, each species exploits a different foraging strategy, each with its own advantages and disadvantages. Although these behavioural differences are relatively small, they allow each species to exploit different prey species and coexist in the same habitats.
References
AmphibiaWeb (2008) Pelobates fuscus: Common spadefoot <http://amphibiaweb.org/species/5270> University of California, Berkeley, CA, USA. Accessed Oct 25, 2019.
Chen, J., Bever, G.S., Yi, H. & Norell, M.A. (2016) A burrowing frog from the late Paleocene of Mongolia uncovers a deep history of spadefoot toads (Pelobatoidea) in East Asia. Nature Scientific Reports, 6: 19209. doi 10.1038/srep19209.
Cogălniceanu, D., Roşioru, D., Székely, P., Székely, D., Buhaciuc, E., Stănescu, F. & Miaud, C.
(2014). Age and body size in populations of two syntopic spadefoot toads (genus Pelobates) at the limit of their ranges. Journal of Herpetology, 48: 537-545.
Degani, G. (2015) The habitats, burrowing behavior, physiology, adaptation and life cycle of spadefoot toads (Pelobates syriacus, Boettger, 1869) at the southern limit of its distribution in Israel. Open Journal of Animal Sciences, 5: 249-257.
Eggert, C. & Guyétant, R. (2003) Reproductive behaviour of spadefoot toads (Pelobates fuscus): daily sex ratios and males’ tactics, ages and physical condition. Canadian Journal of Zoology, 81: 46-51.
Pfenning, D.W. (1992) Polyphenism in spadefoot toads tadpoles as a locally adjusted evolutionarily stable strategy. Evolution, 46 (5): 1408-1420.
Székely, D., Cogălniceanu, D., Székely, P. & Denoël, M. (2017) Out of the ground: coexisting fossorial species differ in their emergence and movement patterns. Zoology, 121: 49-55.
Zeng, C., Gomez-Mestre, I. & Wiens, J.J. (2014) Evolution of rapid development in spadefoot toads is unrelated to arid environments. PLOS ONE, 9: e96637, doi: 10.1371/journal.pone.0096637.