Environmental constraints affect underground reproduction of the common toad (Bufo bufo)

The common toad (Bufo bufo) is a widespread species in Europe, with accidental occurrence in caves. This study reports the first case of breeding of B. bufo in a natural cave in the Carpathian Mountains. The breeding activity was observed at the end of May and the beginning of June. We identified pairs of common toads in axillary amplexus, egg strings and tadpoles up to a distance of 97 m inside the cave. We hypothesized that there is an active selection of some cave sites as breeding habitat and that the egg deposition is not randomly distributed inside the cave. In 25 sample points, we recorded ten cave environmental features and we assessed the relationship between egg strings presence and environmental features by performing a bias reduction in binomial-response generalized linear model. The results revealed that the strongest ecological factor determining the selection of breeding areas inside the cave was water flow velocity. The presence of light, although an important environmental feature, was not the most determining factor in breeding site selection, as toads successfully entered in completely dark environments to lay eggs in hydrologically favourable places. This study provides new information about the cave breeding of Bufo bufo and highlights the environmental features determining the common toad’s breeding site selection.