Adaptive significance of communal oviposition in wood frogs (Rana sylvatica)

Abstract

1. Wood frogs (Rana sylvatica) are one of the earliest anurans to breed in spring. Their breeding behavior is ‘explosive’, with most oviposition occurring within 2 to 6 days. Early breeding may reduce predation and ensure adequate developmental time for larvae, but it is accompanied by the risk of substantial egg mortality due to cold temperature. 2. Males amplex females throughout breeding ponds, and pairs move toward a communal oviposition site. Females usually deposit eggs on submerged twigs or vegetation directly contiguous to other egg masses. Late breeders oviposit at the edge of the growing clump, thus surrounding the egg masses of earlier breeders. 3. Egg masses in the center of a clump are insulated from thermal extremes in the surrounding water. Within a clump, central egg masses were warmer than peripheral egg masses, but egg masses at the edge of a clump were warmer than single egg masses deposited outside the clump. 4. Central egg masses had significantly greater hatching success than peripheral egg masses. Central egg masses also had significantly more eggs than peripheral egg masses, suggesting that larger females obtained central sites. 5. Because central egg masses have the greatest survivorship to hatching, wood frogs should be selected to breed early to obtain optimal oviposition sites. Breeding synchrony may result from this selection for early oviposition, with the ultimate limiting factors being how quickly frogs can emerge in the spring and when ice on ponds melts.