Random mating by size in a population of common toads (Bufo bufo)

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Abstract
The mating system of a population of individually marked common toads (Bufo bufo) was studied during two years at a pond on the island Öland, southern Sweden. The mating system can be described as explosive breeding with scramble competition for mates. Males outnumbered females by approximately 2:1 and competed for the possession of females by guarding them prior to spawning and by displacing already paired males. Data on displacement patterns showed that smaller males were displaced more often than larger males. However, successful males were not larger than the male they displaced. Furthermore, thc mean size of spawning males was not different from the population mean and the size distribution of spawning males did not differ from the size distribution expected by chance. No size assortion could be detected among spawning pairs. Female fecundity showed a strong correlation with increasing body size. Males of all sizes showed capability of fertilizing all eggs of any female. Females apparently did not choose their mates. Though males would enhance their reproductive success by mating with large and more fecund females, this scenario was probably hampered due to a short breeding season and an operational sex ratio close to 2: 1.

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