Sex Ratio and Breeding Population Size in Fowler's Toad, Anaxyrus ( = Bufo) fowleri

Abstract

Sex ratios among anurans at breeding sites are routinely observed to be skewed toward males, which has implications for the strength of sexual selection in these animals. However, the relative numbers of males and females observed at breeding sites also depends upon their relative conspicuousness and the physical sex ratio of the adult population as a whole, which includes animals not present at breeding sites. Using intensive capture–recapture methods, I estimated abundances of both sexes of Fowler's Toads in a population at Long Point, Ontario, over a span of 14 years (1998 to 2011, incl.). Although males greatly outnumbered females at breeding sites, persisted there for longer periods of time, and were more readily re-captured, both sexes could be found in their lakeshore non-breeding habitat with equivalent reliability. Estimates of abundance were calculated for each sex based on 3,162 total captures of 686 females and 982 males. The abundances of males and females each year were not significantly different (P = 0.738), which was consistent with a 1∶1 physical sex ratio. Both sexes also exhibited large, but strongly correlated (P << 0.001; R2 = 0.838), variations in their abundance over the 14 yrs. Only 39% of total estimated males were found at breeding sites. The ratio of males at breeding sites to total males declined significantly (P = 0.002; R2 = 0.542) over the 14-year study, in parallel with a diminishing availability of breeding sites.