Implications of female body-size variation for the reproductive ecology of an anuran amphibian

Animal body size has profound ecological and evolutionary implications for lifespan, access to resources and reproduction. Body size, though, is phenotypically plastic and may be strongly correlated with both fecundity and mate choice. Therefore, temporal variation in body size within populations may correspondingly alter a population's reproductive ecology over time and, if not taken into account, bias our perception of it. Male Fowler's Toads (Anaxyrus fowleri) at Long Point, Ontario, have previously been shown to vary in average body size by as much as 18% over a period of 23years. Here I demonstrate that the females also fluctuate in body size in correlation with their abundance and in synchrony with the males. Clutch size, which averaged 4443 eggs per clutch among 34 females, varied with individual female body size irrespective of temporal variation in average body size or abundance. Body size measurements of females and males in 75 mating pairs over 24years were, overall, significantly correlated. However, when year-to-year variations in mean body size were accounted for statistically, this apparent evidence of size-assortative mating was negated. These results indicate that abundance, body size and fecundity likely are interrelated aspects of population regulation and that many purported observations of size-assortative mating in animals may need to be re-evaluated.