At the invasion front, male cane toads (Rhinella marina) have smaller testes

Abstract

As a colonizing species expands its range, individuals at the invasion front experience different evolutionary pressures than do those at the range-core. For example, low densities at the edge of the range mean that males should rarely experience intense sperm competition from rivals; and investment into reproduction may trade-off with adaptations for more rapid dispersal. Both of these processes are predicted to favour a reduction in testis size at the invasion front. To explore effects of invasion stage in Australian cane toads (Rhinella marina), we collected and dissected 214 adult males from three regions: one in the species' range-core (northeastern Australia), and two from invasion fronts (one in northwestern Australia and one in southeastern Australia). Despite the brief duration of separation between toads in these areas (approx. 85 years), testis masses averaged greater than 30% higher (as a proportion of body mass) in range-core males than in conspecifics sampled from either vanguard of the invasion. Previous work has documented low reproductive frequencies in female cane toads at the invasion front also, consistent with the hypothesis that evolutionary and ecological pressures unleashed by an invasion can favour relatively low resource allocation to reproduction in both sexes.

Keywords: Amphibia; Anura; Bufo marinus; mating system; sexual selection.

Figure 1.

Relative mass of the testis (quantified as residual scores from the general linear regression of testis mass versus body mass) of adult male cane toads (Rhinella marina) from the range-core and two invasion-front regions within Australia. The graph shows mean values and associated standard errors. (Online version in colour.)