Tradeoffs between dispersal and reproduction at an invasion front of cane toads in tropical Australia

Crystal Kelehear,Richard Shine

doi:10.1038/s41598-019-57391-x

Crystal Kelehear, Richard Shine

Open Access

https://doi.org/10.1038/s41598-019-57391-x

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Abstract

Individuals at the leading edge of a biological invasion experience novel evolutionary pressures on mating systems, due to low population densities coupled with tradeoffs between reproduction and dispersal. Our dissections of >1,200 field-collected cane toads (Rhinella marina) at a site in tropical Australia reveal rapid changes in morphological and reproductive traits over a three-year period after the invaders first arrived. As predicted, individuals with dispersal-enhancing traits (longer legs, narrower heads) had reduced reproductive investment (lower gonad mass). Post-invasion, the population was increasingly dominated by individuals with less dispersive phenotypes and a higher investment into reproduction (including, increased expression of sexually dimorphic traits in males). These rapid shifts in morphology and reproductive biology emphasise the impacts of the invasion process on multiple, interlinked aspects of organismal biology.

Highlights

Individuals at the leading edge of a biological invasion experience novel evolutionary pressures on mating systems, due to low population densities coupled with tradeoffs between reproduction and dispersal
We describe the results of studies over a three-year period at a single site, beginning soon after the initial arrival of a colonising species
Spatial comparisons among toad populations suggest a tradeoff between dispersal rate and reproductive output: compared to conspecifics from long-colonised areas, females from invasion-front populations reproduce at low rates even under standardised conditions[20], and males have smaller testes relative to body size[21]

Summary

Introduction

Individuals at the leading edge of a biological invasion experience novel evolutionary pressures on mating systems, due to low population densities coupled with tradeoffs between reproduction and dispersal. Levels of investment into reproduction should depend upon competing priorities, with organisms decreasing reproductive output if investment into other functions yields higher benefits in fitness Such tradeoffs may be especially clear during biological invasions, because of differences in evolutionary forces at the invasion front versus in long-colonised areas[2]. The increased allocation to dispersal is exacerbated by the non-adaptive process of spatial sorting; individuals that allocate more energy into dispersal will likely be close to the range edge, even if more rapid dispersal does not enhance fitness[12] To test these ideas, we need to compare attributes of individuals at an expanding invasion front to conspecifics in longer-colonised areas. Sample size Morphological traits Snout-urostyle length (mm) Body mass (g) Head width (mm) Tibia length (mm) Sex ratio (% male) Reproductive traits Total testes mass (g) Total ovary mass (g) Rate of change (haldanes) Relative head width (all) Relative head width (male) Relative head width (female) Relative tibia length (all) Relative tibia length (male) Relative tibia length (female) Relative testes mass Relative ovary mass

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