Is the behavioural divergence between range-core and range-edge populations of cane toads ( Rhinella marina ) due to evolutionary change or developmental plasticity?

Abstract

Individuals at the leading edge of expanding biological invasions often show distinctive phenotypic traits, in ways that enhance their ability to disperse rapidly and to function effectively in novel environments. Cane toads (Rhinella marina) at the invasion front in Australia exhibit shifts in morphology, physiology and behaviour (directionality of dispersal, boldness, risk-taking). We took a common-garden approach, raising toads from range-core and range-edge populations in captivity, to see if the behavioural divergences observed in wild-caught toads are also evident in common-garden offspring. Captive-raised toads from the invasion vanguard population were more exploratory and bolder (more prone to ‘risky’ behaviours) than toads from the range core, which suggests that these are evolved, genetic traits. Our study highlights the importance of behaviour as being potentially adaptive in invasive populations and adds these behavioural traits to the increasing list of phenotypic traits that have evolved rapidly during the toads' 80-year spread through tropical Australia.

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

“…Many of the evolved differences involve traits that enhance rates of dispersal; for example, invasion-front individuals have higher endurance 33 across a wider range of abiotic conditions 34 , longer legs 14,18 , narrower heads 17 , and invest less energy into immune function 6,35 and reproduction 20,21 . Invasion-front individuals also tend to be bolder and more active 36,37 . Some of the geographically divergent traits are highly heritable, whereas others are influenced by phenotypic plasticity also 19,23,38 . Sampling methods.…”

Diurnal activity in cane toads (Rhinella marina) is geographically widespread

“…The abiotic challenges of heat and desiccation are partially reduced by the large body size of adult cane toads, that confers resistance against rapid changes in temperature or hydration state 24 . Lastly, rapid evolution of behavioural and physiological traits within the toad invasion of Australia suggests that these animals can swiftly adapt to challenges encountered during range expansion [25][26][27][28] .…”

May the (selective) force be with you: Spatial sorting and natural selection exert opposing forces on limb length in an invasive amphibian

“…Our results confirm that short-armed males (as seen in long-colonized areas) can indeed outcompete long-armed rivals (as seen at the invasion front). Field trials of this issue would be of great interest and ideally should use individuals from different populations to incorporate the wide range of morphological, physiological and behavioural divergences between toads that have arisen during their Australian invasion (Brown, Phillips, Dubey, & Shine, 2015;Brown, Phillips, & Shine, 2014;Gruber et al, 2017aGruber et al, , 2017bHudson, Brown, et al, 2016;Hudson, McCurry, et al, 2016;Hudson et al, 2018;Llewellyn, Thompson, Brown, Phillips, & Shine, 2012).…”

“…The anurans have since spread widely across the Australian tropics at an increasingly rapid pace, from 10-15 to 50-65 km per annum (Urban, Phillips, Skelly, & Shine, 2008). That increase in rate of dispersal has been achieved via heritable shifts in toad behaviour and morphology (Gruber, Brown, Whiting, & Shine, 2017a, 2017bHudson, Brown, et al, 2016;Hudson, Brown, Stuart, & Shine, 2018;Hudson, McCurry, et al, 2016). Notably, toads from populations at the expanding range edge have longer forearms than do conspecifics from areas colonized 10-20 years earlier (Hudson, Brown, et al, 2016).…”