The evolution of targeted cannibalism and cannibal-induced defenses in invasive populations of cane toads – pnas.org

Significance

Invasive species are known for their ability to achieve high densities within their introduced range. Hence, invaders often face strong competition from members of their own species. Mechanisms for reducing intraspecific competition may therefore be favored in invasive populations, such as cannibalism, in which individuals kill and eat intraspecific competitors. Here, we find that toad tadpoles from invasive Australian populations have evolved both a strong behavioral attraction to the vulnerable hatchling stage and an increased propensity to cannibalize these younger conspecifics. In response, these toads have also evolved multiple strategies for reducing the duration of the vulnerable period, indicating an evolutionary arms race between the cannibalistic tadpole stage and the vulnerable egg and hatchling stages in invaded habitats.

Biotic conflict can create evolutionary arms races, in which innovation in one group increases selective pressure on another, such that organisms must constantly adapt to maintain the same level of fitness. In some cases, this process is driven by conflict among members of the same species. Intraspecific conflict can be an especially important selective force in high-density invasive populations, which may favor the evolution of strategies for outcompeting or eliminating conspecifics. Cannibalism is one such strategy; by killing and consuming their intraspecific competitors, cannibals enhance their own performance. Cannibalistic behaviors may therefore be favored in invasive populations. Here, we show that cane toad tadpoles (Rhinella marina) from invasive Australian populations have evolved an increased propensity to cannibalize younger conspecifics as well as a unique adaptation to cannibalisma strong attraction to vulnerable hatchlingsthat is absent in the native range. In response, vulnerable conspecifics from invasive populations have evolved both stronger constitutive defenses and greater cannibal-induced plastic responses than their native range counterparts (i.e., rapid prefeeding development and inducible developmental acceleration). These inducible defenses are costly, incurring performance reductions during the subsequent life stage, explaining why plasticity is limited in native populations where hatchlings are not targeted by cannibalistic tadpoles. These results demonstrate the importance of intraspecific conflict in driving rapid evolution, highlight how plasticity can facilitate adaptation following shifts in selective pressure, and show that evolutionary processes can produce mechanisms that regulate invasive populations.

All data are available in the Supplementary Materials. Note that a subset of the invasive range data used here were also used in DeVore et al. 2021; here we incorporate these results into larger datasets to make comparisons between the native and invasive range.

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The evolution of targeted cannibalism and cannibal-induced defenses in invasive populations of cane toads - pnas.org