Shifts in the foraging tactics of crocodiles following invasion by toxic prey

Abstract

Biological invasions can modify the behaviour of vulnerable native species in subtle ways. For example, native predators may learn or evolve to reduce foraging in conditions (habitats, times of day) that expose them to a toxic invasive species. In tropical Australia, freshwater crocodiles (Crocodylus johnstoni) are often fatally poisoned when they ingest invasive cane toads (Rhinella marina). The risk may be greatest if toads are seized on land, where a predator cannot wash away the toxins before they are absorbed into its bloodstream. Hence, toad invasion might induce crocodiles to forage in aquatic habitats only, foregoing terrestrial hunting. To test this idea, we conducted standardised trials of bait presentation to free-ranging crocodiles in sites with and without invasive toads. As anticipated, crocodiles rapidly learned to avoid consuming toads, and shifted to almost exclusively aquatic foraging.

Figure 1

Photographs of freshwater crocodiles taking baits during baiting trials, from the water (A,B); as well as on land (C,D), and showing that sometimes multiple crocodiles visited the bait stations concurrently (E,F; green lines indicate individuals). Photographs taken from remote camera footage and by M. Bruny.

Figure 2

Proportion of chicken versus cane toad baits eaten by freshwater crocodiles (Crocodylus johnstoni) over the trial period by (a) crocodiles from toad-sympatric populations and (b) crocodiles from toad-naïve populations.

Figure 3

Analysis of reactions (consume or reject) of toad-sympatric populations of freshwater crocodiles (Crocodylus johnstoni) to chicken and cane toad baits, using data collected by remotely-triggered video cameras. Rates of consumption of toad versus chicken baits were equal in toad naïve populations (not shown).

Figure 4

Location of baits consumed by freshwater crocodiles (Crocodylus johnstoni) from toad-sympatric and toad-naïve populations.

Figure 5

Freshwater crocodiles feeding on adult cane toads in the water (A,B) and on land (C,D). It is common for C. johnstoni crocodiles to consume prey on land, even if the prey is captured in the water (E: a catfish; F: a small barramundi). Photographs by R. Somaweera.