![]() ![]() ![]() Adult cane toads are chiefly nocturnal in both their native and introduced range, although crepuscular activity is not uncommon 17. Its success as an invader is due to its toxic skin that kills predators 18, 19, 20, 21, 22, but it is also a dietary and habitat generalist with high fecundity (reviewed in 17). The cane toad ( Rhinella marina = ‘ Bufo marinus’) has invaded over 50 countries from its native range in South and Central America 17. Given the serious problems caused by biological invasions, an improved understanding of how behavior contributes to the competitive ability and spread of invasive animals is urgently needed 16. Invasive species are good candidates to search for temporal phase shifts because they often spread into new geographic areas with different selective regimes, manifested in novel predators, competitors or physical environments 15. Finally, life history strategy and physiological condition influenced the likelihood of phase shifts in Salmon 13, 14. Abiotic factors can also cause a phase shift in activity four species of normally diurnal lizards phase shifted to moonlit nights in an open marsh edge, but not in an adjacent forested area where moonlight did not penetrate the forest floor 12. For example, Norway rats shifted their behavior from nocturnal to diurnal in response to predation by foxes 10, and the presence of competitors (polecats and otters) caused mink to shift from predominantly nocturnal to mainly diurnal 11. Physiological constraints such as circadian rhythms as well as behavioral and environmental factors may explain why phase shifts in activity are rare 7, 8, 9.ĭespite their apparent rarity, temporal phase shifts in activity patterns have been demonstrated in vertebrates. Although temporal partitioning between competitors and between predators and their prey is a significant mechanism of coexistence in some ecological communities, relatively few animal species are known to invert their activity patterns into the opposite activity phase as a result of predation or competition 3, 6. Time is an adaptive behavioral resource: animals maintain daily activity patterns that presumably optimize fitness 4, 5. But can novel environments reveal plasticity by causing a shift in a behavior as fundamental and conspicuous as timing of daily activity? An inversion of activity times is not commonly described, despite the fact that it can easily be discerned 3. Such plasticity is not uncommon - frequent behavioral responses to human-induced changed conditions, for example, include alterations in habitat choice, movements, foraging, social behavior and reproductive behavior (reviewed in 2). Plastic behavioral responses can facilitate the establishment of viable populations in novel environments by allowing individuals to survive and reproduce 1. The close proximity of diurnal and nocturnal toads (4–7 km) provided compelling evidence for the abovementioned physical factors as the proximate cause of the behavioral dichotomy, and for a novel (deeply shaded gorges) environment causing the shift to diurnal activity.Ī pivotal response of individual animals to a novel environment is behavioral. Visual encounter surveys confirmed that diurnal activity occurred exclusively at shaded sites, while most nocturnal activity occurred at exposed sites. ![]() ![]() For example, remote camera data (1314 observations of toad activity times over 789 trap days) revealed strictly nocturnal activity at four ‘exposed’ sites (99% of 144 observations over 179 days), compared to mostly diurnal activity at a ‘shaded’ site (78% of 254 observations). Gorges with an east-west axis (day-long northern shadow), narrow gorges and narrow sections of gorges contained toads that were primarily diurnal, while gorges with a north-south axis, wide gorges and wide sections of gorges contained mainly nocturnal toads. Sandstone gorges offered cane toads novel, deeply shaded habitat. Our study can demonstrate, however, that upon reaching novel environments some toad populations became diurnal. Cane toads in Australia are nocturnal, probably because diurnal activity would subject them to intolerably hot and dry conditions in the tropical savannah during the dry season. But can novel environments reveal plasticity by causing a shift in a behavior as fundamental and conspicuous as daily activity? We studied daily activity times near the invasion front of the cane toad ( Rhinella marina), an invasive species that has colonized much of northern Australia. Plastic responses may allow individuals to survive and reproduce in novel environments, and can facilitate the establishment of viable populations. ![]()
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