Most animals that possess potent venom display a wide variety of warning messages to discourage predators. Tiger snakes are large and highly venomous elapids that exhibit these anti-predator behaviours. We compared the anti-predator behaviours of two neighbouring and genetically indistinguishable populations in Western Australia (Herdsman Lake, HL and Carnac Island, CI). CI is free from human, native and feral predation. All of these factors represent a continual threat on HL situated on the mainland. Neither body size, nor sex influenced defensive behaviours. However, we observed a marked inter-population difference among adults in the degree to which anti-predator behaviours were displayed when snakes were continually aggravated: HL snakes exhibited a typical warning signal (flat-neck) and bite, while CI snakes remained very docile. In stark contrast, neonates of both populations exhibited marked anti-predator behaviours and both populations were indistinguishable in terms of the intensity of display. Neonates reared in captivity, hence regularly confronted by human predators, became more defensive in comparison with neonates exposed to natural conditions on CI; similarly several adult CI snakes kept in captivity became more defensive. Our results highlight the extreme behavioural plasticity of snakes. We also hypothesize that CI snakes may become more placid over time as they grow up in an environment free from predation.