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Thursday, September 23, 2021

Cannibal toads, a weird addition to Australia’s wildlife

Researchers note that this is an “evolutionary arms race”, not between two species, but between the different stages of life of the same species.

Written by Ritvik Chaturvedi | Bengaluru |
September 3, 2021 6:48:14 pm
Cane toadCannibalism is one of the strategies by which species contain their numbers and reduce intraspecific competition. (Wikimedia Commons)

Cane toad tadpoles tend to engage in cannibalism or consumption of younger individuals from the same species to enhance their fitness, a recent study has found.

Cane toads (Rhinella marina) are native to South and Central America but were introduced to the Australian mainland and neighbouring islands by humans. It was first introduced to Australia in 1935 to deal with the menace of cane beetles that fed on sugarcane plantations. Although these toads failed to control beetles, they multiplied rapidly in Australia where they had no natural predators. In the course of the last 86 years, thanks to the invasive species, the size of many native species has shrunk rapidly.

This cannibalistic behaviour has been found in invasive populations but not to a very great extent in native ones.

Tough competition

Cannibalism is one of the strategies by which species contain their numbers and reduce intraspecific competition. It is usually observed in invasive species that do not have any natural predators in their new environment.

The authors note that older toads do not exhibit cannibalistic tendencies towards other older toads. Instead, this cannibalistic behaviour is usually exhibited by tadpoles on other eggs and immobile hatchlings, with cannibals reducing the survival of newly laid eggs/hatchlings by as much as 99%.

The study consisted of 514 cannibalism trials in which hatchlings were exposed to tadpoles and it was found that native tadpoles, sourced from French Guiana, show a far lesser propensity towards cannibalism than Australian ones.

Difference in defences

In order to ascertain the difference in defences towards cannibalism between native and invasive hatchlings, a separate experiment was conducted. The defence responses of native and invasive hatchlings in the presence and absence of a cannibal were recorded.

This led to an equally interesting finding that the vulnerable individuals have simultaneously evolved defensive mechanisms to deal with cannibalistic conspecifics (members of the same species).

It was observed that targeted individuals from invasive populations (Australian toads) exhibited some distinctive evolutionary defences against cannibalism. One such defence is the ability to disperse more widely than their native counterparts to evade cannibalism. This enables the species to colonise newer habitats, newer resources that are free of cannibalism.

Quick development

Furthermore, cane toad clutches from invasive populations develop through the pre-tadpole/pre-feeding stages and reach the invulnerable tadpole stage far more rapidly (about five days) than the ones from native populations.

This ability to respond quickly to a changing environment renders an adaptive edge to individuals over those who fail to adapt and are, therefore, eliminated from the population.

Researchers theorise that this difference in behaviour stems from the cost and benefit involved with triggering a defence. In native populations, while the threat of a cannibal does exist, the costs involved in triggering a defence can outweigh the benefits.

Researchers note that this is an “evolutionary arms race”, not between two species, but between the different stages of life of the same species. They maintain that the study could help them understand the emergence of cannibalistic behaviour, as cannibalism has developed in invasive cane toad populations in 86 years, which is a very short period in evolutionary timescale.

– The author is a freelance science communicator. (mail[at]ritvikc[dot]com)

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