Last updated May 8, 2018 at 10:22 am
Hawaiian study provides clue to cane toad evolution
Say what you like about cane toads, but it turns out they are admirably stoic when it comes to the weather.
Research by Australian scientists has discovered that the behaviour of the wildly successful and rather unpleasant invasive species is not affected by climate.
Previous studies have established that certain factors – in particular, an individual’s location in relation to its population distribution range – affect how toads interact with its environment, but bad weather, it turns out, isn’t one of them.
The finding, published in the journal Royal Society Open Science, is important, because identifying behavioural stressors lies at the heart of control strategies.
Enormous range of the toad
In Australia, the cane toad (Rhinella marina) extends from the lush wet tropics of northern Queensland to the much drier climes of north-western Western Australia.
However, the fact that this enormous range is the product of gradual expansion over many years means that the effect of climate is impossible to isolate from other factors, such as natural selection.
To make their finding, therefore, a team led by Jodie Gruber from the University of Sydney chose to leave Australia and investigate cane toad populations on the islands of Hawaii. The advantage of the island populations was threefold.
First, all the toad populations were established through deliberate introduction into sugarcane-growing regions in a single 12-month period beginning in 1932. Second, subsequent range expansion has been limited by the size of the land mass.
Third, and most importantly, the geography of the two islands selected for the study – Oahu and Hawaii itself – mean that they both have a wet side, characterised by abundant rainfall, and a dry side, wherein precipitation is much less.
Using a selection of toads caught from the wet and dry sides of both islands, Gruber and her colleagues conducted a battery of lab tests, assessing the animals’ propensity to take risks, explore, and how they reacted to new experiences.
Dry-weather toad are risk-takers
The researchers found that dry-side Oahu toads were more likely to take risks than dry-side Hawaiian animals, and that over all Oahu toads explored more than their cousins on the other island.
However, there was no indication that behaviour changed between the wet and dry sides of the same island. The scientists conclude that the different outcomes recorded between the islands could be explained by “founder effects, genetic drift, or developmentally plastic responses to ecological factors other than climate”.
The study extends and deepens insights gained by Gruber and her team set out in a 2017 paper, also published in Royal Society Open Science.
In that research, the scientists set out to determine whether cane toads that formed the leading edge of an invasion force, heading into previously toad-free territory, behaved differently to those living in a middle of long-settled areas.
The answer, they discovered was yes – and the reason may well be down to natural selection.
To make this finding, they caught a number of toads from range-edge and range-core populations, then let them spawn and raised the resulting offspring in laboratory conditions.
They found that the offspring, though deprived the environmental challenges of the parents, showed behaviour traits already noted in the older toads. Those raised from range-edge populations were more exploratory and bolder – and more likely to take risks – than the offspring of range-core parents.
The finding, Gruber and colleagues concluded, indicates that behaviour traits “have evolved rapidly during the toads’ 80-year spread through tropical Australia”.