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Evolutionary Trap bites Bufo on the Bum

The fight against the invasive and noxious cane toad, Bufo marinus, has a new weapon with the discovery that meat ants (Iridomyrmex reburrus and I. sanguineus) in Northern Australia are able to overpower and eat juvenile cane toads.

This is because the cane toads evolved in vastly different conditions in the Americas, with completely different survival challenges, and the invading species is now caught in an ‘evolutionary trap’, according to cane toad expert Professor Rick Shine

Prof Shine and his Team Bufo at Sydney University – Georgia Ward-Fear, Matt Greenlees and Greg Brown – measured habitat use and activity patterns in meat ants, metamorph cane toads and metamorphs of seven native frog species (Litoria, Limnodynastes, and Opisthodon ornatus spp) in the laboratory. The study was funded by the Australian Research Council, and was Georgia Ward-Fear’s honours research project.

They found that metamorph toads (ones just metamorphosed from tadpoles) were at risk of attack from the meat ants because of their behaviour. The young cane toads are active during the day and prefer open habitats, unlike native frogs, which are nocturnal and hide in vegetation during the day when the meat ants are seeking food.

Meat ants are found in large numbers around tropical waterholes also frequented by cane toads. They are basically scavengers, Prof Shine said, but the worker ants, averaging 10 mm in length, had often been observed eating small toads.

This led the team to suspect that the toads were not adapted for their habitat – “there might be some kind of mismatch between the invader and its newly invaded range, for example, something about the toads’ behaviour that makes them vulnerable to a predator that poses little danger to native frogs.”

Unlike the native species which breed during the wet seasons, when there are large stretches of water on the Northern Australian coastal floodplains, B. marinus breeds during the dry season. The waterholes are surrounded by large stretches of drying mud, the average temperatures are around 30C and the humidity is very low. Large numbers of metamorphs are constrained to stay close to edges of waterholes or run the risk of desiccation.

It’s hypothesised that the metamorphs feed during the day to avoid being cannibalised by larger cane toads. However, as well as being active in the day, the young toads seem not to possess awareness of the dangers posed by the ants, and so fail to detect and evade approaching ants. What’s more, once under attack, they exhibit poor locomotor ability: short slow hops, reflecting their small size and short limbs; and frequently rely on an ineffective defence mechanism (crypsis) when attacked.

![Meat ant & cane toad](/image/img/Meat ant_cane toad.jpg)

A worker meat ant, Iridomyrmex reburrus, attacking a a juvenile Bufo marinus near a waterhole

In order to test the metamorphs’ mobility skills, the team built a special runway on which they could compare the sprint speed and endurance of the toads against the various species of native frogs.

The animals were encouraged to run by lightly tapping it on the urostyle with a blunt rounded object. “Each time the metamorph ceased hopping, we tapped it lightly on the urostyle. We recorded the time taken to reach the 0·33 m, 0·66 m and 1·0 m marks on the runway, and for statistical analysis we used the single fastest speed over any 0·33 m segment as a measure of sprinting capacity,” said Ward-Fear, lead author of a paper on the research.

“If no hopping response was elicited after five taps, the metamorph was considered exhausted and this time was recorded as the metamorph’s endurance.”

Compared to the native frogs, the toad metamorphs were smaller and slower. “On average, the metamorph cane toads were smaller (in both mass and length) than all other species tested except O. ornatus and L. bicolor. Toads also had the smallest tibia length, both in absolute terms and relative to their body size ,” Ward-Fear noted in her paper.

“Toads recorded the slowest speeds over 33 cm and had lower average endurance levels than did any native species except O. ornatus.”

The combination of these factors – daytime activity, a lack of awareness of the threat from the ants, and smaller, slower hops – puts newly metamorphosed toads at risk of becoming prey rather than predator, a risk the native frogs have evolved to avoid.

“The end result of this mismatch between traits of metamorph cane toads, which evolved in the Americas, and the ecological interaction between metamorph toads and meat ants in tropical Australia, is an ‘evolutionary trap’.” Shine said.

“The characteristics that increased toad survival where they evolved in the Americas are now a disadvantage, because the toads are facing different challenges in Australia – challenges they have not evolved to deal with. Such evolutionary traps should be especially common for invasive species, because so many aspects of their environment differ from those in which the traits of that species evolved.”

“The results are interesting not only because they reveal the cane toad’s Achilles’ heel – a weakness that could be exploited to help control the spread of the toxic toad – but because the same ‘evolutionary trap’ could be used to snare invasive species elsewhere.

Georgia Ward-Fear et al (2009), ‘Maladaptive traits in invasive species: in Australia, cane toads are more vulnerable to predatory ants than are native frogs’, was published online in Functional Ecology, on 31 March 2009.

©Sue Cartledge 2009

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