cane toad research

Our research on the biology, impact and control of cane toads

Beginning in the early 1980's and continuing through to the present day, Rick Shine and his colleagues have conducted a major research program on reptiles and their prey species on the floodplain of the Adelaide River 60 km east of Darwin. That work has focused on issues such as the ways in which year-to-year variation in wet-season rainfall influences the ecology of tropical snakes.

©David Nelson

Cane toads arrived in this area late in the 2004/5 wet-season, and it was obvious that our longterm studies provided a unique backdrop - a once-in-a-lifetime chance to really understand what effects cane toads have on a complex Australian ecosystem. So, we expanded our studies to include the biology and impact of these toxic invaders.

©David Nelson

Although it is still too early for confident evaluation of ultimate impact, we have already learnt a great deal. For example, the toads themselves have changed during the invasion process: invasion front toads are longer-legged, more active and more mobile than their cousins back in long-established populations. Impacts of toad arrival on the native fauna have been less catastrophic than we had feared, except for the deaths of many varanid lizards (goannas), and some species of venomous snakes. Indeed, the impacts of toads seem likely to be less severe and long-lasting than most people have expected. Our results also challenge the effectiveness of current approaches for toad control, and suggest new ecologically-based approaches for reducing toad impact.

Many people ask us about our toad research, so we have set up this website to provide access to that information. After all, the work has been funded by taxpayers, through the Australian Research Council! The website provides links to many of our papers that provide information about toads, with a quick summary of the main results from each paper. If you want a lot more detail, just click on the highlighted link to download the entire paper.

We have also set up another website with a bit less scientific detail, for people that would like to get the "big picture" about cane toads and their impact, but without all the numbers and statistics and so forth that scientists have to put into their papers to back up their conclusions. So if you just want an overview of cane toads, their biology, their ecological impact, and their control, please go to www.canetoadsinoz.com

©David Nelson

A toad by any other name …..

A careful reader will notice something very peculiar about the scientific name of the cane toad, as used in our papers - sometimes we call the toads Bufo marinus, and sometimes Chaunus marinus. Why did this happen? Every species has a latin name that gives its genus and species, because common names are often ambiguous. The latin name is supposed to be better because it doesn't change in different places or at different times...but things don't always work out so neatly! Cane toads have been called "Bufo marinus" for a long time, and that's what we called them in our early papers. But recently some American workers suggested that the toads really belonged in a different genus (Chaunus), based on molecular evidence (DNA sequences). It sounded very convincing, so we started calling the toads Chaunus marinus (and so did a lot of other people). But then another molecular-biology group questioned the first group's conclusions, and suggested yet another name (Rhinella marinus). We decided that it's all too much, and until the mess gets sorted out we'll stick with the tried and true "Bufo marinus".

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"Popular" articles

Some papers are available as pdf files. To read these you will need Adobe Acrobat Reader.
If you have any difficulties downloading files please contact Mel.

The biology, impact and control of cane toads: an overview of the University of Sydney's research program.

We wrote this summary for a Cane Toad Conference held in Brisbane in June 2006; it summarises a lot of the background of Rick’s long-running research program, and some of our findings about toads. It’s a lot less technical than many of the other “scientific” papers, and refers to many of the others.

Phillips, B. L. 2005. The march of toads. Australasian Science 26:14–16.

A broad summary of the evidence for the impact of toads on Australian wildlife, coupled with some of our early findings.

©Michael Crossland

Shine, R. 2007. Toad kill. Australasian Science 28:16-20.

This is an edited transcript of a talk that Rick gave at the Australian Academy of Science; it summarises more recent results about toad biology and impact.

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Scientific papers

Some papers are available as pdf files. To read these you will need Adobe Acrobat Reader.
If you have any difficulties downloading files please contact Mel.

The biology of cane toads

Hagman, M., and R. Shine. 2006. Spawning-site selection by feral cane toads (Bufo marinus) at an invasion front in tropical Australia. Austral Ecology 31:551-558.

Although many people say that toads will breed anywhere, it’s not true. Toads actively select breeding sites that are shallow ponds with long shallow edges and open ground (not thick vegetation) around the water’s edge. This may be helpful if you don't want toads to breed in your backyard pond; just make sure it has steep sides and lots of thick grass growing up to the water's edge.

Phillips, B. L., G. P. Brown, J. Webb, and R. Shine. 2006. Invasion and the evolution of speed in toads. Nature 439:803.

In the first few decades after toads were released in Queensland, they expanded their range at about 10 km per year. That rate then began to increase, and now averages about 55 km per year. We show that toads with relatively long legs (compared to their body length) move faster and further than shorter-legged toads of the same body size; and that the invasion front is composed of very mobile, long-legged toads. Thus, toads have changed dramatically, through rapid evolution, in the course of only about 50 generations in Australia.

Brown, G. P., B. L. Phillips, J. K. Webb, and R. Shine. 2006. Toad on the road: use of roads as dispersal corridors by cane toads (Bufo marinus) at an invasion front in tropical Australia. Biological Conservation 133:88-94.

As noted above, invasion-front toads are in a real hurry to move, and so don’t waste their time struggling through thick vegetation. Instead, they take advantage of the roads (and cattle paths, etc) that we humans have so thoughtfully provided for them. Most of our radio-tracked toads zipped along the highway rather than moving through dense vegetation. When they reached crossroads, their paths of movement often took a right-angle turn because they would rather follow roads, even if the direction isn't exactly perfect, than try bashing through the scrub. By the way, this also means that people tend to overestimate the abundance of toads - because they prefer open spaces, they are much easier to see than are native frogs (that are smaller, and hide in vegetation most of the time).

©Ben Phillips

Phillips, B. L., G. P. Brown, M. Greenlees, J. K. Webb, and R. Shine. 2007. Rapid expansion of the cane toad (Bufo marinus) invasion front in tropical Australia. Austral Ecology 32:169-176

We radio-tracked toads at the invasion front (on the Adelaide River floodplain), by putting small waistbands on the toads and attaching miniature transmitters. Then, Greg was able to go out and locate the toads every day. This is a really useful method, because otherwise the toads are pretty much invisible within the landscape unless they gather beside a pool to start breeding. The tracking showed that the toads move quite amazing distances, and in incredibly consistent directions. We also mapped the advance of the toad front through Kakadu and towards Darwin, and found very similar rates of progress of the front as estimated by the radio-tracking. This is probably one of the most astonishing results from our fieldwork: the invasion-front toads move hundreds of metres every night, an incredible achievement for a small amphibian. Of course, this makes the front almost impossible to stop - if you miss even a single toad, it will be many kilometres past you within a few weeks or perhaps, in a few days.

©Ben Phillips

Semeniuk, M., F. Lemckert, and R. Shine. 2007. Breeding-site selection by cane toads (Bufo marinus) and native frogs in northern New South Wales. Wildlife Research 34:59-66.

Because we had earlier found that toads in Kakadu actively selected particular types of ponds to lay their eggs (an obviously important issue for management, and for discouraging toad breeding), we repeated this study at the other end of the toads’ Australian range, in northeastern New South Wales. Mark found exactly the same kinds of patterns that Mattias had revealed from the Kakadu study, a very encouraging result. It seems like toads are picky about where they breed, all across Australia. Also, the toads tended to like spots quite different from those used by most of the local frogs.

©David Nelson

Urban, M., B. L. Phillips, D. K. Skelly, and R. Shine. 2007. The cane toad's (Chaunus marinus) increasing ability to invade Australia is revealed by a dynamically updated range model. Proceedings of the Royal Society of London Series B 274:1413-1419.

How far will toads eventually spread through Australia? To predict this, Mark Urban and Dave Skelly from Yale University joined forces with us to model toad distributions. The analysis suggested that toads may be adapting to Australian climates, and hence may occupy a much larger area of the continent than was originally thought. However, like all models, treat this one with healthy scepticism: other models give different results. The important point is to recognise that it not safe to assume that toads are only a problem in tropical Australia; even if you live in Sydney, they may soon be coming to a pond near you!

Hagman, M., and R. Shine. 2008. Understanding the toad code: behavioural responses of cane toad (Chaunus marinus) larvae and metamorphs Austral Ecology 33:37-44.

Cane toads are only distantly related to Australian frogs, so perhaps they use different ways to communicate with each other? For example, lots of toad species from around the world send chemical messages to each other when they are attacked by predators. If we can work out those pathways, we might be able to influence cane toads without affecting Aussie frogs. Mattias showed that the tadpoles of cane toads do indeed use such alarm pheromones, and in other work we show that Aussie frogs don't react to them. So perhaps we can use these chemicals to drive toad tadpoles or metamorphs away from their favoured habitats, to places where they are less likely to survive.

Hagman, M., and R. Shine. 2008. Deceptive digits: the functional significance of toe-waving by cannibalistic cane toads (Chaunus marinus). Animal Behaviour 75:123-131.

Cane toads have been studied very intensively, so you don't really expect to see too many "new" behaviours that haven't already been described by other researchers. But Mattias noticed that when he fed his captive toads, they often started to wriggle the longest toes on their back feet - as if it was a lure to attract prey items. We know from field studies that bigger toads sometimes eat large numbers of smaller toads. Sure enough, we found that smaller toads were attracted to the "lure". We built an artificial toe, put it beside a freeze-dried toad, and looked at how effectively we could attract baby toads with it. The colour and movement rates of the REAL toes were ideal for attracting smaller toads to the cannibals. This behaviour is very specialised, and suggests that cannibalism may actually be a fairly important issue in the life of cane toads. One idea we are investigating at present is whether we can exploit this tendency to help use toads to control their own numbers.

Child, T., B. L. Phillips, G. P. Brown, and R. Shine. 2008. The spatial ecology of cane toads (Bufo marinus) in tropical Australia: why do metamorph toads stay near the water? Austral Ecology 33:630-640.

Travis did this study as part of his Honours year. He measured the conditions around ponds in both the wet-season and the dry-season, and looked at where the smallest toads (metamorphs, that are the first stage after the tadpole) are found. It looks as though the grass is greener for a young toad further away from water (there are more bugs to eat, and less chance that your older brother or cousin will try to eat you) but for much of the year, it's simply too dry out there. So, the young toads hang out beside the pond until it rains.

Bowcock, H., G. P. Brown, and R. Shine. 2008. Sexual communication in cane toads (Chaunus marinus): what cues influence the duration of amplexus? Animal Behaviour 75:1571-1579.

Prior to spawning, male frogs grab hold of females and stay attached until the female is ready to release her eggs. This can sometimes take days or even weeks, during which time the male clings to the female's back like a small warty jockey. Females are heavily burdened by such a male, and would benefit by convincing him to let go - but they don't seem able to do this. Male toads often grab each other by accident, and CAN convince the other male to let go by giving a special "release call". Females don't have the same kinds of vocal cords as males, because they don't have to give the loud "advertisement calls". Because they are mute, they aren't able to talk the male into letting go. This may actually end up being useful to know for control purposes - because female toads often drown if they are grabbed by more than one male at the same time, and are in deep water where they can't stay afloat.

Urban, M., B. L. Phillips, D. K. Skelly, and R. Shine. 2008. A toad more traveled: the heterogeneous invasion dynamics of cane toads in Australia. American Naturalist 171:E134-E148.

How fast have cane toads traveled through Australia since their release in 1935, and why have they moved more quickly through some areas than others? We have incredibly detailed information about the rate that toads have spread, because people really notice toads and so, have been able to record when they turn up. We put all this information together with data on climates and landforms etc, to try to work out what factors affect toad invasion speeds. The biggest factor turns out to be the gradual acceleration of the toad front through time - they have evolved to disperse more rapidly (with longer legs and different behaviours) and thus move westwards about five times faster than in the early years of toad invasion.

Hagman, M., and R. Shine. 2008. Australian tadpoles do not avoid chemical cues from invasive cane toads (Bufo marinus). Wildlife Research 35:59-64.

We've shown that chemical substances from injured or crushed toad tadpoles frighten other toad tadpoles. This might be helpful for toad control, if we can force toad tadpoles into particular areas of the pond and so forth. But obviously, we need to make sure that we are not also stressing the tadpoles of native frogs. So - we exposed native tadpoles to the toad's "alarm pheromone". Fortunately, they don't seem to be at all worried by it - unlike the toads.

Child, T., B. L. Phillips, and R. Shine. 2008. Abiotic and biotic influences on the dispersal behaviour of metamorph cane toads (Bufo marinus) in tropical Australia. Journal of Experimental Zoology 309A:215-224.

When cane toads transform from aquatic life (as tadpoles) to terrestrial life (as miniature toads), they are really tiny. This makes them very vulnerable to drying out, or to attack from predators (the tiny toads have less poison than their bigger relatives). So, this "metamorph" stage is perhaps an Achilles Heel - a time in their life when toads are especially vulnerable to control efforts. Understanding where they live at this time, and why they move around to different areas, might help to target these baby toads in our efforts at control. So Travis set up a set of laboratory arenas where he changed aspects like temperature, moisture levels and so forth, to study effects on the baby toads' movements. We found that the young cane toads prefer moist sites, but are deterred from entering such places if a larger cannibal toad is present.

Phillips, B. L., G. P. Brown, J. M. J. Travis, and R. Shine. 2008. Reid's paradox revisited: the evolution of dispersal kernels during range expansion. American Naturalist 172:S34-S48.

There is lots of recent interest in the effects of climate change, and one interesting issue is how quickly animals and plants could move across the landscape to colonise a newly-available area. This happened after the last Ice Age, for example, as glaciers retreated - how quickly could plants and animals move into the new space? Many years ago, Reid noticed a puzzle - species seemed to move much faster than we would expect from the rates that we measure animals and plants dispersing under today's conditions. We show, with some mathematical modeling and studies on movements of radio-tracked toads, that the answer lies in the amount of variation in movement distances. Toads at the invasion front have more variable movement distances per day, and this allows some of them to move much further than we would expect from the "average" values.

Pizzatto, L. and R. Shine. 2008. The behavioral ecology of cannibalism in cane toads (Bufo marinus). Behavioral Ecology and Sociobiology: in press.

Adult cane toads and tiny little metamorphs mostly eat insect prey, but there's a dark side to the dietary habits of intermediate-sized (juvenile) toads. During the long Dry-season when baby toads are restricted to the edge of the pond, and there's not much insect life out in the drier areas, the larger juvenile toads hang around the pond and feed mostly on their smaller brethren. About two-thirds of their diet consists of smaller toads. We recorded behaviours and distributions and numbers of the cannibals and their prey, and ran some simple experiments like dangling dead metamorph toads in front of larger toads to record feeding responses. It's very clear that any tiny toad that moves near the pond's edge at night is very likely to end up inside a larger toad's stomach!

Pizzatto, L., T. Child, and R. Shine. 2008. Why be diurnal? Shifts in activity time enable young cane toads to evade cannibalistic conspecifics. Behavioral Ecology: in press.

Like most species of frogs, adult cane toads move around mostly at night - they spend the day tucked away in a safe shelter. But the tiny little metamorph toads are active by day - which seems very peculiar, since they are at risk of overheating, drying out or being grabbed by a predator at that time. Our experiments show that there is a huge benefit to this daytime activity - the young toads can thereby avoid their older cousins, who otherwise are very happy to eat them.

Alford, R. A., G. P. Brown, L. Schwarzkopf, B. Phillips, and R. Shine. 2008. Comparisons through time and space suggest rapid evolution of dispersal behaviour in an invasive species. Wildlife Research: in press.

In previous work, we showed that cane toads are moving faster and faster through tropical Australia, and that one of the reasons for that acceleration is the evolution of longer legs (that enable toads to move quicker). But of course, how fast a cane toad can disperse depends on its behaviour as well as its running speed. Fortunately, Ross Alford and Lin Schwarzkopf had radio-tracked cane toads in Queensland several years ago, so we were able to combine our results with theirs to see what behaviours have changed over the course of the toads' Australian invasion. As we expected, the invasion-front toads move a lot further every day, and tend to move whenever conditions are suitable - whereas the Queensland toads end up staying a lot closer to home.

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The ecological and evolutionary impact of cane toads

Phillips, B., G. P. Brown, and R. Shine. 2003. Assessing the potential impact of cane toads (Bufo marinus) on Australian snakes. Conservation Biology 17:1738-1747.

In this paper, we look at which species of Australian snake are likely to be at risk from toads, based on their geographic distributions and whether or not they eat frogs (and hence, are likely to try to eat toads). We also show that most (but not all) Aussie snakes are very sensitive to toad poisons. We conclude that a high proportion of Australian snake species might be at risk from toads.

©Ben Phillips

Phillips, B., and R. Shine. 2004. Adapting to an invasive species: toxic cane toads induce morphological change in Australian snakes. Proceedings of the National Academy of Sciences (USA) 101:17150-17155.

If you are a frog-eating snake and toxic cane toads arrive, the best chance of survival may be if you are a big snake with a small head. In this way, you simply cannot open your mouth wide enough to eat a toad large enough to kill you. Ben’s measurements of preserved museum specimens showed that in two frog-eating snake species, that’s exactly the pattern we see. Following toad invasion, average body size increases and relative head size decreases. In combination with some of our other results, the clear message is that Aussie ecosystems can change rapidly to deal with new challenges like the toad invasion.

Phillips, B., G. P. Brown, and R. Shine. 2004. Assessing the potential for an evolutionary response to rapid environmental change: invasive toads and an Australian snake. Evolutionary Ecology Research 6:799-811.

Is there genetic variation for traits (such as tolerance to toad poisons) that might help a population adapt to the arrival of toads? Without such variation, natural selection cannot act. We show that such variation does indeed occur in keelback snakes.

Phillips, B., and R. Shine. 2005. The morphology, and hence impact, of an invasive species (the cane toad, Bufo marinus) changes with time since colonization. Animal Conservation 8:407-413.

The risk that a cane toad poses to a predator depends on how much poison it has within its body – in turn, dependent on how big the toad is, and how big its poison-glands (parotoid glands) are. We show that both of these characteristics shift with time since toads arrived in an area. Toads at the invasion front are bigger, and have bigger toxin glands. We're not sure why these changes occur; they might be due to evolution, or be induced by some aspects of the local environment. Ben is currently running breeding trials with toads, that will answer these and other questions.

Phillips, B. L., and R. Shine. 2006. Allometry and selection in a novel predator-prey system: Australian snakes and the invading cane toad. Oikos 112:122-130.

The vulnerability of a snake predator to a toad depends on the rate that increasing body size enables a snake to eat a bigger toad; and also, the rate that the toad’s total amount of poison increases with its own body size. We measure these size-related changes, to calculate how vulnerable particular kinds of snakes are likely to be.

Phillips, B., and R. Shine. 2006. Spatial and temporal variation in the morphology (and thus, predicted impact) of an invasive species in Australia. Ecography 29:205-212.

Whether or not a predator dies when it eats a toad depends upon the size and toxicity of the toad. This paper maps the variation in space (across Qld) and time (through the year) of toad body size and toad gland size. Both of these traits are critical to the impact that a toad population can have (because body size determines both toxicity, and whether a predator can handle a toad, and gland size determines the toxicity). This paper shows that there is meaningful spatial variation in the toxicity of toads and hence their potential impact on gape-limited predators (such as snakes); toads are more toxic in some areas than in others.

Phillips, B. L., and R. Shine. 2006. An invasive species induces rapid adaptive change in a native predator: cane toads and black snakes in Australia. Proceedings of the Royal Society London Series B 273:1545-1550.

There are many reports of mortality of predators that attack toads. These reports mostly come from areas where toads have just invaded. Our surveys show that predator numbers can indeed fall dramatically due to toad-poisoning, but (encouragingly!) those species do not actually disappear entirely. Indeed, many become common once again (possibly within a decade), and somehow coexist with toads. We show that blacksnakes in toad-infested areas have changed (in feeding behaviour, physiology and relative head size) in ways that enable them to survive despite the toads’ presence. Mother Nature is a tough old lady, and Australian ecosystems have had to cope with many challenges over the last few million years! Toads are just one more challenge.

Smith, J. G., and B. L. Phillips. 2006. Toxic tucker: assessing the potential impact of cane toads on Australia's reptiles. Pacific Conservation Biology 12:40-49.

James and Ben followed up our earlier work on snakes, testing a range of lizards, crocodiles and turtles to see which ones could tolerate toad poisons. The results were similar to the snake work: most species of non-snake Australian reptiles also have very low resistance to toad toxin, although there appears to be much greater variation in toxin resistance between species. Perhaps the most surprising result from this work is that saltwater crocodiles appear to be quite resistant to toad toxin, whereas freshwater crocodiles are highly susceptible.

Greenlees, M. J., G. P. Brown, J. K. Webb, B. L. Phillips, and R. Shine. 2006. Effects of an invasive anuran (the cane toad, Bufo marinus) on the invertebrate fauna of a tropical Australian floodplain. Animal Conservation 9:431-438.

For his Honours year, Matt built outdoor enclosures on the edge of the floodplain, and added frogs to some, toads to others, and no amphibians at all to the remainder. He then surveyed the invertebrates within each enclosure, to actually measure the effect of toads (compared to native frogs) on the numbers and types of insects remaining. To our surprise, toads were not too different from frogs in their effects: both changed invertebrate numbers to about the same degree, though (probably because they are bigger) toads had a bit more effect.

©Ben Phillips

Hagman, M., and R. Shine. 2007. Effects of invasive cane toads on Australian mosquitoes: does the dark cloud have a silver lining? Biological Invasions 9:445-452.

Several people have reported that mosquito numbers have fallen since toads arrived. Why would this happen? Adult toads rarely eat adult mosquitoes, so we thought the effect might occur in the larval stage. Thus, Mattias set up tanks with mosquito larvae either with or without cane toad tadpoles. The toad tadpoles increased mortality in one species of mosquito, and reduced body sizes at emergence in the others. Also, female mosquitoes avoided laying their eggs in ponds containing toad tadpoles. These effects are similar to those we have already shown with native frogs: it seems that mosquitoes don’t really like either frogs OR toads. This doesn't mean that toads will reduce mozzie numbers - that depends on many factors, such as whether the mosquitoes are breeding in mangroves or in freshwater ponds, etc - but it's an interesting example of the fact that invasive species have complex effects - and some people would rate some of those effects as positive, not negative.

Greenlees, M. J., G. P. Brown, J. K. Webb, B. L. Phillips, and R. Shine. 2007. Do invasive cane toads (Chaunus marinus) compete with Australian frogs (Cyclorana australis)? Austral Ecology 32:900-907.

Using the enclosures that he constructed on the floodplain fringe (see above), Matt put combinations of toads and frogs together (versus separately) to see if the presence of a toad reduced the feeding rate or activity level of a native Giant Burrowing Frog. The frogs were less active if toads were present in the arena, but the overall effect on feeding rate was pretty small. It may be that on hot humid nights in the tropics, the number of bugs is so great that there is little effective competition between frogs, or between frogs and toads. Certainly, anybody who has ever spent time out on the Adelaide River floodplain during the wet-season will testify to the fact that it is one of the buggiest places in the universe!

©Greg Brown

Webb, J. K., G. P. Brown, T. Child, M. J. Greenlees, B. L. Phillips, and R. Shine. 2008. A native dasyurid predator (common planigale, Planigale maculata) rapidly learns to avoid toxic cane toads. Austral Ecology: in press.

It seems that one of the toads' main victims in Australia has been the quoll, a medium-sized marsupial carnivore. Quolls try to eat toads (and just about anything else that moves!), and die as a result because of the toad's toxic poisons. We wondered about smaller relatives of the quoll - small mouse-sized marsupials like the planigale. These little blokes are common on the Adelaide River floodplain - what would happen when the toads arrived? Would the planigales attack them and die, like the quolls? To our relief, planigales proved to be surprisingly smart. A few died after they attacked toads, but most did not - and the survivors rapidly learned that toads were best avoided. Thus, planigales should not be too badly affected by the toad invasion.

Pizzatto, L., and R. Shine. 2008. Native Australian frogs avoid the scent of invasive cane toads. Austral Ecology: in press.

Frogs and toads both lose water quickly in dry conditions, so spend most of the day in shelters where they can stay moist. One way that toad invasion might affect native frogs would be to discourage them from using favoured shelter-sites if those spots are already being occupied by cane toads. Ligia tested the responses of small (newly-transformed) native frogs to various smells, and found that the little frogs avoided places that smell like adult cane toads. However, the little frogs also avoided other strong smells, so this may just be a general response not a specific cane-toad-related behaviour. And we doubt that hiding places are too hard to find, so this may not be a big issue for the local frogs unless they are restricted to the pond margins by dry weather, and so are confronted with lots of toads as neighbours.

Llewelyn, J., B. L. Phillips, and R. Shine. 2008. Sublethal costs associated with the consumption of toxic prey by snakes. Austral Ecology: in press.

One of the only Australian snakes that is immune to the cane toads' poison is the keelback, a harmless species that is widely distributed in the tropics. John followed up some earlier work by Ben, showing that although keelbacks can indeed eat toads without dying, the toads are still pretty awful in terms of food quality. First, the toads' poison tends to slow the snake down - so that it takes a long time to eat a toad, and then may be almost paralysed for quite a while. This could be bad news if a predator comes along. Second, even if the snake eats the toad, there doesn't seem to be much net nutritional value - snakes fed on toads tend to lose weight, whereas snakes fed the same amount of frogs grow bigger. So, although keelbacks can eat toads, they don't get much benefit from doing so.

Letnic, M., J. K. Webb, and R. Shine. 2008. Invasive cane toads (Bufo marinus) cause mass mortality of freshwater crocodiles (Crocodylus johnstoni) in tropical Australia. Biological Conservation 141:1773-1782.
Although there are many anecdotal reports of cane toad invasion resulting in the deaths of native predators that try to eat these poisonous amphibians, there is very little real detail on population-level effects. Our surveys in upstream areas of the Daly and Victoria Rivers show a major impact of toad arrival on freshwater crocodile numbers, and on the distribution of body sizes within the crocodile population. Remarkably, cane toads have little or no effect on freshwater crocs at Fogg Dam, perhaps because the toads rarely come to the water's edge where they would meet crocs - instead, there are plenty of puddles around where the toads can rehydrate. In the drier upstream areas around the rivers, though, the toads and crocs are forced into close contact - with devastating results.