Females of this large bodied species can attain snout-vent lengths of over 225 mm, though most adults range from 85 to 150 mm. Adult males are yellowish-brown in color, with the yellow being most pronounced along the sides and the throat. Females and immature males have irregular brown blotches on their dorsal surface. The skin of both sexes is covered by irregularly scattered warts. In sexually active males these warts bear horny spicules. The parotoids are relatively large and often triangular and swollen in appearance. Cranial ridges are well developed. The tympanum is distinct, and the interorbital is concave. The webbing is poorly developed. The nuptial pads in the males are dark on the first 3 fingers (Easteal 1963).
Distribution and Habitat
Country distribution from AmphibiaWeb's database: Anguilla, Belize, Bolivia, Brazil, Cayman Islands, Colombia, Costa Rica, Ecuador, El Salvador, French Guiana, Guatemala, Guyana, Honduras, Mexico, Nicaragua, Panama, Peru, Suriname, Trinidad and Tobago, United States, Venezuela. Introduced: Antigua and Barbuda, Aruba, Australia, Barbados, Dominican Republic, Fiji, Grenada, Guadeloupe, Guam, Haiti, Jamaica, Japan, Martinique, Mauritius, Montserrat, Northern Mariana Islands, Papua New Guinea, Philippines, Puerto Rico, Saint Kitts and Nevis, Saint Lucia, Saint Vincent and the Grenadines, Solomon Islands, Taiwan, Virgin Islands, U.S..
U.S. state distribution from AmphibiaWeb's database: Florida, Hawaii, Texas
Naturally occurring populations are found from the southern tip of Texas and northwestern Mexico to central Brazil. Introduced populations have been established in the Caribbean and Pacific regions. Upper altitudinal limits vary with latitude, from 500 m in Sinaloa, Mexico to 1600 m in Venezuela (Easteal 1963). Bufo marinus was introduced to Northern Queensland, Australia in 1929 and has since spread throughout much of the continent (Lampo and De Leo 1998).
Life History, Abundance, Activity, and Special Behaviors
The mating call is long and has 12 notes/sec. The dominant frequencies are .35 and .7 kHz without frequency modulation to clear harmonics ((Cole et.al. 1968).
Potential Impacts: This toad is highly toxic, secretions from the skin serve as a natural defense against predators. The toxin is stored in the ovum, not the jelly coat. The toxicity of cane toads shifts rapidly during the course of their lifetime. Toad eggs are extremely toxic, while later-stage tadpoles were less toxic. Animals that bite these toads are often seriously affected, many are killed. Domestic dogs and cats often are killed by the toxins. Native wild animals are also affected especially in places where the toads are introduced (especially in places like Australia and in Florida, USA). Marine toads may compete with and prey upon native amphibians (Rabor in Krakauer, 1968).
As an invasive species, cane toads have the potential to bring harmful viral pathogens to native fauna. Scientists found that cane toads carried three of the 14 protozoan species of neotropical origin. Cane toads in Australia also contain nematode lungworm, a parasite endemic to the Americas. While these pathogens and parasites could potentially infect native frog species, observations show that infection has not spread to other native species.
In addition to the direct impacts that cane toads may have on native fauna, the cane toad may also have indirect impacts on food webs and population dynamics. For example, predation on dung beetles by cane toads could reduce dung breakdown rates, affecting many aspects of ecosystem function. Toad invasion can massively reduce the population of predators such as varanid lizards, elapid snakes, freshwater crocodiles, and northern quolls that die from consuming cane toad toxins.
Metamorphosis occurs in tadpoles between 14 and 28 days after hatching, depending on the water temperature. Eggs hatched late in the dry season will metamorphose before the late wet season, when food is most readily available and moisture stress is reduced (Lampo and De Leo 1998). After metamorphosis these toads leave the water and seek diurnal refuges to avoid dehydration and predation. B. marinus uses hollow trees for shelter during the dry season and dense vegetation during the wet season in addition to rock crevices which are used throughout the year (Seebacher and Alford 1999). These toads show more nocturnal activity and travel greater distances per night in Australia than in South America (Seebacher and Alford 1999).
Introduced to many localities in the first half of the 20th century as a means to control sugar cane pests, these toads have undergone a population explosion. They have established widespread ranges in these non-native areas and attain higher densities in Australia than they do in their native range (Lampo and De Leo 1998). This has caused much concern about the effects of B. marinus on native anuran species. The eggs and hatchlings of B. marinus are toxic to many native predators in Australia. The predatory frog species Limnodynastes ornatus showed decreased survivorship where it co-occurred with B. marinus. However Limnodynastes rutella, which is preyed upon by L. ornatus, showed increased survival rates (Crossland 2000). Bufo marinus can inhabit open land near human habitations and sugarcane fields.
B. marinus is an opportunistic feeder, and readily feeds upon land snails as well as centipedes, cockroaches, beetles, grasshoppers, ants, and small field mice. Stomach samples of this species in particular areas of the world show that terrestrial gastropod prey can comprise over 40% of the stomach contents (Hinckley 1963; Bailey 1976; Grant 1996). In laboratory trials under wet conditions, it was found that B. marinus readily consumes a range of camaenid snail species. This makes it unique from other anurans that typically do not prey upon molluscs. While some scientists have proposed that B. marinus consumes vertebrates (e.g., ground-nesting birds such as bee-eaters), this has been found to be very rare. In follow-up studies, cane toads avoided (rather than selected) birds and their eggs as prey (Merops ornatus: Boland 2004).
B. marinus is primarily diurnal in activity, and lacks defense against ant species that are immune to toad toxins. Freshwater crayfish, adult dytiscid diving beetles, dragonfly larvae, and mosquitoes are among other native invertebrates that feed upon the cane toad without ill effects. However, leeches and aquatic snails that feed on larval or adult toads often die from the toxins that are consumed. It should be noted that other invertebrates vary in their susceptibility to toads. Some dytiscid diving beetle larvae can consume hatchlings and tadpoles while others die. Belastomatid giant water bugs can consume some developmental stages of tadpoles without ill-effect, but not others.
Cane toads can serve as an additional food source to vertebrates. Snakes like the keelback (Tropidonophis mairii) and slatey-grey snakes (Stegonotus cucullatus) can consume the cane toads without dying, but keelbacks do show ill-effects. When given a choice, keelback snakes prefer native frogs to cane toads (Llewelyn et al. 2009b). Similarly, raptors consume road-killed toads, but preferentially select native frogs when given a choice. Water rats and introduced black rats frequently consume cane toads.
Most species of Australian freshwater fish often mouth and spit out cane toad early life stages without apparent ill effects. For the fly-specked hardyhead (Craterocephalus stercusmuscarum), the banded and spangled grunters (Amniataba percoides and Leiopotherapon unicolor respectively), the purple-spotted gudgeon (Morgurnda adspersa), glassfish (Family Ambassidae), western rainbowfish (Melanotaenia australis), and black catfish (Neosilurus ater), consuming cane toad eggs or tadpoles are toxic. However, most fish avoid cane toad early life stages because they can detect their noxiousness. For most fish, toad eggs are likely to be more lethal than toad tadpoles.
In 1975, Covacevich and Archer reported that saltwater crocodiles (Crocodylus porosus) could ingest cane toads without ill-effect, while freshwater crocodiles (Crocodylus johnstoni) died after mouthing or ingesting cane toads (Begg et al. 2000). In 1990, Freeland found that C. johnstoni actively hunts and ingests cane toads. It was later reported in 2009 that the deaths of some freshwater crocodiles in the Daly River were due to toad ingestion. A population-level impact study found 34 dead freshwater crocodiles in the Victoria River caused by cane toad ingestion (Letnic et al., 2008). Evidence included a wave of crocodile deaths moving upriver and coinciding with the areas in which cane toads were invading, as well as toad remains found in the stomachs of some of the dead crocodiles.
Turtles also vary in their tolerance for cane toad toxicity. In some cases, turtles ingest toads without ill effect, while other predation attempts by the same species result in fatality. The long-neck turtle (Chelodina sp.) ate a dead cane toad without ill effect, while the saw-shelled turtles (Elseya latisternum) and Krefft's river turtles (Emydura krefftii) consumed toad tadpoles and were also unaffected. In a study by Kruger, long-necked turtles (Chelodina rugosa) seized and spit out toad tadpoles and survived, yet died after consumption of toad eggs.
The area of Australia invaded by cane toads contains high densities of lizards belonging to several phylogenetic lineages. The impact of toads across the lineages is highly nonrandom. Fatal poisoning occurs in three lineages: the Varanidae (goannas), the Scincidae (skinks), and the Agamidae (dragons). Varanids often grow to large body sizes and are therefore more likely to attack a large cane toad more readily than a lizard from most other lineages. Varanids often die after ingesting or mouthing cane toads. Most species of scincid lizards have a relatively small adult body size. Due to their small body size, most Scincid lizards are unlikely to be at risk from consuming a cane toad large enough to kill them. However, some species such as the bluetongue skink (Tiliqua scincoides intermedia) readily consume small toads and die as a result.
Snake species at risk from cane toads are frog-eating species that cannot tolerate toad toxins, can swallow toads large enough to be fatal, and whose geographic distribution overlaps with that of the cane toad. Using this criteria, 49 snake species were found to be potentially at risk from toad invasion. The toad invasion is a potential threat to 70% of the Australian colubrid snakes, 40% of the pythons, and 41% of the elapids. Nine of the species that were deemed "at risk" are currently recognized as threatened species on a federal or state level. Recent work on the feeding patterns of snakes in captivity have shown that species that were previously identified as potentially at risk species were reluctant to take cane toads as prey, or immediately rejected the cane toad after striking it. Colubrids and pythons appear to be less at risk than the elapid snake species, with high mortality rates in death adders (Acanthophis praelongus), black whip snakes (Demansia papuensis), and king brown snakes (Pseudechis australis)(M. Greenlees et al. unpubl. data in Shine 2006).
In 1975, Covacevich and Archer reported that some crows (Corvus sp.) and kookaburras (Dacelo novaeguineae) died after mouthing cane toads, while other individuals of the same species consumed young or road-killed toads without showing any ill effects. At least seven native bird species can eat toads successfully. Reasons for this tolerance may be because they eat only non-toxic parts of the toad or because they are immune to the toxins. In 1997, Dorfman predicted that 76 species in the Kakadu National Park were potentially under threat from cane toads. However, more recent studies by Beckmann and Shine in 2009 have concluded that cane toads appear to have minimal impact on Australian birds. The ability to survive toad invasion may be due to a widespread physiological tolerance of bufotoxins, perhaps reflecting close genetic ties between Australian birds and taxa in Asia, where many bufonid species are similar in toxicity to the cane toad.
In 2004, Webb and Glanznig listed nine species of native mammals and two species of introduced mammals as potentially at risk from cane toad ingestion. Due to previous reports of domestic dogs dying from mouthing cane toads, dingos may also be at risk. Feral cats and pigs also may be negatively affected by cane toads. In captivity, native rodents (Melomys burtoni, Rattus colletti, R. tunneyi) readily killed and consumed small toads, but did not appear to suffer any ill effects. Other rodents (Must domesticus, Pseudomys nanus, Zyzomys argurus) did not attack cane toads as prey at all. Small dasyurid marsupial species, like planigales (Planigale ingrami, P. maculata) and dunnarts (Sminthopsis virginiae) initially attacked toads, became ill as a result, and were later reluctant to attack toads. Relatively few of these dasyurids died as a result of attempting to ingest toads. The species of mammal that is most often a victim of toad invasion is the northern quoll, Dasyurus hallucatus. Lethal ingestion of cane toads is responsible for the local extinction of northern quoll populations in the Mary River region of Kakadu National Park. One study showed that only four of 14 quolls that died from toad ingestion, suggesting that perhaps other factors may be playing a role in quoll extinction. However, more recent studies show that seven out of eight quoll deaths resulted from ingestion of cane toads. In addition, quolls have been killed by toads in areas where very few toads were spotted, suggesting that even in low toad density areas, quolls are still threatened.
Cane toad impacts are affected by attributes of toad biology. Cane toads have a multiphasic life history. Eggs and tadpoles reside in waterbodies, metamorphs are only found in riparian areas, and larger juveniles and adults occupy large areas of drier landscape. In addition, the types and amounts of toxins present in toads vary throughout their lifetime. One specific feature of the cane toad is that their highly toxic eggs are coated in a non-toxic jelly, fooling aquatic predators into consuming them when they would otherwise be able to detect bufotoxins in their prey (Greenlees and Shine 2010).
There also seems to be competitive interactions between cane toads and three species of native tree frogs around waterholes in the Gulf of Carpentaria: Litora pallida, L. rothii, and L. rubella. Evidence suggests that cane toads are likely to have a more adverse impact on larger terrestrial frogs like Cyclorana australis and Limnodynastes convexiusculus. These impacts may be caused by direct behavioral interference rather than competition for food. In a study done in 2009 by Pizzatto and Shine, nine out of ten native frog species avoided sites scented by cane toad chemical cues (all except Litoria rubella), and in 2007, Greenlees et al. found that the presence of cane toads reduced the nocturnal activity levels of the native frog Cyclorana australis. Sexual harassment of female frogs by male toads is also known to occur between native and introduced taxa in other phylogenetic linages (e.g. Valero et al. 2008). In addition to competition involving native anurans, other vertebrate species may also compete with toads for limited resources. In 2004, Boland suggested that cane toads may compete with the nesting burrows of bee-eaters, Merops ornatus.
Cane toads were first introduced into northern Australia in 1936, and have rapidly spread across 1.3 million square kilometers of northern and eastern Australia since (Urban et al., 2007). In northern Australia, toad population expansion has accelerated steadily, from approximately 10 to 55 kilometers per year. On the invasion front, toads move far distances (up to 1.8 km per night), and mostly move in straight lines, actively using cleared areas such as roads as dispersal corridors (Brown et al., 2006; Phillips et al., 2007). This expansion behavior can be compared with toads from older populations. Currently, invasioni front individuals move more often, travel farther per move, and tend to move in straighter lines. However, environmental correlates do not adequately explain these changes (Urban et al., 2008). In order to test whether these differences in behavior resulted from evolution or just plasticity associated with new toad environments, an experiment was created. The dispersal rates of parent populations during the cane toads invasive history was measured and compared to dispersal rates of the offspring. Through this collection method, researchers hoped to minimize environmental effects on on dispersal ability and estimate the heritability of dispersal. 184 toads were radiotracked over two seasons. Toads with larger bodies consistently moved farther than toads with normal or smaller body sizes. The breeding cohort resulted in high levels of variance in daily dispersal of the adult toads. This suggests that differences in birth date can have large influences in dispersal.The estimate of mean within-population genetic variance was large and suggested a narrow sense heritability of 0.24. In addition, there was a log mean daily displacement in this species with a large 95% interval of 0.02-0.72. Thus, there is evidence to suggest that there does seem to be a heritable variance within toad populations for dispersal. This selection on disperse could produce evolutionary change.
Female anurans have been shown to be the choosier sex, selecting mates that show good resource defense techniques or attractive male displays. However, the only way that females can exercise mate choice is by approaching a male with an attractive call or mating site. When a female approaches such sites, she may be intercepted by a "satellite" male that attempts to mate with the female by grasping her in amplexus (Wells 2007). This is particularly common in areas of high density. In a recent study conducted with B. marinus, it was found that perhaps females are not just passive participants in the mating process, but rather have a mechanism of exercising control. Evolutionarily, frogs and toads are able to protect themselves from predators by inflating their bodies. This increased girth may deter predators who may be threatened by an anuran that is too large to ingest. Similarly, female anurans frequently inflate their bodies when being amplexed from males. In this sense, the female cane toads could facilitate male-male competition by reducing the ability of undesirable mates to grasp onto her. In females who were unable to inflate their bodies, the small "satellite" male was aable to grasp onto the female despite takeover attempts by larger rivals. Body size of mates is a factor in female mate choice because fertilization success is highest when the males and females are close in size. Since females are the larger size, choosing a larger male increases the fitness of their offspring. Larger males are often able to displace smaller males, but smaller males rarely displace larger rivals. However, smaller males can resist takeover attempts if the female does not inflate her body. Therefore, this study shows that takeovers by larger males can show female and male tactics in sexual selection.
Trends and Threats
According to the Global Amphibian Assessment (GAA), Bufo marinus is of least concern with regards to extinction and its population is actually increasing. There are currently no significant threats to this species. Some animals that have been introduced to Puerto Rico carry salmonella, putting species that consume them at risk. In Bermuda, survival and development of tadpoles are being negatively affected due to contaminants in ponds and the transfer of accumulated contaminants (Bacon et al. 2006).
Relation to Humans
Humans are affected by the toxic skin secretions. Symptoms include irritation of the skin and burning of the eyes (Wright and Wright, 1949; Krakauer, 1968, 1970; Behler, 1979; Carmichael and Williams, 1991; Conant and Collins, 1991).
Marine toads have probably been introduced more widely than any other amphibian in the world (Behler, 1979; Carmichael and Williams, 1991). They have been introduced as a control agent for insects that damage sugarcane (Riemer, 1959; Krakauer, 1968; King, 1970), however, since they are nocturnal and many sugarcane pests are diurnal, they are not effective biocontrol agents. For information on introductions into Australia, please see Cane toad-Australia
Human perceptions of the cane toad are largely negative, with many fearing its size, appearance, and potential invasive impact on Australian communities. Between 1986 and 1996, the state and federal government spent more than $9,500,000 (Shine et al. 2006) and enormous efforts by volunteers to control local toad populations by collection. However, these negative community perceptions of invasive species impacts are often in error. In actuality, the direct impact of cane toads falls heavily on a small number of native taxa, not a wide spectrum of native fauna. Others suggest a more direct effect on humans by claiming that toads are poisoning waterholes with their toxins and are even leading to drug abuse for humans that become addicted to licking the toads or smoking their dried skins (Clarke et al. 2009). However, there are no data that actually supports these claims.
Recently, humans have thought that a native predator, the meat ant (genus Iridomyrmex), could control the invasive cane toad populations. In an experimental setting, 98% of metamorph toads encountered at least one predator ant species in a high density ant environment, while 87% of metamorph toads encountered at least one predator ant species in a low density ant environment. Most of these toads were attacked, with 82.5% killed at high density and 51% killed at low density. From these data, it appears as though high ant density is more effective at controlling cane toad populations. There are multiple reasons to explain why this may have occurred. First, higher density inevitably leads to more encounters. In addition, at higher ant densities, ants are more likely to swarm onto a prey item. Lastly, higher ant densities increase the body-size threshold at which metamorph toads shift their defense mechanism from the active escape tactic to crypsis. As a result of this higher ant density, more toads of a wider range of body sizes are killed.
In the field, smaller metamorph deaths were most commonly observed. This may be because small metamorphs have a less effective response to ant attack and are also more vulnerable to disease and parasites, such as lungworm (Kelehear 2007), as well as dessication (Child et al. 2008). In addition, the low level of toxins in metamorphs make them even more susceptible to predator attack.
However, it should be noted that meat ants alone are not entirely feasible for controlling cane toad populations. Meat ants are only effective at killing metamorph toads, and may be less effective during the wet-season recruitment events. Increased ant densities might even serve as a food source for adult cane toads. Many are also cautious because using biocontrol in other instances have often vastly negatively affected ecosystems in ways that could not have been predicted. In this instance, the predator is already a native species and is simply being redirected to areas with high populations of cane toads. But reduced or increased populations of ants in certain areas may still affect native fauna, and meat ants can still competitively exclude ecologically similar ant species.
Rick Speare has compiled a bibliography on this species up until 1991:
This species has a diploid chromosome (2n) with 5 large pairs and 6 small ones, which makes 22 chromosomes in total (Cole, Lowe, and Wright 1968).
A Spanish-language species account can be found at this website of Instituto Nacional de Biodiversidad (INBio).
Brown, G. P., Shine, R., Ward-Fear, G. (2010). ''Using a native predator (the meat ant, Iridomyrmex reburrus) to reduce the abundance of an invasive species (the cane toad, Bufo marinus) in tropical Australia.'' Journal of Applied Ecology, 47, 273–280.
Bruning, B., Phillips, B.L., Shine, R. (2010). ''Turgid female toads give males the slip: a new mechanism of female mate choice in the Anura.'' The Royal Society- Biology Letters
Crossland, M. R. (2000). ''Direct and indirect effects of the introduced toad Bufo marinus on populations of native anuran larvae in Australia.'' Ecography, 23(3), 283-290.
Drewes R. C., Roth B. (1981). ''Snail-eating frogs from the Ethiopian highlands: a new anuran specialization.'' Zoological Journal of the Linnean Society, 73, 267-287.
Easteal, S. (1963). ''Bufo marinus.'' Catalogue of American Amphibians and Reptiles. American Society of Ichthyologists and Herpetologists, 395.1-395.4.
Ingram, G. J., Covacevich J. (1990). ''Tropidonophis mairii vs. Bufo marinus. Memoirs of the Queensland Museum.'' , 29(396).
Lampo, M. and De Leo, G. A. (1998). ''The invasion ecology of the toad Bufo marinus: from South America to Australia.'' Ecological Applications, 8(2), 388-396.
Llewelyn J., Phillips B. L., Shine R. (2009). ''Sublethal costs associated with the consumption of toxic prey by snakes.'' Austral Ecology, 34, 179–184.
Llewelyn, J., Schwarzkopf L., Alford R., Shine R. (2009). ''Something different for dinner? Responses of a native Australian predator (the keelback snake) to an invasive prey species (the cane toad).'' Biological Invasions
Maeda, N. and Matsui, M. (1990). Frogs and Toads of Japan, 2nd edition. Bun-Ichi Sogo Shuppan Co., Ltd., Tokyo, Japan.
Phillips, B.L., Brown, G.P., Shine, R. (2010). ''Evolutionarily accelerated invasions, the rate of dispersal evolves upwards during the range advance of cane toads.'' Journal of Evolutionary Biology
Seebacher, F. and Alford, R. A. (1999). ''Movement and microhabitat use of a terrestrial amphibian (Bufo marinus) on a tropical island: seasonal variation and environmental correlates.'' Journal of Herpetology, 33(2), 208-214.
Shine, R. (2006). ''The Ecological Impact of Invasive Cane Toads (Bufo marinus) in Australia'' Quarterly Review of Biology: in press.
Written by Sarah Ng (sarahkng AT berkeley.edu), UC Berkeley
First submitted 2001-01-31
Edited by SN (2010-11-18)
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