Cane Toads, Toadzillas of the Outback

Recently a giant cane toad was found In Australia weighing a massive 2.7 kilos and measured 255mm. The Guiness world record of a pet cane toad from Sweden was 2.65kg  in 1991. The toad was found in Conway National park, located in North Queensland. Rangers had been clearing a path when this giant toad was spotted. Here’s a back story on how this invasive giant came to be and the arms race the native wildlife are facing.

Figure 1: A captive bred cane toad, (Authors own, 2023).

Why Are Cane Toads in Australia?

The cane toad (Bufo marinus or Rhinella marina) are native to South America and Central America. Their defence mechanism is a milky toxin which they secrete from the parotoid glands located on their shoulder pads. This poison is called bufotoxin and contains different chemicals such as bufagin (this affects heart rate) and bufotenine (a hallucinogen). They will breed almost all year round and lay between 8000- 30,000 eggs in long strings of fresh water, the eggs and tadpoles also contain toxins. They will inhabit a range of habitats such as grasslands, edges of rain forest, mangrove swamps and urban or agricultural areas.

Originally they had been released into Australia for sugarcane plantation pest control. The hope was that the toads would eat the cane beetles, and be a natural pest control (Cogger, 2014). Ironically the beetles sat too high on the sugar cane for the toads to predate on and the larvae remain underground out of reach, the toads never predated on the beetles and instead moved to greener pastures fairly quickly. Originally they released 2,400 toads, it is now estimated the population is several millions and are ever expanding the range they populate. Their toxin and size means they have no natural predators in Australia, their diet is very flexible and will consume a variety of food items from insects to food scraps, their reproduction rapid which has led to such a population boom.

They are now considered one of the worst invasive species in the world (National geographic, 2023). The Mitchell’s water monitor (Varanus mitchelli) had been severely impacted by the toads, the species has seen a 97% decline in some areas and is directly linked to the monitors predating on the toads and suffering from the toxins cane toads produce which is vastly different from any of the native species (IUCN, 2018). Their toxins have been known to kill any pet or native species that licks, bites or attempts to eat the toads and they are able to outcompete the native amphibian species within their range.

The Venomous Snakes Arms Race Against the Toads

Figure 2: Dorsal view of the large parotoid glands at the shoulders (Authers own, 2023).

There was hope that the native snake species that have amphibians within the diet would adapt and start to include this abundant species, would species be able to tolerate the toxins and begin to consume the toads. Studies have been carried out on the native frog eating snake populations such as the keelback (Tropidonophis maririi), which showed a stronger resistance to the poison. In the wild they seem to select for native species over the invasive. This pattern was shown even in laboratory settings, they took it a step further and force-fed keelbacks toads which resulted in regurgitations, something that didn’t happen when they were force fed frogs. So despite the high availability, and although the snakes are able to survive the toads toxin, the regurgitator aspect leads to little value in their consumption and therefore  they are widely avoided as prey items (Llewelyn et al., 2010).

An earlier study estimated that as many as 49 Australian snake species had been impacted by the toads presence (Philliphs and Shine, 2004), but it also highlighted the link between head shape and therefore swallowing (gape limit) capacity and head to body mass ratio being linked to the snakes ability to tolerate the toxins. The suggestion was that there would be an evolutionary pressure that those with smaller heads compared to body mass would be less likely to consume a toad large enough to poison them, therefore we would so a morphological change to the population over time. Four species were included in the study, two non vulnerable species (swamp snakes (Hemiapis signata) and keelbacks) and two vulnerable Red-bekkied black snakes (Pseudechis porphyriacus) and green tree snakes (Dendrelaphis punctulatus)). Using preserved specimens and populations outside of the range if the cane toads comparisons were made of the head shapes and body to head ratios. The prediction was true, those species that are vulnerable to the toxins are slowly evolving a reduced head size to avoid consuming cane toads large enough to cause death. 

Take Away Message

It seems that so far, cane toads are here to stay, whether native species will learn to adapt to avoid or co-exist is difficult to predict. It highlights the significant impacts careless releases can make and why it is so important that as a hobby, we take great care in making sure we dont let loose the next toadzilla to rampage on unsuspecting wildlife.

References

Australia’s reptiles threatened by invasive species, climate change – IUCN red list. IUCN. (2018, July 5). Retrieved February 6, 2023, from https://www.iucn.org/news/species/201807/australias-reptiles-threatened-invasive-species-climate-change-iucn-red-list

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, 12(5), 1045–1051. https://doi.org/10.1007/s10530-009-9521-5

Phillips, B. L., & Shine, R. (2004). Adapting to an invasive species: Toxic cane toads induce morphological change in Australian snakes. Proceedings of the National Academy of Sciences, 101(49), 17150–17155. https://doi.org/10.1073/pnas.0406440101