No place like home for a nesting keelback
In Queensland, the keelback snake (Tropidonophis mairii) is probably most famous for its capacity to safely eat the introduced cane toad (Rhinella marina). But there is a lot more to this delightful little freshwater snake than you might think.
The keelback is a small non-venomous snake that rarely exceeds 75-80cm in length. It can be found in coastal areas, usually close to creeks and swamps but also in other environs including suburban backyards. Although they feed primarily on frogs, keelbacks are also known to eat fish, aquatic invertebrates, reptile eggs, occasional small mammals and, yes, some cane toads. Occupying a home range of up to 1km in diameter, the keelback has been recorded travelling up to 900m in a single night. Females lay 5–12 eggs up to three times per year. Young hatchlings are only around 15cm long and apart from the mother’s initial careful selection of an optimal nest site, there is no further parental care for the young snakes.
Cane toad tolerance
While it is reasonably well known that keelbacks can and do eat cane toads, it has only been in the last few years that researchers have investigated why the keelback is able to tolerate toad toxins without major negative impacts while some other Australian snakes have succumbed to these toxic invaders. What they found was that sensitivity to toad toxin was similar between keelback populations that had long been sympatric with toads (~60 years), and keelback populations only recently exposed to toads (~3 years).
This means that tolerance of toad toxins by the keelback is likely due to inbuilt characteristics rather than adaptation to the presence of cane toads. Scientists have concluded that it is the Asian evolutionary origin of the keelback (where its ancestors co-evolved with other toad species with similar toxins) that has resulted in a level of resistance to toad toxin that is around 70 times greater than that present in many other Australian snakes.
However, that doesn’t mean that the keelback will rid Australia of cane toads. While toads can be eaten with relatively minor short term physiological cost, they are definitely not the keelback’s favourite food item. After monitoring the feeding behaviour of keelback hatchlings, researchers found that frogs were overwhelmingly preferred over toads. This was regardless of whether the keelbacks belonged to the above toad-sympatric populations or to the toad natïve populations. This could be due to the cane toad being a substandard meal in a number of ways. Firstly, it can impede the movement capacity of keelbacks for up to six hours following consumption of a cane toad, which leaves the snake more vulnerable to predation. Secondly, the energy required to process the toad’s toxin could be a factor in the reduction in net energy benefit to the snake. In fact, research has shown that the keelback is unlikely to be able to survive on a diet consisting solely of cane toads.
Nest site inheritance
Cases of nest site philopatry (where young return as adults to their place of birth to breed) are well documented in some species such as sea turtles, salmon, migratory birds and sharks, to name a few. A study undertaken by University of Sydney researchers revealed that female keelbacks also return to their place of birth when they are ready to lay eggs. In contrast to the encoded spatial information inherent in some species while they are still within the egg (e.g. sea turtles), their research showed that keelback hatchlings used spatial learning after hatching to enable them to return to their place of birth. In fact, female keelbacks were found to prefer laying their own eggs at sites containing old eggshells, which may even have been the shells they emerged from as hatchlings.
Just when you thought things couldn’t get any more interesting with this little snake, it does have one other quirky little habit: If trapped or handled roughly it will emit a defensive odour that smells just like, well, a fart. However, don’t be tempted to try this at home because while the keelback is inoffensive and non-venomous, it looks almost identical to the highly venomous rough-scaled snake (Tropidechis carinatus). It is worth noting that around 85% of snake bites in Australia are the results of someone trying to directly interfere with the snake. So, as always, leave snakes well alone and just enjoy from a distance.
Written by Julie O'Connor, Sunshine Coast Council senior conservation partnerships officer
Brown, G. P. and Shine, R. 2007. “Like mother, like daughter: inheritance of nest-site location in snakes” Biology Letters 3, 131-133
Llewelyn, J. Phillips, B. L. Brown, G. P Schwarzkopf, L. Alford, R. A. and Shine R. 2011. “Adaptation or preadaptation: why are keelback snakes (Tropidonophis mairii) less vulnerable to invasive cane toads (Bufo marinus) than are other Australian snakes?” Evolutionary Ecology 25, 13-24
Llewelyn, J. Phillips, B. L. and Shine, R. 2009. “Sublethal costs associated with the consumption of toxic prey on snakes” Austral Ecology 34 179 -184.