#BeardedDragon

Curious Case of Self-Inflicted Injury in Bearded Dragons

/ gtattersall

A study from the lab has just been published on a rare behaviour in our bearded dragons.  Missing toes are a reasonably common observation in group housed lizards, with the usual explanation being hungry cage-mates attacking one another, perhaps confusing toes with food or perhaps simply as possible food. However, we report on something quite different here. Over a span of two years, Melanie Denommé (PhD candidate) meticulously observed the behaviour of P. vitticeps housed in laboratory settings and noted that on rare occasions, the lizards would bite their own toes!  We saw this in both juvenile and adult individuals, albeit infrequently. Interestingly, the behaviour occurred in the presence of various environmental conditions, including loose substrates, hot surfaces, and during periods of ecdysis (shedding of skin).

The self-directed toe-biting behaviour involves the lizard assuming a distinctive posture, often resembling a C-shape, before striking at its toes with its tongue. This behaviour, which closely resembles regular eating behaviour, typically lasts for less than a minute. When we saw a call for papers on rare behaviours and proposals for hypotheses for these behaviours from the journal In&Sights, we decided to submit our observations there, and experience the new, open access form of peer review. 

While the function of this behaviour remains unclear, we proposed several hypotheses to elucidate its purpose.

One hypothesis suggests that self-directed toe-biting may serve a grooming or cleaning function, aiding in the removal of loose particulate matter caught between the toes or facilitating ecdysis. However, evidence supporting this hypothesis is inconclusive, as the behaviour was not consistently associated with shedding cycles, and no instances of using the mouth for shedding have been observed in related lizard species.

Another hypothesis posits that self-directed toe-biting may be a maladaptive behaviour resulting from captivity-induced stress or discomfort. This hypothesis is supported by observations of the behaviour occurring after extensive basking or gaping, potentially indicating overheating or discomfort in the lizards. However, the exact stimuli triggering the behaviour remain unclear, and further research is needed to explore its underlying causes.

A third hypothesis suggests that self-directed toe-biting may be a form of self-injurious behaviour linked to perceived stress or neurological factors. While this hypothesis offers insights into potential behavioural abnormalities, no clear correlations between self-directed toe-biting and other stress-related behaviours were observed in the study population.

Throughout the study, we meticulously documented occurrences of self-directed toe-biting using time-lapse cameras. Despite its infrequent occurrence, the behaviour was noted in multiple individuals, suggesting that it may be a natural variation within the species rather than an isolated anomaly.

This study on self-directed toe-biting in captive Pogona vitticeps offers valuable insights into the complexities of animal behaviour and adaptation. By exploring potential hypotheses and documenting observations meticulously, we hope to encourage future research aimed at unravelling the mystery behind this intriguing behaviour and its significance to the lives of these fascinating reptiles.

Illustration of the steps of self-directed toe-biting behaviour. The lizard begins by raising their leg close to their face, then may straighten the leg or curve the head towards the foot. The lizard may also bend their head down to meet their foot. Finally, leading with the tongue, the lizard strikes at their toes repeatedly. The behaviour ends when the foot returns to its original position

Citation

Denommé, M and Tattersall, GJ. 2024. Self-directed toe-biting in captive Pogona vitticeps. In&Vertebrates https://doi.org/10.52732/VOZL2285

Link to Paper

https://inandsight.science/journal/papers/65259e2f890bbf5b40a4ef45

Link to Video Demonstrating the Behaviour

https://doi.org/10.5683/SP3/JQ7UGP

 A time-lapse based video clip of Pogona vitticeps demonstrating toe-biting behaviour. Videos were generated from still images captures at 1 frame per second, with playback at 30 frames/second, so the behaviours are sped up ~30 times.

Dragons like to face the heat

/ gtattersall

At long last, my former MSc student, Ian Black’s first paper has been published!  (Citation below)  Ian graduated last year and moved to Ottawa, but has been maintaining contact and working with me to write his work up (so far, we have a book chapter in review and a second manuscript being edited now).

To many that keep lizards as pets, the results of this study might not be so surprising, as articulated by this meme:

 

From a scientific perspective, however, these results might not have been predicted.  So, what did we show? In a nutshell, we demonstrated that bearded dragons prefer to keep their heads facing toward the heat when given a choice.

To thermoregulatory biologists, this is intriguing.  Why? It is often stated in the herpetology literature that lizards are either heliothermic (i.e. they bask in the sun) or thigmothermic (i.e. they warm up via contact with the substrate), and an implicit corollary is that if a species is known to be heliothermic, it cannot respond to contact based thermal cues (i.e. they cannot sense heat via the skin).  Thus one might not expect them to orient toward or away from this kind of heat source or thermal gradient if they are heliothermic/baskers.

We conducted this study by creating an artificial thermal gradient to allow the lizards the chance to move and select different temperatures throughout the day.

Banner in gradient

Thermal image of a bearded dragon in a temperature gradient (~15C on left, ~42C on right) orienting toward the warm end.

Our paper clearly demonstrates that bearded dragons are very capable of orienting their body along a gradient of temperature, and thus are quite thermally “aware” of the environment around them.  We also demonstrate that both adults and neonates show this behaviour.  Finally, as the lizards choose warmer temperatures later in the day (i.e. move up the gradient), they orient less and less toward the heat, suggesting that they are capable of using their orientation to keep their heads from getting too warm.  Therefore, orientation behaviour is used to fine-tune their thermoregulatory control.  This is analogous to how Galapagos marine iguanas use sky-pointing orientation to maximise solar absorption int he early morning and maximise convective cooling later in the day to prevent overheating (Bartholomew, 1966. Copeia. p 241-250).

 

Please find links to the study below and consider sharing our findings.

Citation and Links

Black, IRG and Tattersall, GJ. 2017.  Thermoregulatory behavior and orientation preference in bearded dragons.  Journal of Thermal Biology. 69: 171-177. doi:doi:​10.​1016/​j.​jtherbio.​2017.​07.​009

For a limited time (until Sept 13, 2017) the paper is available for free for anyone that does not have a subscription here:

https://authors.elsevier.com/a/1VRVT15hXtsbUW

but I will provide permanent links to the pre-print version: Post Review Version or from the Brock University Respository.