A New Study
A new study by Siviter et al ¹ is one of its first to represent data showing the natural variation in environmental factors during embryonic development, and how it may impact upon the development of behavioural traits during the ontogeny of oviparous reptiles. Behavioural traits are often the first characteristic for an individual to alter in the face of environmental change.
Individual differences in sensitivity to environmental cues are predicted to be part of an animal’s personality . These personalities have been shown to be heritable but can be heavily influenced by environmental factors. Some of these various traits include boldness/shyness, proactive/reactive, explorative/less explorative, each of which can have a profound effect upon the decisions made by some animals in an unpredictable environment .
Boldness has been the research topic of choice for a number of years, studying the willingness of an individual to take risks in a novel environment, and has been investigated across various taxa. It has been linked to foraging success, mating success and higher predation and considered to have widespread implications for fitness . A great example of this was compiled in a study by Carter et al (2010). Carter discovered that bolder males of the Namibian rock agama species would spend more time basking and roaming their home range in sight of predators considerably more than the shyer males. They also had larger home ranges and fed more, however they did suffer a higher tail loss, presumably due to increased predation.
Despite the relative abundance of research into animal personality traits, little has been done to investigate the early life experiences which may occur during embryonic development, particularly on the personality of post-hatching animals. A few previous studies suggested that Siviter and his group were heading in the right direction of some interesting and promising research. Rokka et al (2014) found that the hormone composition of magpie (Pica pica) eggs, which is controlled by laying order, affected the exploratory behaviour of the offspring but in opposite directions for either sex. First hatched females were more explorative and less neophobic (fear of something new or unfamiliar), whereas the males were the reverse.
Much of the behavioural work that has investigated shows the impact of incubation temperature on either hunting or anti-predatory behaviour. For example, cold incubated hatchlings of the three-lined skink (Bassiana duperreyi) exhibited more anti-predatory behaviour than hot incubated hatchlings (Flatt et al. 2001). However, the opposite can be said for bearded dragons (Pogona vitticeps). Lizards incubated at hotter temperatures ran faster and were better at foraging than those incubated at cooler temperatures. This is where Siviter decided to expand upon his own research and further investigate how incubation temperatures and boldness were related.
Siviter and colleagues used 13 bearded dragons from the same clutch, they were randomly divided and incubated under two different heat conditions, a ‘cold’ group and a ‘hot’ group. Once hatched they were exposed firstly to a novel object and then a novel environment. At the start of the first experiment (the object), a lizard was placed diagonally opposite facing the object on the other side of the arena. Phase 1 (short term) consisted of two trials separated by 4 or 5 days exposing them to two different objects when they were 10 months old. Phase 2 (long term) trials were a repeat of the experiment but 5 months after phase 1 with another two different objects. Experiment two was identical to the previous with some key changes. Instead of objects, different environments were used. For example, different material was placed on walls and the floor of the arena. Phase 1 trials were conducted when the animals were 12 months old and phase 2 was repeated 3 months later, again with different environments.
There was no long-term impact of incubation temperature on behaviour, however, it did impact upon behaviour in the short term. In the novel object experiment, the ‘hot’ group initially spent more time in close proximity to the object than the ‘cold’ group did, yet the opposite was true when tested in the later phase of the experiment. Siviter found that the results suggested an influence on the development of behavioural traits rather than personality, the lizards from the ‘hot’ group were initially bolder and more explorative than the ‘cold’ group.
The findings of this study highlight the importance of re-testing animals over a long period of time, yet many of the experiments testing animal personality only look at repeatability over short periods. Had long term repeatability not been tested, the results may have been interpreted in a completely different way. The comparison of both the short and long term effects gives the first insight into the impact of incubation temperature and how the behavioural traits are developed along with personality. These results suggest they differ at different stages of the reptile’s ontogeny – their development through to maturity – and most likely influencing their success.
Like these results and Siviters previous research on foraging intake and boldness/exploration, a positive correlation has been observed in a range of animals. Egg incubation temperatures have been shown to affect the growth rates of bearded dragons with heavier lizards being produced from ‘hotter’ temperatures in comparison to smaller lizards from cooler temperatures, which could suggest why the ‘cooler’ group were less inclined to explore due to a much smaller body size and perhaps did not want to expose themselves to any risk of predation. This is also very similar in a study by Tokarz (1985). Tokarz investigated the importance of size in determining dominance in the male brown anoles, discovering that larger males would defend perch sites more successfully, be the first to enter other males’ territory along with more challenge displays than the smaller males. Head nods which are usually only seen in subordinate individuals were also only seen in the smaller males.
These personality traits are intrinsically linked to animal cognition and the process of acquiring knowledge and understanding. Cognition ultimately impacts upon most behaviours (Gregor et al. 2015), and recent research has started to investigate the possibility that variation in cognition is functionally related to variation in personality and that it could be fundamental in understanding individual differences in cognitive performance (Sih Del Giuduce. 2012).
The mechanisms that cause temperature-dependant differences in phenotypes, behavioural traits or personality are still not fully understood. Although Siviter and colleagues produced some useful data that has rarely been collected, there are some important considerations in the interpretation of the data. For example, the sample size was very limited and replication with additional animals is an important next step. Additionally, only one clutch was used in this experiment, perhaps indicating that the results may not be generalised to the population of bearded dragons. Although only the one clutch was used, the results are sufficiently interesting to instigate more research.
As we all know, human-induced environmental change, such as global warming, affects habitats worldwide and the environment that the lizards are incubated in are reflective of the environment they are emerging into. Therefore, a warmer environment may produce animals that are better adapted to survival in that temperature profile . The sensitivity of oviparous reptiles to environmental factors may make them more flexible in dealing with changing environments and their behavioural plasticity may fight these effects allowing time to develop the diversity required for their new environment, as well as being a valuable defence mechanism for tackling human-induced climate change (Holleley et al. 2015).
Behavioural responses need more investigation in a wide range of species to help us understand not only reptilian cognition but also the possible effects of anthropogenic climate changes.