We show that environmental characteristics that are likely to be important under natural conditions indeed affect movement decisions of juvenile common lizards in an experimental laboratory setting. Moreover, we find the back pattern type of the lizards to be a good predictor of movement behaviour under certain circumstances, and that population effects may play a role. In addition, we found high phenotypic plasticity in the movement behaviour with respect to the cover of the surroundings.
Specifically, we found a correlation of cover with back pattern type. Linear individuals seem to be bolder in overcoming the inhibiting effects of a habitat patch without cover, as seen in their faster movement through the channels compared to reticulate individuals. This matches the finding of higher immigration rates of linear individuals, compared to reticulate ones, in natural populations of this species [20
]. This indicates that the different morphs in this species indeed may follow diverging life-history strategies, as there is already some evidence that individuals with linear dorsal patterns grow more slowly and live longer than reticulated morphs, and have a comparatively lower fecundity [20
], Clobert, unpublished data]. Interestingly, the trend observed here of the more explorative and seemingly bolder linear individuals seems counterintuitive, as generally those individuals that are more willing to take risks are also expected to have matching life histories, such as rapid maturity, high fecundity and lower survival probabilities, which is more coherent with the life histories of reticulated lizards. However, it is possible that the measurements applied here do not necessarily reflect boldness and exploratory behaviour, if faster movement does not correspond with increased boldness. Indeed, it might quite be the opposite: linear individuals moved faster in order to avoid dangers such as predation in an unfamiliar environment, which was even exaggerated under unfavourable conditions (i.e. under conditions that were least similar to conditions of their natural habitats).
The fact that dispersal seems to be more common in animals with linear dorsal patterns [20
] also points towards an explanation that is not necessarily linked to bold personality. The fast-lived life histories of the reticulated individuals might in fact be due to differential energy allocation: while linear individuals use their resources to disperse and settle into new habitats early in life, leading to reduced energy levels for growth and reproduction later on, reticulated lizards may make use of their natal sites, allocating all their energy into growth and higher or earlier fecundity with which it is associated.
Even though it is not entirely evident why movement decisions should be correlated with dorsal pattern morphotype, correlational selection for back pattern type and movement or dispersal behaviour constitutes a valid possibility. A correlation between back pattern and flight behaviour has been shown in garter snakes [19
]. Striped garter snakes had higher survival when they took flight without showing a reversal of direction during the predator escape. For spotted snakes there was positive selection for more frequent change of directions during the escape. The adaptive significance of this example of correlational selection could be the perception of being slower when having a striped back pattern [26
]. If the same were true for common lizards, reticulate individuals should be detected easier than linear individuals by visual predators, which could explain the more cautionary behaviour of reticulates. Fitting into this picture, reticulate common lizard females were found to stop more frequently during sprint speed performance trials on a race track, compared to linear individuals [D. Miles, pers.com
.]. The latter study also found that reticulate females had higher endurance when tested on a treadmill. Taken together, the difference in behaviour between linear and reticulate common lizards indicate that there may be two distinct strategies of movement: linear individuals seem to move more between populations and are more likely or faster to make movement decisions, compared to animals bearing reticulate dorsal patterns.
Independent of dorsal pattern, the decision to move into the channel depended on whether the lizards were provided with a light and heat source in the starting container, in combination with humidity in the channel. As predicted, under circumstances where the animals that had been provided with a heat source, they took a longer time to enter the channels, given the channel was humid. Not astonishingly, a cold starting point, where no light and heat is provided, does not constitute a favourable environment and lizards can probably profit by searching for a more favourable patch. Indeed, thermoregulatory behaviour that makes use of available micro-habitat patches that differ in temperature and humidity has been shown in various lizard species [27
]. In addition, individual differences in behaviour and specifically in temperament and personality are expected to affect the propensity to move (and disperse) as shown in a wide range of organisms [2
]. However, here we did not find a strong link between individual personality and the speed to enter or move through a channel. This may indicate that the present design may not be useful to reflect true exploratory behaviour, and thus may be unsuitable to detect differences in personality, such as boldness. It is possible, that due to the acclimation to the laboratory, including maintenance and frequent handling during the trials, the lizards had already learned that the artificial environment in the laboratory is in fact predator-free, and represents relatively risk-limited surroundings. This may have led to a reduced expression of shyness or fear compared to conditions in the field, and potentially resulted in a narrower spectrum of reaction norms of individual differences in movement behaviour under all investigated circumstances, than may be found under natural conditions. However, in other studies on the same species it was found that exploratory and activity-related behavioural traits are correlated with the dispersal decision process [4
]. There is therefore some possibility that these personality traits are either not involved in the transience phase of a dispersal process, or that the artificially created channels and the measurement of movement behaviour within are not related to dispersal decisions in the field. Importantly, we presently assume that the habitat properties that slow down movement as measured here will also affect the dispersal process negatively. However, it has been shown that this is not necessarily true, as boundary effects between differing substrate types and different attractiveness of the various substrates may be based on other criteria than movement facilitation [8
]. This may lead to difficulties for the prediction of dispersal routes or tendency, and it is therefore possible that the here reported factors may not directly affect dispersal behaviour or the dispersal process in the common lizard.
Independent of the conditions in the starting container, lizards moved more quickly into the channel when the channel was not covered. The reason for slower entry into covered channels might be due to the novelty of the environment, paired with the greater inability to assess potential risks associated with moving into a dark refuge area. On the other hand, it is also possible that the lizards moved faster in open channels in their search for a refuge, to escape the open space in the starting container and uncovered channel, representing no protection from potential predators. However, the potential presence of a predator in a refuge or the cost of not being able to forage or bask while in the refuge may have influenced the individuals not to enter covered channels quickly. If the risk of staying outside the refuge is small, as in our design without chemical or visual cues of predators being present, the observed delay of moving into the covered channel may be predicted [31
]. Nonetheless, we also found that individual variation in moving time in covered channels was much higher than in uncovered channels. This increased phenotypic plasticity may be favourable under certain ecological circumstances, and may allow animals that may not be likely to disperse otherwise, to colonise suitable habitat. On the other hand, it may also indicate that the animals may prefer to stay in refuges longer (than crossing open habitat) under certain circumstances, which are dependent on their current metabolic status. This is reflected in the interaction of light at starting conditions, and cover: when no light was provided in the starting container, animals took less time to cross covered channels than open channels. This might indicate that the animals that have not had the opportunity to thermoregulate were less inclined to stay inside the covered space, potentially in search of more favourable conditions (i.e. sunshine and opportunity to bask). Indeed, the fact that movement was faster when no basking opportunity had been provided in general may also point to the fact that the animals may try to evade unfavourable conditions. In cases where it took individuals a long time to cross the channels two factors may have played a major role: the speed of movement itself and the potential reluctance to continue moving (i.e. returning to the starting container after having initiated channel entry). However, we are unable to distinguish between slow rate of movement and returns to the home base from the data recorded in the current study. Even though we are unable to link the respective behaviours with for example dorsal pattern, both slow movement and frequent returns indicate a reluctance to move, and may be linked to less bold behaviour.
All investigated environmental variables had a substantial effect on the time to arrive at the receiving terrarium and finish a trial, once a lizard had inspected the channel. Humidity of the channel substrate had a direct effect, with lizards crossing the channel faster when the substrate was humid. This might indicate that the dry substrate may have posed a greater obstacle, maybe as a generally unfavoured environment, to any exploratory behaviour [see for example [22
]. In general this finding is not surprising, as this lizard species occurs in humid, heterogeneous habitats [21
]. Preferences for conditions that mimic their natural habitats may be imprinted in the animals, or may have evolved as local adaptations for the animals’ exploration and movement decisions. This may also be reflected by the observed population differences.
Interestingly, the presence of substrate in covered channels was not important. Only without cover, lizards took much longer time to arrive at the receiving container when no substrate was in the channel. This makes sense if the primary concern of the lizard is predation: if the terrain is covered and provides shelter from visual predators, moving is relatively safe. If there is no cover, predation risk is much higher and would be exaggerated even more by a surface without good traction (such as smooth plastic, as used here) which would potentially increase the time to escape.