Some like it hot : camera traps unravel the effects of weather conditions and predator presence on the activity levels of two lizards

CITATION: Broeckhoven, C. & Mouton, P. L. F. N., 2015. Some like it hot : camera traps unravel the effects of weather conditions and predator presence on the activity levels of two lizards. PLoS ONE, 10(9):1-15, doi:10.1371/journal.pone.0137428.The original publication is available at http://journals.plos.org/plosoneIt is generally assumed that favourable weather conditions determine the activity levels of lizards, because of their temperature-dependent behavioural performance. Inactivity, however, might have a selective advantage over activity, as it could increase survival by reducing exposure to predators. Consequently, the effects of weather conditions on the activity patterns of lizards should be strongly influenced by the presence of predators. Using remote camera traps, we test the hypothesis that predator presence and weather conditions interact to modulate daily activity levels in two sedentary cordylid lizards, Karusasaurus polyzonus and Ouroborus cataphractus. While both species are closely related and have a fully overlapping distribution, the former is a fast-moving lightly armoured lizard, whereas the latter is a slow-moving heavily armoured lizard. The significant interspecific difference in antipredator morphology and consequently differential vulnerability to aerial and terrestrial predators, allowed us to unravel the effects of predation risk and weather conditions on activity levels. Our results demonstrate that K. polyzonus is predominantly active during summer, when ambient temperatures are favourable enough to permit activity. In contrast, a peak in activity during spring was observed in O. cataphractus, with individuals being inactive during most of summer. While favourable weather conditions had a strong effect on the activity levels of K. polyzonus, no such relationship was present in O. cataphractus. Contrary to our hypothesis, the presence of terrestrial predators does not seem to affect daily activity levels or alter the influence of weather conditions on activity levels. We conclude that inactivity in O. cataphractus appears to be related to seasonal differences in vulnerability to predators, rather than the presence of predators, and highlight the importance of additional selective pressures, such as food abundance, in determining the species’ activity levels.National Research Foundation (NRF) to PFNMNational Geographic Young Explorers Grant (#9090-12) to CBhttp://journals.plos.org/plosone/article?id=10.1371/journal.pone.0137428Publisher's versio

Summary of the results from an ordinal least squares analysis on the activity time-series.

<p>The relationship between each independent variable (i.e. weather condition (PC1) and predator presence) and the dependent variable (i.e. lizard activity), as well as the interaction effect between predator presence and weather condition, is shown. Statistically significant <i>P</i>-values are indicated in bold. Legend: <i>β</i>, regression coefficient; <i>t</i> = <i>t</i>-test statistic.</p><p>Summary of the results from an ordinal least squares analysis on the activity time-series.</p

Frequency of visits by terrestrial and aerial predators to lizard rocks.

<p>The number of days on which terrestrial and aerial predators were detected, is indicated.</p><p>Frequency of visits by terrestrial and aerial predators to lizard rocks.</p

Activity pattern of <i>Ouroborus cataphractus</i>.

<p>A graph of the average index of activity plotted against weather condition (represented by PC1) illustrates that the activity levels of <i>O</i>. <i>cataphractus</i> are not determined by favourable weather conditions.</p

Camera trap photographs illustrating examples of terrestrial and aerial predators present in the habitat.

<p>Top: small grey mongoose (<i>Galerella pulverulenta</i>) and meerkat (<i>Suricata suricatta</i>), bottom: African harrier-hawk (<i>Polyboroides typus</i>) and pied crow (<i>Corvus albus</i>).</p

Example of a camera trap photograph used to calculate activity levels.

<p>Images illustrate active individuals of the heavily armoured <i>Ouroborus cataphractus</i> (top) and the lightly armoured <i>Karusasaurus polyzonus</i> (bottom).</p

Summary of the results from a principal component analysis performed on the weather variables.

<p>Two principal components, explaining 66.3% of the total variation, were retained from the PCA analysis. The first axis was used to represent favourable weather conditions. This axis was positively correlated with temperature and photoperiod and negatively with humidity and barometric pressure. This axis represents a gradient from hot dry days with a long period of sunshine typically observed during summer to cold, humid days with less sunshine characterising winter. The negative correlation with barometric pressure represents thermal lows present in arid environments during the summer. The factor coordinate correlations and eigenvalues of the variables are shown. Values in bold represent loading scores greater than 0.50.</p><p>Summary of the results from a principal component analysis performed on the weather variables.</p

Data from: Enemy at the gates: rapid defensive trait diversification in an adaptive radiation of lizards

Adaptive radiation, the product of rapid diversification of an ancestral species into novel adaptive zones, has become pivotal in our understanding of biodiversity. While it has widely been accepted that predators may drive the process of adaptive radiation by creating ecological opportunity (e.g. enemy-free space), the role of predators as selective agents in defensive trait diversification remains controversial. Using phylogenetic comparative methods, we provide evidence for an ‘early burst’ in the diversification of antipredator phenotypes in Cordylinae, a relatively small adaptive radiation of morphologically diverse southern African lizards. The evolution of body armour appears to have been initially rapid, but slowed down over time, consistent with the ecological niche-filling model. We suggest that the observed ‘early burst’ pattern could be attributed to shifts in vulnerability to different types of predators (i.e. aerial versus terrestrial) associated with thermal habitat partitioning. These results provide empirical evidence supporting the hypothesis that predators or the interaction therewith might be key components of ecological opportunity, though the way in which predators influence morphological diversification requires further study

Morphological trait measurements of cordyline lizards

Measurements of 21 morphological traits of cordyline lizards obtained using micro-CT as well as residuals and PC-scores used as input for phylogenetic comparative analyses