Repeatability of thermal reaction norms for spontaneous locomotor activity in juvenile newts
Introduction
Locomotor activity is a typical attribute of non-sessile animals, which affects their ecology at different scales. At a population level, movement underlies major ecological processes, such as dispersal and migration (Nathan et al., 2008). Within a population, relationships among locomotor activity, energy acquisition, and predation risk may mediate the influence of environmental change on population dynamics (Frid and Dill, 2002). At an individual level, locomotor activity determines both the frequency of interactions with con- and heterospecifics (Dell et al., 2014) and the exposure to abiotic environmental factors (Williams et al., 2008). On the other hand, locomotor activity is a highly flexible trait, which is affected by diverse external and internal factors (Martin, 2003). However, the interactive influence of these factors on locomotor activity has received limited attention.
Among external factors, environmental temperature seems to be the most important determinant of locomotor activity, especially in ectotherms, because of its direct influence on body temperature. Indeed, after pioneering papers (Fry, 1947, Bennett, 1980, Putnam and Bennett, 1981), hundreds of studies have demonstrated the thermal dependence of various locomotor traits, such as maximum velocity, sustainable speed, jump length, or stamina. The thermal dependence of locomotor performance shows the typical pattern of linearly increasing values at body temperatures below thermal optimum, followed by a curved relationship around the thermal optimum, and a fast performance drop above this temperature (Huey and Stevenson, 1979). However, most of this information pertains to forced locomotion, which is largely determined by an ectotherm's morphological and physiological capacity. Spontaneous locomotion, which ectotherms mostly use for routine activities (Reilly et al., 2007), is affected by the individual's internal state and motivation rather than by maximum performance (Hertz et al., 1988). Accordingly, thermal dependence curves for maximum velocity differ between spontaneous activity and forced locomotion (Šamajová and Gvoždík, 2009).
Even when external factors are carefully controlled, locomotor activity varies considerably within a population because of consistent individual differences in behavior, i.e., personality. It is well established that some individuals within a population are consistently more active than others (Bell et al., 2009). However, it is unknown how personality (e.g., individual differences in relative activity) may affect thermal reaction norms for locomotor activity. Although many studies demonstrate thermal dependence of locomotor activity (Chiverton, 1988, Martin et al., 1999), they have often been performed at two experimental temperatures only, which provided insufficient information about the shape of the reaction norm across an ecologically relevant temperature range. If individual variation in thermal reaction norms for locomotor activity is, in fact, repeatable, it remains virtually unknown.
We examined the short-term repeatability of individual thermal reaction norms for spontaneous locomotor activity in juvenile alpine newts, Ichthyosaura alpestris. Newts are ectotherms so their body temperatures, and accordingly locomotor activity, are directly influenced by surrounding environmental (operative) temperatures that vary spatially and temporary in their natural habitat (Hadamová and Gvoždík, 2011). In newly metamorphosed newts, locomotor activity affects their dispersal from water bodies (Pittman et al., 2014) and the avoidance of costly heterospecific interactions (Janča and Gvoždík, 2017). Our two goals were to (1) evaluate individual variation in the magnitude and shape of thermal reaction norms for spontaneous locomotor activity and (2) examine the short-term repeatability of the reaction norms, which is an important assumption for their evolution by natural selection (Angilletta et al., 2002).
Section snippets
Study species and maintenance
The alpine newt, I. alpestris, is a common European newt attaining a total length (TL) of 12 cm (Speybroeck et al., 2016). Its aquatic larvae usually metamorphose during August to September. Some larvae may overwinter and complete their metamorphosis the following spring. Metamorphosed individuals are strictly terrestrial with a crepuscular to nocturnal activity period. Newt dispersal from natal water bodies occurs largely during the juvenile stage (Kupfer and Kneitz, 2000). They feed on
Results
We obtained data from 21 individuals and 166 trials (Gvoždík and Baskiera, 2018). After the habituation period, there was no trend in distance covered during the trial across body temperatures (Fig. A1), and so observed behavior can be classified as spontaneous locomotor activity. All locomotor activity parameters were affected by body temperature, except total activity rate during the first trial series (Fig. 1; Table 1). The effect of body temperature on distance covered, mean velocity, and
Discussion
While individual repeatability and thermal dependence of forced locomotor performance have received ample attention (see Section 1), spontaneous locomotor activity has remained understudied from this view. We showed that the thermal sensitivity of spontaneous locomotor activity depends on the parameter that is used to characterize the activity of individuals. Individual variation in thermal reaction norms affected their magnitude (intercept), not shape. The vertical position of thermal reaction
Acknowledgements
We thank to anonymous reviewers for their comments on the previous versions of this paper. This research was supported by the Czech Science Foundation (Grant no. 17-15480S to LG), the Institute of Vertebrate Biology AS CR (RVO: 68081766 to LG) and BEFO project (Behavioural, Ecological and Phylogenetic Vertebrate Research) of Department of Botany and Zoology, Faculty of Science, Masaryk University in Brno, Czech Republic (MUNI/A/1078/2017 to SB).
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