Drivers of within- and among-individual variation in risk-taking behaviour during reproduction in a long-lived bird

Plastic and selective mechanisms govern parental investment adjustments to predation threat. We investigated the relative importance of plasticity and selection in risk-taking propensity of incubating female common eiders Somateria mollissima facing unprecedented predation in SW Finland, Baltic Sea. Using a 12-year individual-based dataset, we examined within- and among-individual variation in flight initiation distance (FID), in relation to predation risk, nest detectability, individual traits and reproductive investment (NFID = 1009; Nindividual = 559). We expected females nesting in riskier environments (higher predation risk, lower nest concealment) to mitigate environmentally imposed risk by exhibiting longer FIDs, and females investing more in current reproduction (older, in better condition or laying larger clutches) to display shorter FIDs. The target of predation-adult or offspring-affected the mechanisms adapting risk-taking propensity; females plastically increased their FID under higher adult predation risk, while risk-avoiding breeders were predominant on islands with higher nest predation risk. Risk-taking females selected thicker nest cover, consistent with personality-matching habitat choice. Females plastically attenuated their anti-predator response (shorter FIDs) with advancing age, and females in better body condition were more risk-taking, a result explained by selection processes. Future research should consider predator type when investigating the fitness consequences of risk-taking strategies.

The relationships between toad behaviour, antipredator defences, and spatial and sexual variation in predation pressure

BACKGROUND

Animal behaviour is under strong selection. Selection on behaviour, however, might not act in isolation from other fitness-related traits. Since predators represent outstanding selective forces, animal behaviour could covary with antipredator defences, such that individuals better suited against predators could afford facing the costs of riskier behaviours. Moreover, not all individuals undergo equivalent degrees of predation pressure, which can vary across sexes or habitats. Individuals under lower predation pressure might also exhibit riskier behaviours.

METHODS

In this work, I tested these hypotheses on natterjack toads (Epidalea calamita). Specifically, I gauged activity time, exploratory behaviour and boldness in standard laboratory conditions, and assessed whether they correlated with body size and antipredator strategies, namely sprint speed, parotoid gland area and parotoid gland colour contrast. Additionally, I compared these traits between sexes and individuals from an agrosystem and pine grove, since there is evidence that males and agrosystem individuals are subjected to greater predation pressure.

RESULTS

Sprint speed as well as parotoid gland contrast and size appeared unrelated to the behavioural traits studied. In turn, body mass was negatively related to activity time, boldness and exploration. This trend is consistent with the fact that larger toads could be more detectable to their predators, which are mostly gape unconstrained and could easily consume them. As predicted, females exhibited riskier behaviours. Nonetheless, agrosystem toads did not differ from pine grove toads in the behavioural traits measured, despite being under stronger predation pressure.

Toads modulate flight strategy according to distance to refuge

Among antipredator behaviours, escaping and hiding in a refuge are widespread in nature. Frequently, threatened prey flee towards a refuge nearby, if available. Therefore, refuge proximity may affect the fleeing strategy of a prey. In this work, I tested this hypothesis in Epidalea calamita, a cursorial toad that flees by means of intermittent runs. In a linear runway in standardized conditions, toads were recorded while conducting a short-distance (refuge at 70 cm), a medium-distance (refuge at 140 cm, divided in two 70-cm tracks), and a long-distance trial (refuge at 210 cm, divided in three 70-cm tracks), in a random sequence. Video analyses permitted to calculate sprint speed and run rates (number of runs per meter) in each track. Distance to refuge affected toad flight strategy. Toads started flights at a faster speed in the short-distance trials. In the medium- and the long-distance trials, toads accelerated after the first track, seemingly not motivated by refuge proximity. In these trials, run rate was greater in the first tracks. Altogether, these findings suggest that threatened toads respond firstly with slow, intermittent movements, and only shift to less intermittent, faster sprints if the threat persists. However, run rate was lower in the short-distance trial than in the first tracks of the other trials, suggesting straighter (and faster) flight toward the refuge when it is close. The effects of refuge proximity were greater in males, which (jointly with faster sprint speed) could reflect a greater conspicuousness of males to predator resulting in better escape strategies.