A comparison of measures of boldness and their relationships to survival in young fish

Abstract

Boldness is the propensity of an animal to engage in risky behavior. Many variations of novel-object or novel-environment tests have been used to quantify the boldness of animals, although the relationship between test outcomes has rarely been investigated. Furthermore, the relationship of outcomes to any ecological aspect of fitness is generally assumed, rather than measured directly. Our study is the first to compare how the outcomes of the same test of boldness differ among observers and how different tests of boldness relate to the survival of individuals in the field. Newly-metamorphosed lemon damselfish, Pomacentrus moluccensis, were placed onto replicate patches of natural habitat. Individual behavior was quantified using four tests (composed of a total of 12 different measures of behavior): latency to enter a novel environment, activity in a novel environment, and reactions to threatening and benign novel objects. After behavior was quantified, survival was monitored for two days during which time fish were exposed to natural predators. Variation among observers was low for most of the 12 measures, except distance moved and the threat test (reaction to probe thrust), which displayed unacceptable amounts of inter-observer variation. Overall, the results of the behavioral tests suggested that novel environment and novel object tests quantified similar behaviors, yet these behavioral measures were not interchangeable. Multiple measures of behavior within the context of novel environment or object tests were the most robust way to assess boldness and these measures have a complex relationship with survivorship of young fish in the field. Body size and distance ventured from shelter were the only variables that had a direct and positive relationship with survival.

Figure 1. Survival over two nights in the field.

Kaplan-Meier survival analysis with respect to: a) maximum distance moved and b) size (TL) of juvenile Pomacentrus moluccensis on patch reefs in the field. Fish were sequentially ranked for their scores on each trait and two groups (high and low ranked) of twenty fish (21.7% of total) were compared. Solid lines and dashed lines represent the two groups of highest and lowest ranked fish, respectively. Symbols represent presence or absence of individual fish during subsequent mortality surveys.

Figure 2. Eight models of boldness syndrome structure developed based on a priori hypotheses of boldness structure.

Model 1 represents behavioral independence. Model 2 represents a domain-general model of syndrome structure while models 3–8 are more constrained, representing different types of boldness tests. The measured behaviors are represented in rectangular boxes, with shaded boxes representing composite variables. Underlying causal connections (latent variables) resulting in boldness structure are represented in ovals . In order to save space, multiple models are presented with alternative structures denoted by dashed lines labeled with model number (e.g. model 3 excluded size, as denoted with a dashed line labeled 3).

Figure 3. Best fitting structural equation model (SEM).

This SEM shows how behaviors were related within the best fitting model for damselfish. Numbers in parentheses are variances of the different behaviors explained by the SEM structure (R²) for ‘model 6′ (see Fig. 2). Numbers associated with arrows are standardized factor loadings for the effects of the underlying boldness structure on a particular behavior. These represent how behavioral responses are predicted to change based on changes to the underlying boldness structure (e.g. a shift of 1 SD along the distribution of boldness structure for the population would result in a 0.15 SD decrease in height).