Behavioural causes and consequences of sexual size dimorphism in an apex predator species.

Individual differences within populations in a range of phenotypic traits are hypothesised to have important ecological and evolutionary implications. Variation in individual growth rates that result in corresponding variations in body sizes, including size dimorphism, is a particularly widespread feature of many animal populations. The increasing characterisation of consistent individual behavioural variations, unrelated to age or sex, is equally considered to have important fitness consequences. Our understanding of behavioural causes of size dimorphism remains weak, and few studies have investigated the relationship between individual behavioural consistency and growth variations in size dimorphic populations. The overall aim of this thesis is to identify the behavioural drivers that underpin observed growth variations and result in size dimorphism by using pike (Esox lucius) as a model species. The results show that early life growth is an important driver of sexual size dimorphism in this species. A subsequent focus on the juvenile life stages revealed that individual differences in movement and dispersal tendencies were related to growth and body size in wild pike. The findings indicate that intraspecific interactions such as size-dependent interference competition during the first year of life plays a key role in maintaining intraspecific size variation and size dimorphism in the wild population. Experimental work revealed the occurrence of a behavioural syndrome, where the rank order differences in the foraging behaviour between individuals were maintained across time and risk situation. This suggests that individual competitive ability is underpinned by a variation in boldness to forage under risk. The importance of a heterogeneous environment and presence of intraspecific competition pressure for driving habitat and resource segregation, and subsequently sexual size dimorphism, is discussed

Dispersal strategies of juvenile pike (Esox lucius L.): influences and consequences for body size, somatic growth and trophic position

Individual variability in dispersal strategies, where some individuals disperse and others remain resident, is a common phenomenon across many species. Despite its important ecological consequences, however, the mechanisms and individual advantages of dispersal remain poorly understood. Here, riverine Northern pike (Esox lucius) juveniles (young-of-the-year and age 1+ year) were used to investigate the influence of body size and trophic position (at capture) on the dispersal from off-channel natal habitats, and the subsequent consequences for body sizes, specific growth rate and trophic position (at recapture). Individuals that dispersed into the river (‘dispersers’) were not significantly different in body size or trophic position than those remaining on nursery grounds (‘stayers’). Once in the river, however, the dispersers grew significantly faster than stayers and, on recapture, were significantly larger, but with no significant differences in their trophic positions. Early dispersal into the river was therefore not facilitated by dietary shifts to piscivory and the attainment of larger body sizes of individuals whilst in their natal habitats. These results suggest that there are long-term benefits for individuals dispersing early from natal areas via elevated growth rates and, potentially, higher fitness, with the underlying mechanisms potentially relating to competitive displacement

Behavioural syndrome in a solitary predator is independent of body size and growth rate.

Models explaining behavioural syndromes often focus on state-dependency, linking behavioural variation to individual differences in other phenotypic features. Empirical studies are, however, rare. Here, we tested for a size and growth-dependent stable behavioural syndrome in the juvenile-stages of a solitary apex predator (pike, Esox lucius), shown as repeatable foraging behaviour across risk. Pike swimming activity, latency to prey attack, number of successful and unsuccessful prey attacks was measured during the presence/absence of visual contact with a competitor or predator. Foraging behaviour across risks was considered an appropriate indicator of boldness in this solitary predator where a trade-off between foraging behaviour and threat avoidance has been reported. Support was found for a behavioural syndrome, where the rank order differences in the foraging behaviour between individuals were maintained across time and risk situation. However, individual behaviour was independent of body size and growth in conditions of high food availability, showing no evidence to support the state-dependent personality hypothesis. The importance of a combination of spatial and temporal environmental variation for generating growth differences is highlighted

Stable Vascular Connections and Remodeling Require Full Expression of VE-Cadherin in Zebrafish Embryos

BACKGROUND: VE-cadherin is an endothelial specific, transmembrane protein, that clusters at adherens junctions where it promotes homotypic cell-cell adhesion. VE-cadherin null mutation in the mouse results in early fetal lethality due to altered vascular development. However, the mechanism of action of VE-cadherin is complex and, in the mouse embryo, it is difficult to define the specific steps of vascular development in which this protein is involved. METHODOLOGY AND PRINCIPAL FINDINGS: In order to study the role VE-cadherin in the development of the vascular system in a more suitable model, we knocked down the expression of the coding gene in zebrafish. The novel findings reported here are: 1) partial reduction of VE-cadherin expression using low doses of morpholinos causes vascular fragility, head hemorrhages and increase in permeability; this has not been described before and suggests that the total amount of the protein expressed is an important determinant of vascular stability; 2) concentrations of morpholinos which abrogate VE-cadherin expression prevent vessels to establish successful reciprocal contacts and, as a consequence, vascular sprouting activity is not inhibited. This likely explains the observed vascular hyper-sprouting and the presence of several small, collapsing vessels; 3) the common cardinal vein lacks a correct connection with the endocardium leaving the heart separated from the rest of the circulatory system. The lack of closure of the circulatory loop has never been described before and may explain some downstream defects of the phenotype such as the lack of a correct vascular remodeling. CONCLUSIONS AND SIGNIFICANCE: Our observations identify several steps of vascular development in which VE-cadherin plays an essential role. While it does not appear to regulate vascular patterning it is implicated in vascular connection and inhibition of sprouting activity. These processes require stable cell-cell junctions which are defective in absence of VE-cadherin. Notably, also partial modifications in VE-cadherin expression prevent the formation of a stable vasculature. This suggests that partial internalization or change of function of this protein may strongly affect vascular stability and organization

The repeatability (R) of behavioural measures in juvenile pike (n = 34) across context using one trial of each treatment conducted closest in time: (a) trials 1 (n = 3), (b) trials 2 (n = 3), (c) trials 3 (n = 3), (d) trials 4 (n = 3), and (e) all trials (n = 14).

<p>Generalised linear mixed-effects and linear mixed-effects models (rptR package in R, <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0031619#pone.0031619-Grimm1" target="_blank">[33]</a>) with fish identity fitted as random effect and the behavioural measure as dependent factor were used for calculating repeatabilities, standard errors, 95% confidence intervals (CIs) and P-values. Latency to prey attack and swimming activity were log-transformed to achieve normality.</p

Mean behavioural measurements (± SE) of juvenile pike (n = 34) in each trial of the (a) control, (b) competitor and (c) predator treatment.

<p>Mean behavioural measurements (± SE) of juvenile pike (n = 34) in each trial of the (a) control, (b) competitor and (c) predator treatment.</p

Mean number of prey captured in the predator treatment per individual pike (n = 34) and their metrics.

<p>(a) Specific growth rate, (b) initial body mass, (c) final body mass. Correlations were investigated using Spearman's ranking tests.</p

The repeatabilities (R) of behavioural measures in juvenile pike (n = 34) within each experimental situation: (a) control (no risk), (b) competition (low risk), (c) predation (high risk).

<p>Generalised linear mixed-effects and linear mixed-effects models (rptR package in R <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0031619#pone.0031619-Grimm1" target="_blank">[33]</a>) with fish identity fitted as random effect and the behavioural measure as dependent factor were used for calculating repeatabilities, standard errors, 95% confidence intervals (CIs) and P-values. Latency to prey attack and swimming activity were log-transformed to achieve normality.</p