Behavioural plasticity adjusts rapidly to repeated temperature increases: mean, among- and within-individual responses

Animal personality

Risk is one of the main drivers in shaping prey phenotypes around the world. Individuals that accurately assess risk have a better chance of mounting a suitable response to a threat cue and hence gain a selective advantage. Phenotypic plasticity in behaviour is advantageous especially for organisms that transition between stages of a complex life history, as it is hard to predict future risk. For coral reef fish, one such stage is known as settlement. At this stage, naive juveniles are exposed to various levels of risk. Individuals that settle on high-risk habitats may develop a risk-adverse phenotype whereby novel cues are initially labelled as risky. There are many effects to having a risk-adverse phenotype that assist prey to increase their probability of survival. However, more studies need to be done to further understand the risk-adverse phenotype. This thesis aims to investigate: (1) How do high-risk conditions affect the fast-start escape response in juvenile coral reef fish; (2) How do high-risk conditions affect the morphology of juvenile coral reef fish. High-risk conditions were created by exposing individuals to conspecific chemical alarm cues three times a day for four days. Low-risk (control) conditions were created by exposing individuals to saltwater three times a day for four days. Chapter 2 investigated the effects of high-risk conditions on the escape responses of a species of juvenile damselfish, the spiny chromis, Acanthochromis polyacanthus (Pomacentridae). Juvenile A. polyacanthus were caught using hand net on SCUBA and transported back to the laboratory. Individuals were then exposed to either a high-/low-risk treatment before being individually tested in a fast-start arena. Prior to the commencement of the test, individuals were exposed to either chemical alarm cues or saltwater to investigate the effect of acute stressors. All fish were filmed at 420 frames per second using a camera pointed at a mirror tilted at a 45 angle underneath the arena. This resulted in a silhouette of the moving fish. A principal component analysis (PCA) was used to analyse how variables measured (response latency, response duration, response distance, mean response speed, maximum response speed, maximum acceleration) differed between treatments. An analysis of variance (ANOVA) was also used to analyse how escape method and turning angle differed between treatments. Risk (background or acute) affected escape responses in two ways. Firstly, the method of escape used by individuals (i.e., C-start or backing away from the threat). Secondly, escape responses were enhanced by individuals exposed to high-risk (with/without acute risk) and low-risk with additional acute risk. Background risk and acute risk acted in a simple additive manner, as seen by the lack of interaction between the two factors. Results showed that escape responses are amplified as the level of perceived risk increases. Chapter 3 investigated the effects of high-risk conditions on the morphology of a species of juvenile damselfish, the ambon damselfish, Pomacentrus amboinesis (Pomacentridae). Naive fish leaving the pelagic phase to settle on reefs were caught by light traps and transported back to the laboratory where they were exposed to high-/low-risk conditions. Following background risk conditioning, individuals were taught to recognise a novel odour from a predator, the Brown dottyback, Pseudochromis fuscus, as risky or safe. Individuals were then housed in plastic containers for 28 days. During this period, individuals were exposed to the predator odour three times a day. Photographs were taken every seven days to document the change in body depth, standard length, ocellus area, ocellus diameter and eye diameter. Results found that individuals exposed to high-risk conditions grew larger ocelli after four days of exposure. At 28 days post risk history treatment, high-risk individuals not only had larger ocelli but significantly smaller eyes and a slightly larger body. Results suggest that exposure to predation risk at a critical time may place these fish on a certain growth trajectory based on experiences during their period of settlement. This study demonstrates the importance of a risk-adverse phenotype for coral reef fishes settling onto high-risk habitats. As naive juveniles are particularly vulnerable to predation at this stage in life, the risk-adverse phenotype increases the probability of survival. This is achieved by not only treating novel cues as a risk, but also altering morphology and escape responses to aid in the pre- and post-attack stages of an interaction with predators.

Sex-dependent risk-taking behaviour towards different predatory stimuli in the collared flycatcher

In the evolutionary transition from solitary to group living, it should be adaptive for animals to respond to the environment and choose when to socialize to reduce conflict and maximize access to resources. Due to the associated proximate mechanisms (e.g. neural network, endocrine system), it is likely that this behavior varies between individuals according to genetic and non-genetic factors. We used long-term behavioral and genetic data from a population of eastern water dragons (Intellagama lesueurii) to explore variation in plasticity of social behavior, in response to sex ratio and density. To do so, we modeled individual variation in social reaction norms, which describe individuals' mean behavior and behavioral responses to changes in their environment, and partitioned variance into genetic and non-genetic components. We found that reaction norms were repeatable over multiple years, suggesting that individuals consistently differed in their behavioral responses to changes in the social environment. Despite high repeatability of reaction norm components, trait heritability was below our limit of detection based on power analyses (h2 < 0.12), leading to very little power to detect heritability of plasticity. This was in contrast to a relatively greater amount of variance associated with environmental effects. This could suggest that mechanisms such as social learning and frequency-dependence may shape variance in reaction norms, which will be testable as the dataset grows.

In response to environmental stressors, organisms often demonstrate flexible responses in morphology, life history or behaviour. However, it is currently unclear if such plastic responses are coordinated or operate independently of one another. In vertebrates, this may partly result from studies examining population- or species-level mean responses, as opposed to finer grained analyses of individuals or families. We measured predator-specific morphological and coloration plasticity in 42 families of tadpoles of the treefrog Dendropsophus ebraccatus and behavioural plasticity from 18 of these families, allowing us to examine the correlation between three predator-induced plastic responses. For all three plastic responses, tadpoles showed strong opposing responses to each of two predators, providing the appearance of covariation in plasticity. However, the examination of individual families revealed a strong correlation between morphological and coloration plasticity, but no correlations between either morphology or colour and behavioural plasticity. Thus, our analysis shows that some aspects of the plastic phenotype develop together while others function independently. This highlights the importance of examining individual- and family-level variation for understanding the adaptive significance of developmental plasticity, which is crucial for a holistic appreciation of phenotypic plasticity and its importance in ecology and evolution.