Variety: The essential spice of life

31 May 2019 - By: Alex Evans

Variety: The essential spice of life

By Alex Evans

Whether from environmental, social, or internal sources, stress plays a major role in the life and death of wild animals. However, not all individuals react in the same way to stress and there is often a great deal of variation within species that goes overlooked. During the “Intraspecific variation in responses to stress” session at the 2018 SEB Annual Meeting, researchers from around the world cast the spotlight on these important intra-individual differences.

On the clock

An organism’s biological (or circadian) clock plays a vital role in keeping them ticking, at least in a figurative sense. These clocks take the form of complex interactions between many biological systems that generally form 24-hour repeats, allowing their hosts to physiologically anticipate regular changes in the environment resulting from the changes between night and day. Whilst these clocks sound like they should be running regularly for all animals, there can be a surprising amount of variation between individuals that can have interesting effects on the ways that they cope with stress. Christian Tudorache, an animal physiologist from Leiden University in the Netherlands, presented his work exploring the intricate inner workings of biological clocks in zebrafish and how differences in their rhythmicity can have interesting consequences on their stress response behaviours.

“Like biological clocks, animal coping styles are also determined by the interaction of various biological functions and incorporate the anticipation of environmental changes,” says Christian. “Based on these similarities and the results of an RNA-sequencing experiment that found strong differences in the expression of clock genes between ‘proactive’ and ‘reactive’ coping styles or personality types, we decided to look into the interactions between clock function and other levels of biological function.”

Based on these explorative RNAsequencing results, Christian had a number of hypotheses for the underlying relationship between clock and coping style, including differences in gene expression that were linked to shifts in the phase or amplitude of the biological clock. “We tested these hypotheses on three levels of biological function,” explains Christian.

“Firstly, on the molecular level by measuring gene expression with qPCR [quantitative polymerase chain reaction]. Secondly, on the endocrine level by investigating hormone production using ELISA [enzyme-linked immunosorbent assays]. Finally, on the behavioural level by studying locomotion activity by means of an automated observation software.” After analysing the results of all three tests, Christian found that the amplitude and rhythm of the circadian clock correlated strongly with the coping style of the zebrafish. “We discovered a strong rhythmicity in proactive fish and a total absence of rhythmicity in reactive fish,” Christian says. “These results are exciting because they not only show another fundamental aspect of variability linked to individual personality types,1, 2 but they also suggest that the circadian clock may play a part in the divergent survival strategies observed in nature.”

Christian goes on to explain how these differences can have significant benefits to survival in a variety of stressful scenarios. “We know that proactive individuals thrive under constant conditions whilst reactive animals generally prosper under conditions that are more unpredictable,” he says. “A certain degree of variability in individuals of different personality types and clock rhythmicity will have advantages for the survival of the population as a whole.” Building on the work presented during the session, Christian also hopes to deeper explore the underlying molecular mechanisms at play, as well as the more observable behavioural traits tied to biological clocks. “It would be interesting to see if we can manipulate personality by changing the rhythmicity of the clock, or, vice versa, alter the rhythmicity of an individual by adjusting its personality.”

Stress to Impress

From cancer to the common cold, illness plays a natural role in all animal life, but the way in which organisms respond to their illnesses can vary on a much more personal level. For some fish, social pressures also play a surprising role in how these responses manifest themselves, as marine ecophysiologist Sandra Binning (University of Montreal, Canada) has discovered.

Building on fellow postdoc Patricia Lopes’s work investigating the social aspects of sickness behaviours in zebra finches, Sandra wanted to examine how these social pressures affect responses in individuals representing a wide range of personality types, from extremely bold to incredibly shy. Owing to the importance of social signalling in some wild fish communities, Sandra was primarily interested in how typical social behaviours would be affected by illness. “Sicknessassociated behaviours such as lethargy and isolation may be good for an individual in the absence of competitors, predators, or mates, but can be problematic for territorial challenges and predation risk,” explains Sandra. “We really wanted to learn to what extent these animals can choose when and where to ‘feel sick’ according to the potential stakes.”

“We wanted to answer two important questions with this research,” says Sandra. “Firstly, does sickness affect all individuals in the same way? And how does an individual’s ‘normal’ behaviour affect their expression of sickness behaviours in different social contexts?” To address these questions, Sandra and her team visited the Lizard Island Research Station on the Great Barrier Reef in Australia to study the Ambon damselfish, a small coral reef fish.
S. Binning
Sandra Binning diving in the Great Barrier Reef Photo: Dominique Roche

“Ambon damsels are a great species for behaviour studies because they adjust well to captivity and display a large range of behavioural variation, both within and among individuals.” They divided the fish into two equal-sized groups. By injecting one group with a substance called lipopolysaccharide (LPS), the team were able to trigger an immune response in the fish that stimulated their physiological response to illness without the use of real bacterial or viral infections. The other group were treated with a saline control, and the behaviours exhibited by both groups before and after injection were compared in social and non-social contexts. “We measured each fish’s exploration of a novel object, which we considered to be our asocial context, and their level of aggression towards a perceived intruder, which was our social context.”

After comparing the treated and control fish, Sandra and her team revealed that, contrary to Patricia’s work with the zebra finches, the Ambon damsels treated with LPS were much less exploratory and less aggressive than the control group. “We also found that bolder and more aggressive fish showed a dramatic change in behaviour following the LPS injection, whilst there was no noticeable difference in the behaviour of the more shy and docile fish,” says Sandra. These results are important in furthering our understanding of how intra-individual responses to disease or parasitism may vary in wild fish populations.

As echoed by many other speakers during this session, Sandra is keen to stress the significance of natural variation that is often left unexplored. “Historically, experimental biologists have tended to ignore natural variation among individuals and focus on mean-level responses,” she explains. “Whilst this approach is valid, it’s also important to recognise the important insights we can gain from understanding how and why individuals respond differently to the same treatments.”

From head to tail

The ‘personalities’ of animals are often reflected in the ways that they handle new and potentially stressful situations. Whilst the behaviours associated with these personalities can be quite distinct, the neurological processes at work are much less obvious. Svante Winberg, a researcher of behavioural neuroendocrinology at the University of Uppsala, Sweden, is working to shed some light on how signals from the brain interact with social and environmental factors to manifest into a variety of personality types.

“I have always been interested in intraspecific variance within behaviour and what makes individuals different,” says Svante. “In my early work on fish dominance hierarchies as a PhD student, I found that monoaminergic systems in the brain (e.g. serotonin and dopamine) were very important in determining social rank and behaviours.” Previous research into behavioural stress responses has shown that individual mammals and fish generally tend towards two polar personality types: proactive, where behaviours are often aggressive, and reactive, which are often more timid in stressful situations. These personality types are associated with neurological signals, which is the area of research that most interests Svante. “Proactive individuals are characterised by responding to stress with low cortisol but high plasma catecholamines, whereas the reactive ones show the opposite pattern.” Interestingly, Svante points out that the learning styles of each personality type differ too. “Proactive animals are fast learners and do not respond to environmental cues, hence their name, whereas reactive animals show much more plastic behaviours.”

“The hypothesis is that proactive fish will have an advantage in a stable and predictable environment since they are aggressive and form behavioural routines,” he explains. “On the other hand, reactive fish will do better in variable and unpredictable environments where there is an advantage to be plastic in behaviour.” It is not just wild fish populations that have been investigated; human interactions with fish can also strongly influence their personality types. “Domestication, as occurs in aquaculture, tends to result in a shift towards a more proactive phenotype as the bolder wild fish are more likely to be caught,” says Svante. “However, in aquaculture aggressive behaviour can be a serious problem and these studies on stress coping and personality traits are of great importance to aquaculture and animal welfare research.”

Whilst Svante’s earlier work focused on identifying links between brain monoaminergic systems and personality traits in rainbow trout,3 his more recent efforts have focused on a staple laboratory study species, the zebrafish.4 “They are small, easy to reproduce with a short generation time, and have a well-annotated genome,” explains Svante. “We have been able to create lines of zebrafish with different personalities through selective breeding and we are hoping to identify any genes of importance using whole genome sequencing (GWAS) and any effects of environmental factors.” As well as advancing our understanding of addiction and affective disorders in humans, these zebrafish have the potential to serve as useful models in examining these disorders further.

Ace in Shoal

In wild animals, leadership qualities are often expressed through individual responses to difficult situations. More specifically, these qualities can influence the roles of dominance and subordination within highly social animal groups. As a researcher with interests in both animal physiology and behaviour, Brett Culbert (University of Guelph, Canada) finds that these social hierarchies offer an interesting intersection of research between the two fields. “Within a social hierarchy, individuals have quite different behavioural and physiological profiles depending on their position in the hierarchy,” explains Brett. He adds that whilst social hierarchies exist throughout the natural world, he feels that fish offer an interesting perspective on the topic: “There is such diversity in the behaviour of fishes, and many species live incredibly social lifestyles!” Brett’s study species for this line of work, a cooperatively breeding cichlid (Neolamprologus pulcher), is indeed a great candidate for investigating the effects of stressful changes on social dynamics. “Individuals of this species live their entire lives in groups and subordinates help raise the offspring of dominants—just like in lions, meerkats, and many bird species,” says Brett. “However, they tend to receive far less attention than either their avian or mammalian counterparts.”
Neolamprologus pulcher
Brett Culbert’s study species, Neolamprologus pulcher Photo : Brett Culbert

Brett’s first experiments in this area explored the role of physiological stress in the ascension of subordinate male cichlids to dominance within their social groups.5 “We accomplished this in the laboratory by removing dominant males from their social group— allowing subordinate males to ascend to the dominant position,” he explains. “We found that ascenders quickly became behaviourally dominant but continued to display higher levels of the stress hormone cortisol compared to the dominants that they succeeded.” One potential reason for these lingering levels of cortisol is that it can take time for the hierarchy to become stabilised and staying on high alert may be beneficial until the new roles have become established.

Through a second series of experiments, Brett aimed to investigate how different social situations influenced the recovery of the cichlids following a stressor. “We found that the cortisol levels of individuals that recovered with their group members were lower than those of individuals that recovered in the absence of their groupmates,” he explains. “We are currently performing follow-up experiments to tease apart what is driving this difference, but it seems that individuals feel safer and recover quicker when they are with their social group.” Whilst these results primarily demonstrate the importance of social factors when looking into the specific relationships between physiology and behaviour, they also emphasise the more general importance of taking species sociality into consideration when designing experiments. Taking this work forwards, Brett hopes that it may help to shed some light on how social pressures have shaped the evolution of intraspecific variation in stress responses. “N. pulcher has many close relatives that live in almost identical habitats; however, some are highly social whilst others are not,” explains Brett. “Understanding how these different species respond physiologically to social challenges may help us learn more about the evolution of these contrasting social lifestyles.”


1. Tudorache C, Schaaf M, Slabbekoorn H. 2013. Covariation between behaviour and physiology indicators of coping style in zebrafish (Danio rerio). J Endocrinol, 219, 251–258.

2. Tudorache C, Slabbekoorn H, Robbers Y, et al. 2018. Biological clock function is linked to proactive and reactive personality types. BMC Biol, 16, 148.

3. Øverli Ø, Winberg S, Pottinger TG. 2005. Behavioral and neuroendocrine correlates of selection for stress responsiveness in rainbow trout—a review. Integr Comp Biol, 45, 463–474.

4. Teles MC, Dahlbom SJ, Winberg S, Oliveira RF. 2013. Social modulation of brain monoamine levels in zebrafish. Behav Brain Res, 253, 17–24.

5. Culbert BM, Gilmour KM, Balshine S. 2018. Stress axis regulation during social ascension in a group-living cichlid fish. Horm Behav, 103, 121–128.

Category: Animal Biology
Alex evans

Alex Evans

Alex Evans is a Research Postgraduate at the University of Leeds investigating the energetics of bird flight. In his spare time, Alex enjoys writing about the natural world, contributing to the Bird Brained Science blog and exploring other avenues of science communication.

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