Disscussion
We report for the first time personality in a natural population of the damselfly C. splendens measured in the wild. We show cross-context repeatability in most of the traits studied. Traits related to both courtship and boldness axes showed repeatability values close to the average value of behavioural repeatability across over 750 studies of various behavioural traits and taxa (Bell et al., 2009). Our research responds to the need to study personality in natural field conditions in order to assess ecologically relevant situations and contexts (e.g. Archard and Braithwaite, 2010; Hertel et al., 2020). Further, our study indicates that C. splendens is likely suitable to become a model organism in behavioural studies.
Generally, individuals’ latency to approach a rival and number of bites are one of the most commonly used indicators of aggressiveness and show high repeatability in most studies (Keiser et al., 2018). In our case, absence or low repeatability for both reaction to intruder and number of bites (Tab.2), are in contrast to that trend, and are also in contrast to an earlier meta-analysis (Bell et al., 2009). However, in crickets, Fitzsimmons and Bertram (2013) showed low repeatability of aggression scores (quantified from the duration and frequency of agonistic behaviours during contest). The authors suggested that the trait plasticity was an effect of social environment as well as the insect physiology (Fitzsimmons and Bertram, 2013). In our study, the intruder male was chosen randomly, and we did not take measures of their physiological condition, which might have been useful for a deeper understanding of our result. Also, we propose that future studies should be using a mirror (e.g. Balzarini et al., 2014) instead of an actual rival, and that this would bring more controlled and comparable metrics of aggression. In the case of banded demoiselle, the time needed for reaction to rival male might also be strongly influenced by the social environment (Bell et al., 2009). In this species antagonistic behaviours depend on whether the potential rival male is a neighbour, a wandering male or an actual opponent. It has been shown that neighbouring territorial males avoid contest (Briffa and Weiss, 2010; Golab et al., 2017; Gordon, 1997; Maynard Smith and Parker, 1976). Also, non-territorial C. splendens males show a characteristic wandering behaviour, that is, patrol larger sections of a river looking for territories or mates. During this activity non-territorial males may either pass a given territory, approach the resident and retreat immediately or approach and initiate a conflict of varying intensity (Koskimäki et al., 2009; Panov and Opaev, 2013). Resident males have to evaluate which of the three type of males he is facing for every interaction and react adequately to the situation.
With regard to biting an intruder, based on our data and earlier studies, we suggest that this trait probably is unsuitable for personality measures. The biting behaviour requires a situation when the head (mandible) of the resident male approaches the part of the opponent’s body that is ‘suitable for biting’. In aerial contest (Marden and Waage, 1990) biting the intruder may arise by chance, depending of the direction/intensity/frequency of the chasing damselflies movements. Also, some parts of the body may simply be harder to bite, for instance the centre of a wing area. Additionally, despite the fact that dragonflies have one of the most advanced visual systems among insects (Bybee et al., 2012) and can compute flight trajectory of their prey (Olberg, 2012) there is no evidence that odonates would be able to compute their opponents body movements during fighting in order to precisely bite one another. Summarizing the above, we conclude that the time for reaction to the opponent and number of bites in C. splendens are plastic traits, depending on either social, environmental or physical factors, or a combination of them all and hence are not useful for personality studies in calopterygids.
In contrast, the number of hits seemed to be a better proxy of aggression in C. splendens in the wild. Generally,Calopteryx spp. males compete for resources during aerial contests (Marden and Waage, 1990), but most of the disputes are brief pursuit flights after which an intruder is chased away. During escalated longer aerial fights some collision or hitting can occur (Rüppell et al., 2005; Golab pers. observ.). The above discussed results illustrates the importance of choosing the right test for estimating a personality trait (Sih and Bell, 2008) and developing multiple proxies for a given behavioural axis might be crucial to identify the most suitable test for the targeted trait in a given species (Carter et al., 2013).
The time a resident male needed to react to an approaching female showed individual consistency (Tab. 2). The trait may appear similar to the reaction to an intruder, which was not repeatable, but when reacting to a female the resident male wants to attract rather than chase away (Corbet, 1999). In addition, the two traits related to courtship: dive display and engagement were also consistent and, as such, potentially a good metric for personality. Consistent engagement to mating displays has also been shown for instance in male guppies (Biro et al., 2016; Magellan and Magurran, 2007). This is all in accordance with theory that predicts consistency in mating behaviours since it reduces cognitive costs for potential mates (Dall et al., 2004). Also, given that personality traits are heritable (Korsten et al., 2013; Réale et al., 2007) and the offspring environment is predictable, female (choosing partner) can assess what behavioural traits would be adaptive for her offspring and choose a sexual partner of a beneficial behavioural profile. But on the other hand, highly variable or unpredictable environments would favour behavioural plasticity rather than consistency (Dingemanse et al., 2010). Hence, there are studies showing no personality in mating-related behaviours as for example research on subdominant reindeers, whose propensity to enter/visit mating group is based on proximate factors such as the group sex ratio and a day of mating season (Strong, 2015).
Among insects one of the most advanced personality research in the wild has been conducted on crickets. In an interesting and large-scale project “Wild Crickets” (https://www.wildcrickets.org/) a group of researchers studied personality both in the field and in the laboratory. They found that individual behavioural consistency is steady over adult lifetimes (Fisher et al., 2015a) and that personality in captivity not always predicts personality in nature (Fisher et al., 2015b). This is in line with another study on crickets Gryllus campestris showing that handling procedure in translocation experiments may bias repeatability estimates (Niemelä and Dingemanse, 2017). In our study we did not compare field with lab-based experiments. However, since there is growing evidence that gene expression can be significantly modified by environmental factors (Niemelä and Dingemanse, 2014) and artificial conditions can impose additional unnatural stressors (Archard and Braithwaite, 2010), we conclude that field studies are superior, in ecological relevance, compared to lab-based experiments on most animals including adult calopterygid damselflies. More specifically, the methods presented in this study are particularly promising for studying adult damselfly behaviour since they reduced handling trauma, did not influence natural/free behaviour of damselflies during the observations (Golab – pers. obs.) and prevented damselflies from adjusting/habituating to the procedure (Archard and Braithwaite, 2010; Hilfert-Rüppell, 1999).
One common difficulty when studying animal personality in the field is controlling environmental heterogeneity (Bell, 2004; Dingemanse and Réale, 2013; Quinn et al., 2009). In our experiments environmental factors were natural and many were standardized for: microclimate (sunlight penetration, air temperature, wind speed), water quality (current, temperature velocity), composition and structure of macrophytes (Gibbons and Pain, 1992; Guillermo-Ferreira and Del-Claro, 2011; Hilfert and Ruppell, 1997; Hilfert-Rüppell, 1999; Siva-Jothy et al., 1995, 1995; Siva-Jothy and Hooper, 1995) and predation (Golab and Sniegula, pers. obs., details in methods). This adds a robustness to our results that often can be lacking in animal personality field-studies due to varying environmental conditions.
Here we emphasise that Calopteryx spp. express other ecologically important behaviours, beyond the “Big Five”(Keiser et al., 2018), that could be potentially useful for future personality studies (Koski, 2014). These traits include: territory patrolling (Corbet, 2004; Golab et al., 2017), gathering of non-territorial males (Golab et al., 2013) and a very elaborated repertoire of courtship behaviours (Corbet, 2004; Rüppell et al., 2005).
To summarize, our work is the first that demonstrate behavioural repeatability in an adult damselfly in the wild. Our results suggest that adult C. splendens is a very promising model organism for studying insect personality under ecologically relevant natural conditions. The species has an elaborate repertoire of behaviours that can be easily observed and measured swiftly using only the naked eye. In addition, they also have a strong site fidelity which enables controlled and relevant manipulations of key environmental parameters. We suggest that our study represents the natural variability that exists in studied behaviours of this species. Two of the traits related to aggressiveness were not consistent and should hence not be useful for personality tests or experiments. This emphasises the need of proper trait selection when aiming to understand ecological implications of differences in individual behaviour.