The correlation between subordinate fish eye colour and received attacks: a negative social feedback mechanism for the reduction of aggression during the formation of dominance hierarchies (2023)

Social rank functions to facilitate coping responses to socially stressful situations and conditions. The evolution of social status appears to be inseparably connected to the evolution of stress. Stress, aggression, reward, and decision-making neurocircuitries overlap and interact to produce status-linked relationships, which are common among both male and female populations. Behavioral consequences stemming from social status and rank relationships are molded by aggressive interactions, which are inherently stressful. It seems likely that the balance of regulatory elements in pro- and anti-stress neurocircuitries results in rapid but brief stress responses that are advantageous to social dominance. These systems further produce, in coordination with reward and aggression circuitries, rapid adaptive responding during opportunities that arise to acquire food, mates, perch sites, territorial space, shelter and other resources. Rapid acquisition of resources and aggressive postures produces dominant individuals, who temporarily have distinct fitness advantages. For these reasons also, change in social status can occur rapidly. Social subordination results in slower and more chronic neural and endocrine reactions, a suite of unique defensive behaviors, and an increased propensity for anxious and depressive behavior and affect. These two behavioral phenotypes are but distinct ends of a spectrum, however, they may give us insights into the troubling mechanisms underlying the myriad of stress-related disorders to which they appear to be evolutionarily linked.

Aggression is an extremely complex behaviour and female aggression is understudied when compared to males. Despite the fact that it has been suggested that conflict among females may be more frequently resolved peacefully, in many species females show high levels of aggression. We used Cichlasoma dimerus to describe dynamics and conflict outcome in intrasexual agonistic encounters. We performed encounters of two sex-matched animals in a neutral arena and we recorded agonistic interactions during one hour. All aggressive and submissive behaviours were described and quantified to perform the ethogram. Encounters followed three phases: pre-contest, contest and post-resolution. Latency, time of resolution and frequency of aggressive displays did not differ between sexes. Relative variations in size between female opponents better explained aggression outcome in each contest, since higher levels of aggression occurred in dyads of more similar fish. However, this was not observed in males, suggesting that probably morphological characteristics could be less relevant in male conflict resolution. Altogether these results suggest that in this ethological context, C. dimerus females are as aggressive as males and that they have similar motivation towards territorial aggression, emphasizing the need of deepening the study of aggression in females and not only in males.

This study aims to describe a Bayesian Hierarchical Linear Model (HLM) approach for longitudinal designs in fish’s experimental aggressive behavior studies as an alternative to classical methods In particular, we discuss the advantages of Bayesian analysis in dealing with combined variables, non-statistically significant results and required sample size using an experiment of angelfish (Pterophyllum scalare) species as case study. Groups of 3 individuals were subjected to daily observations recorded for 10min during 5days. The frequencies of attacks, displays and the total attacks (attacks+displays) of each record were modeled using Monte Carlo Markov chains. In addition, a Bayesian HLM was performed for measuring the rate of increase/decrease of the aggressive behavior during the time and to assess the probability of difference among days. Results highlighted that using the combined variable of total attacks could lead to biased conclusions as displays and attacks showed an opposite pattern in the experiment. Moreover, depending of the study, this difference in pattern can happen more clearly or more subtly. Subtle changes cannot be detected when p-values are implemented. On the contrary, Bayesian methods provide a clear description of the changes even when patterns are subtle. Additionally, results showed that the number of replicates (15 or 11) invariant the study conclusions as well that using a small sample size could be more evident within the overlapping days, that includes the social rank stability. Therefore, Bayesian analysis seems to be a richer and an adequate statistical approach for fish’s aggressive behavior longitudinal designs.

Pigmentation often signals status and in general melanin-based pigmentation is indicative of aggression and stress resilience in vertebrates. This is evident in the salmonids Atlantic salmon (Salmo salar) and rainbow trout (Oncorhynchus mykiss) where more melanin spotted individuals are more stress resilient. However, in the salmonid Arctic charr (Salvelinus alpinus) it seems as if it is carotenoid-based pigmentation that signals aggression and stress resilience. In our study, social stress effects on carotenoid-based spots, and behavioural and physiological stress responses were investigated. Socially stressed individuals have more spots, and behavioural stress responses were associated with spots. Some of the results concerning physiological stress responses, such as plasma cortisol levels and monoaminergic activity, are associated with spottiness. Further, the earlier proposed lateralization of spots, with left side connected to stress responsiveness and right side to aggression, is to some extent validated although not conclusively. In conclusion, this study provides further evidence that more stressed charr have more carotenoid spots, and for the first time monoaminergic activity is shown to be connected with carotenoid pigmentation.

Social conflicts are usually solved by agonistic interactions where animals use cues to signal dominance or subordinance. Pigmentation change is a common cue used for signalling. In our study, the involvement of carotenoid-based pigmentation in signalling was investigated in juvenile Arctic charr (Salvelinus alpinus). Size-matched pairs were analysed for pigmentation both before and after being tested for competitive ability. We found that dominant individuals had fewer carotenoid-based spots on the right and left sides as well as lower plasma cortisol levels compared to subordinate individuals. Further, the number of spots on both sides was positively associated with plasma cortisol levels. These results indicate that carotenoid-based pigmentation in Arctic charr signals dominance and stress coping style. Further, it also appears as if carotenoid-based pigmentation is lateralized in Arctic charr, and that the right side signals aggression and dominance whereas the left side signals stress responsiveness.

We expand the use of eye darkening (ED) to indicate non-social stress in the fish Nile tilapia Oreochromis niloticus (L.). ED is easily estimated, not requiring any sophisticated equipment, and is non-invasive, facilitating the collection of several measures of stress over time. In the current study, we showed the following: (i) high- and low-ED occur spontaneously, indicating different fish reactions to adjustments to a novel environment; (ii) fish confinement or air exposure clearly increases ED (air exposure is a stronger stressor than confinement), and the time to restore basal values indicates the severity of the impact of the stressor on the fish (this response is not affected by period of the day, e.g., morning or afternoon); and (iii) in adults, females were more responsive (slower recovery) to 2-min air exposure than to 30-min confinement.

Pharmacology Biochemistry and Behavior, Volume 157, 2017, pp. 1-8

Changes in the expression of the dopamine transporter (DAT), or the sensitivity of dopamine receptors, are associated with aging and substance abuse and may underlie some of the symptoms common to both conditions. In this study, we explored the role of the dopaminergic system in the anxiogenic effects of aging and acute cocaine exposure by comparing the behavioral phenotypes of wild type (WT) and DAT knockout zebrafish (DAT-KO) of different ages. To determine the involvement of specific dopamine receptors in anxiety states, antagonists to D1 (SCH23390) and D2/D3 (sulpiride) were employed. We established that DAT-KO results in a chronic anxiety-like state, seen as an increase in bottom-dwelling and thigmotaxis. Similar effects were produced by aging and acute cocaine administration, both leading to reduction in DAT mRNA abundance (qPCR). Inhibition of D1 activity counteracted the anxiety-like effects associated with DAT deficit, independent of its origin. Inhibition of D2/D3 receptors reduced anxiety in young DAT-KO, and enhanced the anxiogenic effects of cocaine in WT, but did not affect aged WT or DAT-KO fish. These findings provide new evidence that the dopaminergic system plays a critical role in anxiety-like states, and suggest that adult zebrafish provide a sensitive diurnal vertebrate model for elucidating the molecular mechanisms of anxiety and a platform for anxiolytic drug screens.

Animal Behaviour, Volume 105, 2015, pp. 55-62

Conflict is an inherent part of group living, and the mediation of conflict is essential for the stability of social groups. Response to within-group social conflict should depend on the external social environment. Individuals in dense social neighbourhoods have greater opportunities to disperse and join a nearby group compared to individuals in sparse social neighbourhoods with few nearby groups. To explore the influence of the social neighbourhood on responses to conflict, we experimentally perturbed groups of wild Neolamprologus pulcher, a cooperatively breeding cichlid fish, by temporarily removing a subordinate individual. Such removals typically increase the amount of within-group aggression. As predicted, aggression towards the returning subordinate and the rate of eviction from the group increased with the density of neighbouring social groups. Furthermore, we predicted that the returning subordinate could improve its likelihood of reacceptance into the group by displaying submissively. To test this prediction, we attempted to manipulate submissive behaviour by injecting the removed individuals with isotocin, a nonapeptide hormone that has been shown in the laboratory to increase the expression of submissive behaviour in this species. As predicted, subordinates that received isotocin showed more submission when returned to their group. However, contrary to our prediction, these isotocin-treated fish received more aggression from their group-mates and were more likely to be evicted than fish receiving a saline control injection. Our results emphasize the importance of the social neighbourhood in determining within-group dynamics but surprisingly contradict the notion that submissive behaviour reduces aggression and facilitates group stability.

Hormones and Behavior, Volume 65, Issue 4, 2014, pp. 340-344

Living animals exploit information released from dead animals to conduct adaptive biological responses. For instance, a recently published study has shown that avoidance behavior is triggered by death-associated odors in zebrafish. Stress can clearly act as an adaptive response that allows an organism to deal with an imminent threat. However, it has not been demonstrated whether these chemical cues are stressful for fish. Here, we confirmed that dead zebrafish scents induce defensive behavior in live conspecifics. Additionally, we show for the first time in fish that these scents increase cortisol in conspecifics. To reach this conclusion, firstly, we exposed zebrafish to multi-sensorial cues (e.g., visual, tactile, chemical cues) from dead conspecifics that displayed defensive behaviors and increased cortisol. Also, when we limited zebrafish to chemical cues from dead conspecifics, similar responses arose. These responses coincide with the decaying destruction of epidermal cells, indicating that defensive and stress responses could take place as an effect of substances emanating from decaying flesh, as well as alarm substance released due to rupture of epidermal cells. Taken together, these results illustrate that living zebrafish utilize cues from dead conspecific to avoid or to cope with danger and ensure survival.

General and Comparative Endocrinology, Volume 221, 2015, pp. 64-74

The family Cichlidae is well-known for pair-formation, parental care, territoriality, elaborate courtship and social organization. Do cichlids use chemical communication to mediate any of these behaviours? Early studies suggest that parent cichlids can discriminate between conspecific and heterospecific wrigglers (but not eggs) using olfactory cues. Some species are able to discriminate between their own brood and other conspecific broods based on olfaction. The young recognise conspecific adults (although not necessarily their parents) through the odorants they release. In both scenarios, protection of the young from predation is the likely selective force. Some male cichlids use urinary pheromones during courtship and spawning to attract females and induce ovulation. Females – in their turn – may base their mate-choice in part on assessment of those self-same pheromones. The same pheromonal system may be involved in establishing and maintaining the social hierarchies in lek-breeding cichlids. Individual recognition is also mediated by chemical communication. Finally, there is ample behavioural evidence that cichlids – like ostariophysan fish – release alarm cues that alert conspecifics to predation danger. Although the effects of these cues may be similar (e.g., increased shelter use, tighter schooling), they are different substances which remain to be identified. Cichlids, then, use chemical communication associated with many different behaviours. However, given the diversity of cichlids, little is known about the mechanisms of chemical communication or the chemical identity of the cues involved. The aim of this mini-review is to persuade those working with cichlids to consider the involvement of chemical communication, and those working in chemical communication to consider using cichlids.

Hormones and Behavior, Volume 84, 2016, pp. 57-63

The teleost fish nonapeptides, arginine vasotocin (AVT) and isotocin (IT), have been implicated in the regulation of social behavior. These peptides are expected to be involved in acute and transient changes in social context, in order to be efficient in modulating the expression of social behavior according to changes in the social environment. Here we tested the hypothesis that short-term social interactions are related to changes in the level of both nonapeptides across different brain regions. For this purpose we exposed male zebrafish to two types of social interactions: (1) real opponent interactions, from which a Winner and a Loser emerged; and (2) mirror-elicited interactions, that produced individuals that did not experience a change in social status despite expressing similar levels of aggressive behavior to those of participants in real-opponent fights. Non-interacting individuals were used as a reference group. Each social phenotype (i.e. Winners, Losers, Mirror-fighters) presented a specific brain profile of nonapeptides when compared to the reference group. Moreover, the comparison between the different social phenotypes allowed to address the specific aspects of the interaction (e.g. assessment of opponent aggressive behavior vs. self-assessment of expressed aggressive behavior) that are linked with neuropeptide responses. Overall, agonistic interactions seem to be more associated with the changes in brain AVT than IT, which highlights the preferential role of AVT in the regulation of aggressive behavior already described for other species.

Behavioural Brain Research, Volume 362, 2019, pp. 332-337

The elevated plus maze is a prominent and well-documented test for studying anxiety in rodents. Fish are becoming more prevalent in studies of anxiety, yet the elevated plus maze has not been adapted and validated for fish. In the present study, we created an aquatic version of the elevated plus maze called the ‘submerged plus maze,’ which is shaped like a plus symbol with four arms alternating between black and transparent walls. We used convict cichlid fish (Amatitlania nigrofasciata) and administered diazepam to validate the apparatus for studying anxiety-like behaviour. After diazepam exposure, fish spent more time in and entered more open arms than after vehicle exposure, consistent with the effect of benzodiazepines on rodents in the elevated plus maze. The submerged plus maze maintains construct validity for testing anxiety in convict cichlid fish.