Social isolation affects intra-specific interaction behaviour and reduces the size of the cerebellar brain region in juvenile Atlantic salmon Salmo salar

Ontogeny and social context regulate the circadian activity patterns of Lake Malawi cichlids

Activity patterns tend to be highly stereotyped and critical for executing many different behaviors including foraging, social interactions, and predator avoidance. Differences in the circadian timing of locomotor activity and rest periods can facilitate habitat partitioning and the exploitation of novel niches. As a consequence, closely related species often display highly divergent activity patterns, suggesting that shifts from diurnal to nocturnal behavior, or vice versa, are critical for survival. In Africa's Lake Malawi alone, there are over 500 species of cichlids, which inhabit diverse environments and exhibit extensive phenotypic variation. We have previously identified a substantial range in activity patterns across adult Lake Malawi cichlid species, from strongly diurnal to strongly nocturnal. In many species, including fishes, ecological pressures differ dramatically across life-history stages, raising the possibility that activity patterns may change over ontogeny. To determine if rest-activity patterns change across life stages, we compared the locomotor patterns of six Lake Malawi cichlid species. While total rest and activity did not change between early juvenile and adult stages, rest-activity patterns did, with juveniles displaying distinct activity rhythms that are more robust than adults. One distinct difference between juveniles and adults is the emergence of complex social behavior. To determine whether social context is required for activity rhythms, we next measured locomotor behavior in group-housed adult fish. We found that when normal social interactions were allowed, locomotor activity patterns were restored, supporting the notion that social interactions promote circadian regulation of activity in adult fish. These findings reveal a previously unidentified link between developmental stage and social interactions in the circadian timing of cichlid activity.

Multiple faces of stress in the zebrafish (Danio rerio) brain

The changing expressions of certain genes as a consequence of exposure to stressors has not been studied in detail in the fish brain. Therefore, a stress trial with zebrafish was conducted, aiming at identifying relevant gene regulation pathways in different regions of the brain. As acute stressors within this trial, feed rewarding, feed restriction, and air exposure have been used. The gene expression data from the experimental fish brains have been analyzed by means of principal component analyses (PCAs), whereby the individual genes have been compiled according to the regulation pathways in the brain. The results did not indicate a mutual response across the treatment and gender groups. To evaluate whether a similar sample structure belonging to a large sample size would have allowed the classification of the gene expression patterns according to the treatments, the data have been bootstrapped and used for building random forest models. These revealed a high accuracy of the classifications, but different genes in the female and male zebrafish were found to have contributed to the classification algorithms the most. These analyses showed that less than eight genes are, in most cases, sufficient for an accurate classification. Moreover, mainly genes belonging to the stress axis, to the isotocin regulation pathways, or to the serotonergic pathways had the strongest influence on the outcome of the classification models.

Coercive mating has no impact on spatial learning, cognitive flexibility, and fecundity in female porthole livebearers (Poeciliopsis gracilis)

Coercive mating is a sexual selection strategy that is likely to influence female cognition. Female harassment levels have been linked to altered brain gene expression patterns and brain size evolution, suggesting females may respond to coercive mating by investing energy into "outsmarting" males. However, females exposed to coercive males have decreased foraging efficiency and likely increased stress levels, suggesting their brain function might instead be impaired. While it is therefore likely that coercive mating impacts female cognitive abilities, a direct test of this idea is currently lacking. In this study, we investigate the impact of coercive mating on female spatial memory and cognitive flexibility in a species with prevalent coercive mating. We compared the performance of female porthole livebearers (Poeciliopsis gracilis), which had been previously housed alone or with a coercive male, in both a spatial food localization task and a reversal learning task. While we found that both single and paired fish exhibited high proficiency in learning both tasks, we found no differences in learning ability between females that had or had not experienced coercive mating. In addition, our study found that the presence of a coercive male had no impact on female fecundity, but did influence female mass and standard length. Several studies have assumed that the presence of males, particularly coercive males, may affect the cognitive performance of female fish. However, our study shows that for some species females adapted to coercive mating regimes may be unaffected by male presence with regards to some cognitive tasks.

Chemical and Visual Cues as Modulators of the Stress Response to Social Isolation in the Marine Medaka, Oryzias melastigma

The marine medaka is emerging as a potential behavioral model organism for ocean studies, namely on marine ecotoxicology. However, not much is known on the behavior of the species and behavioral assays lack standardization. This study assesses the marine medaka as a potential model for chemical communication. We investigated how short exposure to visual and chemical cues mediated the stress response to social isolation with the light/dark preference test (LDPT) and the open field test (OFT). After a 5-day isolation period, and 1 h before testing, isolated fish were randomly assigned to one of four groups: (1) placed in visual contact with conspecifics; (2) exposed to a flow of holding water from a group of conspecifics; (3) exposed to both visual and chemical cues from conspecifics; or (4) not exposed to any stimuli (controls). During the LDPT, the distance traveled and transitions between zones were more pronounced in animals exposed to the conspecific's chemical stimuli. The time spent in each area did not differ between the groups, but a clear preference for the bright area in all animals indicates robust phototaxis. During the OFT, animals exposed only to chemical cues initially traveled more than those exposed to visual or both stimuli, and displayed lower thigmotaxis. Taken together, results show that chemical cues play a significant role in exploratory behavior in this species and confirm the LDPT and OFT as suitable tests for investigating chemical communication in this species.