Buzinski, O. D. | bioRxiv Science

bioRxiv2025

The effect of environmental enrichment on social dominance and welfare in a cichlid fish

Environmental enrichment can have complex, contradictory effects on aggression and animal welfare. Although increasing enrichment may reduce aggression by limiting the visibility of competitors, it may also intensify territorial defense and harm the welfare of subordinates who are the target of territorial aggression. However, it remains unclear how variation in the complexity of an enrichment structure defended by a single dominant individual influences welfare outcomes for subordinates. In the cichlid fish Astatotilapia burtoni, dominant males defend structures as mating territories, whereas subordinate males do not. To establish social hierarchies, we housed two differently sized males (larger become dominant) and six females in one compartment containing one defendable structure. We manipulated structure complexity by placing 1, 2, or 3 halves of terracotta pots clumped together. Increasing cave number did not enhance territoriality: dominant males showed similar aggression rates, relative gonad sizes, and testosterone levels across treatments. Cave number did not significantly affect mortality, body condition, growth rate, or fin damage in subordinate males. Although relative gonad size and testosterone levels were generally higher in dominants, these differences were not always significant in the 2 and/or 3-cave treatments, suggesting weaker physiological differentiation between social states. The behavioral data supported this pattern, with status-specific differences in chase rates declining with increasing cave number. Overall, increasing environmental enrichment had little effect on welfare, but resulted in reduced physiological and behavioral distinctions between dominant and subordinate males. Therefore, careful consideration of enrichment strategies is essential for accurately interpreting status-specific outcomes in laboratory settings.

animal behavior and cognition↗

bioRxiv2025

Territory proximity influences aggression and modulates the relationship between social dominance and oxidative stress in a highly social African cichlid fish

High social rank has many benefits, including priority access to mates and resources. However, maintaining high rank or social dominance can be physiologically costly, particularly in species that engage in agonistic competition to maintain their dominance status. Previous studies have rarely manipulated the intensity of agonistic competition in controlled settings to isolate the cost of social dominance. We utilized males of the cichlid fish Astatotilapia burtoni to investigate how the degree of territoriality influences patterns of oxidative stress. In this species, high-rank males compete for territories to attract mates. We housed males individually with a defendable structure and used clear tank dividers to allow visual access to a size-matched male neighbor housed in an identical setup to manipulate territoriality. Defendable structures were placed either close to the divider (proximal treatment) or further away (distal treatment). We found that males in the distal treatment expressed higher levels of territorial aggression than those in the proximal treatment. While males in the distal treatment did not display higher levels of oxidative stress, territory proximity influenced the relationships between physiological markers of dominance (i.e., testosterone, gonad size) and oxidative balance. Although our work is not consistent with higher competition increasing the physiological cost of high rank, it suggests that the level of competition influences the regulation of oxidative balance relative to social dominance characteristics. We suggest that manipulating the degree of territoriality in a controlled setting provides a unique opportunity to further our understanding of how individuals manage the trade-offs associated with high rank. Summary StatementManipulating the amount of space between territories results in variability in the degree of territorial aggression and the interaction of physiological markers of social dominance and oxidative stress.

animal behavior and cognition↗