Phenotypic integration (i.e. the degree of covariation among traits) is an important and ubiquitous feature of multicellular organisms that shapes functionality within and among structures. Integration can permit or limit the accumulation of morphological variation by partitioning traits into distinct subunits (i.e. modules) and then varying the degree of association among these subunits (Goswami et al., 2014; Klingenberg, 2008). Recent research has demonstrated that differences among populations in the pattern and magnitude of phenotypic integration can have major consequences for taxonomic diversification, rates of morphological evolution, and the ability of a population to respond to selection (

Animals display remarkable diversity in rest and activity patterns that are regulated by endogenous foraging strategies, social behaviors, and predator avoidance. Alteration in the circadian timing of activity or the duration of rest-wake cycles provide a central mechanism for animals to exploit novel niches. The diversity of the 3000+ cichlid species throughout the world provides a unique opportunity to examine variation in locomotor activity and rest. Lake Malawi alone is home to over 500 species of cichlids that display divergent behaviors and inhabit well-defined niches throughout the lake. These species are presumed to be diurnal, though this has never been tested systematically. Here, we measure locomotor activity across the circadian cycle in 11 Lake Malawi cichlid species. We document surprising variability in the circadian time of locomotor activity and the duration of rest. In particular, we identify a single species, Tropheops sp. “red cheek” that is nocturnal. Nocturnal behavior was maintained when fish were provided shelter, but not under constant darkness, suggesting it results from acute response to light rather than an endogenous circadian rhythm. Finally, we show that nocturnality is associated with increased eye size after correcting for evolutionary history, suggesting a link between visual processing and nighttime activity. Together, these findings identify diversity of locomotor behavior in Lake Malawi cichlids and provide a system for investigating the molecular and neural basis underlying variation in nocturnal activity.

Animals display remarkable diversity in rest and activity patterns that are regulated by endogenous foraging strategies, social behaviors, and predator avoidance. Alteration in the circadian timing of activity or the duration of rest-wake cycles provide a central mechanism for animals to exploit novel niches. The diversity of the 3000 cichlid species throughout the world provides a unique opportunity to examine variation in locomotor activity and rest. Lake Malawi alone is home to over 500 species of cichlids that display divergent behaviors and inhabit well-defined niches throughout the lake. These species are presumed to be diurnal, though this has never been tested systematically. Here, we measure locomotor activity across the circadian cycle in 12 cichlid species from divergent lineages and distinct habitats. We document surprising variability in the circadian time of locomotor activity and the duration of rest. In particular, we identify a single species, Tropheops sp. red cheek that is nocturnal. Nocturnal behavior was maintained when fish were provided shelter, but not under constant darkness, suggesting it results from acute response to light rather than an endogenous circadian rhythm. Finally, we show that nocturnality is associated with increased eye size, suggesting a link between visual processing and nighttime activity. Together, these findings identify diversity of locomotor behavior in Lake Malawi cichlids and provide a system for investigating the molecular and neural basis underlying the evolution of nocturnal activity.

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