Resilience to the daily light cycle of multimodal communication in the Malawian cichlid fish Maylandia zebra

Multimodal communication is a widely studied phenomenon across the animal kingdom, generally involving the visual, chemical and acoustic channels. However, most studies do not consider the impact of the daily light intensity variations: for a multitude of species that are dependent on visual cues, the light changes constitute a challenge, which may be solved by a shift from visual to non-visual signalling. In this paper, we examine the impact of light intensity on the visual and acoustic communication of fish during aggressive interactions. First, we describe the aggressive interactions within a stable group of males of the cichlid Maylandia zebra submitted to a light-dark (LD) cycle including 3 phases: low-light (mimicking dusk), daylight, and darkness (night). Under low-light and darkness, the number of aggressions is lower compared to daylight. The presence of aggressive acts in darkness suggests that the triggering of aggression relies on chemical and/or mechanical channels, taking the place of the visual channel under daylight and low-light conditions. Aggressive acts are more frequently accompanied by sounds in darkness (19.5%) than in daylight (8.5%) or low-light (8.5%), suggesting that the relative importance of visual and auditory cues in social interactions depends on environmental constraints affecting each sensory modality. The number of pulses per sound is significantly lower in sounds that accompany nocturnal aggressive acts. We then studied the impact of visual and visual + acoustic stimuli on aggressive interactions occurring under daylight, low-light and darkness conditions, using a fish (opponent) as visual stimulus and sounds from conspecifics as acoustic stimulus. Under the darkness conditions, stimuli have no effect, as the triggering effect of light is absent, and as mechanical and chemical channels cannot play a role due to glass separation between the opponent fish and the test fish. Under low-light conditions, visual + acoustic stimulus has a greater lowering impact on the aggressiveness of the tested males than under daylight conditions. Together, these results demonstrate that senders and receivers optimise their communication during aggressive interactions by adjusting the relative importance of auditory and visual channels according to ambient light conditions. This phenomenon is likely to be widespread in fish and other animals as many species experience a daily light cycle in their natural environment. Furthermore, in the context of increased anthropogenic change, phenotypic plasticity could provide a short-term mean of compensating for disturbances such as increased turbidity that disturbs visual communication.