Tracking the startle response of guppies Poecilia reticulata in three dimensions

Habituation underpins preference for mates with novel phenotypes in the guppy

Populations harbour enormous genetic diversity in ecologically important traits. Understanding the processes that maintain this variation is a long-standing challenge in evolutionary biology. Recent evidence indicates that a mating preference for novel sexual signals can be a powerful force maintaining genetic diversity. However, the proximate underpinnings of this preference, and its generality, remain unclear. Here, we test the hypothesis that preference for novel sexual signals is underpinned by habituation, a nearly ubiquitous form of learning whereby individuals become less responsive to repetitive stimuli. We use the Trinidadian guppy ( Poecilia reticulata), in which male colour patterns are diverse yet heritable. We show that repeated exposure to males with a given colour pattern reduces female interest in males with that pattern, and that interest recovers following brief isolation. These results fulfil two core criteria of habituation: responsiveness decline and spontaneous recovery. To distinguish habituation from sensory adaptation and fatigue, we also demonstrate stimulus specificity and dishabituation. These results provide the first evidence that habituation causes a preference for novel sexual signals, addressing the mechanistic underpinnings of this mating preference. Given the pervasiveness of habituation among taxa and sensory contexts, our findings suggest that preference for novelty may play an underappreciated role in mate choice and the maintenance of genetic variation.

Is the Capacity for Vocal Learning in Vertebrates Rooted in Fish Schooling Behavior?

The capacity to learn and reproduce vocal sounds has evolved in phylogenetically distant tetrapod lineages. Vocal learners in all these lineages express similar neural circuitry and genetic factors when perceiving, processing, and reproducing vocalization, suggesting that brain pathways for vocal learning evolved within strong constraints from a common ancestor, potentially fish. We hypothesize that the auditory-motor circuits and genes involved in entrainment have their origins in fish schooling behavior and respiratory-motor coupling. In this acoustic advantages hypothesis, aural costs and benefits played a key role in shaping a wide variety of traits, which could readily be exapted for entrainment and vocal learning, including social grouping, group movement, and respiratory-motor coupling. Specifically, incidental sounds of locomotion and respiration (ISLR) may have reinforced synchronization by communicating important spatial and temporal information between school-members and extending windows of silence to improve situational awareness. This process would be mutually reinforcing. Neurons in the telencephalon, which were initially involved in linking ISLR with forelimbs, could have switched functions to serve vocal machinery (e.g. mouth, beak, tongue, larynx, syrinx). While previous vocal learning hypotheses invoke transmission of neurons from visual tasks (gestures) to the auditory channel, this hypothesis involves the auditory channel from the onset. Acoustic benefits of locomotor-respiratory coordination in fish may have selected for genetic factors and brain circuitry capable of synchronizing respiratory and limb movements, predisposing tetrapod lines to synchronized movement, vocalization, and vocal learning. We discuss how the capacity to entrain is manifest in fish, amphibians, birds, and mammals, and propose predictions to test our acoustic advantages hypothesis.