Why bilaterally symmetrical organisms express handedness remains an important question in evolutionary biology. In some species, anatomical asymmetries have evolved that accompany behavioral handedness, yet we know remarkably little about causal links between asymmetric morphological traits and behavior. Here, we explore if a dextral or sinistral orientation of the male intromittent organ predicts side preferences in male behaviors. Our study addresses this question in the Costa Rican livebearing fish, Xenophallus umbratilis. This fish has a bilaterally symmetrical body plan, with one exception—the male anal fin (gonopodium), used to inseminate females, terminates with a distinct left- or right-handed corkscrew morphology. We used a detour assay to test males for side biases in approach behavior when exposed to four different stimuli (predator, potential mate, novel object, empty tank control). We found that left morph males preferred using their right eye to view potential mates, predators, and the control, and that right morph males preferred to use their left eye to view potential mates and predators, and their right eye to view the control. Males of both morphs displayed no eye bias when approaching the novel object. Our results suggest that there is a strong link between behavior and gonopodium orientation, with right and left morph males responding with opposite directional behaviors when presented with the same stimuli. This presents the intriguing possibility that mating preferences—in this case constrained by gonopodial morphology—could be driving lateralized decision making in a variety of non-mating behaviors.

Evolutionary biologists have grown increasingly interested in laterality, a phenomenon where bilaterally symmetrical organisms show a side bias in some trait. Lateralized behavior is particularly interesting because it is not necessarily tied to morphological asymmetry. What causes lateralized behavior remains largely unknown, although previous research in fishes suggest that fish might favor one eye over another to view potential food sources, mates, and to assess predation risk. Here we test the hypothesis that a history of predation risk predicts lateralized behavior in the livebearing fish Brachyrhaphis rhabdophora. To do this, we used a detour assay to test for eye bias when a focal fish approached various stimuli (predator, potential mate, novel object, and empty tank control). Contrary to our predictions, we found no differences in lateralized behavior between fish from populations that co-occurred with fish predators relative to those that do not co-occur with predators. In fact, we found no evidence for behavioral lateralization at all in response to any of the stimuli. We explore several possible explanations for why lateralized behavior is absent in this species, especially considering a large body of work in other livebearing fishes that shows that lateralized behavior does occur.

Xenophallus umbratilis is a freshwater livebearing fish that exhibits unique antisymmetry in the male gonopodium, which terminates in either a dextral or sinistral twist. This asymmetry in the gonopodium suggests that males might exhibit side-biased behavior when interacting with females to mate. We conducted two assays to assess the laterality of male and female mating interactions based on gonopodial morphology. We observed lateralized mating behavior in one test where males with sinistral gonopodial morphology interacted with a single female. However, we did not find lateralized mating behavior in males with dextral gonopodial morphology. We also examined male and female positioning in trials that placed a single female with five males, all with the same morphology. These trials also showed no evidence of lateralized body positioning.