The perceptual effects of signal components: black sword margins are crucial for signal size discrimination in green swordtails Xiphophorus hellerii - research paper

Aug 26, 2026

Male green swordtails communicate with a caudal fin signal composed of colour, sword length, and a black melanic margin, yet the role of female sensory and perceptual processing in shaping complex signal evolution remains unresolved. Using a two-choice assay with animated courting males varying in size and margin coloration, the study quantifies female mate preferences. Females preferred larger males only when the black margin was present, while margin absence eliminated size-based preference despite equal assessment time, indicating black margins enable perceptual discrimination of signal size.

LanguageEnglish

CategoryResearch & Report

FormatPDF

Pages9

Reading Time23 min

AccessFree to download

Introduction

Signal components and sexual selection hypotheses

Receiver sensory processes and perceptual processing

Size discrimination and black coloration signals

Cite this article: Caves EM, Kelley LA. 2025 The perceptual effects of signal components: black sword margins are crucial for signal size discrimination in green swordtails Xiphophorus hellerii. Proc. R. Soc. B 292: 20242137. [https://doi.org/10.1098/rspb.2024.2137](https://doi.org/10.1098/rspb.2024.2137) [Received: 6 September 2024](Received: 6 September 2024) [Accepted: 16 December 2024](Accepted: 16 December 2024) Subject Category: Behaviour Subject Areas: behaviour, evolution, cognition Keywords: perceptual processing, signal evolution, visual ecology, mate choice, swordtail, melanic coloration Authors for correspondence: Eleanor M. Caves e-mail: [eleanor_caves@brown.edu](eleanor_caves@brown.edu)[ ](eleanor_caves@brown.edu)Laura A. Kelley e-mail: [l.a.kelley@exeter.ac.uk](l.a.kelley@exeter.ac.uk) Electronic supplementary material is available online at [https://doi.org/10.6084/](https://doi.org/10.6084/)[ ](https://doi.org/10.6084/)[m9.figshare.c.7599504](m9.figshare.c.7599504.)[.](m9.figshare.c.7599504.) The perceptual effects of signal components: black sword margins are crucial for signal size discrimination in green swordtails Xiphophorus hellerii Eleanor M. Caves1,2,3 and Laura A. Kelley3 1Department of Ecology, Evolution, and Organismal Biology, Brown University, Providence, RI 02912, USA 2Department of Ecology, Evolution, and Marine Biology, University of California, Santa Barbara, Santa Barbara, CA 93106, USA 3Centre for Ecology and Conservation, University of Exeter, Penryn, UK EMC, 0000-0003-3497-5925; LAK, 0000-0003-0700-1471 The signals that mediate mate choice can be complex, comprising multiple components, and understanding how complex signals evolve under sexual selection has been the focus of much study. However, open questions still remain about the role of the female’s sensory and perceptual processes in shaping the evolution of complex signals. Male green swordtails Xiphophorus hellerii have an elongated caudal fin that comprises colour, length and a black melanic margin; females prefer males with larger bodies, longer swords and complete black sword margins. Here, we used a two-choice assay to quantify female preferences for animations of courting males of different sizes with or without sword margin coloration, and found that, when a black melanic margin was present, females exhibited preferences for larger males. However, when the margin was absent, females did not show size-based mate preference, though females spent equal time assessing males in both treatments. Our results suggest that the presence/absence of the black sword margin is an important predictor of female preference, specifically a female’s ability to discriminate between potential mates of different sizes, pointing to a novel size discrimination function of black margins in animal signals, which in many species involve patterns or structures with dark edges. 1. Introduction A major focus in the study of sexual selection has been on the signals that females use to assess potential mates. Assessment signals can be complex, comprising multiple components such as pattern, colour, motion and sound, and much research has attempted to reveal why signals have certain components. Since the early 1990s, two main hypotheses have emerged that posited that assessment signal form might evolve due to selective pressures imposed by the signal receiver’s sensory system. The ‘receiver bias’ hypothesis [1,2] stated that signals should evolve forms that exploit the innate properties, or biases, of the receiver’s sensory system and brain, to increase the stimulation of a sensory system by a signal. Building on these ideas, the ‘sensory drive’ hypothesis [3] posited that, in addition to signal traits evolving particular forms due to sensory biases, both signal form and sensory capability should additionally be under selection to optimize transmission and detection in the environment where signalling takes place.

FAQs

What question does the study address about complex mate-choice signals?

It examines how female perceptual processes influence the evolution of complex signals, focusing on whether specific signal components support accurate assessment of signal traits.

How was female preference tested in this study?

A two-choice assay measured female preferences for animations of courting males differing in body size and presence or absence of the black sword margin.

What main result links the black sword margin to mate choice?

Females showed preference for larger males only when the black melanic margin was present; without the margin, females displayed no size-based preference even though they spent equal time assessing males.

Aug 26, 2026

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