Comparative evaluation over time during mate choice in the green swordtail Xiphophorus hellerii

During mate choice, choosers are exposed to a variety of sexual signals varying in both magnitude and the environment in which they are experienced. Previous work assumes that choosers evaluate signal variation from potential mates that are simultaneously viewed and compared. However, this is an extreme scenario, and it is important to also consider sequential presentation of mates, as most animals likely experience both scenarios during mate choice. Using green swordtail fish (Xiphophorus hellerii), we assessed whether female preferences for larger males differed when males of different sizes were experienced simultaneously or sequentially. We also investigated the perceptual mechanisms of comparison in both contexts, given recent research suggesting that female preferences are often nonlinear. We found that females consistently preferred larger males, irrespective of whether males were experienced simultaneously or sequentially. However, female preferences were stronger for a male of a given size when viewed under simultaneous, compared with sequential, conditions. During sequential presentation, females compared information from both previously and currently presented males, and interest did not decay with subsequent presentations. Previous research has demonstrated that female green swordtails assessing males simultaneously attend to the relative size difference between males, but we found no evidence of any comparative size assessment. Our study demonstrates that when designing mate choice experiments, it is important to consider how females encounter potential mates in the wild, highlighting that there are clear differences in preferences due to methods of mate presentation and that stochastic adjusting of internal standards of quality frequently occur.