In most animals, the origins of mating preferences are not clear. The 'sensory-bias' hypothesis proposes that biases in female sensory or neural systems are important in triggering sexual selection and in determining which male traits will become elaborated into sexual ornaments. Subsequently, other mechanisms can evolve for discriminating between high-and low-quality mates. Female guppies (Poecilia reticulata) generally show a preference for males with larger, more chromatic orange spots. It has been proposed that this preference originated because it enabled females to obtain high-quality mates. We present evidence for an alternative hypothesis, that the origin of the preference is a pleiotropic effect of a sensory bias for the colour orange, which might have arisen in the context of food detection. In field and laboratory experiments, adult guppies of both sexes were more responsive to orange-coloured objects than to objects of other colours, even outside a mating context. Across populations, variation in attraction to orange objects explained 94% of the inter-population variation in female mate preference for orange coloration on males. This is one of the first studies to show both an association between a potential trigger of a mate-choice preference and a sexually selected trait, and also that an innate attraction to a coloured inanimate object explains almost all of the observed variation in female mate choice. These results support the 'sensorybias' hypothesis for the evolution of mating preferences.

The suitability of calcein as a marker for population studies depends on (1) the assumption that marked individuals have unaltered viability, (2) the fidelity of the calcein label, and (3) the facility with which calcein can be used. We examined the effects of calcein on survival, growth, and the timing and size at sexual maturity of least killifish Heterandria formosa and present a new method for detecting calcein. To test the assumption that marked individuals have unaltered viability, juvenile least killifish were immersed for 24 h in a 250‐mg/L solution of calcein. A control group of same‐aged juveniles was immersed in the same volume of water for 24 h without calcein. All individuals were then removed and reared individually in separate containers. Upon examination under an epifluorescent microscope, all calcein‐treated fish showed fluorescent green marks on their scales and fin rays, whereas controls did not. The calcein treatment had no significant effect on growth and survival through 9 weeks nor on the age and size at maturity. We also designed a portable detector (uses InGaN blue LEDs as a light source) for distinguishing calcein‐marked individuals; using either this new detector or a standard epifluorescent microscope, the fluorescent mark was visible on the test fish for up to 5 weeks in the laboratory, although some individuals required remarking (due to fading) at 2–3 weeks postimmersion. The calcein tag was also visible in the vertebrae of ethanol‐preserved specimens for up to 6 years, provided specimens were stored in the dark.