Comparative evaluation and its implications for mate choice Melissa Bateson 1 and Susan D. Healy 2 1 School of Biology and Psychology, University of Newcastle upon Tyne, Henry Wellcome Building for Neuroecology, Framlington Place, Newcastle upon Tyne, UK, NE2 4HH 2 Institute of Evolutionary Biology, School of Biological Sciences, University of Edinburgh, Kings Buildings, West Mains Road, Edinburgh, UK, EH9 3JT Experiments on decision making by humans show that the choices that we make can be very labile. The magni- tude of our preferences, and even our rank ordering of options, can vary according to the number and type of alternatives available for comparison. This apparent irrationality has been argued to result from our use of decision heuristics that have evolved to enable us to choose quickly and efficiently between options differing in multiple attributes. Here, we argue that, because there is also selective pressure for animals to make mating decisions quickly, and because potential mates also differ in multiple attributes, similar decision heur- istics might have evolved for mate choice. Following this reasoning, the attractiveness of a given mate will depend on the others with whom he or she is being compared, rather than being an absolute function of his or her underlying quality. We describe some of the rami- fications of such comparative evaluation, and argue that it could offer new insights into some of the biggest out- standing problems in mate choice and sexual selection. Mate choice as information processing Mate choice can be viewed as an information-processing problem [1]. Females have to perceive cues from males; they have to remember this information from one male to the next; and, finally, they have to use the information that they have obtained to make mating decisions (Figure 1). In contrast to the many studies identifying the cues that females attend to in males and the underlying qualities signalled by these cues [2], there has been less empirical work directed at understanding e.g. • Parasite load • Development • Nutrition • Relatedness • Good genes’ • MHC genes • Territory quality e.g. • Colour • Body size • Tail length • Symmetry • Song • Display rate • Odour • Courtship gifts e.g. • Discrimination • Weber’s Law • Peak shift Traits (male quality) Cues Perception Learning and memory Decision mechanisms e.g. • Best of n • Threshold • Sensory bias Male biology Female cognition Female choice TRENDS in Ecology & Evolution e.g. • Capacity • Retention time • Accuracy Figure 1. The flow of information from the male to the behavioural output of the female. The assumption is that male quality translates into his traits, which then serve as cues for the female [42]. Her choice is based on her ability first to perceive the trait(s) of the male and then to use that information to make her decision [43]. The information from one male will be compared with that she has received from other males, and this comparison will be affected by her cognitive abilities, including her memory, and her ability to process information from multiple sources simultaneously. Corresponding author: Bateson, M. (Melissa.Bateson@ncl.ac.uk). Available online 8 September 2005 Opinion TRENDS in Ecology and Evolution Vol.20 No.12 December 2005 www.sciencedirect.com 0169-5347/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.tree.2005.08.013