How animals discriminate between stimulus magnitudes: a meta-analysis

To maximize their fitness, animals must often discriminate between stimuli differing in magnitude (such as size, intensity, or number). Weber's Law of proportional processing states that stimuli are compared based on the proportional difference in magnitude, rather than the absolute difference. Weber's Law implies that when stimulus magnitudes are higher, it becomes harder to discriminate small differences between stimuli, leading to more discrimination errors. More generally, we can refer to a correlation between stimulus magnitude and discrimination error frequency as a magnitude effect, with Weber's law being a special case of the magnitude effect. However, the strength and prevalence of the magnitude effect across species have never previously been examined. Here, we conducted a meta-analysis to quantify the strength of the magnitude effect across studies, finding that, on average, perception followed Weber's Law. However, the strength of the magnitude effect varied widely, and this variation was not explained by any biological or methodological differences between studies that we examined. Our findings suggest that although its strength varies considerably, the magnitude effect is commonplace, and this sensory bias is therefore likely to affect signal evolution across diverse systems. Better discrimination at lower magnitudes might result in signalers evolving lower magnitude signals when being discriminated is beneficial, and higher magnitude signals when being discriminated is costly. Furthermore, selection for higher magnitude signals (eg sexual ornaments) may be weakened, because receivers are less able to discriminate as signal magnitudes increase.