Signal costs, and associated trade-offs, are widely thought of as fundamental to the evolution and maintenance of reliable signals. Contrasting this view, the amplifier mechanism postulates that signals may be deemed reliable by design rather than by costs. Although the idea is not new, investigation of amplifier signals in nature remains scant. Here we show that displays and body markings on the Mediterranean tarantula, Lycosa tarantula (L.) (Lycosidae), are consistent with interpretation as amplifiers of size and body condition. The size of tibia and abdomen ventral markings (which alternate between dark and pale patterns) presented to rivals during contests are highly correlated with body size and condition, respectively. Abdomen markings have the additional feature of placement on a region especially influenced by abdomen distension and therefore has greater proportional variation than the underlying amplified trait. In spite of the differences in size and in body patterns between spiders of two isolated and ecologically different populations, correlation between body marking and dimensions, and relatively high proportional variation of the abdomen markings, was maintained, probably reflecting the adaptive value of these traits. We conducted a feeding experiment which showed that the abdominal markings enlarged (stretched with abdomen distension) at a higher rate if spiders were provided with more food, further supporting the idea that this marking amplifies body condition.