Back to the Meadow Brown: eyespot variation and field temperature in a classic butterfly polymorphism

Since the classic work of E.B. Ford, alternate hypotheses have focused on explaining eyespot variation in the Meadow Brown butterfly strictly as a genetic polymorphism and the role of temperature in this classic example of natural selection has therefore been overlooked. Here we use large and continuous field collections from three sites in the UK to examine the effect of field temperature on total eyespot variation using the same presence/absence scoring as Ford. We show that higher developmental temperatures in the field lead to the disappearance of the spots visible while the butterfly is at rest, explaining Ford’s original observation that hindwing spotting declines across the season as temperatures increase. Analysis of wing damage supports the historical hypothesis that hindwing spots confuse aerial predators. However, as hindwing spotting declines over the season, a ‘trade-off’ is suggested between their role in deflecting predators early in the season and their later developmental cost. In contrast, the large forewing eyespot is always present, scales with forewing length and its variation is best explained by day of the year rather than developmental temperature. As this large forewing spot is thought to be involved in ‘startling’ predators, its constant presence is therefore likely required for defence. We model annual total spot variation with phenological data from the UK and derive predictions as to how spot patterns will continue to change under increasing summer temperatures, predicting that spotting will continue to decrease both across a single season and year or year as our climate warms. Summary statement We show that a long-held example of ‘genetic’ polymorphism, eyespot variation is the Meadow Brown butterfly, is correlated with field temperature during butterfly development.