Dark ventral colouration indicates head size in a semi‐colonial gecko

Abstract Several animals present ornamental colouration which acts as signals in sexual or other social contexts by indicating some trait of the bearer. This colouration is often present only in one sex and may be produced by a varied combination of structural elements and several pigments. Given that colours may be produced in different ways, the traits the colouration is indicating, and the costs or trade?offs associated, may differ according to the colour and the species studied. Here, I study a dark ventral colouration present in males of the gecko Quedenfeldtia trachyblepharus, a high‐altitude, semi‐colonial lizard endemic to the High Atlas in North Africa. I tested for the correlation between the extension of the dark colouration and several morphological traits (body size, head size and limb length) in the gecko, as well as parasitism by mites and age. No relationship was found between dark colouration and parasitism, age and limb length, but the extension of the dark colouration was positively correlated with head size controlled for body size. Given that head size is usually a good indicator of fighting capacity, and this gecko shows a strong social structure, the findings suggest that the dark colouration is used in agonistic encounters among males, allowing for evaluation of the fighting capacity of the rivals.