The blood orange is a variety of orange (Citrus × sinensis) with crimson, almost-blood-colored flesh. The fruit is smaller than an average orange; its skin is usually pitted, but can be smooth. The distinctive dark flesh color is due to the presence of anthocyanins, a family of antioxidant pigments common to many flowers and fruit, but uncommon in citrus fruits. The flesh develops its characteristic maroon color when the fruit develops with low temperatures during the night. Sometimes there is dark coloring on the exterior of the rind as well, depending on the variety of blood orange. The skin can be tougher and harder to peel than that of other oranges. While all oranges are likely of hybrid origin between the pomelo and the tangerine, blood oranges originated as a mutation of the sweet orange.