The article explores the relationship between oxidative stress, genetic variation, and the evolution of sexual signals in animals. It argues that female mate choice can promote the evolution of male ornaments that reliably indicate the male's level of oxidative stress, which is a key factor in fitness-related traits. The immune and detoxication systems, characterized by allelic polymorphisms, generate reactive metabolites and free radicals, which can cause oxidative stress. This stress is sensitive to the expression of sexual ornaments, such as carotenoid pigmentation in birds and fishes, and can be a reliable indicator of an individual's health and condition. The article discusses the genetic basis of these systems, including the major histocompatibility complex (MHC) and biotransformation enzymes, and their role in maintaining genetic variation. It also highlights the importance of antioxidant defenses in mitigating the negative effects of oxidative stress. The authors provide examples of sexual ornaments, such as song repertoire size in birds, comb size in roosters, and carotenoid pigmentation in males, that are particularly sensitive to oxidative stress. They conclude that female mate choice can enhance offspring fitness by selecting males with ornaments that indicate high genetic quality and resistance to oxidative stress.The article explores the relationship between oxidative stress, genetic variation, and the evolution of sexual signals in animals. It argues that female mate choice can promote the evolution of male ornaments that reliably indicate the male's level of oxidative stress, which is a key factor in fitness-related traits. The immune and detoxication systems, characterized by allelic polymorphisms, generate reactive metabolites and free radicals, which can cause oxidative stress. This stress is sensitive to the expression of sexual ornaments, such as carotenoid pigmentation in birds and fishes, and can be a reliable indicator of an individual's health and condition. The article discusses the genetic basis of these systems, including the major histocompatibility complex (MHC) and biotransformation enzymes, and their role in maintaining genetic variation. It also highlights the importance of antioxidant defenses in mitigating the negative effects of oxidative stress. The authors provide examples of sexual ornaments, such as song repertoire size in birds, comb size in roosters, and carotenoid pigmentation in males, that are particularly sensitive to oxidative stress. They conclude that female mate choice can enhance offspring fitness by selecting males with ornaments that indicate high genetic quality and resistance to oxidative stress.