Sex differences now become more frequent. The males are in some cases distinguished by bright colours and ornamental appendages. During their amours and duels certain male fishes flash with beautiful and glowing colours. Reptiles exhibit the same form of sexual-passion, and jealous combat of rival males. The rattle of certain snakes is supposed to act as a love-call. Snakes of different sexes appear to feel some affection for each other when confined together in cages. Romanes relates the interesting fact that when a cobra is killed, its mate is often found on the spot a day or two afterwards. Darwin cites an instance of the pairing in spring of a Chinese species of lizard, where the couples appear to have considerable fondness for one another. If one is captured, the other drops from the tree to the ground and allows itself to be caught, presumably from despair.[45] A further development is reached by those animals among whom what has well been called "the note of physical fondness" is first sounded. We find the males playing with the female, wooing and caressing her, it may be dancing with her. The love-play is often extraordinary,[46] as, for instance, in the well-known case of the stickleback. Not only does the male woo the female with passionate dances, but by means of its own secretions it builds a nest in the river weeds. The males at this season are transformed, glowing with brilliant colours, and literally putting on a wedding garment of love. The stickleback is passionate, polygamous and very jealous of rivals. His guardianship of the nest and vigilance in protecting the young cannot be observed without admiration.