male and female sex differ in size and appearance, a phenomenon called sexual dimorphism. Sexual dimorphism in animals is often associated with sexual selection—the mating competition between individuals of one sex vis-à-vis the opposite sex. In many cases, the male of a species is larger than the female. Mammal species with extreme sexual size dimorphism tend to have highly polygynous mating systems—presumably due to selection for success in competition with other males—such as the elephant seals. Other examples demonstrate that it is the preference of females that drives sexual dimorphism, such as in the case of the stalk-eyed fly. Females are the larger sex in a majority of animals. For instance, female southern black widow spiders are typically twice as long as the males. This size disparity may be associated with the cost of producing egg cells, which requires more nutrition than producing sperm: larger females are able to produce more eggs. Sexual dimorphism can be extreme, with males, such as some anglerfish, living parasitically on the female. Some plant species also exhibit dimorphism in which the females are significantly larger than the males, such as in the moss genus Dicranum and the liverwort genus Sphaerocarpos. There is some evidence that, in these genera, the dimorphism may be tied to a sex chromosome, or to chemical signalling from females. In birds, males often have a more colourful appearance and may have features (like the long tail of male peacocks) that would seem to put them at a disadvantage (e.g. bright colors would seem to make a bird more visible to predators). One proposed explanation for this is the handicap principle. This hypothesis argues that, by demonstrating he can survive with such handicaps, the male is advertising his genetic fitness to females—traits that will benefit daughters as well, who will not be encumbered with such handicaps. Sexual characteristics Sex differences in behavior The sexes across gonochoric species usually differ in behavior. In most animal species females invest more in parental care, although in some species, such as some coucals, the males invest more parental care. Females also tend to be more choosy for who they mate with, such as most bird species. Males tend to be more competitive for mating than females. See also Mating type Sexing Sex allocation Sex and gender distinction Sex assignment Sex organ References Further reading N.B.: One of many books by this pioneering authority on aspects