Two sexes of one species can look like different animals
Sexual dimorphism means males and females of a species differ in form beyond the reproductive organs themselves. Peacock trains, raptor size reversals, and yellow-tinged blue tit nestlings show how ornament, color, and body size diverge—and how mate choice can amplify the split.
The differences can be in size, weight, colour, markings, behaviour or cognition, and they turn up in most species with separate sexes, which covers most animals and some plants. Colour gaps have their own name, sexual dichromatism, and are common in birds and reptiles. Competition for mates tends to exaggerate such traits, because showier individuals leave more offspring. The peacock is the textbook case: a male's enormous iridescent train and a female's muted brown make the pair look like separate species.
Subtler cases abound. Male blue tit nestlings are yellower than females, probably thanks to eating green caterpillars rich in the carotenoids lutein and zeaxanthin, and the sexes also differ in ultraviolet colours invisible to us. Some frogs change colour permanently, while males of Ranoidea lesueuri flush only for the breeding season. Female guppies choose brightly spotted males over dull ones, fitting the sexy son idea that females favour elaborate partners. Male redlip blennies grow a gland that secretes antimicrobial compounds, which they rub inside the nest to protect eggs from infection.
Sometimes the usual pattern flips. In reversed dimorphism the female is bigger or flashier, as in birds of prey, spiders, leopard seals and some waders. Many insects show females larger than males despite fierce male rivalry; in the red mason bee, males measure 8 to 10 mm and females 10 to 12. Male dung beetles such as Onthophagus taurus grow horns that females lack, and some insects have females that stay larva-shaped or flightless as adults.
Plants play too. About 6 percent of flowering species have separate male and female individuals. Insect-pollinated sexes usually look alike so pollinators keep visiting, but male Catasetum orchids slam pollen packets onto euglossine bees, which then shun male flowers and head for females. Wind-pollinated plants such as Leucadendron rubrum are more often dimorphic, since males are selected to scatter pollen and females to catch it, and male hemp photosynthesises faster during growth but females overtake them at maturity.
Source: Sexual dimorphism
