feathers may have served as insulation, helping females incubate their eggs, but if proto-bird courtship combined displays of forelimb feathers with energetic jumps, then the transition to flight could have been relatively smooth. Sexual selection may sometimes generate features that help cause a species' extinction, as has been suggested for the giant antlers of the Irish elk (Megaloceros giganteus) that became extinct in Pleistocene Europe. Or it may do the opposite, driving species divergence—sometimes through elaborate changes in genitalia—such that new species emerge. In different taxa Sexual selection is widely distributed among the eukaryotes, occurring in plants, fungi, and animals. Since Darwin's pioneering observations on humans, it has been studied intensively among the insects, spiders, amphibians, scaled reptiles, birds, and mammals, revealing many distinctive behaviours and physical adaptations. In mammals Darwin conjectured that heritable traits such as beards, hairlessness, and steatopygia in different human populations are results of sexual selection in humans. Humans are sexually dimorphic; females select males using factors including voice pitch, facial shape, muscularity, and height. Among the many instances of sexual selection in mammals is extreme sexual dimorphism, with males as much as six times heavier than females, and male fighting for dominance among elephant seals. Dominant males establish large harems of several dozen females; unsuccessful males may attempt to copulate with a harem male's females if the dominant male is inattentive. This forces the harem male to defend his territory continuously, not feeding for as much as three months. Also seen in mammals is sex-role reversal, as in the highly social meerkats, where a large female is dominant within a pack, and female–female competition is observed. The dominant female produces most of the offspring; the subordinate females are nonbreeding, providing altruistic care to the young. In arthropods Sexual selection occurs in a wide range of spider species, both before and after copulation. Post-copulatory sexual selection involves sperm competition and cryptic female choice. Sperm competition occurs where the sperm of more than one male competes to fertilise the egg of the female. Cryptic female choice involves the expelling of a male's sperm during or after copulations. Many forms of sexual selection exist among the insects. Parental care is often provided by female insects, as in bees, but male parental care is found in belostomatid water bugs, where the male, after fertilizing the eggs, allows the female to glue her eggs onto his back. He broods them