In human paleolithic populations, males were no doubt vital for hunting and protection. Many bird species rear the young jointly. However, most invertebrate species are not raised by parents at all, with larvae developing amongst the plankton. In addition, males and females in many vertebrate species spend resources to attract and compete for mates. Sexual selection may appear to favor traits that may reduce the fitness of an organism, such as brightly colored plumage in birds of paradise, which makes them more visible to predators. Thus, sexual reproduction appears to be highly inefficient.