is another factor leading to assortative mating in this species. Evidence for size-related assortative mating has also been found in the mangrove snail, Littoraria ardouiniana and in the Japanese common toad, Bufo japonicus. The second common type of assortative mating occurs with respect to coloration. This type of assortative mating is more common in socially monogamous bird species such as the eastern bluebirds (Sialia sialis) and western bluebirds (Sialia mexicana). In both species more brightly colored males mated with more brightly colored females and less brightly colored individuals paired with one another. Eastern bluebirds also mate assortatively for territorial aggression due to fierce competition for a limited number of nesting sites with tree swallows. Two highly aggressive individuals are better equipped to protect their nest, encouraging assortative mating between such individuals. Assortative mating with respect to two common color morphs: striped and unstriped also exists in a polymorphic population of eastern red-backed salamanders (Plethodon cinereus). Assortative mating is also found in many socially monogamous species of birds. Monogamous species are often involved in bi-parental care of their offspring. Since males are equally invested in the offspring as the mother, both genders are expected to display mate choice, a phenomenon termed as mutual mate choice. Mutual mate choice occurs when both males and females are searching for a mate that will maximize their fitness. In birds, female and male ornamentation can indicate better overall condition or such individuals might have better genes, or be better suited as parents. In humans Assortative mating in humans has been widely observed and studied, and can be broken down into two types of human assortative mating. These are genetic assortative mating (assortative mating with mate choice based on genetic type and phenotypical expression) and social assortative mating (assortative mating with mate choice based on social, cultural, and other societal factors) Genetic assortative mating is well studied and documented. In 1903 Pearson and colleagues reported strong correlations in height, span of arms, and the length of the left forearm between husband and wife in 1000 couples. Assortative mating with regards to appearance does not end there. Males prefer female faces that resemble their own when provided images of three women, with one image modified to resemble their own. However, the same result does not apply to females selecting male faces. Genetically related individuals (3rd or 4th cousin level) exhibit higher fitness than unrelated individuals. Assortative mating