Mammalian genital enhancement Although it is not known if females have been observed to exhibit these characteristics, males of various species, in addition to size, possess various characteristics which may have an effect on potential sexual selection. In general, birds are well known for their spectacular plumage displays. In contrast, mammals, like many fish and invertebrates, cannot fly and therefore exhibit elaborate coloration and display of the male genitalia. It is known that many mammals have developed elaborate penile erections to promote species diversity, and there have been many instances of male-male competition for sexual status and dominance. These developments are found in a variety of species and are more prevalent in polygamous or promiscuous species, rather than monogamous species. There are two primary mammalian genitalia displays that have been identified as sexual selection indicators. These include the penis spines of rodents, as well as the intromittent spines of carnivorans (members of the family Felidae). These features are known to improve male genital fitness because they increase the effectiveness of sperm movement, fertilization, and ejaculation. Penile spines Penile spines are present in many small rodents, including mice, rats, gerbils, and degus. These spines are found on the penile tissues that can be erect or flaccid. They are made of keratin, and when erect, the length of the spines can reach from 3 mm to 13 mm in length, and from 4 mm to 15 mm in width. They can also include multiple spines that are located on the dorsal side of the glans of the penis. It is suggested that the role of penile spines is to protect sperm during their passage through the female reproductive tract. They also make it easier for male to hold the ejaculate within the sperm-storage organs for a period of time, which protects their germ cells from possible phagocytosis and destruction. Since female rodents can reach sexual maturity between 18 and 30 days old, it is thought that they would not need to be promiscuous to mate, which would explain the absence of female displays such as sexual swellings. Therefore, females and males are faced with the task of finding a mate in which their respective genes are passed down to the next generation. Many studies have shown that females tend to have a preference for the most colorful or otherwise ornamental males. However, not only do females need to choose between males, they also must decide whether or not to mate with that male. If a male is in high demand, then it would be in a female's best interest to mate with a male with a larger size or penis spine size to ensure that her offspring would have a strong fighting ability for future reproductive success. This is advantageous because larger males are usually more aggressive and have stronger sperm. Although the exact role of these spines in improving male reproductive success is not clear, it is possible that these features function in the same way that female displays in other species, which can be the result of female preferences for certain traits. Some studies indicate that males with large penile spines possess a greater competitive ability for access to females, which suggest that females can choose mates based on penile spine size and quality. Carnivoran intromittent spines Spines have been described in carnivorans as well, including, felids, canids, hyaenids, canids, and mephitids. These intromittent spines are found on the penis shaft, and are known to be found among the Canidae, Felidae, Hyaenidae, and Mephitidae families. One of the reasons why penile spines are found in some families but not in others may be due to their function. For example, the intromittent spines of felids and canids are used to anchor the penis into the vaginal folds during copulation and are used for species identification. Therefore, the spines may have evolved to serve as a physical barrier to prevent the entry of competing males into the vagina, for the purpose of protecting paternity of the current male. The spines are also known to be in contact with the female's reproductive tract, and therefore they are likely to play an important role in the maintenance of sperm motility. This is a distinct difference in that other mammals, such as cows and dogs, do not possess spines on their intromittent organs. The spines are also related to the size and quality of the male's genitalia. It is known that both factors contribute to the male's physical health, reproductive success, and overall longevity. Since penile spines are important in maintaining sperm and ensuring paternity, they are considered to be a reproductive trait used in pre-copulatory male–male competition. Therefore, they are considered an example of precopulatory selection. These spines are also a feature that can be found in male mice, rats, gerbils, degus, guinea pigs, hedgehogs, and rabbits. Human penis Although the human penis has not developed any intromittent spines, it has been found to have a long structure that is erectile. Furthermore, humans have also developed various other pre-copulatory and post-copulatory traits, which may be indicative of sexual selection, including increased penis size, elongated, mushroom-shaped heads of the glans, and a baculum (bone found in some male mammals) found within the soft tissue. The baculum serves an important role in increasing the penetration of the female reproductive tract and is considered an adaptation to increase male sexual and reproductive success. Prevalence Studies of pre-copulatory traits have only been conducted in a few species of mammals, and this number is limited to rats, gerbils, degus, guinea pigs, rabbits, hedgehogs, and mice. It is also important to note that the prevalence of the penile spines and bacula are not common in males. In many species, the spines and bacula are only found among the dominant males, which are usually larger and more dominant. This is a common trend observed in mammals. Sexual selection in humans Research has shown that promiscuous females tend to have larger penises than females with less sexual experience, which suggests that sexual selection is an important factor in the evolution of the human penis. Although the correlation is not completely clear, penis size is considered an indicator of reproductive success. It is believed that sexual selection has played a significant role in human evolution because many different human traits have developed over time. These traits include the size of the human brain, as well as many other physical and mental characteristics. It is believed that the evolution of various human characteristics are a result of sexual selection, because men and women with certain features tend to have greater reproductive success than those without these traits. See also Human penis Human female orgasm Human male sexuality Genetic sexual attraction Sperm competition in humans References External links Category:Mammal reproductive system Category:Sexual selection Category:Evolutionary biology Category:Animal sexuality