Sexual dimorphism in non-human primates
Encyclopedia
Sexual dimorphism in non-human primates has long been observed in the primate
Primate
A primate is a mammal of the order Primates , which contains prosimians and simians. Primates arose from ancestors that lived in the trees of tropical forests; many primate characteristics represent adaptations to life in this challenging three-dimensional environment...

 order
Order (biology)
In scientific classification used in biology, the order is# a taxonomic rank used in the classification of organisms. Other well-known ranks are life, domain, kingdom, phylum, class, family, genus, and species, with order fitting in between class and family...

, with numerous studies performed to document and explain the phenomenon. Recent studies have mainly used the technique of comparative analysis
Comparative analysis
Comparative analysis is type of analysis used in various of sciences and in different modifications:Criminology and forensics:* Comparative contextual analysis - criminology* Comparative bullet-lead analysis - forensicsSociology:...

 to examine both the variation in the expression of the dimorphism among primates and the fundamental causes of sexual dimorphism
Sexual dimorphism
Sexual dimorphism is a phenotypic difference between males and females of the same species. Examples of such differences include differences in morphology, ornamentation, and behavior.-Examples:-Ornamentation / coloration:...

. Primates usually have dimorphism in body mass and canine tooth size along with pelage and skin colour. The dimorphism in primates has been attributed to many factors:
  • Mating system
    Mating system
    A mating system is a way in which a group is structured in relation to sexual behaviour. The precise meaning depends upon the context. With respect to higher animals, it specifies which males mate with which females, under which circumstances; recognised animal mating systems include monogamy,...

     – Polygynous species are more sexually dimorphic than monogamous species, such as the New World monkeys. It is also associated with greater male gonad
    Gonad
    The gonad is the organ that makes gametes. The gonads in males are the testes and the gonads in females are the ovaries. The product, gametes, are haploid germ cells. For example, spermatozoon and egg cells are gametes...

    al investment than what is found in closely related monogamous species.
  • Size – Larger species are more sexually dimorphic than smaller species.
  • Habitat – Terrestrial
    Terrestrial animal
    Terrestrial animals are animals that live predominantly or entirely on land , as compared with aquatic animals, which live predominantly or entirely in the water , or amphibians, which rely on a combination of aquatic and terrestrial habitats...

     species tend to be more sexually dimorphic than arboreal species. It is possible that male-male competition in terrestrial species is more dependent on body size.
  • Diet – Frugivore
    Frugivore
    A frugivore is a fruit eater. It can be any type of herbivore or omnivore where fruit is a preferred food type. Because approximately 20% of all mammalian herbivores also eat fruit, frugivory is considered to be common among mammals. Since frugivores eat a lot of fruit they are highly dependent...

    s, for reasons that are not clear, are slightly more sexually dimorphic than folivore
    Folivore
    In zoology, a folivore is a herbivore that specializes in eating leaves. Mature leaves contain a high proportion of hard-to-digest cellulose, less energy than other types of foods, and often toxic compounds. For this reason folivorous animals tend to have long digestive tracts and slow metabolisms....

    s. Energy may be less of a constraint. or maybe females may be more clumped around an area of fruit trees.
  • Copulatory plugs
    Mating plug
    right|thumbnail|A mating plug in a female [[Richardson's ground squirrel]] A mating plug, also known as a copulation plug, sperm plug, vaginal plug, or sphragis, is gelatinous secretion used in the mating of some species. It is deposited by a male into a female genital tract and later hardens into...

     - Copulatory plugs are negatively associated with the degree of sexual dimorphism in primates suggesting they may be used as an alternative form of mate guarding, thus reducing selection pressure on large male size in some species.

.

Comparative analyses have substantiated the sexual selection
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...

 hypotheses, and have generated a more complete understanding of the relationship between sexual selection, natural selection
Natural selection
Natural selection is the nonrandom process by which biologic traits become either more or less common in a population as a function of differential reproduction of their bearers. It is a key mechanism of evolution....

, and mating systems in primates. Studies are helping to find the relative contribution of the various selective and non-selective mechanisms in sexual dimorphism evolution and expression. These studies have shown that dimorphism is the product of changes in both male and female traits. Ontogenic scaling, where relative extension of a common growth trajectory occurs, may show some insight into the relationship between sexual dimorphism and growth patterns.

Sexual dimorphism in the primate face

Sex-linked skeletal and dental
Tooth
Teeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...

 variation in primates has been the focus of considerable anthropological and anatomical research since 2002. This area is currently being investigated as sexual dimorphism is one of the major potential sources of variation
Variation
- Physics :* Magnetic variation, difference between magnetic north and true north, measured as an angle* Variation , any perturbation of the mean motion or orbit of a planet or satellite, particularly of the moon- Mathematics :* Bounded variation...

 that must be taken into account when assessing the number of species represented in a fossil sample and the second is that there have been some strong links between sexual dimorphism and ecological variables in extant species. Thus, by establishing the nature of sexual dimorphism in fossil species it has been possible to make conclusions about their social systems and habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...

 preferences.

Sexual dimorphism in primate teeth

Primates, such as baboon
Baboon
Baboons are African and Arabian Old World monkeys belonging to the genus Papio, part of the subfamily Cercopithecinae. There are five species, which are some of the largest non-hominoid members of the primate order; only the mandrill and the drill are larger...

s, are well known for their highly sexually dimorphic canine teeth, with males possessing canines that are up to 4 times taller than those of females. Primate canine dimorphism has been extensively documented, with the consensus that large male primate canines serve as weapons for intrasexual competition, and some evidence that large female canines also have this function. Primate canine teeth are as large or larger as similarly sized carnivores; they also seem to have been selected for their strength and length does not necessarily impact on canine function. In deciduous teeth it has been shown that Cercopithecidae, with the exception of Papio cynocephalus, showed no significant sexual size dimorphism and that it would be problematic to use deciduous teeth in species determination. Phylogenetic comparative methods have been used to analyze the consequences of sexual selection on canine size and canine size dimorphism in primates. The analyses indicated that in Haplorrhini
Haplorrhini
The haplorhines, the "dry-nosed" primates , are members of the Haplorhini clade: the prosimian tarsiers and the anthropoids...

the degree of sexual selection is correlated with canine size dimorphism, as well as with canine size in both sexes. Matched-pairs analyses on canine size after correcting for the effects of body size showed significant effects of sexual selection on relative canine size, indicating that canine size is more important in haplorrhine male-male competition than body size.
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