Sexual dimorphism describes the morphological, physiological, and behavioral differences between males and females of the same species. Most primates are sexually dimorphic for different biological characteristics, such as body size, canine tooth size, craniofacial structure, skeletal dimensions, pelage color and markings, and vocalization. However, such sex differences are primarily limited to the anthropoid primates; most of the strepsirrhine primates (lemurs and lorises) and tarsiers are monomorphic. Sexual dimorphism can manifest itself in many different forms. In male and female primates there are obvious physical differences such as body size or canine size. Dimorphism can also be seen in skeletal features such as the shape of the pelvis or the robustness of the skeleton. There are two mating systems in the sexual selection of primates.
Types
Body size Extant primates exhibit a broad range of variation in sexual size dimorphism (SSD), or sexual divergence in body size. It ranges from species such as gibbons and strepsirrhines (including Madagascar's lemurs) in which males and females have almost the same body sizes to species such as chimpanzees and bonobos in which males' body sizes are larger than females' body sizes. In extreme cases, males have body sizes that are almost twice as large as those of females, as in some species including gorillas, orangutans, mandrills, hamadryas baboons, and proboscis monkeys. Patterns of size dimorphism exhibited in primates may correspond to the intensity of competition between members of the same sex for access to mates–intrasexual competition, counteracted by fecundity selection on the other sex. Some callitrichine and strepsirrhine primates are, however, characterized by the reverse dimorphism, a phenomenon in which females are larger than males.
Tooth size Canine sexual dimorphism is one particular type of sexual dimorphism, in which males of a species have larger canines than females. Within primates, the male and female canine tooth size varies among different taxonomic subgroups, yet canine dimorphism is most extensively found in catarrhines among haplorhine primates. For example, in many baboons and macaques, the size of male canines is more than twice as large as that of female canines. It is rare, yet females in some species are known to have larger canines than males, such as the eastern brown mouse lemur (Microcebus rufus). Sexual dimorphism in canine tooth size is relatively weak or absent in extant strepsirrhine primates. The South American titi monkeys (Callicebus moloch), for instance, do not exhibit any differences in the size of canine teeth between the sexes. Among different types of teeth constituting the dentition of primates, canines exhibit the greatest degree of variation in tooth size, whereas incisors have less variation and cheek teeth have the least. A canine dimorphism is also more widely seen in maxillary canines than in mandibular canines.
Craniofacial structure
Craniofacial sex differentiation among anthropoid primates varies in a wide range and is known to arise primarily through ontogenetic processes. Studies on hominids have shown that, in general, males tend to have a greater increase of facial volume than of neurocranial volume, a more obliquely oriented foramen magnum, and a more pronounced rearrangement of the nuchal region. The breadth, length and height of the neurocranium in adult male macaques, guenons, orangutans and gorillas are about nine percent larger than the neurocranial dimensions in adult females, whereas in spider monkeys and gibbons the sex differences is on a general average about 4 to 5 percent. In orangutans, males and females share similarities in facial dimensions and growth in terms of orbits, nasal width, and facial width. They tend to have some significant differences, however, in various facial heights (e.g., height of the anterior face, premaxilla, and nose).
Skeletal structure Primates also exhibit sexual dimorphism in skeletal structures. In general, skeletal dimorphism in primates is primarily known as a product of body mass dimorphism. Hence, males have proportionally larger skeletons compared to females due to their larger body masses. Larger and more robust skeletal structures in males is also attributable to better developed muscle scarring, and more intense cresting of bones compared to those of females. Male gorillas, for example, possess large sagittal and nuchal crests, which correspond to their large temporalis muscles and nuchal musculature. Also, an unusual skeletal dimorphism includes enlarged, hollow hyoid bones found in males of gibbons and howler monkeys, which contribute to the resonation of their voices.
Pelage color and markings
Sex differences in pelage, such as capes of hair, beards, or crests, and skin can be found in several species among adult primates. Several species (e.g., Lemur macaco, Pithecia pithecia, Alouatta caraya) show an extensive dimorphism in pelage colors or patterning. For example, in mandrills (Mandrillus sphinx), males display extensive red and blue coloration on their face, rump and genitalia as compared to females. Male mandrills also possess a yellow beard, nuchal crest of hair, and pronounced boney paranasal ridges, all of which are absent or vestigial in females. Studies have shown that male color in mandrills serves as a badge of social status in the species.
Temporary sexual dimorphism Some sexual dimorphic traits in primates are known to appear on a temporary basis. In squirrel monkeys (Saimiri sciureus), males can gain fat as much as 25 percent of the body mass only during the breeding season, specifically in their upper torso, arms, and shoulders. This seasonal phenomenon, known as "male fattening," is associated with both male–male competition and female choice for larger males. Orangutan males tend to gain weight and develop large cheek flanges, when they achieve dominance over other group members.
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