Sexual dimorphism in non-human primates
Sexual dimorphism can manifest itself in many different forms. In male and female primates there are obvious physical difference 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.[3] 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.
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,
Tooth size
Canine sexual dimorphism is one particular type of sexual dimorphism, in which males of a species have larger
Among different types of teeth constituting the dentition of primates, canines exhibit the greatest degree of variation in tooth size, whereas
Craniofacial structure
Craniofacial sex differentiation among anthropoid primates varies in a wide range and is known to arise primarily through
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.
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.[2] 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.[5] Studies have shown that male color in mandrills serves as a badge of social status in the species.[13]
Temporary sexual dimorphism
Some sexual dimorphic traits in primates are known to appear on a temporary basis. In
Orangutan males tend to gain weight and develop large cheek flanges, when they achieve dominance over other group members.[14]Vocalization
In many adult primates, dimorphism in the vocal repertoire can appear in both call production (e.g., calls with a particular set of acoustic traits) and usage (e.g., call frequency and context-specificity) between the sexes.
Ultimate mechanisms
Ultimate mechanisms explain the evolutionary history and functional significance of the sexual dimorphism expressed among primates.[18]
Intrasexual selection
A prime example of intrasexual selection can be found in baboons. Male baboons are known to violently fight and threaten each other over females and show high levels of sexual dimorphism in body and canine size, both of which are assumed to aid in combat.[2] The “winners” of such interactions mate with the desired female and produce offspring, passing their traits to the next generation, while unsuccessful males are excluded from mating.[2] As a result, traits beneficial to fighting are selected for in the population over time.[2]
Intrasexual selection also operates through female–female competition.[2] Female howler monkeys, for example, experience frequent agonistic encounters both within and between coalitions.[2] One possible evolutionary explanation for female–female competition in red howler monkeys is its role as a counter-strategy to infanticide through group size regulation (by evicting other females).[2][19] Instances of female–female competition such as this could potentially select for greater body and/or canine size in females, as well as reduce the pressure for those same traits in males by limiting the occurrence of male–male competition (as group size regulation reduces the likelihood of threats/takeovers by immigrant males), overall reducing dimorphism.[2]
Intersexual selection
In rhesus macaques, red facial coloration is attractive to females to the point of influencing the reproductive success of high-ranking males.[20] To be deemed a sexually selected trait said trait must be heritable and confer a reproductive advantage.[20] In this example, facial redness is heritable, but only increases a male's reproductive success if he is also high-ranking, and rank is not determined by facial redness (dominance in rhesus macaques is not competition-based but rather queue-based).[20][21] While this trait is believed to be the result of intersexual selection, such examples demonstrate the complex nature of determining evolutionary explanations for sexually dimorphic characteristics.
Paternity confusion is another component of female choice. By actively seeking out matings with newly immigrated males, females produce offspring whose fathers are unknown. This is beneficial to females because it allows them to sire offspring without the risk of infanticide.[2] These “sneaky matings” mean that even if a male “wins” the opportunity to mate with a female, the father of her infant is not necessarily determined by the outcome of male–male competition, thus limiting the reproductive benefits associated with such competition and dampening the pressure for sexually selected dimorphic traits.[2]
Mating systems
In
The lack of a clear relationship between mating system and intensity of sexual dimorphism in strepsirrhines remains a mystery, with some explanations ranging from ecological constraints to selection for speed and agility to unique instances of female social dominance (such as in lemurs) reducing dimorphism.[2] One study offers a challenge to the argument that environmental constraints are the main factor driving monomorphism on Madagascar but fails to isolate specific factors to substitute this theory; simply put, there is no consensus on why strepsirrhines do not follow similar patterns to haplorhines.[4]
Phylogeny
Similar magnitudes of body weight dimorphism have been observed in all species within several taxonomic groups such as callitrichids, hylobatids, Cercopithecus, and Macaca.[22] Such correlation between phylogenetic relatedness and sexual dimorphism across different groups reflects similarities in their behaviors and ecological conditions, but not in independent adaptations. This idea is referred to as “phylogenetic niche conservatism."[22]
Terrestriality
Niche divergence
It has been hypothesized that niche divergence between the sexes attributes to the evolution of size dimorphism in primates.[2] Males and females are known to have different preferences for ecological habitat due to different reproductive activities, which could possibly lead to dietary differences, followed by dimorphic morphological traits.[24] This niche divergence hypothesis, however, has never been strongly supported due to the lack of compelling data.[2]
See also
References
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- )
- ^ .
- ^ a b Dixson, A.; Dixson, B.; Anderson, M. (2005). "Sexual selection and the evolution of visually conspicuous sexually dimorphic traits in male monkeys, apes, and human beings". Annual Review of Sex Research. 16: 1–19.
- ^ Gordon, A. D. (2006). "Scaling of size and dimorphism in Primates II: Macroevolution". International Journal of Primatology. 27 (1): 63–105.
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- ^ Gingerich, P. D.; Schoeninger, M. J. (1979). "Patterns of tooth size variability in the dentition of primates" (PDF). American Journal of Physical Anthropology. 51 (3): 457–465.
- ^ .
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- ^ Wood BA. 1976. The nature and basis of sexual dimorphism in the primate skeleton. Journal of Zoology 180(1):15-34.
- ^ Setchell JM, Wickings EJ. 2005. Dominance, status signals, and coloration in male mandrills (Mandrillus sphinx). Ethology 111:25-50.
- ^ a b Plavcan JM. 2012. Sexual size dimorphism, canine dimorphism, and male–male competition in primates: where do humans fit in? Human Nature 23:45-67.
- ^ Stone AI. 2014. Is fatter sexier? Reproductive strategies of male squirrel monkeys (Saimiri sciureus). International Journal of Primatology 35:628-642.
- ^ a b Bouchet H, Blois-Heulin C, Lemasson A. 2012. Age- and sex-specific patterns of vocal behavior in De Brazza's monkeys (Cercopithecus neglectus). American Journal of Primatology 74:12-28.
- ^ Bouchet H, Pellier A, Blois-Heulin C, Lemasson A. 2010. Sex differences in the vocal repertoire of adult red-capped mangabeys (Cercocebus torquatus): A multi-level acoustic analysis. American Journal of Primatology 72:360-375.
- ^ Watkins GG. 1996. Proximate causes of sexual size dimorphism in the Iguanian lizard Microlophus occipitalis. Ecology 77(5):1473-1482.
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- ^ PMID 25253459.
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- ^ a b c Plavcan JM, Van Schaik CP. 1997. Intrasexual competition and body weight dimorphism in anthropoid primates. American Journal of Physical Anthropology 103:37-68.
- ^ Cardini A, Elton S. 2008. Variation in guenon skulls (II): sexual dimorphism. Journal of Human Evolution 54:638-647.
- ^ Shine R. 1989. Ecological causes for the evolution of sexual dimorphism: a review of the evidence. The Quarterly Review of Biology 64(4):419-461.
Academic resources
This article includes a list of general references, but it lacks sufficient corresponding inline citations. (February 2023) |
- The Differences between the sexes. Short, R. V. (Roger Valentine), 1930-, Balaban, E. (Evan), International Conference on Comparative Physiology (11th : 1992 : Crans, Switzerland). Cambridge: Cambridge University Press. 1994. OCLC28708379.
- Plavcan, J. Michael (2001). "Sexual dimorphism in primate evolution". American Journal of Physical Anthropology. 116 (S33): 25–53. ISSN0002-9483.
- Larsen, C. S. "Equality for the Sexes in Human Evolution? Early Hominid Sexual Dimorphism and Implications for Mating Systems and Social Behavior." Proceedings of the National Academy of Sciences, vol. 100, no. 16, 2003, pp. 9103–9104., .
- Leigh, Steven R. “Socioecology and the Ontogeny of Sexual Size Dimorphism in Anthropoid Primates.” American Journal of Physical Anthropology, vol. 97, no. 4, 1995, pp. 339–356., .
- Scaglion, Richard. "On Australopithecine Sexual Dimorphism". Current Anthropology, vol. 19, no. 1, 1978, pp. 153–154., doi:10.1086/202026.
- "Sexual Dimorphism". The American Naturalist, vol. 37, no. 437, 1903, pp. 349–349., doi:10.1086/278295.