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Monkeys, Apes and Humans
Anthropology 1500
Department of Anthropology, University of Missouri-Columbia

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Sexual Dimorphism and Mating System

Sexual dimorphism in body size (as well as canine size) is related to mating system. Mate competition is more intense in polygynous mating systems than monogamous systems. Size, generally, is critical in mate competition in polygynous systems.

Species

Mating System

Body size dimorphism

Humans

Variety of mating systems

Low (male 1.1x of female)

Gorillas

Polygynous one male

High (male 1.5x female)

Orangs

Polygynous, solitary

High (male 2x female)

Gibbons

Monogamous

Slight 1.02

Chimps

Polygynous multi male

Moderate 1.3

Bonobos

Polygynous multi male

Moderate 1.2

(Low sexual dimorphism in humans suggest that body size might not have been important in mate competition in humans but we will discuss this further when we cover human evolution.)

Factors related to sexual dimorphism

(1) Intrasexual competition associated with mating system (see above table.) Could include mating system, age of maturity, parental care by males,

(2) Arboreality v. terrestriality. Generally, terrestrial polygynous species (e.g., baboons) are more sexually dimorphic than arboreal polygynous species (e.g., black and white colobus). Why? Large body size probably does not help arboreal males when they fight in trees.

(3) Diet. Frugivores, for reasons that are not clear, are slightly more sexually dimorphic than folivores. Energy may be less of a constraint. Females may be more clumped.

(4) Species body size. Bigger primates are more sexually dimorphic than smaller species. One researcher suggested that as certain primate species became bigger during their evolutionary history, males evolved to be bigger faster than females. This idea has not received much support, however.

 

Gorilla

bonobo

 
 
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