- Morphological differences between males and females in:
o Body size and shape
o Color: sexual dichromatism
- Not all primates show sexual dimorphism
o Sexual monomorhism: sexes are the same size, color, etc.
- In many primates, if sexes differ in body size, usually the males are bigger
o Why? Male-male physically aggressive contest competition over
females. Larger body size in males has been “selected for” because
this trait has been adaptive in these species
2 types of competition
- Contest competition: when a resource can be monopolized by a single
individual or group and can be fought over
o Hanuman langurs
o Size and strength is an advantage
- Scramble competition: when a resource is exploited by the individual that
arrives and uses it first
o Small nocturnal prosimians that don’t live in social groups
o Gelada baboons, aye-aye, Howler monkey, gorilla
o It can become a contest competition
Sexual dimorphism and mating systems
- Which mating system is most frequently associated with sexual dimorphism
in the form of larger males?
o Polygyny i.e. gelada baboon, Hamadryas baboon
o Polygynandrous species sometimes have sexual dimorphism (with
larger males) i.e. common chimps
o Monogamous species are usually sexually monomorphic in body size.
i.e. titi monkey
Sexual dimorphism and taxonomic groups
- Most apes are sexually dimorphic in body size, with larger males
o Except for gibbons and siamangs. They do have sexual dichromatism
but pretty much the same size.
- Many OW monkeys are sexually dimorph9ic in body size with larger males
- Many NW monkeys and prosimians are NOT sexually dimorphic in body size.
o Exception: in some nocturnal prosimians with dispersed mating
systems, males are larger than females. i.e. galagos
Sexual dimorphism in canine size
- Males with larger canines than females – indicates male-male physically
aggressive contest competition. o Many polygynous and polygynandrous mating systems
Secondary sexual characteristics
- External traits that help dis