Primate Mating Behaviour Flashcards
How can promiscuous/polygynous mating systems lead to sperm competition?
Promiscuous vs Polygynous Mating Systems
• In promiscuous polyandrous mating systems (chimps and baboons common), females mate with many males → sperm competition results
• In poygynous mating systems, males monopolise access to several females, but in primates, mating incidents are low, as he guards relatively few females and these are usually rearing offspring and are in lactational anoestrous
• In seasonally breeding polygynous species, males indulge in scramble competition for access to females over a limited period (ie, Antechinus) → sperm competiton results.
• In non-seasonal primates with stable polygynous systems, female fertility is low and sperm competition reduces.
From sexual selection to sperm competition:
• Sperm competition arises when one female mates with several males in an ovulatory cycle
• Consequence – competition for sperm access to ova
• Selection occurs deep in female reproductive tract
• Female promiscuity gives litters of mixed paternity
• Materna/paternal genome conflict → evolution of genomic imprinting?
Describe primate social and mating structures
Gorillas:
• Polygynous – silverback has harem of 3-4 females
• Each adult female comes into estrous once every 4 years → rest of time lactational anestrous or pregnant
• Male may only get chance to copulate once a year, since mating is confined to estrous
• Male gorillas weight 250kg, yet testis only 10g → don’t need large testis as don’t mate often.
Chimpanzees:
• Polyandrous – females mate with multiple males while she is in estrus
• Sperm competition results
• Dominant males have high copulation frequency
• Several females may come into oestrous at the same time, favouring the evolution of large tesits size and production of many sperm
• If the female is to mate with many males, it pays to advertise her receptive state with pronounced estrous swelling at the time of ovulation.
Orang-utan:
• Males are solitary, defend large territories
• Females occupy overlapping ranges
• Prolonged period of offspring care and lactation → come into estrous once every 4 years
• Solitary. non-promiscuous nature means females do not advertise their ovulation
Bonobo:
• Males inherit social status from their mothers, remain in maternal area for life
• Females have prolonged period of estrous swelling, well beyond period of fertility
• Highly promiscuous – will even mate with males from outside the community, but most infants are sired by resident males.
Humans:
• Only species of mammal in which all signs of ovulation are suppressed
• Only species to mate over entire ovarian cycle, irrespective of fertility
Describe the ‘males view of the female’ sexually selected traits
- Gorilla female has small, externally visible breasts and small genitalia
- Orangutan isn’t much difference, genitalia slightly smaller
- Chimpanzee advertises her reproductive state – has pronounced genital swelling → estrogen responsible. Relatively small breast tissue.
- Humans have a switch – small genitalia, but large breast tissue → estrogen responsive. Breast tissue not necessarily lactating – suggesting it is more of a badge for sexual selection
→ So in humans, breast is the estrogen responsive sexually selected characteristic, in chimps it is the genitalia.
• Switch from perineal to breast has been associated with the evolution of bi-pedalism
Describe the ‘female view of the male’ sexually selected traits
- Gorilla – colour of back shows dominance. Small testis and small penis
- Orangutan – larger testis. penis is still small
- Chimps – very large testis (due to sperm competition selective pressure). Slightly larger penis. No male can have dominant access to other females, so this fives rise to larger amounts of germ cells to help with sperm selection.
- Humans – testis aren’t very different to orang-utan, however has a very large penis relative to other great apes. This enlargement is associated with the greater mating frequency.
Variation in shape of primate penises
• All primates have an os penis (baculum)
• The exception is man – this is the result of gaining something much larger, that depends on hydraulic pressure
• Reported cases of penis ossification occurring following trauma
• Speculated that the loss of a bone in humans is because humans evolved a mating system in which a male tends to accompany a particular female to try and ensure paternity of her children. This lead to frequent matings of a short duration
• Primates with a baculum only infrequently encounter females, but matings are of longer duration, maximising the chance of fathering.
• Richard Dawkins suggested loss of a baculum is a result of sexual selection of females towards males – looking for honest signs of good health in a prospective man.
What are the effects of changing social status in male primates?
Effect on Badge
• Badge signals sex steroid production
Mandrils:
• With a change in rank, within 5-6 months you get an increase in the facial and genital brightness of the new alpha male.
• Takes longer for the previous alpha male to los the facial and genital brightness – sex steroid effects have a long latency.
Effect on Testis Size
• When you gain rank, you gain testis size and increase testosterone production
• When you lose rank, you lose testis size but don’t really lose testosterone production
Time course of effects:
• In the months preceding the deaft of the dominant male, all the subordinate males are showing increase in the bright colouration of the skin. They are all competing with each other to be the dominant male.
• Eventually one will win, and take over alpha status. It retains its bright colouration, whereas the rest lose it.
What are the badges of fertility in female primates?
• Estrous swelling in perineal region of chimpanzees → estrogen dependent and advertisement of fertility.
- Series of swellings prior to menarche
- Following menarch, number of long anovulatory cycles for 2 or 3 years
- Finally female conceives
- Still goes through a couple more perineal cycles before pregnancy is firmly established, then you get a prolonged period without.
- Female reproductive cycle in chimps is 36 days – exhibit pronounced swelling for around 10 days.
- Estrogen levels rise and fall along with the swelling
- Even though these females may mate multiple times throughout their cycle, dominant males focus mating around the time of maximum fertility → dominants copulate most around the time of ovulation, so most likely to father.
- Suggested that males track fertility through the estrous swelling – may be able to retain a memory for each female about what size indicates max fertility.
Examples of sexually selected attractiveness traits in humans
Dixon et al
Mens preferences for Womens Breast Morphology in NZ, Samoa and PNG:
• Men from PNG preferred larger breasts to a greater extent than man from Samoa and NZ → provides evidence to support the hypothesis that men from subsistence living cultures have greater preference to cues indicative of caloric reserves.
• Symmetrical breasts most attractive to men in each culture
• Large areola preferred in PNG, smaller in NZ → seems to be culture specific
• Darker areola pigmentation preferred over light → suggests pigmentation indicative of sexual maturity preferred.
Male preference for female waist to hip ratio and body mass index in the highlands of PNG
• Men consistently chose females with a lower WHR (hourglass figure) as more attractive
Eye Tracking of Mens Preferences for Female Breast Size and Areola Pigmentation
• Sexual selection via mate choice long been implicated in the evolution of permanently enlarged breasts in women
• Men rated images of medium or large breasts more attractive
• Dark/medium areolar pigmentation rated more attractive
• Variations in breast size had no significant effect on eye tracking measures (initial visual fixation, number of fixations and dwell times) → so visual attention to these traits does not correlate with attraction.
How does the structure of the germ cell reflect mating system?
Calculation of residual testis size:
• Log testis size plotted against log body size
• The residual is the mathematical distance from the best fit line
• Baboon and chimp have the relatively largest residual testis size, and the relatively largest sperm midpiece volume
• Humans have relatively small testis size (gorillas even lower), and a smaller sperm midpiece volume
➢ This is a consequence of sperm selection
➢ In those species where sperm selection is an issue, they have evolved a large midpiece – you want the sperm to be bigger, and have more motility.
Also:
• In promiscuous polyandrous societies with sperm competition. there are much more ejaculations per hour than in monogamous or polygynous species.
