Most differences between sexes can be traced back to the fundamental difference, in other words, the size of gametes- how much parental investment each sex does. Females make large food-rich gametes. When fertilization became internal, then females had to gestate them, so they had even more to gain from protecting their kids. So then females invested even more. Also, when the kids are born, the female has more of an opportunity for further investment since she is there when they're born but the male is probably gone. So then females began lactating and carrying the babies around. So then they ended up doing a LOT of investment. Also, since it's internal, females are sure the baby is theirs. Males can never be sure that the infant they're protect is theirs. If they make a mistake, then they're contributing to some other guy's genes. But they still have the opportunity to increase their reproductive success by seeking further matings. So, females are really choosy about who to mate with but males are really indiscriminate.
There are sex-role reversed species, but none occur in primates. Today we will discuss male-male competition, and then female choice will be later.
To determine factor #2, you could just count males and count females, but a better predictor is the Operational Sex Ratio which is the number of sexually active males to the number of receptive females. If all the females come into estrous at the same time, then there will be an equal ratio of active males to receptive females. If the females come into estrous at different times, then there's only one female ready at a time, so there are lots of males competing for her and male-male competition will be more intense.
Factor #1 is usually more important, but in some species #2 can be important.
For example, hamadryas baboons. Body size is advantageous in fights and if fighting is important in gaining access to females, then males will tend to get bigger. In the monogamous owl monkey there's little sexual dimorphism. Also, their investment is more equal. The sex ratio in a breeding group seems to predict the dimorphism. As operational sex ratio goes up, so does the sexual dimorphism. The closer a groups is to being monogamous, the less differences there are between the sexes.
The more intense sexual selection is, the larger male canines become compared to females'.
In this case, sexual selection is leading not to differences between sexes within a species, but differences in males between species. Sandy Harcourt is the one who first pointed out that a 400 lb. gorilla has smaller testes than an 80 lb. chimp. How come??
Usually, competition between males is to gain matings, but just having sex more doesn't automatically give you more kids. In species whose females have sex with several males, a female might have several males' sperm in her reproductive tract. A guy who releases more sperm will be more likely than the other guys to fertilize her, so they develop bigger and bigger testicles.
Competition between sperm while in the reproductive tract is called sperm competition (original, huh?). There are some interesting examples in other animals where sperm do all kinds of neat things. However, in primates it's pretty much just a matter of numbers. So, some species compete before copulation and some compete after copulation. In gorillas, all competition is over by the time mating actually occurs. The male has fought for the right to the women. In muirikis, woolly spider monkeys, or chimps, multiple males live together and they don't fight. They share access to females, so they compete after the copulation has taken place.
See the graph on pg. 40 of Krebs and Davies which has body weight plotted against testes weight. It is separated out by multi-male, monogamous, and single-male breeding systems. Farthest below the line are the monogamous guys. The highest are multi-male systems. In between are single-male/polygynous systems. This is because the ruling male can't always control access to all the females and other males sometimes sneak in so there's still a little encouragement to develop bigger testes. Based upon their testes size, humans seem to be tending towards monogamy but they are a little heavier than strictly monogamous species, showing that we're lightly polygynous. This is backed up by data from traditional cultures.
People have done a lot of studying on this topic since sometimes things don't work out as predicted, like in another group of savannah baboons studied by Barb Smuts. When she compared the rank of males vs. their numbers of consorts, higher guys did the best, medium guys did the worst, and lower guys did in between.
It turns out that the lowest guys were pursuing alternate strategies. They'd hang out with a chosen female, protecting her and her offspring. Then, when those females came into estrous, they'd sometimes prefer those sensitive guys to the big strong dumb ones.In 27 studies of male mating success, 23 showed a relationship between dominance and mating success, while 4 showed no correlation. However, a lot of the negative results came from studies in captivity.
Now, since DNA fingerprinting has enabled people to better study exactly who is fathering babies, we have come to find out that sometimes the guy who has more copulations isn't always the one who fathers more children. When you look at actual paternity, usually the more dominant guy does have more children. Since females are in estrous for several days, sometimes the last guy to have had her is the lucky father. Sometimes females get more choosy when they're more receptive, so while they let lower guys do it earlier, when the time is more critical, they get together with the more dominant guys.
One reason is evolutionary equilibrium between two strategies- they're equally effective. This results from a situation where the success rate of individuals using a certain strategy depends on how rare they are in the population (frequency dependance.) Rarer is more successful. Like if everyone is becoming lawyers, then you'll do well to become a doctor, since there'll be a demand for it. If the frequency changes, then so will the best strategy.
The other reason there might be alternative strategies in a population is
called the 'best of a bad job' scenario. Some guys who
don't have what it takes to pursue most effective strategy so they pursue
a less effective strategy which is better than doing nothing. For
instance, to make more money you should become a CEO, but if you don't
have the education or contacts for it, then becoming a Macdonald's worker
is better than sitting around making no money at all.
The alternative strategies are forcing copulation or not doing so. Males become sexually mature before they reach full body size. So, only some active males have secondary sex characteristics. These guys will occasionally force sex, but usually females will mate willingly since these secondary sexual characteristics turn them on. However, there are also a bunch of guys running around who are sexually mature but aren't adult-sized. When they find an estrous female, they latch on to her, follow her around, and force sex repeatedly. Apparently this is making the best of a bad job- they're not that attractive to females, so they have to force it. No paternity studies have been done to see how the success rates compare for these tactics. Some people suggest that actually the forcing males are not younger, they're just different phenotypes who don't get as large and don't develop the face flanges.2. Chimpanzees
From the work of Caroline Tutin. There are 3 different strategies;
Opportunistic matings. If a receptive female comes along, males will mate with her. This method is almost haphazard. Data from paternity tests shows that the opportunistic method gives almost no kids, but 75% of the copulations.
Possessive matings. If a female is in estrous, a high-ranking male may begin following her and restricting access to her. 25% of the copulations are possessive, but they only give about 10% of the kids.
Consortships. One male and one female leave the group together for hours if not days, mating repeatedly. Almost no copulations are from consortships, but they yield like 90% of the kids.
Note that in the consortship, sometimes the female is willing, but other times the male will use violence or threats of violence to separate her from the group.