What do primates need to have in their home range to be able to stick around? Food is always good. Water is, for some species. Some who live in the tropical rainforest can lick the dew off leaves, but in arid areas it's really important to have a water source. Shelter is not an issue for some species like ones who live in rainforest canopy, but for baboons who sleep on sleeping cliffs to escape from predators, their possible ranges are affected by the availability of cliffs. Also, the last thing you need are mates! You've got to be around the other sex or your territory will be uninhabited within one generation.
This brings us to the question, How long is long enough to watch a group? How do you make your recommendation measurable? You could watch them until they don't go anywhere new anymore, but that's a little imprecise. A more consistent way is to call it at a year. This is good 'cause sometimes there's seasonal variation.
Gibbons: Monogamous groups with small territories. They have aggressive defense involving displays, chases, and fights. They sing to defend their territory. When two couples meet up, they'll display, sing, and chase. Between-group aggression is sex-specific; males fight males, and the females fight females.
Mangabey: (cercocebus) Large home ranges with almost complete overlap but exclusively-used core areas. Males utter a distinct call upon contact and groups mutually avoid each other. They use a call called a whoop gobble to let others know they're there. If someone hears it, they move away and they all avoid each other. This is unless there is a specific resource that they might all want to feed at like a good fruit tree.
Gorillas: Large home ranges with almost complete
Adult males are the primary participants in aggression. Males initiate aggressive displays including chest-beating, branch-breaking, and charges. The females and young are rarely involved in conflicts. Our professor says that while he has never seen an adult male charging and displaying, everyone else who has agrees that it's quite intimidating!
Why don't all primates defend territory? It seems pretty nice since you get exclusive use and all, right? As long as there is competition for resources, there is the potential for territory defense but the problem is, it has to be economically defendable; the benefits of defense must outweigh the costs.
Check out the diagram from Krebs and Davies on home range size plotted against the costs and benefits. Costs increase linearly as the home range size increases, but benefits are a curve which will level off eventually. This is because you only need so much; getting more access to food won't mean anything if that's all you can eat. So, eventually the two lines cross. In the region where the benefits are greater than costs, that's where you'd expect defense of territory to happen. However, if benefits are always lower than costs then you don't expect territory defense to happen. People in primatology don't really use this type of approach quantitatively since in primates it's all too complex and too hard to measure well, but it is useful conceptually to help you think about relative costs and benefits.
One obvious cost for primate territoriality is related to the necessary size of home range for your food type. If it needs to be huge then it's too hard to defend it, but if you only need a smaller one then its easier to defend. Another factor is how hard it is to get around in your habitat. If you're a quick animal then maybe it's practical for you to defend your territory, but if you are really slow then don't even bother.
Some guy said, if d=diameter of home range and drl=day range length, you'll see territoriality when drl > d. In other words, if you can cross your whole territory in a day, then you'll defend it 'cause it won't cost you too much energy and time, but if it takes a long time to get through your territory then you won't.
In other species there's another solution- if groups are constantly
encountering each other it will take a lot of energy fighting and chasing
so if they could avoid it all, it would be better for everyone. Instead,
some species use loud calls for mutual avoidance. A prime example of this
is the howler monkey. When they get up, before they go anywhere they sit
around and howl really loud and listen to who else is yelling. Then they
go off for the day's foraging in a direction where there wasn't anyone
yelling back. Likewise, the mangabeys give out whoop-gobble to warn
other groups of their presence, and they listen to whoop-gobbles to avoid
others. This has been proven in playback experiments. Scientists recorded
whoop-gobbles and then set up speakers. They played it back at different
distances from groups of mangabeys. When the speakers were close to the
group, they moved away, but when they were farther away, 200 meters or
more, then they didn't do anything.
For males, however, it always pays off to get more female access, so they always have conflict. This is very consistent. You can make a reliable prediction that males will fight other males.
Males, on the other hand, produce tiny little gametes which are little
more than DNA with tails. They can move around and are so easy to make
they represent almost no energy or effort by the male.
If a female makes a bad choice of mate and her offspring dies because of it, then she has incurred a substantial loss of investment. If the male makes a bad choice of mate, however, he can simply turn around and mate again, none the poorer.
"a struggle between the individuals of one sex, generally male, for the possession of the other sex."This is similar to natural selection- they act on the same mechanism, but sexual selection refers only to those traits which affect mating success.
From flies: They put a bunch of tagged flies together and watched who mated with whom. All the females mated either one or two times. The males, however, ranged from no times to four times.
From red-tailed deer: 36% of the females had no matings at all, while 45% of the males had no matings at all. However, the females had quite a narrow range of number of offspring living to maturity, while the males' numbers went all the way up to 24!
From elephant seals: Most females had one or two offspring, but the males either had none or they had 100-200!
From red colobus: Some males had up to 30 offspring, while females all had 0, 1, or 2.
From Kipsigis: (Polygynous traditional pastoralists in Kenya) The females had 15 kids at the most, while several males had over 25 kids.
To sum up, males have much more varied reproductive success than females.
This is all well and good for things like flies where the female lays the egg, the male fertilizes it, they both fly away and that's the end of their respective investments in the offspring. In primates, there's still tremendous investment to be done after the laying of the egg.
There's gestation- energy that must be spent carrying
the developing child inside your own body.
There's lactation- energy spent providing food for the child for the first stages of its life.
There's carrying- energy spent lugging the child around from one place to another which, as anyone who has carried around a two-year-old can tell you, mounts up real quick.
Parental investment as defined by Trivers:
"Any investment by the parent in an individual offspring that increases the offspring's chance of surviving (and hence reproductive success) at the cost of the parent's ability to invest in other offspring."
So, basically, males can increase their reproductive success by getting more women, while females can increase their reproductive success by turning food into eggs as a faster rate. Remember, "Where one sex invests considerably more than the other, members of the latter will compete among themselves to mate with members of the former." Of course, this is a more generalized account of things- it's not always males doing chasing and females being choosy. If the investment by each sex changes, then the competition should change as well. So, if males were to invest more than the females, we would expect the females to compete among themselves for the males.
There are all kinds of species with all different levels of parental involvement. There is a South American frog, denderbates, called the poison dart frog. The females lay their eggs on the backs of the males. The males then climb up and find little pools of water in the crotches of trees where they put the eggs. The males will climb back up the trees every day to check on the progress of the tadpoles, bringing them food and water, and even moving them if the pool looks like it is drying up. Meanwhile, the female goes blithely along finding guys who will let her lay her eggs on their backs. The males act all coy, and the females will fight viciously among themselves if they encounter each other.
There are no role-reversal species in primates. Nor in mammals, for that matter, since having breasts predisposes the females to invest more than the males. However, there are all different degrees of parental investment. The more monogamous the species, the less sexual selection there is and the less difference between the sexes there is in reproductive success. The more polygynous a species is, the more sexual selection there is and the more differences in reproductive success there is.
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.