Monday, October 7 -- Spacing Patterns and Territoriality

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Also relevant to this lecture is ch. 22 in Smuts et al. which this lecture is partially based on.

The Use of Space

Most animals, besides our fun-loving friend the barnacle, move around and use space. If you were an animal, there are several spatial-use categories you might fall under.
You might wander nomadically, like the cedar waxwing. They don't often come back to same place twice. This type of spatial use is rare; most animals don't do that. Why? Because it's safer when you know territory, otherwise you'll waste energy looking for food, water, and safe spots. Most animals who wander nomadically do so 'cause they have to 'cause there's limited resources.
You might wander seasonally. This is seen in African ungulates like the zebra, for example, and also birds of course. You may ask, "Why bother? Migration is dangerous, difficult, and expensive in terms of energy." (Or you might not ask but I'll tell you anyway.) They do it because of seasonal changes; habitats become too cold or too dry, and food availability varies. Primates don't do this too much 'cause they're in the tropics so they're not too affected by seasonality.
Most animals stick in one place. The advantage to this is local knowledge. You know where the dangers are, where the food and water are, when and where not to go, and you also know other individuals or groups of the same species in the area. (Don't confuse staying in one place with the issue of dispersal; to avoid inbreeding, at least one sex must move once in its life, but just because a species disperses doesn't mean that they're nomadic. But dispersal comes later.)

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.

A Reminder on Terminology:

day range: The distance a group or individual moves in a day.
home range: The area it stays within during a set period of time like a year.
core area: The part of its home range that it stays in mostly. The definition of a core area is a little less precise than the others- there are no rules on measuring how long or how much they have to use it for it to be a core.

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.


Once you've decided to be an animal who stays in one place, how do you deal with other groups who want to go where you are? You might want to defend your space and its resources. This behavior is called territoriality.
Note that an animal or group's "area of exclusive use" isn't the same as area that they range over- ranges can overlap a lot or they can overlap hardly at all.

Four examples: Vervets, Gibbons, Mangabeys, and Gorillas

Vervets: They're terrestrial savannah dwellers with small territories. When groups meet, there's sometimes friendly interactions especially involving juveniles, but usually aggressive defense. Females take an active role in conflict. They have a special call for group-group conflicts. Conflicts involve chases, hitting, and sometimes biting.

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 overlap.
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!

What causes variations in...

...home range size?

Remember some things which affect home range size; the group size affects how much resources they need available. Also the quality of their food and how available it is. If you eat leaves, you don't need to look so hard for them so you don't need so much space. An example of this is seen when you compare orangutans and gibbons. They're good to compare 'cause they live in pretty much the same habitat and both eat the same type of food. However, orangutans' body size is much larger. Gibbons have smaller home rages because they just don't need as much space to find their food. Also, compare mangabeys; different kinds have different sized groups. Their body sizes are pretty constant but there's a strong correlation between group size and home range size. You can also compare colobines; the black and white colobus is pretty much folivorous and has a small home range but the red colobus, who lives in a similar habitat but includes more fruit in its diet, tends to have larger home ranges.

...encounters between groups?

Let's say your group has a set home range. When other primates are encountered, what do you do? Some groups do have encounters without conflict, like squirrel monkeys or colobus monkeys. Sometimes maybe they'll even play together, but since resources are often limited, they're competing so there's conflict. There are different ways to do this. One way is to set up a territory and defend it. In this system, who wins and who loses depends not so much on the characteristics of the group but where the conflict took place- the home team usually wins. Four representatives (one from each group of primates) who exhibit this kind of territoriality are: prosimian- indri; new world monkey- titi monkey; old world monkey- vervet; and ape- gibbon. Also, chimps defend community boundaries which are a little different.

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.

Variation in intergroup behavior


Ok, so we're in a place that is not beneficial to defend. We encounter another group; what type of conflict will there be? Usually one group will be dominant over another- the outcome will not depend on the location of conflict, but on the competitors. In a number of encounters between the same two groups, one will usually win in every encounter.

Intergroup dominance

What determines which group is dominant? Generally it is the size of the group or the size of the fighters. It might also depend on the fighting abilities of the group members, but usually it's group size and number of adult males. An example of this is the macaques- the outcome of macaque conflicts depends on group dominance.

Mutual avoidance

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.

Individual differences in intergroup behavior

May be caused by difference in locale, since there is variation in geography. However, within the same location there's a lot of difference among individuals; sometimes even when the adults are going to fight, the kids get together first and play. If you think this is weird, remember that one sex is going to have to disperse, so this way they get to know people from the neighboring groups that maybe they're going to have to join when they hit puberty.
Intergroup behavior also varies between the sexes. We will do more on this later, but males and females have different agendas; A female's output is limited by access to food/resources, while males are limited only by access to mates. This means that females only fight when resources are limited and defendable. Depending on the situation, sometimes they'll fight and sometimes they won't fight. Female participation depends also on the composition of group. In some species, the females stay where they're born, but in others they disperse and live with strangers. So the females who stay make a sharp distinction between members of their groups and nonmembers so they fight more. In dispersing species, the females could care less if someone is attacking the group. Thus, both ecology and population structure both predict whether females will join in the fight.

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.

Wednesday, October 9 -- Sexual Selection

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Examples of Sex Differences

Morphology: There is, of course, genitalia. Also, in some species, like the saki monkey, the fur on males and females is a different color. Another common difference is in body size. There is some sexual dimorphism in humans, and more in baboons. You may remember that in orangutans, females and babies have regular-looking faces, while the males have the face flanges filled with fat. In the proboscis monkey, the males have the huge nose. Many males have morphology related to fighting or defense- the large canines, the fluffy manes, the antlers in ungulates.
Behavior: Males put more time and effort into pursuing prospective mates, while females put more time and effort into raising the young. Females put little or no effort into seeking mates, and males put little or no effort into raising the children.

But what's the real difference between males and females?

Almost all of the above-mentioned sexual differences can be traced back to one basic difference between the sexes; the size of the gametes produced by each sex. Females produce a large, food-rich gamete which is immobile. This represents a lot of effort on her part because she had to supply all the energy put into making this gamete. These are called eggs.

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.

Sexual Selection

Darwin, when trying to explain everything as an adaptation to the environment, was puzzled by seeming impediments. The most famous of these types of things is the peacock's tail. In primates, one may wonder about the brightly-painted face of the male mandrill. These would seem to hinder the owner by making him more visible to predators, or int he case of the peacock, making it more difficult to move around easily. He finally decided that they were the effects of:
"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.

The Two Kinds of Sexual Selection

Favors the ability of one sex, usually male, to compete directly with one another for fertilizations, for example by fighting.
Favors traits in one sex which attract the other.

Parental Involvement and Sexual Selection

Why does sexual selection have different effects in males and in females?

Variance in Reproductive Success

Females in a species usually all have medium reproductive success. Males are generally very successful or not at all successful. For instance, look at this data:

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.

Differences in Parental Investment

Females put a lot into each egg. This limits their possible number of kids. Males, however, are only limited by how many females they can get their hands on. Thus, female's reproductive success is dependent upon how much resources she can get ahold of. Male's reproductive success is dependent upon the females. Therefore, males compete for access to 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."

In general, in mammals, females do most of the care. Very few are monogamous, but in those that are, males do more of the parental care. Also, the variation in reproductive success between the sexes isn't as great as when there's polygyny.

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.

Role-Reversal Species

These do exist! There are species where males invest more than females in the offspring. From this, you would predict that females reproductive success was greater than males'. You'd also predict that females would compete among themselves for access to males. You would further predict that males would be choosy about their mates. While rare, these species do occur, mostly in insects and fish. Male seahorses, for example, have pouches into which the female lays the eggs. He then carries the offspring to maturity while the female goes off laying more eggs. In birds, this is seen in the jacana, or lily-trotter.

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.

Friday, October 11 -- Male-Male Competition

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Sexual Selection Theory: The Recap

There are a lot of differences between sexes, and most of them are the results of sexual selection. For example, the proboscis monkey. Sexual selection is a special form of natural selection. It's the struggle by one sex for mating with other sex. In primates it's always males competing.

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.

Sexual selection takes two forms;

Intrasexual selection: Male-male competition for access to females.
Intersexual selection: Female mate choice.

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.

Male-Male Competition

What determines the intensity of male-male competition?

Male-male competition is more intense when...
1. Female parental investment greatly exceeds that of males.
2. There are many more males then females.

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.

Morphological Effects of Male-male Competition

From (Krebs and Davies, pp 34-40)

Sexual dimorphism in body size

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.

Canine size

The more intense sexual selection is, the larger male canines become compared to females'.

Testes size

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.

Dominance and reproduction

Males seem to be really interested in establishing dominance even when there's not a resource to be fighting over at the moment, while females don't show such behavior. Steve Altman studied some baboons and said that male rank determined access to females. Male #1 had over 75% of the consorting days while #2 had like 17%, male #3 had 10 and #4 had like 7. males 5-7 had none.

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.

Alternative mating tactics

Like we saw earlier in the baboons, sometimes different males will adopt different strategies. This can be for two different reasons. (This is in detail in Krebs and Davies ch. 10, Alternative Mating Strategies.)

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.

Two examples of primate populations with alternative mating strategies:

1. Orangutans
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.

Discussion --

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Today we turned in our papers, discussed the paper for next week, and reviewed primate taxonomy questions.

Let me know your thoughts:
Last modified: October, 1996