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Sexual Dimorphism and Mating Patterns of Primate Species

SexualDimorphism and Mating Patterns of Primate Species


Primatesare social beings that spend most of their lives living incommunities or groups. The semi-terrestrial species of primates suchas baboons have been found to keep themselves in large social groupswhich help them protect one another against predators and also duringthe periods of food shortage. However, leaf feeders such as theColumbus monkey are not perturbed by the state of food in theirecosystem. Such animals do not depend on the fruit bearing periodsfor their survival and the continuous presence of leaves enables themto survive with or without the social groups (Grueteret al. 2012).

Theprimate communities interact with members of other communities thatare around them in the ecosystem. However, in many cases, thecommunities stick within their locale and hardly move to otherregions beyond their home range. The localization of the animals in aparticular region can easily lead to rapid depletion of the resourcesin that particular habitat.&nbsp It is also common that communitiesare unwelcoming to the outsiders whom they show aggression andtherefore, most of the species do not interact especially the females(Grueteret al. 2012).In contrast, chimpanzees show a different trend in socializing withthe outsiders. When different communities of chimpanzees meet, theydevelop a warm and friendly moment that lasts for several hours andsome of the females end up shifting camps for the usual reason,seeking mates. Sometimes the encounters between different chimpanzeetroops might turn into a genocidal one.

Compositionof Non-human Primate

Theprimate social groups show composition variations, however, thespecies have very little unevenness. Therefore,in many occasions, many non-human primate species can only be amember of one of the following six basic patterns:

  1. A female and her offspring – this pattern is rare among the primates but is dominant among the orangutans and also a few troops of nocturnal prosimians. The females lead their lives in their own group while the males too have their own groups or they live in solitude. However, the male and the females come together during the mating period and thereafter, they are separated. Furthermore, the young females and males are separated into their gender groups immediately they reach sexual maturity (Grueter et al. 2012).

  2. Monogamous pattern – this family type is composed of one male who is the father, a female who is the mother and the offspring. When the children in the family are grown, they are separated from the parents and they go form their own nuclear families. However, this pattern of mating is very rare among the nonhuman primates and common among the human beings (Grueter et al. 2012). The mating system is common among the prosimians, minor groups of Asian apes and the New world monkeys.

  3. Polyandrous pattern – the family begins with a monogamous unit of the family where another adult male may join the family and give hand in child rearing. When another male joins the family, the female might mate with both males and bare children (Grueter et al. 2012). Because the monkeys bear twins more often, it is the males who help in carrying the children on their backs and therefore, the polyandrous mating pattern is very rare in nonhuman primates.

  4. Polygyny Pattern- is a mating pattern where one male lives with not less than one females and mates with them. The behavioral pattern is not promiscuous but the single male and his ladies form a child bearing and mating groups. This mating system is common among the humans especially in Africa, native America, and south Asia. It is also common among many nonhuman primates such as gorillas, hamadryas baboon, howler monkey among others (Grueter et al. 2012). However, it cannot be assumed that the mating pattern is perfect among the primates. For instance, in the geladas, the females are the controllers of the group even though the males are bigger, more aggressive, and seem stronger. The females ensure that the non-family members are kept away from the families by pushing them away (Grueter et al. 2012). The females have their way of selecting a suitable male mate in or around the group. The chosen male is one who is not abusive and easily cooperates with the females in child rearing and protection of the territory. However, the relationship formed is only for a short period of time.

Onemale and several females pattern of mating may change in its formwhen hit with pressure from predation. The normal way of life amongthe hamadryas baboons is that the male herds the females of thegroup, maintain order and protects the community from predators(Grueteret al. 2012). However, gorillashave a different pattern in which they are less concerned wit thepredators. The male in the community is dominant over the females andthe children. When the young males reach sexual maturity, they arepushed away by the silverback male who is dominant. The males whohave been chased away looks for their mates and form their familieswith one dominant male and several females and children.

  1. Multimale – multifemale pattern – this is a pattern of mating in the semiterrestrial nonhuman primates where there are no stable heterosexual relationships among the mates. The males and the females have a number of mates among them. The pattern is common among the baboons, macaques, and the new world monkeys. The members have a hierarchy of dominance amongst the males and the females. The hierarchy is formed such that the members of a community know how to interact with everyone (Grueter et al. 2012). This helps in eliminating conflicts among the members and promotes submissiveness.

  2. Fission- fusion pattern – is one in which the composition of the community keeps changing throughout the year due to various factors. Males have the characteristic of wondering alone and joins different groups in hunting and leaves after a short time. On the other hand, the females keep shifting from community to community during the estrus period as they seek for mates (Grueter et al. 2012). However, the males show stability in that as they wonder they do not go very far from their communities and they leave for home quite faster from other communities. This form of mating pattern is common among the chimpanzees.

However,it is not only the chimpanzees that show the fission-fusion matingpattern. The rhesus macaques also have their own. Among the macaques,the male keeps wandering from one group to the other in search ofmates. However, they are not normally welcomed and they are forced towalk from group to group until they find where they can fit (Grueteret al. 2012). The chimpanzeesshow peace and are quite hospitable to the non-group members.However, the males show hostility with the aim of stamping theirdominance in the group. On the other hand, the males show support forone another in terms of sharing meet and allogrooming (Grueteret al. 2012).

Whenthe average male of a species differs from the average female, thespecies is said to be sexuallydimorphic. Humans, for example, are sexually dimorphic regarding body size, menbeing on average about 10% taller than women. In this exercise, youwill analyze data on sexual dimorphism in non-human primates.

Sexualdimorphism can get measured in a variety of ways. In the exercisebelow, the following statistic has gotten used:

D= log10(averagemale weight) – log10(average female weight)

Wherelog10isthe logarithm to base 10. For a detailed discussion of this statisticand others

Likeit, the appendix can get consulted. If you are not interested in adetailed discussion, then the following paragraph is sufficient.

A1-gram difference between males and females would be more significantamong mice than among elephants. It makes more sense to give equalweight to equal percentage differences. The above referenceillustrates the function of the logarithms in the definition above ofD. If males are 1% larger than females, then D = 0:00995 for mice aswell as elephants.


Answerthe questions below using the data in Table 2.1 and Figure 2.1

1.In which species is sexual dimorphism most pronounced? In otherwords, which species

hasthe largest value of D?

Answer:Papio hamadryas whichhas a D value of 0.29

2.Fill in the graph in Figure 2.1. Plot each polygynous species as aplus sign

(+)And each monogamous species as a circle (●). Species without sexualdimorphism

shouldfall on the 45° line. If males are larger than females, then thespecies will plot

abovethis line, and the vertical distance above the line measures sexualdimorphism.

Asan example, the point for the Talapoin Monkey (Miopithecus talapoin)is already

plottedfor you.

Logbase ten of male weight versus log base ten of female weight.Polygynous are indicated by plus sign while circles indicatemonogynous. The Talapoin Monkey (Miopethecustalapoin)is already plotted for you

3.In a couple of sentences, summarize three patterns that you see inthe graph.

  1. Polygenous – the pattern of mating gets characterized with several females and one male primate mating who produce offspring with largely varying traits (Grueter et al 2012). The pattern is common in most of the primate species which are denoted by the crosses as in the plot diagram below. The polygynous type of mating contributes to the speciation of the primates as the offspring produced in each generation is more diverse compared to the parents and the generations before them. On the other hand, the system can change to dission-fusione in which the composition of the community keeps changing throughout the year due to various factors. Males have the characteristic of wondering alone and joins different groups in hunting and leaves after a short time. On the other hand, the females keep shifting from community to community during the estrus period as they seek for mates (Grueter et al. 2012). However, the males show stability in that as they wander not very far from their communities and leaves the for home quite faster from other communities. This form of mating pattern is common among the chimpanzees.

Monogynous-the characteristic of the mating pattern in the monogynous case iswhereby a male and a female who mate to produce some offspring. Theoffspring then mate with each other to form the nuclear family(Grueteret al. 2012).The incident is only evident in 5 primate species in the data plotand data set given. The characteristic means that the monogynous is avery rare species of the nonhuman primates.

Nonaligned– the pattern of mating in the nonaligned type is also called themultiple male-multiple female patterns. The pattern is characterizedby many males and females coexisting and mating freely. The offspringalso follow the trend, and this leads to low variation as most of thetraits are from a group of males and females who are of the samelineage (Grueteret al. 2012).The mentioned trait is the most common of the mating traits in thenonhuman primates. The primates form a social order made of alphamales and alpha females who are in charge of giving birth.

Table2.1: Sizes and mating systems of primate species1


Mating system

Log10Male WT (kg)

Log10Female WT (kg)

Lemur mongoz

Propithecus verreauxi

Galago alleni

Tarsus bancanus

Alouatta villosa

Callicebus moloch

Ateles geoffroyi

Cercopithecus aethiops Cercopithecus neglectus Cercopithecus ascanius Miopithecus talapoin

Cercocebus albigena

Macaca nemestrina

Papio hamadryas

Colobus badius

Presbytis entellus

Presbytis obscurus

Presbytis senex

Hylobates agilis

Hylobates hoolock

Hylobates lar

Symphalangus syndactylus Pongo pygmaeus

Pan troglodytes

Gorilla gorilla












































































P=polygynous M= monogamous

Dataare from Steven

AppendixA: What is a logarithm?

Weall know that

102= 100

Writingcan also express this same fact as

log10100= 2

Inwords, this reads “the logarithm to base 10 of 100 equals 2.&quotBoth equations mean the

samething: if you raise 10 to the 2nd power you get 100.

Similarly,103= 1000, so log101000 = 3. Here are some other numbers and their logs:


log10 x













Noticethe pattern. Asxgrows, log10xgrows too, but much more slowly. Logarithmsare useful when we are more interested in proportional differencesthan in absolute ones.For example, a one-ounce difference is large if we are comparingmice, but small if we are comparing elephants. Thus, we would notwant to use ‘ounces&quot as the horizontal axis of a graph thatincluded values for animals ranging in size from mouse to elephant.The graph is easier to interpret if we use “log ounces&quotinstead because a proportional difference of (say) 10% between miceoccupies the same space on the graph as a 10% difference betweenelephants. The graph explains why logarithmic scales are used sooften in illustrations.

AppendixB: Details about measuring sexual dimorphism

Howdoes one measure sexual dimorphism? To make this question concrete,let us take




Body WT (kg)


Body WT (kg)

Agile gibbon (Hylobates agilis)



Talapoin Monkey (Miopithecus talapoin)



Whichspecies is most dimorphic? The answer to this question depends on howwe measure sexual dimorphism. One possibility is to use thedifference between male and female weight values simply. Using thementioned measure, the Gibbon is most dimorphic since males of thatspecies are (on average) 0.32 kg larger than females (5.82-5.5)whereas male talapoin monkeys are only 0.3 kg larger than theirfemales (1.4-1.1). This measure of dimorphism is unsatisfactory,however, because it ignores the overall difference in size betweenthe two species. Surely a 1 gram difference is more important incomparisons among mice than in comparisons among apes.

Toincorporate the effect of overall body size, we use a differentmeasure of di-

morphism.One possibility is the ratio, R, of male to female size. For thegibbon in our


R= maleweight = 5.82 = 1.058


Forthe talapoin monkey, R= 1:27. Thus, male agile gibbons are only 5.8% larger than females,but male talapoin monkeys are 27% larger. By this measure, the monkeyis more dimorphic than the gibbon. The mentioned aspect seems a morenatural measure of sexual dimorphism since it automatically scalesthe result to body size.

Thereis one remaining problem, which arises when we try to make a graph ofsexual

dimorphismagainst body size. If the horizontal axis of our graph got measuredin (say)

Kilograms,then two animals that differed by 1 gram would be separated by thesame distance on our graph, whether they were mice or elephants. Aswe argued above, a 1-gram

Thedifference is more significant among small animals than among largeones. Byplotting

dimorphismagainst raw body size, we would exaggerate differences among largeanimals and obscure those among small ones.

Toavoid this difficulty, we work with logarithms. (If you are not surewhat a logarithm is seen Appendix A, above.) In the exercise above,you were asked to make a graph with the logarithm of average femaleweight on the horizontal axis and the logarithm of average maleweight on the vertical axis. If the average male and female weightsare equal, then species will fall on a line that gets drawn from theorigin at an angle of 45°. If males are larger than females, thenthat species will plot above the 45° line. The vertical distance, D,from the 45° line to a species`s point on the graph is equal to

D= log10(averagemale weight) – log10(averagefemale weight)

Forexample, in the case of the agile gibbon, the vertical distance tothe 45° degree line is

Dgibbon= log105:82 – log105:5 = 0.765 -0.740 = 0.025

Inthe case of the talapoin monkey, the vertical distance to the 45°degree line is

Dmonkey= log101.4 – log101.1 = 0.146 – 0.041 = 0.105

Thisnew quantity, D, is another way to measure sexual dimorphism. Sinceit gets based on logarithms, it is automatically scaled to body sizeand will not obscure differences within

smallspecies or exaggerate those within large ones. We now have twomeasures of sexual dimorphism, R, and D, both of which take body sizeinto account in a natural fashion. It turns out thatD= log10R.For example, in the case of the Gibbon,

Dgibbon= log10Rgibbon= log101.058 = 0.025

Wecan calculate Deither as the log of the ratio of male and female sizes or as thedifference of the logs. The answer is the same in either case. Thetwo measures of dimorphism, RandDwill never disagree. If species A is more dimorphic than species Baccording to R, then it is also more dimorphic according to D. Thus,both statistics provide the same information, and it does not mattermuch which we use. In this exercise, you have been asked to use Drather thanRbecause this makes it easier to interpret the graph.


StevenJ. C. Gaulin and Lee Douglas Sailer. (1984) Sexual dimorphism inweight among the

primates:The relative impact of allometry and sexual selection. InternationalJournal of Primatology 5(6):515-535.

Grueter,C. C., Chapais, B., &amp Zinner, D. (2012). Evolution of multilevelsocial systems in

nonhumanprimates and humans.&nbspInternationalJournal of Primatology,&nbsp33(5),1002-1037.

1Data obtained from : J. C. Gaulin and Lee Douglas Sailer. (1984) Sexual dimorphism in weight among the primates: The relative impact of allometry and sexual selection. International Journal of Primatology 5(6):515-535. Pg. 525 &amp 526