Uakari monkey Monkeys, Apes and Humans human baby
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monkey baby  Anthropology 1500 orang baby
Monkeys, Apes and Humans
Anthropology 1500
Department of Anthropology, University of Missouri-Columbia

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Writing Intensive Tutoring

The WITS Lab offers free services to improve your writing. They can help you with brainstorming, outlining, and organizing your writing assignments. To use these services you should make an appointment at least five days ahead. If you schedule an appointment and find that you don't need help, cancel your appointment. The writing tutor for this class is John Wagner.

Convergence evolution produces analogous traits in species. In similar niches, natural selection produces similar traits (e.g., big eyes in Owl monkeys and tarsiers). Shared ancestry produces homologous traits.

Gametes don't make exact copies, hence, they are not replicators. They are vehicles carrying replicators, genes.

Example of sympatric species: baboons and chimps, hanuman langurs and humans, rhesus macaques and humans.

Example of competitive exclusion: Darwin's finches.

Homologous traits in chimps and humans (due to shared ancestry): two eyes, two legs, lack of a tail.

Proximate explanations involve environmental cues and physiological mechanisms. Evolutionary function explains why natural selection promoted a trait or behavior, i.e., the adaptive rationale.

Individual level v. group level adaptations. Two reasons why individual selection is a better explanation than group selection are: (a) Individuals reproduce faster than whole groups or species. (b) Mutations promoting group altruism to the detriment of the individual can not be passed on.

Adaptive radiation. Lemurs speciated (evolved into many species) when they colonized different niches on Madagascar.

Why live in groups (cont.)

There are many disadvantages and few advantages to living in groups. Conditions favoring group-living are: localized resources (groups form incidentally), cooperative food-getting (e.g., wolves), and protection from predators.

Hypotheses for Group-living

Hypotheses are possible explanations for a research problem. To determine which hypothesis is the best explanation we devise and test predictions. To test the predator explanation, we predict that species most vulnerable to predators, because of their body size and niche, are most likely to live in groups. Some researchers argue that group-living can be due to competition with others of your own species. For example, primates may form groups (usually kin-related) to defend fruit trees from other groups. To test these predictions we collect evidence that supports or falsifies our explanations. We accept a hypothesis that is not falsified by evidence or that receives the most support. Here is an example.


Hanuman langurs have a puzzling behavior. Adult males kill infants. For primatologists, the research problem is: why do males go to the trouble of killing infants? Here are some hypotheses and their predictions:



Population Control: To control their numbers, langurs kill infants. This prevents overpopulation which threatens group survival. (Good-for-the group)

Infanticide should occur when population density is high. It should not occur when population density is low.

Competition for Resources: Males kill infants to make more resources available to their kin and descendants.

Infanticide should occur when resources are scarce. Infanticide is less likely to occur when resources are plentiful. Also, males do not kill their own kin or offspring.

Cannibalism: Males kill infants and consume them as food.

Males should be observed to eat infants they kill.

Social Pathology: Langur infanticide is abnormal. They live in an unnatural habitat, disturbed by humans.

Infanticide should occur in langur populations living close to humans. Infanticide should be rare in langurs living away from humans.

Reproductive Competition (AKA the Sexual Selection Hypothesis): Langur males kill infants of other males so that females can become fertile sooner and bear their offspring.

Males should kill infants fathered by other males, not their own. Females should become receptive sooner. Killers should have probability of fathering subsequent offspring.


The last hypothesis requires more explanation. Hanuman langurs live in single-male groups. Females and their dependent infants live in "harems" with one dominant adult male. Other adult males live in bachelor groups. The harem male often loses his harem; he is usurp by a male in a bachelor group. Infanticide occurs when the new male takes control of the group. This a critical piece of evidence. It neither supports nor falsifies other explanations but it is best explained by the reproductive competition hypothesis.

These observations support the reproductive competition explanation.

  • First, females don't give up their infants without a fight, i.e., they don't "sacrifice" their infants for the good of the group. Females may deter the male if they cooperate or if their infants are less vulnerable to attack.
  • Second, infanticide occurs in many primate species. These species have similar social systems as hanuman langurs—single-male societies and take-overs. Males in these species kill infants likely to be unrelated to them.
  • Third, in the reproductive competition explanation, males kill infants so that they can reproduce faster. Females nursing babies are infertile. When males kill their infants, females return to fertility. Males are more likely to kill infants that are being nursed than weaned infants. This makes sense because mothers of weaned infants are likely to be fertile.
  • Lastly, males that kill infants are ones most likely to have sexual access to females. Males in bachelor groups are less likely to kill infants.

Infanticide in langurs is probably a male reproductive strategy. But females have countermeasures. When a new male takes over the harem, a pregnant female may have a sham estrus. She is not fertile—she is carrying the offspring of the previous harem male—but she dupes the new male. By mating with the new male, the pregnant female can make the male think that he has sired her infant. Sham estrus is a countertactic against infanticide.

Related evidence from other species

Infanticide has been studied in mice. Males kill infants unlikely to be their offspring. If a male copulates with a female 58 days ago ± 2 days, he does not kill the pup. This behavior indicates that natural selection has fine-tuned infanticidal behavior and paternity recognition in mice.A related behavior observed in mice is the Bruce Effect. When a pregnant mouse smells the urine of a strange male, she aborts her litter. This makes sense as a new male is likely to kill her litter. By aborting her litter she is conserving energy and will be better able to allocate it to future reproduction.

In humans, adults are more likely to abuse stepchildren than their own children. Stepparents and stepchildren are unrelated and, in some conditions, it makes evolutionary sense for stepparents to disfavor the offspring of others. (Disclaimer: Even if there is an evolutionary basis for child abuse in humans it does NOT follow that the behavior is justified or moral OR INEVITABLE OR PREPROGRAMMED.)




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