Article Review #2: The Ecological Importance of Trichromatic Vision to Primates

This article, written by Nathaniel Dominy and Peter Lucas, analyses the relationship between chromatic vision and food source selection in various primate species. In particular, Dominy and Lucas sought to determine the evolutionary force that led to trichromatic vision in howler monkeys and a number of cattarhine primates. Most primate species possess dichromatic vision, which means that they have retinal cones designed to discriminate between yellow and blue light. However, these trichromatic primates also have cones that distinguish between red and green light. For some time, primatologists believed that this additional type of retinal cone evolved in order to help primates separate ripe fruits from unripe fruits. In this journal, Dominy and Lucas propose that trichromatic vision actually developed to help primates find young reddish leaves, which are more nutritious and less tough than mature leaves. In order to support their hypothesis, Dominy and Lucas observed a number of primates located in the Kibale National Park in Uganda. Dominy’s and Lucas’ team logged an impressive 1,170 hours of observations and collected about 700 different types of foliage that had been partially consumed by the primates. Next, the team measured the specific wavelengths of the half-eaten food using a spectrometer, along with the nutritional value and relative toughness of the foliage. The food samples that were collected contained all sorts of leaves and fruits with various levels of ripeness. Overall, Dominy’s and Lucas’ hypothesis that trichromatic vision was more closely related to leaf selection than fruit selection was supported. They found no significant difference in ripeness between the fruits consumed by trichromatic primates and dichromatic primates. However, their data demonstrated that the trichromatic primates were much more efficient in locating the younger red leaves, which are more nutritious.

This scientific article contained several characteristics that added to its credibility and readability. First, Dominy and Lucas compiled an astounding amount of data. They spent almost a year in the field collecting the half-eaten food of primates. In total, they examined 624 pieces of food that were taken from 118 different species of plants. They also increased the legitimacy of their data by switching target individuals every ten minutes of feeding. I was also impressed by Dominy’s and Lucas’ ability to offer possible explanations for their hypothesis, without trying to force its validity. For example, they suggested that the dichromatic primates were just as able to locate ripe fruits as the trichromatic primates because they relied more heavily on olfaction and touch. Another reason I approve of Dominy’s and Lucas’ article is the unique nature of its subject. I have never considered the different types of vision primates possess, let alone their functional differences. Unfortunately, this article did contain one slight weakness. As far as I could tell, the authors never mentioned the specific primate species that they studied in the Kibale National Park. They gave examples of dichromatic and trichromatic primates but failed to identify the specific species that they observed. Overall, I support this article because the uniqueness of the subject matter and substantial amount of evidence provided far outweigh this small weakness. Image Image


The Influence of Food Sources on Social Structure

The availability of resources often plays a critical role in determining the social structure and activity of many primate species. I think the importance of this link can best by demonstrated by the existence of the niche species concept. Through the niche species concept, some taxonomists argue that the food resources and habitat of an animal is the single most significant factor in defining a species. Although I think this concept is a little over the top, I do think that the availability of food governs a large portion of primates social organizations and general behavior.

                For primates, survival hinges almost entirely on their ability to consume calories, so it should not come as a surprise that the pursuit of food resources affects many facets of their lives. One such facet dominated by food availability is the social organization of primate groups. Living in a group possesses several benefits, mainly protection and mating opportunities. However, just like Thomas Malthus famously stated regarding human population, a group of primates cannot outgrow the amount of food it has access to. Thus, the primary food resources of primates plays large role on group size. For example, chimpanzees, which mainly rely on fruits for sustenance, forage in small groups of around ten members which corresponds the limited amount of fruit that an area could offer. On the other hand, the grass-eating geladas of Ethiopia are less limited by food resources and often travel in groups close to a hundred in number. The availability of food also affects gender roles within groups. For instance, in a male-dominant society, limited resources might encourage females to seek pair bonding. That way, females are less affected by the discrimination they might face with feeding opportunities. Similarly, food availability could influence the sexual dimorphism of a species. With inadequate food sources, the dominant gender would consistently have more access to calories and could consequently grow bigger of many generations. Furthermore, competition over food resources could lead to more aggressive behaviors within societies and between separate groups. Altogether, access to food governs nearly every aspect of primate social organizations.

                The availability of food resources are especially important for females in primate societies. This stems mainly from the needs of females to support the offspring they have depending on them. Since primates care for their young for a longer period than most other animals, this creates a significant stress for females. In addition, to having to provide for their newborns, female primates also have to intake a larger number of calories when they are pregnant. Unfortunately, most primate societies are dominated by males, so females’ usually get last priority on feeding when a new source is discovered. For these reasons, the lives of females are influenced even more strongly by the availability of food.

                Due to the large impact of food availability on the characteristics of primate societies, many species that are separated be vast distances, yet depend on similar food resources exhibit common social organizations. For example, both chimpanzees, found in central Africa, and spider monkeys, found in Latin America, display analogous social structures due to their common dependence on fruiting trees. Both species forage for food in small, fission-fusion groups that change on a daily basis. Furthermore, both primates belong to a larger community that inhabit a common home range. Additionally, female spider monkeys and female chimpanzees tend to stick to a smaller range of area than the males. In both cases, the primates’ small group sizes likely derives from the relative scarcity of ripe fruits. The fission-fusion style of living could also be attributed to the fluctuating number of fruit trees in certain areas. The difference in gender migration for both species relates back to food availability as well. In both situations, the males can probably afford the travel further in search of food, while the females are limited to a smaller range by their offspring and greater need to conserve calories.

                On the other hand, some species that live fairly close geographically, display greatly different social structures, due to their reliance on different food sources. For example, the bonobos of Africa live relatively close to the chimpanzees, but have a vastly different social organization. Chimpanzees have male-dominated society filled with aggression, while the bonobos are female-dominated and surprisingly peaceful. Many primatologists attributed this difference to the bonobos access to large, stable sources of fruit along with the addition of some herbaceous food sources. With more stability, the bonobos do not need to be as aggressive as the chimpanzees to secure a sufficient number of calories. Despite the chimpanzees proximity to bonobos, their variance in food sources has led them to develop distinctive social organizations.




Spider Monkey                                                 Chimpanzee                                              Bonobo

Examining Bonobo Sexuality

In an article entitled, ‘The Other “Closest Living Relative”’, Parish and De waals compare the behaviors of bonobos and chimpanzees.  Then, the two authors go on discuss the evolutionary implications of the social differences between these closely related apes. Although bonobos and chimpanzees share many commonalities, they each possess distinct societal structures and mating patterns. First, bonobos live in female-dominant groups, mate indiscriminately, and are fairly non-violent. On the other hand, chimpanzees live in male-dominant societies, mate less frequently, and often engage in violent conflicts.

                Bonobos have crazy sex lives. It appears that bonobos, unlike most other primates, participate in sexual activities for many reasons outside reproductive interests. For example, bonobos have sex to reduce tension between individuals, to settle debts, and even in exchange for food. These occasions add up, and to an observer, it might seem that sex constitutes most of bonobo activity. Interestingly, bonobos practice two sexual acts that are entirely absent in chimpanzee mating. First, bonobo females often engage in genito-genital rubbing, in which females embrace each other and rub their genital swellings together. At the moment, it does not appear that this serves any reproductive purpose and is simply performed for pleasure. Furthermore, female bonobos have been frequently been recorded having sex with males from other groups, in the presence of males from their own groups. While female chimpanzees also partake in intra-group mating, they always do so in reclusion. This variance probably stems from the different gender roles in the two societies. For example, in male-dominant chimpanzee societies, a female chimpanzee would probably be violently attacked by a male in her group if they witnessed her having sex with a male from another group. In contrast, female bonobos are not intimidated by the males in their groups, and mate with whom they please. Altogether, bonobos have extremely active sex lives that appear to be largely driven by pleasure.

                The unique sex lives and gender dominance of bonobos may have developed for several evolutionary purposes. In bonobo societies females enjoy several advantages, such as feeding priorities and protection from violence. Overall, these increased resources may have allowed bonobo females to reproduce at a younger age.  This lower age of fertility could have served as a reproductive advantage, and reinforced female dominance over time. Furthermore, increased resources for females could have lowered the amount of time female bonobos spent between births. Female dominance also probably led to a decrease in infanticide over time. According to Parish and De waals, evolutionary processes also could have influenced the active sex lives of bonobos. Since it appears that bonobos frequently mate in order to settle disputes and relieve stress, it could be interfered that the frequency of their mating patterns increased over time to create a more amicable society. Overall, the female dominancy and active sex lives of bonobos hold many reproductive advantages that could have driven the evolution of bonobo society over many years.

                The social and sexual organization of bonobo society possesses several features that are fairly unique to the primate family. Although bonobos display philopatry, females still dominant every aspect of their societies. Not only is this female dominance rare primate societies, but bonobos tendency to exhibit philopatry also suggests that familial relations play a less significant role in their societies than most other primates. In addition, bonobos engage in far less violence and competition than most other primate species. This can probably be attributed to their female dominant society; since females generally possess less testosterone than males, a chemical known to encourage aggression. Altogether, bonobo societies maintain several unique characteristics that all seem to be related to the dominance of females within their groups.

                I personally believe that studying bonobos is extremely relevant to understanding behaviors in our own species. For many years, chimpanzees have served as the main subject for trying to decipher the evolution of humans. Logically, this makes sense. Chimpanzees are very closely related to us and their societies exhibit many of the same traits as our own. However, I think the importance of studying bonobos has been overlooked for far too long. Although their groups are not arranged quite as similarly to ours as chimps, they are practically just as similar to us genetically. I think some of the idiosyncrasies of bonobo behavior could play a very important role in understanding some of the subtleties of our own culture than may have been overlooked.Image

Article Review #1

In 1994, Mirendwicz and Schultz published a scientific article in the Journal of Neurophysiology entitled “The Importance of Unpredictability for Reward Responses in Primate Dopamine Neurons’. This article outlined Mirendwicz’s and Schultz’s experiment that measured chemical signaling in the brains of two macaque monkeys when they were exposed to different types of rewarding stimuli. The two macaques were placed in a box that contained a key, lever, microphone, and a liquid dispenser. At first, the macaques received a small amount of apple juice at random intervals. While the macaques lapped up their sweet treats, their neuronal activity was measured by moveable microelectrodes embedded in their ventroanterior midbrains. After allowing the monkeys to enjoy their free rewards for a small period of time, the experimenters upped the ante. The microphone in the room began to play a 100 Hz sound at random intervals and, in order to receive their apple-y goodness, the Macaques had to turn the key and press the lever within two seconds of the sound. Then after an extended period of time, the Macaques began to receive random dispenses of apple juice, without having to complete the task, all whilst having their neuronal activity measured.


The measurements of the Macaque’s dopamine neurons revealed several important findings. As expected, the activity of the dopamine neurons spiked, to fairly consistent levels, shortly after they received the apple juice when they did not have to perform the task. Interestingly, when the macaques were presented with the sound and first discovered that they had to perform the task to receive the juice, their dopamine neurons became significantly more active than they were during the random dispensing. However, after the macaques mastered the task, their dopamine neurons became slightly active at the sound of the microphone but did not respond to the actual intake of the juice. Once the random ejection of juice resumed, their dopamine activity returned to levels of the original phase. This variance in dopamine activity has several curious implications, which Mirendwicz and Schultz noted.

Mirendwicz and Schultz suggested that the reduction in the macaque’s dopamine levels during the conditioned phase of the activity could serve an evolutionary purpose. For example, receiving a larger amount of dopamine after completing a new, rewarding task could encourage learning and innovation. To me, this conclusions seems perfectly logical. It could have developed over many years in order to reward primates for discovering novel techniques of gathering food and attracting potential mates. I can also see evidence of this adaptation in human beings. For instance, I know that I am personally more excited when I learn something new and receive some unexpected benefit rather than some conditioned reward like a grade on a test.

Altogether, I think Mirendwicz’s and Schultz’s experiment was a very worthwhile investigation into a phenomenon that may seem obvious, but has an extremely interesting evolutionary function. Furthermore, the techniques used in this experiment were fairly sophisticated, especially considering the time at which it was performed, and produced extremely objective results. If I were forced to come up with a criticism of the study, it would have to revolve around the limited subject range. I wish the experimenters analyzed a larger number of primates, ideally across a variety of species. It would be interesting to see if all primates display this trait and to investigate if the extent to which each species exhibits this behavior correlates to overall brain size. However, overall, I think the novelty and evolutionary implications of this experiment greatly outweigh its limitations.

Primate Hybridizations

This week’s readings in Strier’s Primate Behavioral Ecology and Cortez-Ortiz’s scientific article, “Hybridization in Large-Bodied New World Primates”, both discuss the circumstances surrounding primate species that have distinct behavioral differences, yet occupy the same niche and mate together. These instances of hybridization have massive implications for our current system of classifying species and for the manner in which we view the evolutionary past. According to three out of the four main species concepts, the biological concept, the recognition concept, and the ecological concept, many of these hybridized individuals suggest that the two parent species should actually be classified as the same species. However, since the parent species often possess varying behaviors and appearances, this challenges our typical idea of a species, that individuals within a species display strictly uniform characteristics. Furthermore, the existence of hybridization in nature complicates the divergent manner in which we have assumed most species evolved.

                Cortez-Ortiz’s article examines the hybridization of two types of howler monkeys, A. palliate and A. pigra. These two types of howler monkey display morphological and behavioral distinctions and occupy separate niches, for the most part. However, Cortez-Ortiz focuses on a particular area in Tabasco, Mexico, where the two species habitats overlap and hybridization occurs. Despite the theory that these two monkeys diverged about three million years ago, the two types of howler monkeys still recognize each our as potential mates and even produce offspring! Although the male hybrid offspring are not able to reproduce viable offspring, the female hybrids are in fact fertile. These hybrid offspring display traits inherited from both of their parent species, such as mixed fur coloring. The fact that female hybrid individuals can reproduce with either of their parent species, at least to some extent, suggests that over time convergent evolution could occur and the degree of hybridization could increase. However, this possibility also depends heavily on the success of the mixed traits within the particular environment. Cortez-Ortiz makes a point of attributing the sympatry of the two howler monkeys to human deforestation within the Yucatan peninsula. At first, this might suggest that convergent evolution did not occur prior to human existence. This would be nice, as it would reinforce our current concept of a primarily divergent evolutionary path. Unfortunately, I don’t think this is the case because natural biographical pressures, such as changes in climate and food sources, have always existed.

                In Primate Behavioral Ecology Strier focuses on a well-studied instance of hybridization in Ethiopia’s Awash National Park. In this particular area, the habitats of the Hamadryas baboon and the Anubis, or Olive, Baboon overlap and hybridization occurs. Although these two species can produce viable offspring of both sexes, they have such different social organizations that makes long-term hybridization seem unlikely. The Anubis Baboon lives in a multi-male multi-female troop that is primarily matriarchal. On the other hand, the Hamadryas Baboon lives in a troop with one, extremely dominant male, and several females. This distinction causes some hybrid males to try to exert dominance over hybrid females and Anubis females, but the females usually don’t fall in line. Strier noted that hybrid individuals had higher reproductive rates with other hybrids than either of the parent species. However, I think with the relatively small number of current hybrids, the clashing social structures will prevent the hybrid species from gaining a significant foothold in the area.

                The questions primate hybridizations creates regarding the definition of species have yet to be completely answered by leading experts. Although most authorities maintain that hybrid parents are separate species, due to their varying traits, this highlights flaws in our concepts of species. I personally think that the term species should be regarded as a more general grouping of individuals. Furthermore, I believe we should try to become less focused on rigid compartmentalization and more focused on the implications convergent evolution may have had evolutionary history.


 Hamadryas Baboon (                         Anubis Baboon (

Proboscis Monkey

              In my opinion the proboscis monkey is one of the most unique animals on earth. The proboscis monkey, or N. I. larvatus, belongs to the subfamily Colobinae along with other old world primates such as the gray langur and snub-nosed monkey. The proboscis can be easily identified from its primate relatives by its’ hilariously oversized nose that often reaches four inches in length! The proboscis also has a fairly recognizable coat with long reddish brown fur on its’ back and shorter grayish fur on its’ underside. To add to their comic appearance, proboscis monkeys sport nice potbellies that remind me of my fifty-year old uncle with an addiction to Budweiser heavies. With their portly midsections, males can weigh up to sixty pounds; however, there is a significant amount of sexual dimorphism within the species and females normally don’t exceed thirty pounds.


            The proboscis’ habitat is limited to the island of Borneo, with most individuals living on the southern end of the island. The proboscis’ is arboreal and favors low-lying forests that have water sources nearby. It navigates this tricky environment by walking on four legs and leaping from branch to branch. Interestingly, the proboscis has also been recorded leaping into water sources and swimming long distances underwater. The monkeys also have slightly webbed feet to aid their aquatic tendencies.


            The proboscis monkeys’ diet consists of mainly fruits and leaves. For half the year, the monkeys prefer fruits because they are readily available and highly caloric. For the other half of the year the monkeys rely on leaves for sustenance, often chewing the leaves in a cud to aid digestion. Occasionally, proboscis monkeys will consume insects when they can be captured easily.

            Usually, proboscis monkeys live in groups composed of one male and three to eight females, along with their offspring. At nighttime, several of these groups typically come together and sleep in adjacent trees, probably for protection purposes. Within groups, there is normally very little aggression and only rarely intragroup conflict. After about five to eight years, these groups will generally disband and reform with different members.

            Unfortunately, the proboscis monkey is listed on the IUCN endangered species list. Over the past couple decades their numbers have declined significantly, mostly due to logging and habitat degradation.

Interdisciplinary Primatology

            The study of primates pertains to such a wide variety of disciplines for a number of reasons. First, primates have a rich evolutionary history that plays an important role in understanding those primates that currently inhabit the earth. This draws in disciplines such as archaeology, osteology, and, paleontology. Then, the most fundamental level of gathering information about primates incorporates fields like biology, ecology, zoology, and even chemistry. The next phase in primatology, analyzing the information gathered, requires aspects from an incredibly large number of studies. For example, components of psychology, physiology, and phonology could all be necessary to decipher a piece of information that initially pertained to primates. Lastly, many disciplines become involved with primatology by taking analysis of primate behaviors and applying them to other areas. Several of these studies, like sociology and anthropology, overlap with primatology because of the close relationship between humans and other primates. Overall, the nature of primates and multistep process in studying them revolves around an immense web of disciplinary studies, but is most directly labelled as primatology.


            I think the approach to studying primatological data in the fields of anthropology, psychology, and biology is the same on the most fundamental level. Basically, those three fields are all taking in information about primates and trying to identify some unique feature that has not already been discovered. All three fields also need their work to contain a solid amount of reproducibility and sample size. However, I believe the lens with which each field views the primate data can differ significantly. For example, anthropologists and psychologists probably both have greater tendencies to anthropomorphize than a scientific studying the data for strict biological purposes. Furthermore, anthropologists and psychologists probably focus on more ephemeral aspects of the primate information, while biologists likely focus on the physicalities.  Altogether, I think psychologists, anthropologists, and biologists all approach primatological data in a similar manner but view the information quite differently.


            Studying non-human primates reveals a huge amount of information about ourselves mainly because of our close evolutionary relationship to them. For example, because we shared common ancestors so recently with other primates, studying them could help reveal why we evolved how we did. From a psychologist’s perspective, studying primate behaviors could provide insight into certain human actions that are hard to study in humans without influencing the results. Since we share roughly ninety-nine percent of our DNA with non-human primates we have an incredible amount in common with them, and often times studying primates can reveal something about ourselves that we would not have been able to uncover otherwise.


           If I were a primatologist, I would take an approach similar to the paths taken by most other primatologists. However, I would not get too hung up on anthropomorphizing. If I saw two primates wrap the arms around each other and embrace, I wouldn’t hesitate to call it hug. Sure, the two primates might not be hugging because they feel affection for each other, like humans usually do, but, I don’t think we should shy away from using the most accurate word we can to describe a behavior just because humans do it too. However, using words usually applied to humans also requires readers to understand that non-human primates are unique animals and may be performing an action for a different reason than we do. In general, I would follow the same approach most primatologists use, but would not be too concerned with anthropomorphizing.