john hawks weblog

paleoanthropology, genetics and evolution

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primates

  • Incisors

    Mon, 2011-10-17 23:41 -- John Hawks
    Synopsis: 
    Laboratory exercise introducing incisors, including lemur tooth combs.

    The incisors are the front teeth. They are basically flat and have a blade-like occlusal surface. Each quadrant has two incisors.

    In humans and other primates, the upper central incisor (called the I1) is typically larger, the lateral (the I2) smaller.

    At this station you'll find casts of several primates, including some prosimians with tooth combs. Examine these mandibles. Some of the tooth combs include four teeth, and some six. The tooth combs with six teeth include the two incisors (I1 and I2) and the lower canines. The four-tooth combs are missing either the lateral incisor or the canine. Specialists disagree on this point. What do you think?

    Study terms: 
    incisor
    canine
    lemur
    Tags: 
    Anthropology 105
    explainer
    laboratory
    teeth
    primates

  • Primate classification and phylogeny

    Wed, 2011-10-12 13:07 -- John Hawks

    Our relationship to other kinds of primates is in part reflected by the pattern of similarities and differences we share with them. This pattern of similarities and differences is also used to classify different primate species into groups. There are six major branches of primates, classified as superfamilies. These include:

    Lemuroidea
    including the lemurs of Madagascar
    Lorisoidea
    including lorises and galagos
    Tarsioidea
    with the tarsiers
    Ceboidea
    or New World monkeys
    Cercopithecoidea
    or Old World monkeys
    Hominoidea
    including apes and humans

    The last three of these, hominoids, cercopithecoids and ceboids, share a common ancestor that lived sometime before 55 million years ago. These three superfamilies form a single branch, or clade, on the primate evolutionary tree.

    Scientists have often grouped these superfamilies into two major categories, called grades, which express the broad adaptations shared by different groups. Grades are not necessarily evolutionary lineages, but are meant to express the ways that different lineages share common sets of adaptations. One of these grades, the prosimians, includes lemurs, lorises, galagos, and tarsiers. Prosimians share a basic set of adaptations with other primates, including binocular vision, a slower rate of reproduction than many mammal groups, nails on the toes and fingers instead of claws, and other adaptations to life in the trees.

    Other primates include the monkeys, apes, and humans, who are grouped as anthropoids. Unlike the prosimians, the anthropoids form a single evolutionary lineage, or clade, because monkeys, apes, and humans are more closely related to each other than to any other living group. The evolutionary relationship of the anthropoids leads them to share many derived features, including

    Classification and phylogeny of living primate superfamilies

    Lemuroidea

    The lemurs are living and fossil primates of Madagascar. Lemurs share several derived features with lorises, including a set of closely-spaced and projecting lower incisors and canines, called a tooth comb, and a claw rather than a nail on the second toe. Both these features are used for grooming, and are so specialized relative to other primates, that lemurs and lorises may be classified as sister groups in a single clade, called the Strepsirrhini, a relationship supported by the close genetic relationship of the two superfamilies. Lemurs and lorises also share general features that were most likely present in the earliest primates, including a single postorbital bar instead of a bony enclosure for the eye orbits, and an external nose membrane connected to the upper lip.

    Today, the lemurs include five distinct families, all limited to Madagascar. These range in size from the mouse lemurs, which are the smallest living primates at only 60 grams, to the indri, approaching 10 kg. The most divergent living lemuroid is the aye-aye, a nocturnal creature with long bony clawed fingers and perpetually growing incisors, both supporting an adaptation to finding and eating grubs inside of wooden branches. The other lemuroids show a broad range of dietary and locomotor adaptations. Some species, like the sifaka, primarily leap using long hindlimbs and cling to vertical branches. Others are arboreal quadrupeds, or spend substantial time on the ground.

    In the recent past, a greater diversity of lemurs existed on Madagascar than remain today. Large extinct lemurs such as Megaladapis reached over a hundred kilograms at their largest, exceeding the size of female gorillas. Some large extinct lemurs appear to have had a sloth-like adaptation for below-branch suspension and feeding, and most of the larger forms primarily ate leaves. These lemur species existed within the past fifteen hundred years, and were likely driven to extinction by humans, who reached the island within that time period. The discovery of lemur skeletal remains in association with human archaeological sites confirms this extinction hypothesis.

    Lorisoidea

    The living lorisoids include galagos and lorises. Galagos, or bushbabies, are small prosimians weighing for the most part a fraction of a kilogram. Nocturnal creatures with large eyes and ears, and long tails, galagos are found across West Africa and into the central and southern portions of the continent. Galagos eat mainly fruits, insects, and gums, and some species are mostly quadrupedal, while others are adapted to leaping and a vertical posture. Lorises share a similar diet and a nocturnal activity pattern with the galagos, but differ in their relatively slow and deliberate style of foraging. Several species of lorises are distributed across Africa, South Asia, and Southeast Asia.

    Tarsioidea

    Tarsiers are small primates from the islands of Southeast Asia: Java, Borneo, Sulawesi, and the Philippines. Averaging slightly greater than 100 grams, tarsiers have several distinctive skeletal adaptations, including very long legs and ankles, immense eyes, and large hands and feet. These features support the adaptation of tarsiers of clinging to vertical branches and leaping between them, as well as their nocturnal activity pattern. Tarsiers eat insects, lizards, and other small vertebrates.

    The tarsiers share several features with anthropoid primates that may indicate a close phylogenetic relationship between the two. Unlike other prosimians, tarsiers lack a moist external nose, and they have a wide postorbital plate instead of a narrow bar. These and several more subtle cranial features link tarsiers with monkeys and apes. Many paleontologists believe that anthropoids may have originated from an Eocene group, called the omomyids. This group shares many features with living tarsiers and which may represent the common ancestor of both groups of living primates. If true, then the tarsiers are the closest primate relatives to the anthropoids, and the two groups form a clade called the Haplorrhini.

    Ceboidea

    Ceboidea includes the American, or New World, monkeys. Much of the earliest record of primate evolution, dating to greater than 40 million years ago during the Paleocene and Eocene, is from North America. These lineages apparently became extinct in North America by Oligocene times, and the New World monkeys that appear in the Oligocene of South America derive from an early Old World anthropoid lineage. The earliest anthropoid primates now known are from East Asia, which may have been the original source location for the anthropoids. Present-day New World monkeys possess three upper and lower premolars on each side, like most prosimians and the earliest anthropoids. This primitive dental formula distinguishes the New World monkeys from both the Old World monkeys and apes, which have only two premolars rather than three. Because of their round, forward-facing nostrils, the New World monkeys are called Platyrrhini, or flat-nosed, in contrast to the more narrow noses of the cercopithecoids and hominoids, which together are called Catarrhini, or downward-nosed. These differences imply that the platyrrhines existed as a lineage apart from catarrhines at least as early as 35 million years ago, when the first fossil catarrhines lived.

    South America was an island continent until around 5 million years ago, when the connection to North America arose. The first fossil New World monkeys date to the Late Oligocene, some 30 million years ago, meaning that these primates reached South America by an ocean crossing, probably an accidental journey on rafting vegetation from Africa. Once they reached South America, these monkeys underwent an impressive diversification. There are five living subfamilies of New World monkeys, with a total of sixteen genera. These vary from the relatively tiny callitrichines, including the marmosets and tamarins, to the relatively large atelines, including spider monkeys and howler monkeys, but most species range from one to five kilograms in mass. Several New World monkeys, including spider monkeys, howlers, and capuchin monkeys, have prehensile tails.

    Cercopithecoidea

    The cercopithecoids, or Old World monkeys, include two major groups. The cercopithecines, including macaques, baboons, guenons, and mangabeys, have a broad diet based on fruits, leaves, seeds, nuts, and insects. The colobines, including African colobus monkeys and Asian langurs and proboscis monkeys, tend to specialize to a greater degree on leaves in their diets. Old World monkey species have a diversity of anatomical adaptations to support these general patterns. The dietary diversity of cercopithecines is supported by broader incisors and low-crowned molars, typical of fruit eaters, and pouch-like cheeks for stashing food. Colobines have high-crowned molars for shearing leaves, and a large specialized gut for digesting leafy matter.

    A key adaptation shared by all living cercopithecoids is the distinctive shape of their molars. These teeth have cusps aligned into two high ridges extending across the tooth from side to side, a pattern called bilophodont. The ridges on the top and the bottom teeth interlock in opposing sawtooth patterns, creating a strong shearing action, ideal for reducing leaves and other fibrous plant matter, such as fruit rinds. This dental pattern is first found in fossil monkeys from the Early to Middle Miocene of East Africa. The living varieties of cercopithecoids arose later and underwent a major adaptive radiation during the Pliocene. Today the cercopithecoids include some of the most successful varieties of primates because of their geographic extent, the number of their species, and their dense populations.

    Hominoidea

    The hominoids include the living apes and humans, and their fossil relatives. The living great apes belong to four species, including orangutans, gorillas, chimpanzees, and bonobos. The gibbons and siamangs are in the hylobatid family, and include several different species sometimes called lesser apes. Humans and their extinct relatives are the hominins.

    Tags: 
    primates
    taxonomy
    phylogeny

  • Primate extractive foraging and tool use

    Tue, 2011-09-20 17:08 -- John Hawks
    Synopsis: 
    Many kinds of primates make and use tools, or find other ways to defeat the natural defenses of their foods.

    An important difference among some primate species is their ability to get foods that are hidden or protected by natural defenses. A little cleverness may yield foods that are inaccessible to other animals.

    For example, gorillas eat a high proportion of leaves and stems of terrestrial plants, especially in mountainous habitat where fruits are scarce. These herbaceous plant parts often have defenses such as stinging hairs or thorns. Such defenses are meant to deter animals like gorillas from eating the plants, and they are effective — it hurts to eat plants that sting! But gorillas can make use of these plants by following special methods to neutralize the defenses. One kind of sting-covered nettle leaves is commonly eaten by mountain gorillas, which carefully roll stacks of leaves in a way that encapsulates the stings inside a single leaf where they do not hurt so much to chew [1].

    Some primates make and use tools for extractive foraging, including chimpanzees, bonobos, orangutans and capuchin monkeys. A tool can be any kind of natural object that is altered by an individual and used for a purpose. Capuchins use and alter sticks to probe holes for insects [2]. Some groups of capuchins have developed a way of cracking nuts by using large stones [3]. Capuchins are small monkeys, so it is quite impressive to see one lift a stone bigger than his head, then toss it down forcefully to break open a nut. Other capuchins gather around to watch and pick up the shattered fragments of nutmeats. Younger capuchins seem to choose to watch the most skilled nutcrackers, which gives them a basis for learning through this social event [4].

    Chimpanzees use both simple and complex tools. The most celebrated chimpanzee tool is the termite stick. This is simply a stick or leaf stem that has been stripped by the chimpanzee, forming a long probe. This is inserted into termite or ant nests where the insects crawl onto the stick. Then, the chimpanzee pulls the stick out and licks off the termites [5].

    A more elaborate version of this behavior, probing into holes for a hidden resource, can be used to obtain honey. Honey is an important resource for chimpanzees in many parts of their range, and is produced both by bees that live in trees or hollow logs, and by bees who live in burrows underground. Finding the entrance to an underground hive is a simple matter of watching where the bees go. But the brood and honey chambers of these burrows may be a meter or more underground, and removed some distance from the entrance. Chimpanzees must dig quite a long tunnel in some cases to get the honey, and for this they use several different wooden tools to probe, soften and break up the ground, and dig [6].

    Chimpanzees also crack nuts across some parts of their habitat, and this is one of their most complex tool-using behaviors [7]. Different groups use different techniques for cracking nuts. Generally, a chimpanzee puts a nut on a large stone or log. Then, the chimpanzee uses a hammerstone or log to strike the nut. This may take several blows, and the effectiveness depends on the orientation of both the nut and hammer. Chimpanzees return to favored stone platforms or tree roots over many years, so that this technological element is a persistent feature of chimpanzee societies. Archaeologists have studied this behavior to try to see what traces may remain from using stone in this way, and have even found evidence of chimpanzee nutcracking from thousands of years ago [8]. Some chimpanzees do not crack nuts at all, even those who have nuts in their environment. For example, the chimpanzees at Loango, Gabon, do not crack nuts but use complex sets of tools to probe underground bee hives for honey [9].

    Chimpanzees and other apes use tools for purposes other than foraging. For example, some chimpanzees clip a leaf with their lips or teeth as a signal to other individuals---perhaps an invitation to groom or to play. Leaves and leaf stems are used extensively for wiping the body and probing teeth. Leaves are also used to soak up water and squeeze it into the mouth, like a sponge. These and other simple uses of natural objects vary among populations of chimpanzees extensively. Tool use therefore suggests that chimpanzees are interacting with some aspects of the material world in part through their mental adaptations for social behavior, as they absorb behavioral and technological knowledge from other individuals.

    Other hominoids use tools less extensively than chimpanzees but show similar abilities to perform complex tasks. Like chimpanzees, orangutans can be trained to use many kinds of human tools, even extending to complex tasks. But their natural use of tools is very limited, perhaps linked to the relative lack of extractive foraging opportunities in their arboreal existence [10]. Likewise, bonobos use leaves in some ways similar to chimpanzees, but extractive foraging is not common [11]. Experiments in naturalistic settings show that chimpanzees tend to use their existing cultural knowledge to solve new problems. For example, chimpanzee groups where sticks are a common solution to problems tend to use sticks to probe for novel foods, while those who use more leaves in other contexts will more likely probe with fingers than with sticks [12]. The familiarity with tool use may help develop new tool-using behaviors, even if the cognitive potential for tool use is widely shared among primates that don't use them.

    References

    1. Citekey Byrne:1993 not found
    2. Phillips PC. 1998. The Language of Gene Interaction. Genetics 149:1167–1171.
    3. Anderson JR. 1990. Use of objects as hammers to open nuts by capuchin monkeys (Cebus apella). Folia primatologica; international journal of primatology 54:138-45.
    4. Ottoni EB, de Resende BD, and Izar P. 2005. Watching the best nutcrackers: what capuchin monkeys (Cebus apella) know about others' tool-using skills. Animal cognition 8:215-9.
    5. Goodall J. 1986. The Chimpanzees of Gombe: Patterns of Behavior. Cambridge, MA.
    6. Sanz CM, and Morgan DB. 2009. Flexible and Persistent Tool-using Strategies in Honey-gathering by Wild Chimpanzees. International Journal of Primatology 30:411 - 427.
    7. Boesch C, Marchesi P, Marchesi N, Fruth B, and Joulian édéric. 1994. Is nut cracking in wild chimpanzees a cultural behaviour?. Journal of Human Evolution 26:325 - 338.
    8. Citekey Mercader:2002 not found
    9. Boesch C, Head J, and Robbins MM. 2009. Complex tool sets for honey extraction among chimpanzees in Loango National Park, Gabon. Journal of human evolution 56:560-9.
    10. van Schaik CP, Ancrenaz M, Borgen G, Galdikas B, Knott CD, Singleton I, Suzuki A, Utami SS, and Merrill M. 2003. Orangutan cultures and the evolution of material culture. Science (New York, N.Y.) 299:102-5.
    11. Hohmann G, and Fruth B. 2003. Culture in Bonobos? Between‐Species and Within‐Species Variation in Behavior. Current Anthropology 44:563 - 571.
    12. Gruber T, Muller MN, Strimling P, Wrangham R, and Zuberbühler K. 2009. Wild chimpanzees rely on cultural knowledge to solve an experimental honey acquisition task. Current biology : CB 19:1806-10.
    Study terms: 
    social learning
    tool
    extractive foraging
    termite fishing
    capuchins
    chimpanzees
    Tags: 
    tool use
    primates
    behavior
    culture
    learning
    social learning

  • Primate vertebral numbers

    Sun, 2011-09-18 20:36 -- John Hawks
    Synopsis: 
    A laboratory exercise to explore the numbers of vertebrae in different primates.

    Between the skull and the sacrum, humans have 24 vertebrae. Well, most humans, anyway. Sometimes humans have a few more or less.

    Humans vary in the length of the lumbar region, the number of vertebrae between the lowest ribs and the pelvis. The typical number is five, but some people have only four. Rarely, people have six lumbar vertebrae.

    Non-human primates also vary in the number of lumbar vertebrae. This variation is connected to locomotion. Species with vertical, suspensory postures have relatively short lumbar columns. Chimpanzees, gorillas and orangutans have fewer lumbar vertebrae than humans. Quadrupedal primates, including most monkeys and prosimians, have longer lumbar columns than humans.

    What to do: This station has several skeletons of different kinds of primates — both New World and Old World monkeys and apes. Determine the number of lumbar vertebrae in each of these primates. Do these primates vary in the other segments? Do they ,for instance, have the same number of ribs?

    Anatomy of the vertebral column
    Study questions: 
    1. Consider the number of lumbar vertebrae in gorillas and orangutans. Explain how these apes each have relatively few lumbar vertebrae and humans have more than either. What do you suppose was the number of vertebrae in the common ancestors of these apes and humans?
    2. Why do quadrupeds have a longer lumber spine?
    3. Why do you think there is very little variation in the cervical spine?
    Study terms: 
    lumbar
    cervical
    thoracic
    posture
    locomotion
    New World monkeys
    Old World monkeys
    apes
    hominoids
    Tags: 
    Anthropology 105
    explainer
    laboratory
    anatomy
    primates
    vertebrae

  • Meet Daubentonia madagascarensis

    Wed, 2011-09-07 09:21 -- John Hawks
    Synopsis: 
    A laboratory station at which students encounter the skull and mandible of the aye-aye

    The aye-aye is possibly the world's strangest primate. The species is native to Madagascar, and falls into the family of all primates from that island, the lemurs. But the aye-aye is a very specialized lemur, with anatomical features and behaviors not found in other lemurs.

    Aye-ayes hunt for insects, using their fingers to tap on branches and locate grubs and insects that have burrowed into the bark and wood. Their middle finger is slender and elongated, with a claw on the end. They use this to probe inside insect burrows and take them out.

    Like some other lemurs, aye-ayes are nocturnal creatures, active at night. They are highly endangered and survive only in two forest preserves.

    The skull and mandible of the aye-aye are very distinctive compared to most other primates, even other lemurs.

    Study questions: 
    1. Inspect the dentition, or teeth, of the aye-aye and compare them to the other primates at this station. Do they have the same number of teeth?
    2. Nocturnal mammals tend to have larger eyes than diurnal mammals, which are active during the day. How can you compare the orbit size of the aye-aye to the other primates at this station? Are there others you think are likely to be nocturnal?
    Study terms: 
    nocturnal
    aye-aye
    lemur
    Tags: 
    primates
    Anthropology 105
    conservation

  • Will a Jurassic placental mammal make the molecular clock make sense?

    Wed, 2011-08-24 17:19 -- John Hawks

    A new paper in Nature by Zhe-Xi Luo and colleagues [1] reports the discovery of a 160-million-year-old early mammal, Juramaia, which they attribute to the placental mammal lineage. The news aspect is that this extends the chronology of fossil placental and marsupial mammals (the sister clade of placentals) by some 40 million years. That's a big chunk of time, but it's a really nice fossil which seems pretty clear in its morphology.

    I'm reading this closely because of the effect on the interpretation of mutation rates and the molecular clock. Obviously, if the earliest evidence for placental mammals used to be 120 million years ago, and now it's 160, that should affect the way we approach the genetic divergence of mammal lineages. In particular, when it comes to primates, some modern lineages are represented by fossils relatively early in the Cenozoic, suggesting that the common ancestor of all the primates may have been much earlier, deep in the Cretaceous period. But there is no fossil evidence of that ancestor, and until recently molecular comparisons seemed to suggest a recent chronology with a common ancestor just before the Cretaceous-Tertiary (K-T) boundary. That is, until direct estimates of the human mutation rate started to suggest a much lower rate of mutations per generation than had previously been assumed.

    I've written about these issues several times, both with respect to hominins and other primates. For example my (unfinished) series from 2010:

    "Were there Cretaceous anthropoids? Part 1. The problem in a nutshell"

    "Were there Cretaceous anthropoids? Part 2: What is an anthropoid?"

    "Were there Cretaceous anthropoids? Part 3: Ghost lineages"

    And last month's "More on the mutation rate", pointing to my review from late last year, "What is the human mutation rate?" It's a key scientific problem right now, and genetic evidence may be approaching the point of a solution. Finding older and older fossils tends to confirm a lower rate of mutations, and a long chronology for the extant lineages.

    The current paper by Luo and colleagues addresses the molecular clock and suggests how a 160-million-year-old placental mammal may affect things:

    Timing of the divergence of marsupials and placentals is critical for calibrating the rates of evolution in therian mammals, especially for molecular evolutionary studies and comparative genomics 2, 10, 13. Previously, some molecular time estimates for marsupial and placental divergence postulated significantly older windows for this divergence than the then-oldest fossil records3, 7. However, these and other previous molecular estimates differed widely. Several were compatible with relatively young placental intraordinal divergences (for example, ref. 10), and just about all showed wide error margins (reviewed by ref. 13). Regarding the marsupial–placental split, recent molecular rate studies provided estimates of 147.7 ± 5.5 Myr (ref. 11), or 160 Myr (median) with a 95% highest posterior distribution of 143–178 Myr (ref. 12), or a window of 193–186 Myr (ref. 9). This new eutherian fossil age is now similar to the age of placentals at 160 Myr with 95% posterior distribution from 143 to 178 Myr by the latest molecular estimate12. The age of Juramaia has now set the minimal divergence time by the fossil to coincide with the range of molecular time estimates, serving as a corroboration of the newest fossil record with the molecular clock of evolution. The 160-Myr-old Juramaia also has important implications for mammalian evolution as a whole. Eutherian mammals are nested in the more inclusive Mesozoic boreosphenidan clade (Fig. 3, node 1), for which the previously earliest record had been entirely Early Cretaceous1, 27. The eutherian Juramaia requires that the ghost-lineages of boreosphenid and cladotherian mammals would also extend to the Middle Jurassic. Therefore the magnitude of the mammalian faunal turnover from the Early to Middle Jurassic is greater than previously known, and the Early–Middle Jurassic is a critical transition for the appearance of more of the derived mammalian clades1, 2.

    Reference 10 from that quote is a paper by Kitazoe and colleagues (2007) [2]. In that paper, the divergence of New World and Old World monkeys is up over 55 million years ago, and the divergence of anthropoids and strepsirrhines was around 85 million years. In other words, that "fast" chronology paper predicted anthropoids at or around the K-T boundary. Reference 11 is by Bininda-Emonds and colleagues (2007) [3], in which primates were estimated to have originated 91 million years ago, with a haplorhine-strepsirrhine divergence 87 million years ago. The other references here don't discuss within-primate divergences together with the more ancient mammalian representatives. I discussed more focused comparisons of primate divergences last year ("Were there Cretaceous anthropoids? Part 1. The problem in a nutshell").

    It looks to me like an earlier origin of placental mammals will elevate the likely divergence dates for primates to some degree, which will make a difference to interpretation of fossils like Altiatlasius or Algeripithecus. I think it's consistent with a lower mutation rate within the hominoids also, but it's unclear whether we need the within-family rate of change to be consistent with the longer term rate of change among orders of mammals.

    References

    1. Luo Z-X, Yuan C-X, Meng Q-J, and Ji Q. 2011. A Jurassic eutherian mammal and divergence of marsupials and placentals. Nature 476:442 - 445.
    2. Kitazoe Y, Kishino H, Waddell PJ, Nakajima N, Okabayashi T, Watabe T, and Okuhara Y. 2007. Robust Time Estimation Reconciles Views of the Antiquity of Placental Mammals. PLoS ONE 2:e384.
    3. Bininda-Emonds ORP, Cardillo M, Jones KE, MacPhee RDE, Beck RMD, Grenyer R, Price SA, Vos RA, Gittleman JL, Purvis A, et al. 2007. The delayed rise of present-day mammals. Nature 446:507 - 512.
    Tags: 
    primates
    molecular clock
    mutation rate
    mammals
    Cretaceous
    Synopsis: 
    The discovery of the 160-million-year-old Juramaia suggests a lower mutation rate and longer chronology for primates.

  • Monkey numerical distractions

    Fri, 2011-04-15 08:20 -- John Hawks

    This study has been out for a few weeks, and I've been meaning to put up a short comment about it: "Representational format determines numerical competence in monkeys", by Vanessa Schmitt and Julia Fischer [1]. The abstract:

    A range of animal species possess an evolutionarily ancient system for representing number, which provides the foundation for simple arithmetical operations such as addition and numerical comparisons. Surprisingly, non-human primates tested in ecologically, highly valid quantity discrimination tasks using edible items often show a relatively low performance, suggesting that stimulus salience interferes with rational decision making. Here we show that quantity discrimination was indeed significantly enhanced when monkeys were tested with inedible items compared with food items (84 versus 69% correct). More importantly, when monkeys were tested with food, but rewarded with other food items, the accuracy was equally high (86%). The results indicate that the internal representation of the stimuli, not their physical quality, determined performance. Reward replacement apparently facilitated representation of the food items as signifiers for other foods, which in turn supported a higher acuity in decision making.

    This seems so obvious in retrospect. An experimenter has to provide some kind of motivation or there will be no experiment. Providing food rewards in psychology tests on animals will conflate numerical cognition with food, rewards, and motivation. I'm surprised that a simple substitution of inedible items turned out to be so successful in relaxing this cognitive bias.

    As I'm thinking about the "numbers as technology" theme, I keep returning to the idea that most interesting technologies are cobbled together from heterogeneous parts. Cognitive technology is no exception. In this experiment, we see the interference between the food/reward aspects of cognition and the representation of number. To have an effective practice of number as applied to food items, an individual would have to overcome this interference.

    We might tinker with the system in different ways -- for example, we could set up a new system of behavioral rewards or we could change neurotransmitter regulation to decrease food salience. What is the dividing line between technical and natural solutions? Imagine a pill that improves monkey math by inhibiting dopamine receptors. The same inhibition might emerge by mutations to dopamine receptors -- a natural tweak that alters the threshold of technical interventions. A new reward system might seem purely technical -- in the experiment, it worked to substitute different kinds of food treats in different contexts. But then, "different" is itself a function of perception, which can be changed by changing visual and olfactory receptors. "Technical" is a matter of arranging heterogeneous things in such a way that their natural course of action achieves a desired end.

    References

    1. Schmitt V, and Fischer J. 2011. Representational format determines numerical competence in monkeys. Nature Communications [Internet] 2:257+. Available from: http://dx.doi.org/10.1038/ncomms1262
    Tags: 
    technology
    mathematics
    psychology
    cognition
    primates

  • The real "junk" DNA

    Wed, 2011-03-09 22:47 -- John Hawks

    Let me be honest: when I started doing paleoanthropology, I really did not expect I'd be talking about Neandertal penises.

    And yet, here I am. Cory McLean and colleagues [1] combine a straightforward genomic analysis of human-specific deletions with a couple of transgenic mice, and take us straight to penis spines.

    You see, most primates, and indeed many mammals, have at least some spines on their penises. "Spine" means more or less what you would expect: little projections that are covered in hard material, generally keratin, curving toward the base of the penis. These spines are sometimes called "horny papillae."

    No, I cannot make this stuff up.

    The morphology of these spines varies among primates. They overlie sensory receptors, and they intensify or enhance sensations accompanying intromission of the penis. Like a KY commercial, except they don't enhance sensations for the female. The net effect in some species is to reduce how long it takes the male to ejaculate. For example, a 1991 paper [2] by A. F. Dixson...

    No, I cannot make this stuff up.

    ...removed the penile spines of several male marmosets, finding that they took twice as long to achieve penile intromission after starting pelvic thrusts. Of course, "twice as long" in marmosets only means 15 seconds. The spineless males took 2 seconds to ejaculate, compared to only 1.73 seconds for those who had a "sham surgery" -- that is, they got the same depilatory spine-removal procedure without the active ingredient. That's some evidence in favor of the idea that losing penile spines might be related to longer coital duration.

    But penile spines don't always mean fast sex. Galagos have penises covered in long hook-like spines, which they use in virtual sex marathon sessions lasting two hours or more. Prosimians tend to have much more elaborated spines, in contrast chimpanzees' spicules are comparatively minor -- in a broad comparison across primates, Harcourt and Gardiner [3] rated chimpanzees along with humans as having insignificant penile spinosity.

    Let me just say that the comparative data don't convince me of an adaptive model for loss of penile spines in humans. Evidence from mutilated monkeys is not all that persuasive. I mean, really, how fast do you think you would manage after the "operation"? More important, the differences among hominoids run against the hypothesis -- gibbons have the spiniest penises among the apes, despite their monogamous, pair-bonded social habits.

    And I'll pause to savor the surreality: I'm here making value judgments about genital cacti.

    One thing that is definitely well-known about these penile spines is that their development depends on testosterone. Castrated monkeys do not develop the characteristic spines, and they lose them if already present. The androgen receptor (AR) locus is surrounded by promoter/enhancer sequences that are tissue-specific, capable of being flipped on or off as development proceeds within different parts of the body.

    Within this system, the genetics in humans and chimpanzees are simple: A long (60 kilobase) deletion of DNA in the human lineage has knocked out a 5 kb conserved region that enhances AR. That enhancer is specific to the follicles around the developing facial whiskers (vibrissae) and in the skin layers of the penis. This specificity was discovered in transgenic mice, in which a reporter gene is inserted with the enhancer, and embryos display expression of the reporter wherever the enhancer is active. Very straightforward, very cool science.

    One more thing: The chimpanzee version can drive expression when implanted into transgenic human foreskin fibroblasts. That indicates that the overall genetic system to make penile spines is still there lurking in our genomes. If we could turn on the gene at the right time, replacing the function of the enhancer, we can still grow penile spines.

    Just saying -- there may be a market there. Maybe the "male enhancement" companies will hit that next. I can only imagine what the wrapper on the NASCAR circuit will look like. OK, I know, don't encourage them. It's bad enough that we have labs full of foreskin tissue with chimpanzee genes floating around.

    I couldn't make this stuff up if I tried.

    Finding the deletion was straightforward genomics: They scraped the human genome for parts missing from chimpanzees and macaques, and then extracted from that set all deletions that included sequence conserved in other mammals. Others have done similar comparisons for conservation and human-specific changes; this is a clever twist on the same problem. It does fit an ongoing theme -- many essential aspects of humans may involve the loss of genes or functionality from our ape ancestors.

    Ok, so where do Neandertals fit in? They have the sequence deletion just like the rest of us do. If that deletion rules out chimpanzee-like spiky penises, then Neandertals could glide like the rest of us.

    All in all, it's a nice short paper, and very straightforward. The only questionable part to me is the social model. The genetics and expression data are solid.

    Speaking of Neandertals and the androgen receptor (AR) locus, my genome appears to have a Neandertal-derived haplotype across that gene. I'll expose this fact at greater length later, but I thought it worth sharing that the current paper is not the end of the story. Neandertals may not have had penis spines, but some functional polymorphisms in testosterone response might still have come into our population from them or other ancient people.

    UPDATE (2011-03-11): Eric Michael Johnson gives us the real dirt on this story ("Penis spines, pearly papules and Pope Benedict's balls"). He points out the relatively small extent of these features of the chimpanzee penis compared to other primates, and adds detail about the lack of association between their presence and sexual system in hominoids.

    He also reveals a shocking fact: a fairly large fraction of men still have the chimpanzee-like pearly papules.

    Scicurious also takes on the topic "Friday Weird Science: Penis Spines, what are they REALLY?", reviewing the original Osman Hill study of chimpanzee penis morphology. I think the Nature paper is very misleading in its use of galago illustrations for these spines, the chimpanzee version is comparatively minor.

    References

    1. McLean CY, Reno PL, Pollen AA, Bassan AI, Capellini TD, Guenther C, Indjeian VB, Lim X, Menke DB, Schaar BT, et al. 2011. Human-specific loss of regulatory DNA and the evolution of human-specific traits. Nature [Internet] 471:216–219. Available from: http://dx.doi.org/10.1038/nature09774
    2. Dixson AF. 1991. Penile spines affect copulatory behaviour in a primate (Callithrix jacchus). Physiology & behavior [Internet] 49:557–562. Available from: http://view.ncbi.nlm.nih.gov/pubmed/2062934
    3. Harcourt AH, and Gardiner J. 1994. Sexual Selection and Genital Anatomy of Male Primates. Proceedings: Biological Sciences [Internet] 255. Available from: http://dx.doi.org/10.2307/49837
    Tags: 
    chimpanzees
    Neandertal DNA
    mating
    primates

  • Quote: Barbara J. King on anthropology

    Sun, 2011-02-20 16:46 -- John Hawks

    Barbara J. King has written a short essay about why she loves anthropology:

    When I do anthropology, it always starts with agitated questions. No matter how modest my contribution, as I work, I feel connected to anthropologists past and present, people who, in Papua New Guinea or Paris, in Berlin or Boston, trained themselves to see the rhino lumbering in their path. To capture from our peripheral vision something strange and exciting about human meaning-making or its evolution, to move it front and center into our minds and join those minds up with others, is a challenge and a joy.

    Tags: 
    primates
    quotes
    history of anthropology

  • Spider monkey followers

    Wed, 2011-01-26 09:57 -- John Hawks

    Anthony Di Fiore writes in the NY Times "Notes from the field" feature about his work with spider monkeys in Ecuador: "Spider monkey fathers and sons".

    One trick we’ve learned for locating samples is to listen for the buzz of a shiny emerald dung beetle, since they often find a fresh sample within a few moments of it hitting the ground. Conveniently, the dung beetles sometimes roll up a nice bolus of poop, kicking out embedded seeds, which we then shamelessly steal.

    Tags: 
    field sites
    primates

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Neandertals

For years, I've worked on their bones. Now I'm working on their genes. Read more about the science studying these ancient people.

Denisova

From a finger bone of an ancient human came the record of a completely unexpected population. My lab is working on the science of the Denisova genome.

Acceleration

The advent of agriculture caused natural selection to speed up greatly in humans. We're uncovering some of the ways that populations have rapidly changed during the last 10,000 years.

Malapa

Just outside Johannesburg, the Malapa site is producing some of the most exciting finds in human evolution. This site is the headquarters of the Malapa Soft Tissue Project.