200 – 300 MYA, our ancestors were classified as “mammal-like reptiles”. The animals at the earlier end of this spectrum were closely related to reptiles, and those at the more recent end were nearly mammals, with a long continuous transition in between. In popular perception, the differences between these classes are cosmetic. Reptiles are covered in tough flat scales, while mammals have fur. Reptiles sprawl with legs jutting out to the side, unlike mammals with erect legs directly beneath their bodies. Reptilian teeth are uniform and single-cusped, like points on a saw blade. Thankfully, mammals do not flash smiles of pointy triangles.
These traits don’t just make us look better. They evolved together around one invisible but profound adaptation: warm-bloodedness. The term “warm-blooded” does not tell the whole story. It’s not just important that mammalian body temperature is relatively high, but that it’s constant. A reptile is at the mercy of the environment. It is much more vulnerable to overheating or freezing to death if it does not hide in shelter. Except in the worst extremes, a mammal is safe and can stay active in all weather. This advantage is obvious.
Another benefit of warm-bloodedness is chemical predictability. At the cellular level, biology is governed by enzyme reactions. Enzyme activity is temperature-dependent. In a cold-blooded animal, a certain enzyme might be reactive one day and inert the next, abundant one day and then depleted. A well-regulated temperature helps sustain enzymes, and therefore life functions, at a nice steady pace. This allows for much more efficient evolution of the genes regulating enzymes and cell activity.
A reptile’s body heat literally “goes with the flow”. The mammalian body must be able to resist temperature gradients with the outer environment. When it is hot outside, the mammal must be able to release heat. Venting is accomplished by exposure of vessel-rich skin to the air, or the evaporation of small amounts of water from the body. The body must also be able to retain heat in cold weather. That is the most important function of fur, an outer insulator. Sweat glands, body fat, and fur evolved along with warm-bloodedness. They are extremely efficient solutions, as they resist the temperature gradient without expending energy.
In fact, energy is the major cost associated with warm-bloodedness. Pound for pound, a mammal requires five to ten times as much energy (food) as a reptile! 1 That is why warm-bloodedness did not evolve overnight. The body needed time to evolve the capability to catch and digest that much food.
This growing appetite explains certain skeletal trends among the mammal-like reptiles. A reptile does not chew its food. It uses its teeth only to catch and kill prey, which it then swallows whole and lets its digestive system slowly do the rest. A mammal must be able to chew its food to make digestion faster and easier. Jaws strengthened. Teeth became more robust, and they specialized into incisors and molars. Meanwhile, our ancestors developed better legs to forage for food. Vertical limbs can take longer strides and navigate diverse terrain. With sprawling legs, reptiles need more muscular energy just to support their weight. Mammals can save this energy for running.
Soft anatomy, too, was overhauled for higher energy requirements. Muscles improved in strength and stamina. The heart evolved a fourth chamber to keep oxygen-rich blood separated from de-oxygenated blood. A diaphragm developed to assist with breathing. The adaptation that directly caused warm-bloodedness was a simple increase in the number of mitochondria per cell.
Mammal-like reptiles are of particular interest because they were our “bottleneck” ancestors. They survived the P-T extinction, while most tetrapods around them died. This may be testimony to their warm-bloodedness, or it may be simple luck. Whatever it was, nature selected these creatures to pass their traits along into the Mesozoic Era. After recovery, mammals became successful worldwide. Warm-bloodedness is organic climate control. It permitted our mammalian ancestors to thrive in all habitats.
Our ancestors, technically known as synapsids, had a very interesting relationship with bird ancestors, the diapsids. In the Permian Period, synapsids ruled the land as the largest and most diverse animals, while smaller diapsids scurried around beneath their feet. After the P-T event, the roles were reversed. Most synapsids went extinct. The survivors shrank and evolved into tiny mammals, as diapsids grew and evolved into dinosaurs.
The evolution of the earliest mammals, then, was guided by some crucial environmental factors. It was a post-extinction world; life was sparse and the planet was hot. Dinosaurs were unbeatable in their roles as large creatures and carnivores. Rather than competing with dinosaurs for the top of the food chain, Mesozoic mammals specialized in lower niches. They were small burrowers and tree-climbers. Based on their teeth, they mostly ate insects and worms. In turn, they must have made perfect bite-sized snacks for large reptiles, so hiding was a top concern. They turned to a nocturnal lifestyle.
The earliest fossils that are classified unequivocally as mammals are from the late Triassic period, a little more than 200 MYA. Early mammals resembled rodents. They are identified as mammals primarily by features of the teeth and skull. Reptiles have a main jaw bone with a number of secondary jaw bones behind it. During the reptile-mammal transition, the main jaw bone grew and the secondary jaw bones shrank and became detached. The mammalian jaw formed a hinge at a different location, further forward in the skull. Two tiny secondary jaw bones found their way into the middle ear and became adapted for the purpose of hearing! Today, we call them the anvil and the stirrup. They give mammals the ability to hear a much broader range of frequencies than other animals. 2
Good hearing is vital for nocturnal animals. So are smell and touch. Night life requires the ability to detect food, family, friends, and foe by any means possible. Fur and whiskers help animals feel their way around in the dark. Early mammals developed an advanced olfactory (smell) system. The nasal cavities were filled with convoluted structures that captured chemicals from the air. The parts of the brain responsible for processing smell, as well as tactile sensations from whiskers, grew significantly. 3
The word “mammal” itself is borrowed from the mammary glands. It’s impossible to know for sure, but there is reason to believe that nursing evolved right around the time of the first true mammals. 4 The production of milk may have first been useful for keeping eggs moist. 5 The first mammals laid eggs like their reptilian ancestors. Over time, eggs stayed longer inside the mother’s body until eventually the young were born live. 6 Mammals that have a uterus and give live birth are called placentals. Most modern mammals worldwide are placentals. Those that still lay eggs are found only in Australia. Almost all mammals that have a pouch for their young are also in Australia. This is the first sign of continental isolation. Pangaea was starting to disassemble as mammals diversified in the late Mesozoic.
Dinosaurs went extinct at the end of the Mesozoic Era. Mammals, with their competition gone, soon began to flourish and diversify, and went on to become masters of the Cenozoic Era. Birds carried on the dinosaur line. This was yet another role reversal of what had happened at the beginning of the Mesozoic, when dinosaurs had crowded out the mammal-like reptiles!
In the classic Linnaean scheme, humans belong to the class of mammals and the order of primates. Our ancestors evolved into primates right around the Mesozoic-Cenozoic boundary. 7 The earliest primates lived in the northern continents, and it is likely that they originated in Asia. 8 Primitive examples of primates are lemurs and tarsiers, not much larger than the rodent-sized Cretaceous mammals from which they descended. They were omnivorous, eating meat as well as fruits and other plant matter. Early primates lived in trees. Their hands and feet had grasping fingers, which were effective for holding onto branches, catching insects, and picking fruit.
With dinosaurs out of the way, some primates, simians, became active during the day. As daylight gave them much more visual information to process, simians began a trend toward better eyesight. Their eyes were close together in the front of the face. This arrangement narrowed their field of vision but provided good depth perception. Three dimensional vision was very useful for living in a treetop environment, where judging leaps could be a matter of life and death.
The first simians resembled miniature monkeys. 9 One Cenozoic trend was bodily growth. By about 35 MYA, our line the catarrhines emerged. 10 The range of catarrhines was restricted to Africa and Southern Asia. They preferred tropical forests and did not occupy Europe. The continents were far too spread apart by this time for them to make it to Australia or the Americas. The primitive catarrhines are thus known as “Old World Monkeys”.
The word “catarrhine” describes Old World Monkeys’ “downward nose”, to contrast them with New World monkeys, which had nostrils pointed to the sides. Instead of claws, catarrhines had flat nails on all fingers and toes. The catarrhine dentition has come down to us almost unchanged. In each quarter-jaw, there are two incisors, a canine, two bicuspids, and three molars. Early catarrhines retained the sharp, long canine teeth that get their name from dogs. Catarrhines did away with the prehensile tail, which had been almost like a fifth limb for older primates. Primates like lemurs and spider monkeys can use their tails to grasp branches. Catarrhines came up with an even better solution, the opposable thumb. As we know, this proved to be extremely valuable much later for the use of tools and text messaging.
Vision continued to advance. Other mammals see only greens and blues. Catarrhines were sensitive to red light as well. With three primary colors, they were able to perceive a much more vivid picture of the world. Some hypotheses speculate that this trichromatic vision provided valuable advantages in the visually busy world of treetops. In a chaotic canopy of branches, vines, and flowers, it could be useful to discern ripe fruit and leaves from the background clutter. 11 The sense of sight proved so valuable for catarrhines that they downgraded their sense of smell to make better use of skull and brain facilities. 12 That kind of evolutionary tradeoff is not uncommon. When a new feature develops and grows, it often does so at the expense of another.
Being mammal means so much more than just having hair and warm blood. The mammalian brain is more advanced than that of any other animal. Mammals, especially primates, are inherently social. The evolution of pregnancy and live birth created radically new social paradigms, like gender roles and the mother-infant bond. All of these elements of mammalian nature are tied together with complex emotions. The lifestyle of even the lowliest placental mammal, the mouse, is far closer to human than the life of the lizard or fish.
Brains and consciousness were introduced in Chapter 9. The neuron or nerve cell is nearly as old as animal life. Nerves transfer information from one part of the body to another. Individual neurons relay information in the form of electrical signals. Those signals have a two-way interaction with chemicals called neurotransmitters and hormones within the nervous tissue. The brain and spinal cord form the central nervous system (CNS), where signals from various parts of the body converge and interconnect. Response signals originate in the CNS and travel back to the muscles and organs. Some neural circuits operate on auto-pilot, like the circuits that control heartbeat and reflexes. Others are used to integrate the outer senses (sight, sound, smell, touch, and taste) and the inner body senses (hunger, pain, balance, etc) to create a sense of consciousness.
The brain also generates emotions, which mediate the animal’s reactions to the world around it.
What is an emotion? It would be impossible to define emotions to a computer or an animal that did not experience them. Objectively, they are electrochemical signals in particular parts of the brain. Of course, you and I both know that they produce subjective inner feelings, sometimes but not always accompanied by physical changes. We inwardly perceive many emotions as strongly positive or negative, so they guide us toward or away from specific situations. If you love someone, you want to spend time with them, and if you are scared of an animal or an angry person, you will hide or run away. Emotions thus serve the function of making choices for us – sometimes literally life-or-death choices. Many people like to think of emotions as magical or spiritual. Mental feelings are certainly mysterious, but they have physical causes in our own bodies. We also like to think that we are in control of our emotions, not the other way around. In reality, even human beings, the most free-thinking animals in the world, follow our emotional impulses more slavishly than we realize. Our pre-human ancestors’ lives were even more emotionally determined, because their rational faculties were less developed.
The earliest emotions evolved as survival directives. The half-joking summary of reptilian mentality is, “If it’s smaller than me, I’ll eat it. If it’s the same size as me, I’ll mate with it. If it’s bigger than me, I’ll run away.” 13 A reptile’s inner life is exceedingly simple. It does not torture itself over what it should have done yesterday or how to get along with its neighbors. On the other hand, it also does not enjoy the happiness of friendship or a life well lived.
Interestingly, many of the most powerful mammalian emotions involve parts of the brain that already existed in reptiles, and are regulated by neurotransmitters that are also inherited from reptiles. For example, mammals feel a rush of euphoria when the neurotransmitter dopamine is released from one part of the brain’s limbic system into another part. Reptiles also have dopamine and a limbic system, but they do not show any signs of emotional rushes. It seems that we must attribute most of our emotionality to the biological difference between the reptile and mammal brain. There is one key difference.
When you visualize a brain, you probably think of the human brain with its wrinkly exterior. That outer layer is called the neocortex (“new covering”) because it evolved within the last 200 – 300 million years, well after the earliest brains. Although it is only about a millimeter thick, it is highly complex and covers the brain’s entire surface. It is very prominent in humans, smaller and smoother for simpler mammals. Reptiles and birds have a less developed version of the neocortex. 14 It is absent altogether in our more distant cousins, though some evidence suggests that it is the product of genes that had been latent since the Proterozoic Eon (Chapter 9). 15
The neocortex serves at least three crucial functions. It is heavily involved in sensory processing. It may have evolved originally to handle the cognitive demands of the senses of smell, touch, and vision. 16 Second, the neocortex is very effective at motor control. The more complex the movement, the more brain surface is required. Primate hand-eye coordination and facial expressions are especially demanding.
In primates, the neocortex also began to assume a prominent role in social intelligence. As social groups grew, so did the neocortex. 17 It grew much faster than the skull, which is why it began folding and wrinkling to accommodate its increased surface area. The very front tip of the neocortex, the prefrontal cortex, is especially dedicated to social and emotional processing. The richness of primate emotion, then, comes from associating activity in the “reptilian” brain to its social context as understood by the intelligence of the neocortex. 1 The diagram shows how this was just one central theme in the evolution of early mammals and primates.
Reptiles do not have an infancy life phase. They hatch as miniature adults ready to take on the world, and their mothers are nowhere to be seen. As mammals and primates became more biologically complex, it took longer for the young to reach maturity. Infants relied on their mothers’ milk for sustenance, and their large brains needed time to develop. Parental care was absolutely essential for survival.
Basal mammals such as rats exhibit a transitional level of maternal care. A mother rat recognizes her entire litter by smell. She will stay with the litter, but if one infant is removed, she might not notice. She watches over her young just for a few weeks until they can regulate their own body temperature, and then goes her own way for a new litter. 18 On the other extreme, simians spend 25 – 30% of their lives in infancy. These mothers have to look after their children for years or decades.
Something had to compel mammalian mothers to undertake such a responsibility. Although it is difficult for us to believe, it is not a given that an animal mother will care very much about her children. Many egg-laying animals abandon their nests, and some species are known to eat their own young! 19 To a fish, this might make sense. If a mother fish spawns 100 young and is feeling hungry, she can afford to weed out some of the slow weak ones and leave the hardier offspring to survive and carry on her genes.
Compared to egg-layers, a mammal mother does not have very many offspring. Pregnancy is a big deal; it is highly demanding on the body. A female can only carry one or a small number of children at a time. At a minimum, the mother must carry each child to term through a full gestation period, not to mention nursing and other child care, before her next pregnancy. Her body is only up to the task during her window of young adulthood. Even the most fecund mammal like a rodent will have maybe 40 children in her lifetime. A typical monkey will bear about ten. In the wild, each individual child’s odds of success are not very high. For a primate mother, then, every baby matters.
Live childbirth and prolonged infancy created the pressure for maternal love. Evolution will favor the mother who protects her children – and a female who loves her children will protect them. The emotions of love, trust, and bonding between a mother and child are associated with the neurotransmitter oxytocin, which is unique to mammals. It is released during pregnancy and childbirth into the bloodstream of both mother and child, and it assists with nursing. 20 Again, it might strike us as hollow that love is chemical, but in animal studies, motherly devotion can be turned on and off with the flip of an oxytocin switch. 21
In many mammal species, mother / child families are self-supporting social units. For most primates, these single-parent families form the core of larger communities. Our primate ancestors became increasingly social animals through the Paleogene Period. Simians are more social than prosimians, 22 and catarrhines all live in social groups. 23 The exact nature of the social unit varies by species. It depends on other factors in the environment such as availability of food and danger from predators. For instance, monkeys who eat fruit require larger groups than leaf-eaters in order to forage effectively. Most Old World monkeys live in social units with multiple adult females. 24 Extended female relatives help each other raise children and forage for food. Males can help expand the territory or defend it from other families, and keep a look out for predators. 25
Without the hormonal influences of pregnancy, mammal fathers do not get as emotionally attached to their offspring, and do not generally participate in the daily activities of raising their young. All across the animal kingdom, males and females fulfill different roles for their species. The difference derives from the nature of eggs as large (“expensive”) and sperm as numerous (“competitive”) a fact that has been true since before sex cells even had bodies to carry them around. Gender roles are exaggerated by the mammalian life cycle. Since a mother is limited to a relatively small number of children, the best strategy for her genes is to choose mates judiciously and supervise each child safely to adulthood. A competitive male can rely more on the law of large numbers – his genes will become predominant if he sires more children than other males. His options are determined by his social structure and his status within the community.
Social life added a whole new dimension to the challenges of the environment. In addition to predators, primates now had to figure each other out. Cooperation, competition, and compromise were paramount to success. Since the neocortex is so heavily devoted to social skills, it is reasonable to assume that it evolved in response to these needs. Monkeys were able to recognize individuals by facial features instead of by smell. This was doubly important, because facial expressions were sophisticated ways to communicate emotions. Monkeys began to understand concepts such as friendship 26 and fairness 27 , guilt, cheating and punishment. 28 A degree of memory is essential for the social construct of reciprocal altruism. 29 If A does a favor for B, B should remember the favor and return it before too long. If B does not return the favor, then A knows that B is a cheater and will stop giving favors. Monkeys exhibit some innate sense for these rules of reciprocal altruism. 30 It’s not chess, but it’s certainly a step up from reptiles eating their own children.
Male social life is especially dynamic. Female hierarchies tend to be pretty rigid. Males much more frequently engage in competition and change status. Status can have clear survival benefits such as access to the best food, mates, and trees. A male monkey’s status depends on his fighting skills as well as his social skills, 31 conferring benefits for higher intelligence.
Whatever the advantages, the primate brain clearly evolved to reward social success and to avoid social falling. 32 When an individual connects with his peers in a positive way, a burst of neurotransmitters surges through his brain to associate that moment with an emotional reward and to encourage him to do it again. We call it happiness.
- McCluskey, Elwood, “Temperature regulation in tetrapod vertebrates: ectotherms vs. endotherms”, Origins 9(2):98 – 100 (1982), http://www.grisda.org/origins/09098.htm (accessed 4/17/16) ↩
- Heffner and Heffner, “Hearing ranges of laboratory animals”, Journal of the American Association for Laboratory Animal Science Vol. 46 no. 1 (January 2007), http://www.psychology.utoledo.edu/images/users/74/21.JAALAS_Revised.pdf (accessed 4/23/16) ↩
- Rowe et al, “Fossil evidence on Origin of the Mammalian Brain”, Science Vol. 32, Issue 6032 (5/20/2011), pp. 955 – 957, abstract available at http://science.sciencemag.org/content/332/6032/955 (accessed 4/24/16) ↩
- Kielan-Jawarowska et al, Mammals from the Age of Dinosaurs, Columbia University Press (2004), ISBN 9780231119184, Ch. 4. ↩
- Oftedal, “The origin of lactation as a water source for parchment-shelled eggs”, Journal of Mammary Gland Biology and Neoplasia, 2002 Jul;7(3):253-66, http://www.ncbi.nlm.nih.gov/pubmed/12751890# (accessed 4/24/16) ↩
- A species of skink is currently in the process of making this transition! See Stewart et al, “Uterine and eggshell structure and histochemistry in a lizard with prolonged uterine egg retention (Lacertilia, Scincidae, Saiphos)”, Journal of Morphology vol. 271 issue 11 (Nov. 2010) pp. 1342 – 1351, http://onlinelibrary.wiley.com/doi/10.1002/jmor.10877/abstract;jsessionid=B7666761E2EBEC658C4B47DD712EB3C0.d02t01 (accessed 4/24/16) ↩
- Steiper, M., & Seiffert, E., “Evidence for a convergent slowdown in primate molecular rates and its implications for the timing of early primate evolution”, Proceedings of the National Academy of Sciences vol. 9 no. 16 (4/17/2012), pp. 6006 – 6011, http://www.pnas.org/content/109/16/6006.full (accessed 4/25/16) ↩
- Fleagle and Gilbert, “Primate Evolution”, All the World’s Primates, 2011, http://alltheworldsprimates.org/john_fleagle_public.aspx (accessed 4/24/16) ↩
- Beard, Chris, “Searching for our primate ancestors in China”, 1996, http://www.carnegiemuseums.org/cmp/cmag/bk_issue/1996/marapr/beard.htm (accessed 4/25/16) ↩
- Schrago and Russo, “Timing the Origin of New World Monkeys”, Mol Biol Evol (2003) 20 (10): 1620-1625, http://mbe.oxfordjournals.org/content/20/10/1620.full (accessed 4/25/16) ↩
- This is a somewhat controversial hypothesis, but there is evidence in favor of it. See e.g. Osorio and Vorobyev, “Colour vision as an adaptation to frugivory in primates”, Proc. Biol. Sci. 1996 May 22;263(1370):593-9, abstract available at http://www.ncbi.nlm.nih.gov/pubmed/8677259 (accessed 4/25/16) and Dominy and Lucas, “Ecological importance of trichromatic vision to primates”, Nature 410, 363-366 (15 March 2001), abstract available at http://www.nature.com/nature/journal/v410/n6826/full/410363a0.html (accessed 4/25/16) ↩
- The olfactory cortex occupies 65% of total cortex in insectivorous mammals and less than 5% in catarrhines (“Comparison of brain structure volumes in Insectivora and Primates: I. Neocortex”, Frahm et al, “I. Neocortex” and Stephan et al, “II. Accessory olfactory bulb (AOB),” both from SJ. Hirnforsch. 23, 1982). The visual cortex occupies as much as 50% of the total neocortex in some catarrhines (Barton, “Visual specialization and brain evolution in primates”, Proc. R. Soc. B 265, 1933-1937, 1998, doi:10.1098/rspb.1998.0523). ↩
- The earliest version of this statement I can find online is by Midas Dekkers in his 1994 book Dearest Pet: On Bestiality, Verso, 1994, ISBN 0 86091 462 3, p. 32. ↩
- Ulinski, Philip, “The Cerebral Cortex of Reptiles”, Ch. 5, Comparative Structure and Evolution of Cerebral Cortex, Part I, Springer Science + Business Media New York (1990), p. 139, http://link.springer.com/chapter/10.1007/978-1-4757-9622-3_5 (accessed 5/07/16) ↩
- Tomer, R; Denes, AS; Tessmar-Raible, K; Arendt, D (2010). “Profiling by image registration reveals common origin of annelid mushroom bodies and vertebrate pallium”. Cell 142 (5): 800–809. ↩
- Rowe et al, “Fossil evidence on Origin of the Mammalian Brain”, Science Vol. 32, Issue 6032 (5/20/2011), pp. 955 – 957, abstract available at http://science.sciencemag.org/content/332/6032/955 (accessed 4/24/16) ↩
- Dunbar, Robin (1992), “Neocortex size as a constraint on group size in primates”, J Hum Evol 20:469-493, https://www.researchgate.net/publication/222461154_Neocortex_Size_As_A_Constraint_On_Group_Size_In_Primates (accessed 5/14/16). ↩
- Broad et al, “Mother-infant bonding and the evolution of mammalian social relationships”, Philos Trans R Soc Lond B Biol Sci. 2006 Dec 29; 361(1476): 2199 at 2201 , http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1764844/ (accessed 5/11/16) ↩
- Klug and Bonsall, “When to care for, abandon, or eat your offspring: the evolution of parental care and filial cannibalism,” The American Naturalist 2007 Dec;170(6):886-901, doi: 10.1086/522936, http://www.ncbi.nlm.nih.gov/pubmed/18171171 (accessed 5/04/16) ↩
- Broad et al, “Mother-infant bonding and the evolution of mammalian social relationships”, Philos Trans R Soc Lond B Biol Sci. 2006 Dec 29; 361(1476): 2199–2214 , http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1764844/ (accessed 5/11/16) ↩
- van Leengoed, E., Kerker, E., and Swanson, H. H. (1987). “Inhibition of post-partum maternal behaviour in the rat by injecting an oxytocin antagonist into the cerebral ventricles”. J. Endocrinol. 112, 275–282, http://joe.endocrinology-journals.org/content/112/2/275.abstract (accessed 5/15/16). Olazábal DE, Young LJ. “Oxytocin receptors in the nucleus accumbens facilitate “spontaneous” maternal behavior in adult female prairie voles.” Neuroscience. 2006 Aug 25;141(2):559-68. Epub 2006 May 24, http://www.ncbi.nlm.nih.gov/pubmed/16725274 (accessed 5/15/16) ↩
- Tomasello and Call, Primate Cognition, Oxford University Press, 1997, ISBN 0-19-510623-7, eBook edition location 195. ↩
- Tomasello and Call, Primate Cognition, Oxford University Press, 1997, ISBN 0-19-510623-7, eBook edition location 209. ↩
- O’Neil, Dennis, “Social Structure”, Palomar College Biological Anthropology Tutorials, 2012, http://anthro.palomar.edu/behavior/behave_2.htm (accessed 5/15/16) ↩
- At least indirectly while keeping a jealous lookout for competing males. See Baldellou and Henzi, “Vigilance, predator detection and the presence of supernumerary males in vervet monkey troops”, Animal Behaviour vol. 43 issue 3 (March 1992) 451 – 461, abstract at http://www.sciencedirect.com/science/article/pii/S0003347205801046 (accessed 5/21/16) ↩
- Young et al, “Responses to social and environmental stress are attenuated by strong male bonds in male macaques”, PNAS vol. 111 no. 51, 11/11/14, http://www.pnas.org/content/111/51/18195.abstract (accessed 5/21/16) ↩
- Van Wolkenten et al, “Inequity responses of monkeys modified by effort”, PNAS vol. 104 no. 47, 11/20/2007, http://www.pnas.org/content/104/47/18854.full.pdf (accessed 5/21/16) ↩
- Hauser, “Costs of deception: cheaters are punished in rhesus monkeys (Macaca mulatta)”, PNAS vol. 89 no. 24 (12/15/1992) 12137 – 12139, http://www.pnas.org/content/89/24/12137.short (accessed 5/21/16) ↩
- Trivers, R.L. (1971). “The evolution of reciprocal altruism”. Quarterly Review of Biology 46: 35–57. doi:10.1086/406755, https://www.researchgate.net/publication/230818222_The_Evolution_of_Reciprocal_Altruism (accessed 5/22/16) ↩
- Seyfarth and Cheney, “Grooming, alliances and reciprocal altruism in vervet monkeys”, Nature 308, 541 – 543 (4/05/1984), doi:10.1038/308541a0, http://www.nature.com/nature/journal/v308/n5959/pdf/308541a0.pdf (abstract accessed 5/21/16) ↩
- Cawthon Lang KA. 2005 July 20. Primate Factsheets: Rhesus macaque (Macaca mulatta) Behavior . http://pin.primate.wisc.edu/factsheets/entry/rhesus_macaque/behav . Accessed 2016 May 21. ↩
- See several examples cited in Buss, David, “The Evolution of Happiness”, American Psychologist Vol 55 no. 1, 15 – 23, January 2000, http://people.uncw.edu/bruce/psy%20292/pdfs/happiness.pdf ↩
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