Carnivora (Land and Marine Carnivores)
CarnivoraFamily: Dogs, Wolves, Coyotes, Jackals, and Foxes
Dogs and Cats
Family: Raccoons and Relatives
Family: Weasels, Badgers, Skunks, and Otters
Family: Civets, Genets, and Linsangs
Family: Mongooses and Fossa
Family: Aardwolf and Hyenas
Family: Eared Seals, Fur Seals, and Sea Lions
Family: True Seals
(Land and marine carnivores)
Number of families 12
Number of genera, species 114 genera; 264 species
The order Carnivora is one of the 20 orders of mammals. The Carnivora are a diverse group of animals, living in almost any habitat, including the oceans, with over 260 species. Most carnivores are land animals, some like the otters spend much of their lives in water and about 30 species, the seals and their relatives, are marine, only leaving the sea once a year to breed.
Despite the name, not all Carnivora live exclusively on meat. Bears, jackals, and foxes are omnivorous, surviving on a diet of meat and fruits, the aardwolf eats almost nothing else but termites, and the giant panda lives almost entirely on bamboo shoots. The unifying feature of the Carnivora is a set of scissor-like teeth set back in the mouth and used for shearing through meat, called the carnassials. However, it is not quite as simple as that, as some of the modern day Carnivora like the giant panda and the aardwolf do not possess carnassials. It is sufficient qualification for a species to be included in the Carnivora if its evolutionary ancestors did.
Eating meat has many advantages over a diet of vegetable matter, particularly grass. Meat is easy to digest and rich in protein. On the other hand, meat is more difficult to procure than vegetable matter. Swift-footed and wary prey have to be caught and killed before they can be eaten and "static meat" in the form of carrion is usually widespread and scarce. It is not surprising, therefore, to find that carnivores are often highly intelligent animals with sophisticated social systems.
Evolution and systematics
The evolutionary history and systematics of the Carnivora are clouded in controversy, as the fossil record is patchy and incomplete. In spite of this limitation it is remarkable what paleontologists, evolutionary biologists and geneticists have managed to uncover in the way of the early history of mammals. A major breakthrough has been the development of accurate methods to date fossils.
About 65 million years ago (mya) the dinosaurs, which were the dominant animals on Earth, underwent a rapid and mass extinction. At this time the mammals were small shrew-like creatures. With the extinction of the dinosaurs many ecological vacancies, known as niches, opened up, including that of predator, and the mammals quickly filled many of them. The early mammalian predators were marsupials, mammals whose young develop in a pouch, the ancestor of which was a small, opossum-like creature with a pointed snout and large ears. These early marsupial carnivorous creatures soon evolved into all shapes and sizes and dominated the southern continents for 30 million years.
Meanwhile, placental mammals were evolving in the northern continents. Instead of their young developing in a pouch after being born, placental mammals grow their young inside them, in a womb. One of these placental mammals was a squirrel-sized creature called Cimolestes that lived on insects. A very important feature possessed by Cimolestes was a flattening of the cheek teeth providing the beginning's of a
scissor action. Over several millions of years these teeth became refined to slice meat in what became the carnassial shear. This feature was inherited by two separate groups of animals. One gave rise to the modern Carnivora, the other to a group known as the Creodonts. At first the Creodonts dominated as the earth's meat eaters. In the fossil record from 55 to 35 mya a number of cat-, dog-, bear- and hyena-like animals are found, some even with saber teeth, but none of these were true Carnivora. Then the fossil record shows a change; more Carnivora species are found and fewer and fewer Creodonts.
It is not known for sure why this replacement of Creodonts by Carnivora took place. The carnassial shear in the Carnivora was situated more to the front of the mouth than in the Creodonts. This meant that the teeth further back in the mouth could still be used for feeding on other foods, for example on vegetable matter. Perhaps the Carnivora could be more flexible in diet and therefore exploit more ecological niches, both meat eating and vegetable, than the Creodonts, who had no teeth behind their carnassial shear and so could only eat meat. Support for this idea comes from evidence of climatic change during the demise of the Creodonts. The earth became cooler and more seasonal. This may have led to a situation where prey became less available, but fruit crops and insects more abundant due to the seasonal bloom.
The early Carnivora, known as miacids, were small and rather unspectacular, many resembling the genets of today. The major division into dog- and cat-like Carnivora took place some 55 mya and all the modern carnivore families had evolved by 7 mya. Among the cat-like Carnivora were the sabertoothed cats that dominated the carnivore scene from 26 to 2 mya. As the Carnivora moved south, they out-competed the marsupial predators mentioned earlier. Today, only a handful of their descendants such as the Tasmanian devil and quoll survive in Australia. Perhaps the best known was the thylacine or Tasmanian wolf that was exterminated about 70 years ago by bounty hunters.
Traditionally Carnivora are divided on the basis of their anatomy and behavior into two suborders, terrestrial carnivores (Fissipedia) and marine carnivores (Pinnipedia). This subdivision is incorrect, for blood serum analyses have shown that the pinnipeds are closely related to bears and evolved from a single bear-like ancestor. Today most scientists involved in the field of carnivore classification recognize 10 families in the two major divisions; the cat-like and the dog-like Carnivora. The former are the Viverridae (civets and genets), Herpestidae (mongooses), Felidae (cats) and Hyaenidae (hyenas). The latter are the Ursidae (bears), Otariidae (eared seals—fur seals and sea lions—although the most recent classification puts the walrus in a separate family, the Odobenidae), Canidae (dogs), Procyonidae (a collection of mainly South American carnivores including the raccoons and coatis and a taxonomic group that is still surrounded by much controversy), Mustelidae (otters, badgers, skunks, weasels and polecats), and Phocidae (true seals—elephant seals, monk seals, leopard seals, etc.).
Carnivores come in all shapes and sizes, ranging in size from the 1.76 oz (50 g) least weasel (Mustela nivalis) to the 48,000 times heavier, 5,300 lb (2,400 kg) southern elephant seal (Mirounga leonina). Most are so distinctive that even laypeople can easily distinguish the various families, even though the order has considerable diversity. Bears, dogs, hyenas, mongooses, martens and weasels, cats, and even viverrids are readily recognizable, although the marine families and procyonids are more difficult to tell apart.
The pinnipeds have streamlined, oval-shaped bodies with limbs modified as flippers. Terrestrial carnivores either walk on the soles of their feet (plantigrade) or on their toes (digitigrade). The limbs of evolutionarily ancient carnivores underwent a fusion of bones in the feet that probably originally provided a firm basis for flexion at the midcarpal joint. This gave them the flexibility to climb, grapple with prey, or absorb the shock of running and leaping. Another skeletal characteristic is an undeveloped collar-bone or clavicle. The main function of the well-developed clavicle in primates is to allow attachment of muscles to give the necessary flexibility of lateral movement to the limbs. This is not necessary for the back and forth movement of the limbs needed for a long stride for running as is the case for most carnivores. With the exception of the hyenas, carnivores possess an elongated penis bone known as the baculum to prolong copulation. This is probably especially important in species where ovulation is induced by copulation. Modified skin glands often located in the anal region secrete substances as a means of communication and information exchange between members of the same species.
The typical dental formula for carnivores is (I3/3 C1/1 P4/4 M3/3) × 2 = 44, with variation in the number of molars and premolars. The canines are usually large and the carnassial
shear, the modified fourth upper premolar and the lower first molar, with high cusps and sharp tips, is adapted to cutting and slicing meat. The typical carnivore skull shows a powerful jaw for the capture of prey and tearing up of meat, and the skull often has a sagittal and/or occipital crest to enlarge the area for muscle attachment.
Carnivores are found throughout the world, although many islands do not have indigenous populations. Antarctica and Australasia have no naturally occurring terrestrial carnivores, although the dingo (Canis familiaris dingo) has lived in Australasia for at least 3,500 years, having been brought there by Asian seafarers. Introduced carnivores, feral cats in particular, occur on many islands and are often a conservation management problem as they prey on indigenous fauna naive to predation.
Carnivores have a very wide habitat tolerance and are found in all habitats both on land and sea. Only the tops of the highest mountains, the most extreme deserts and ocean depths are devoid of carnivores. Although terrestrial carnivores spend most of their time on the ground, leopards, (Panthera pardus) and martens (Martes spp.) are adept at climbing trees, otters are at home in rivers and lakes, polar bears (Ursus maritimus) live much of their lives on sea ice, and the least weasel is able to hunt underground or under snow. The marine carnivores breed on land and forage in the sea. The elephant seal can stay under water for up to two hours and dive to a depth of 5,000 ft (1,500 m).
The large number of species, wide habitat tolerance, diverse diets and well developed brains of carnivores have combined to lead to the evolution of a wide range of behaviors and social systems. Only the higher primates have more complex behavior patterns and social systems than the social carnivores. This flexibility in behavior within the order can be seen between species and, perhaps most interestingly, within a species as it adapts to different environmental demands.
Many carnivores are solitary in that when they move about looking for food they do so on their own, or at most as a mother with her dependent offspring. However, detailed studies of theses so-called solitary species have revealed that although they may appear to be solitary, they share a territory
with others of their kind and cooperate and communicate with their fellow group members.
The civets and genets (Viverridae) are a good example of the solitary template for carnivores from which the array of social systems seen in the order probably evolved. Solitary males live in comparatively large territories that encompass the smaller territories of several females. However, with palm civets, subordinate, usually younger males, occupy small areas within the dominant male's territory, avoiding contact with the dominant male as he moves through the area.
Bears are also mainly solitary, however, flexibility in behavior allows concentrations of brown and polar bears to collect at food sources. For example, brown bears (Ursus arctus) gather during the salmon migration on the northwestern seaboard of North America and polar bears may gather at a whale carcass in the Arctic Circle. Somewhat surprisingly, polar bears also concentrate during times of food shortage. During summer and fall when the ice has broken up, a number of males may fast together in peace at certain preferred sites along the coast. Testosterone levels are low and there is no food to compete for.
Many of the 37 species of cat are truly solitary and only one, the lion (Panthera leo), is highly social. Lions live in prides of 2–12 related females and their young. The members of a pride do not stay together all the time but they defend a common territory and are friendly towards each other when they meet. Males form coalitions, usually of 2–4, but up to 7. Males join prides, but their tenure is variable and they may be displaced by a stronger coalition, or themselves move on to another pride. Pride and territory size is variable with respect to resources, as is the association between the females and the males. In open areas males spend much time with the pride, probably because the females and their cubs are more detectable by strange males that may kill the cubs. In wooded savannas the males can leave the pride and look for other females to mate with when the cubs are quite small. It is easier to hide them from infanticidal males in the thicker bush. The cheetah (Acinonyx jubatus) is the only other cat whose adult individuals form long-lasting relationships, in that cheetah males also form coalitions of 2–3 individuals that cooperatively defend a territory.
The basic social system of dogs is different from the cats and is based on monogamy. However, canids show far more flexibility in their social systems both within and between species than the cats. African wild dogs (Lycaon pictus) and dholes (Cuon alpinus) live in very tight and close knit packs that always hunt together, but where the alpha pair are the breeders. Gray wolves (Canis lupus) may do the same, or go off in pairs. Red foxes (Vulpes vulpes) often forage alone, but may not always maintain a monogamous mating system and sometimes a territory may comprise one adult male and several vixens.
The mustelids, with over 50 species, are the largest carnivore family. They appear to be predominantly solitary, although sea otters may occur in "rafts" of several hundreds. The European badger (Meles meles) is one of the best studied carnivores and illustrates the fact that so many carnivores are
"blatantly solitary, but secretly social rafts" and has evolved a tendency to live in groups. Several badgers, mainly close relatives, may share a territory and live together in a large set, but forage on their own. The honey badger (Mellivora capensis) is another species that has been found to have a rather different social system than was thought before a detailed study was conducted. This time what was considered to be social, is in fact a solitary trait. Although sometimes seen traveling in pairs, a larger male and smaller female, these are not mated pairs, but mother and son. The single cub is dependant on its mother until it is larger than her. Males do however sometimes come together in groups of up to six and have very large overlapping home ranges when the solitary living females come on heat.
Mongooses show a very wide diversity of social systems. Most tend to be solitary, but three species, banded mongoose (Mungos mungo), dwarf mongoose (Helogale parvula), and meerkat (Suricata suricatta) have evolved complex and different social systems. In dwarf mongooses the dominant pair are most likely to breed, whereas in the banded and meerkat groups several females do so. One of the larger mongooses, the nocturnal white-tailed (Ichneumia albicauda) is another seemingly solitary species that exhibits a degree of sociality as several females have been found to have overlapping ranges.
The hyenas, with only four species, are the smallest carnivore family. Three species, the brown hyena (Parahyaena brunnea), spotted hyena (Crocuta crocuta), and aardwolf (Proteles cristatus) have been well studied and have shown a remarkable degree of diversity and flexibility in social systems. The spotted hyena is highly social living in female dominated clans of 5–80 individuals, living in fiercely defended clan territories that may be as large as 400 mi2 (1,000 km2), or as small as 16 mi2 (40 km2), depending on resources. In the Serengeti, with its migratory prey system, the clan system is flexible so that the hyenas can commute from their territories through other hyena territories to get to the feeding grounds. The brown hyena always forages on its own yet may share a territory with as many as 14 other hyenas. All clan members carry food to the den to feed cubs, not just the parents.
The aardwolf is monogamous, yet during the mating season some males may be cuckolded by their mates, who may copulate with neighbors.
The procyonids have not been well studied, and although some species like the ringtail appears to be solitary, most appear to move in large groups. It is thought likely that all species maintain complex social relationships within and among the sexes. This is an important family for understanding sociality in carnivores and more studies are needed.
Why these differences in social system, and particularly why do some species form groups? An obvious answer is that carnivores form groups in order to cooperate in hunting. While this may be partly true, it does not explain why, for example, invertebrate-eating meerkats are so social. Even in the case of large prey hunters like lions and spotted hyenas it has been found that hunting group size is not necessarily related to hunting success, nor that this strategy leads to the acquisition of more food than solitary hunting. For the smaller species it has been suggested that being in a group helps prevent predation by increased vigilance and cooperative defense. While this is also sometimes true—meerkat individuals take turns in guarding while the rest of the group is foraging—it does not explain why other species like European badgers, red foxes, and brown hyenas forage solitarily yet sometimes live in groups.
The evidence suggests that these and many other group-living carnivores are influenced by the dispersion pattern of their food. For many carnivores, food is often irregularly dispersed in patches and some patches moreover are richer than others. Territory size is influenced by the distance between the patches, and the number of animals living in the territory by the richness of the patches. This is known as the Resource Dispersion Hypothesis (RDH) and has been found to explain group size and territory size in a number of carnivores. It also explains why group size and territory size are not related. A group of brown hyenas living in an area with a large number of rich food patches close together will have a small territory and contain more members than one living where food patches are poor and widely dispersed. Similarly, in conditions where food patches are poor but close together both group size and territory size will be small. Once there is enough food in a territory to support several individuals it makes sense to share these with close relatives rather than a bunch of strangers. Any coincidental benefits that accrue then will be shared by relatives and also they can assist each other, for example by helping to feed each others young. For lions, the major advantage accruing to females living in the pride is the cooperative defense of their cubs against infanticidal males.
Feeding ecology and diet
Carnivores do not eat only meat. In fact they have a varied diet and comparatively few are exclusively meat eaters. Some, such as the bamboo specialist giant panda (Ailuropoda
melanoleuca), the frugiverous palm civets, kinkajou (Potos flavus), and raccoons, hardly ever eat meat. Mustelids are probably the most exclusively meat eating family, weasels and their allies being known as fierce and combative predators capable of killing prey up to 10 times their body weight and otters living mainly of fish, crayfish, crabs, and frogs. However, European badgers rely mainly on earthworms. Mongooses live mainly off insects, although some species are known as snake killers. Cats too are mainly carnivorous, the large cats are probably the most spectacular of all predators. Bears, viverrids, dogs, and hyenas are more omnivorous, although all, except viverrids, have meat-eating specialists amongst their ranks. Polar bears, African wild dogs, and spotted hyenas rarely divert from a meat diet, but brown bears, brown hyenas, and jackals are all truly omnivorous. The aardwolf is another strict specialist feeding almost exclusively on snouted harvester termites of the genus Trinervitermes. The marine carnivores feed on a variety of marine animals including fish, mollusks, crustaceans, penguins and, particularly in the case of the leopard seal, other seals. The world's most abundant mammal after humans, the crab-eater seal (Lobodon carcinophagus), feeds mainly on krill, and one of the giants, the walrus (Odobenus rosmarus), mainly eats mollusks.
A characteristic of most of the food eaten by carnivores is that it is of high quality, but difficult to obtain, therefore, they have to make full use of their opportunities. Many carnivores live under what has been called a feast or famine regimen. They are able to gorge themselves when the opportunity is presented, a spotted hyena can eat a third of its body weight in one sitting, and are also able to go for long periods without eating. Hibernating bears are the most extreme in this regard and are able to survive for half a year without eating,
drawing on fat reserves built up during the bountiful summer. If more food is found than an individual can consume, some species will cache the remains. Brown hyenas will scatter hoard ostrich eggs under bushes and in thick grass clumps should they find an unattended nest. Canids actually bury their excess food and show an uncanny ability for relocating it.
The impact that predators have on their prey is a complicated subject of great controversy and emotion as it often clashes with our own interests. Certain important principles need to be taken into account. Predators do not kill at will, or even the first prey they come across. They have to pit their skills and stamina against formidable opponents. The kill is the culmination of a range of behavioral strategies that may have taken hours or even days to succeed. The relationship between predator and prey is a delicate balance, an evolutionary arms race, where neither has managed to gain the upper hand. Ecologically speaking, predation is an important process that contributes to the dynamic nature of ecosystems. Predators help to keep prey numbers in check and often to dampen drastic fluctuations. They may weed out the less fit members of the prey population by selecting the old and infirm. They also often select males over females from the prey population, thereby lessening their impact as most prey species are polygamous; i.e. one male mates with several females. Furthermore, the impact they have on the prey populations is often mediated by environmental conditions such as droughts in Africa and severe winters in north America and Europe.
Mating systems are the most complex and variable aspects of social behavior. Carnivores give birth to altricial young that are dependent on adults for their survival for an extended period. Much of their behavior is therefore centered around not only producing young but also raising them. There are two basic types of mating system in carnivores; monogamy where a male mates with one female, and polygyny where males mate with several females and/or vice versa. Monogamy is the least common of the two systems and is practiced by all canids, and also in the aardwolf and some mongooses, although in most species the rules are broken. Either a male attracts more than one female to the territory, or cuckoldry occurs. Monogamous systems are characterized by both sexes and often older offspring helping to raise young by feeding and guarding, and by a lack of sexual dimorphism. An extreme case is found in pack living animals such as African wild dogs and dwarf mongooses where normally only one pair breeds while the other sexually mature adults abstain and help to raise the young. In polygynous species the males are usually larger than the females and often are equipped with spectacular adornments to attract females, like the lion and elephant seal. Cooperation in raising young is less common but does occur in some social polygynous species, for example female lions suckle each others' cubs.
Many carnivores range widely and spend their time alone and so it is important for the females to advertise when they are ready to mate. Scent marking through urination and anal secretions is widespread in carnivores, and is the obvious mechanism to achieve this. Even then the best male may have difficulty in being at the right place at the right time. One way that a female can ensure that she mates with the best available male is to adopt a reproductive strategy known as induced ovulation. The females come into estrus, but do not shed eggs until stimulated to do so by copulation. The other strategy is called spontaneous ovulation, where the eggs are shed in a cycle that is unaffected by mating. Although there are exceptions, spontaneous ovulators are likely to be more social species than are induced ovulators.
Smaller animals have faster metabolic rates and breed faster than larger animals. The females of the smallest carnivore, the least weasel, are sexually mature at three months. Litter size is usually six, so if she lives long enough—the average life expectancy is less than one year—a female can potentially produce 30 descendants a year. This is achieved by producing six in her first litter, another six in her second, plus six offspring from each of the three daughters she would be expected to produce in her first litter. Males are not sexually mature in their first year. At the other end of the scale, lions may only produce a litter of three or four cubs in three and a half years as the cubs only become independent at about three years of age. If, however, the female loses all her cubs she will quickly come into estrus again. African wild dogs have higher metabolic rates than would be predicted from their size and their populations turn over rapidly. This is reflected in their high reproductive potential. They are seasonal breeders that produce large litters, the record is 21 for a single female.
Pinnipeds, so well adapted to a life in the sea, must come to breeding grounds on land in summer in order to reproduce. The males arrive slightly earlier than the females and set up territories. The females arrive shortly before giving birth to a single pup that was conceived the previous season. The lactation period is very short and intense, not more than six weeks, in the true seals. The pups are weaned and deserted abruptly and the females mate, before going back to sea for another year. In eared seals, the female comes into season and mates about one week after giving birth. Lactation lasts 4–6 months during which time the mother makes periodic feeding forays into the sea.
Conservation is the wise use of resources on a sustainable basis. The Species Survival Commission of the World Conservation Union (IUCN) is divided into a number of taxonomic or functional specialist groups. In the case of carnivores these are mainly based on families such as the Canid, Cat, and Hyena Specialist Groups. These groups have produced a series of status surveys and action plans that assess the conservation status of the relevant species and make recommendations for their conservation. The conservation status of each species is assessed and placed into one of a number of categories depending on its status, the most important of which are Extinct, Extinct in the Wild, Critically Endangered, Endangered, and Vulnerable. The 2002 IUCN Red List of Threatened Species lists 120 carnivores of which three, the Falkland Island Wolf (Dusicyon australis), the sea mink (Mustela macrodon), and the Barbados raccoon (Procyon gloveralleni), are classified as Extinct; one, the black-footed ferret (Mustela nigripes), as Extinct in the Wild; and five, the red wolf (Canis rufus), the Ethiopian wolf (Canis simensis), the Iberian lynx (Lynx pardinus), the Mediterranean monk seal (Monachus monachus), and the Malabar civet (Viverra civettina), as Critically Endangered. Thirty-two species comprising three viverrids, five mongooses, four cats, one bear, two eared seals, one canid, eight procyonids, seven mustelids, and one true seal are classified as Endangered, and 40 including five viverrids, four mongooses, 12 cats, three bears, five eared seals, two canids, eight mustelids, and one true seal are Vulnerable. The only family with no members classified as Endangered or Vulnerable is the small hyena family. The remaining listed species are classified as either Data Deficient (19), meaning that there is not enough known about them to be sure of their status, or as Near Threatened (20), which is the lowest category of threat. Thus, besides the three recently extinct species, nearly half the living carnivores are under some sort of threat of extinction and 65% are Endangered or Vulnerable. Clearly, the situation is serious. Although protected areas are a vital component of the conservation action plans, many species and populations do not enjoy the security of protected area management and other ways need to be found for humans and wildlife to coexist. Innovative solutions such as protecting livestock from predation, the use of guard dogs to protect livestock, and education of local people have met with limited success.
Significance to humans
Human relationships with carnivores are extreme and of mixed emotions. On the one hand we respect and revere them. Indeed two species, the wolf and the wild cat, have been domesticated and become our closest animal companions. In the case of the domestic dog, we have also developed and trained many breeds to work for us as hunting dogs, herd dogs, and guide dogs. Carnivores are also important to us aesthetically and economically. We admire their hunting ability and their striking beauty. Many symbols of royalty and heraldry are carnivores. They are a prime attraction for ecotourists, especially where they can be viewed in their natural habitat. Through the ages man has also hunted carnivores for food, medicine, and their pelts, and today they are also hunted for recreational purposes as trophies, often at great expense.
On the other hand, humans and carnivores have long been in conflict because of similar ecological interests. Our ancestors on the African plains competed for food with the larger carnivores. With the development of agriculture and animal husbandry this conflict increased as carnivores of all sizes tended to prey on animals that we had domesticated and were important for us economically. In addition, large carnivores sometimes kill people. Animals that compete most with each other display most aggression towards each other. Moreover, the larger and more powerful ones have a negative impact on the smaller and less powerful competitors—lions influence cheetah and wild dog numbers and wolves impact on coyotes. Humans as the supreme carnivorous animal (not carnivore, which is a taxonomic term) have impacted all their competitors and carnivores have suffered from the brutal and efficient actions of humans as much as, if not more than, any other group of animals. With the human population explosion and the development of more efficient mechanisms for killing, this carnage has accelerated: shooting, trapping, poisoning, and over harvesting have taken a very heavy toll on many carnivore species. Even through the domestication of dogs and cats, their wild ancestors are threatened through crossbreeding with them and spreading disease.
In an attempt to redress the imbalance, a network of governmental and nongovernmental organizations have been established throughout the world and millions of dollars have been spent and are being spent on research, protection and management programs, compensation schemes, and education. Although there have been some successes the situation is serious and a major human effort is required if more of these magnificent and important animals are not to go the same way as the Falkland Island wolf, the sea mink, and the Barbados raccoon.
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Gus Mills, PhD