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Post by brobear on Mar 22, 2017 11:32:43 GMT -5
The one thing that stands out instantly about the physical build of a grizzly is that here is a mammal designed for brute force. A more thorough study will show that for so cumbersome an animal, the grizzly is also surprisingly quick and agile. www.allgrizzly.org/front-limbs Front limbs Of all the morphologic features that typify bears, the front limbs and associated skeletal infrastructure are the most distinctive. They are also diagnostic of the bear life strategy (see Life strategy). No other terrestrial vertebrate of its size--certainly no other large carnivore--has front limbs that are as flexible, powerfully built, and mounted with such dexterous paws. Nor do any comparable-sized carnivores have such out-sized claws...claws which are clearly "designed" to be powered by the muscular arms and shoulders to either climb trees, extract food from a durable matrix (i.e., dig), or grapple with and subdue large prey such as seals, moose, and elk. What follows is a summary of the evidence produced over the years elaborating on and substantiating the preceding thumbnail sketch. You will have to forgive me for the abundance that follows, but it is reflective of the extent to which I see this aspect of bear morphology as key to understanding the overall bear life strategy--as well as niche. The results at left come from a seminal paper published by Alberto Martin-Serra and his colleagues in 2014. His analyses delve into patterns of variation in post-cranial skeletal structure among carnivores. Some of the results are relatively arcane, as is often the case in morphology. But the overall results insofar as bears are concerned are pretty straight-forward. Each of the graphs at left represents major coordinated trends in the shape of two important front-limb bones--the radius at top, and the scapula (or shoulder blade) in the bottom two graphs. This variation is after accounting for the effects of body size, as such. Each dot represents an individual specimen, with those of bears denoted by brown dots and those of all other carnivore species by gray dots. The bones illustrated along each axis portray the shape that is associated with extreme values along each gradient. "PC I" invariably represents the dominant theme of variation, "PC II" the secondary theme, and "PC III" the tertiary theme. The typifying characteristics of the quadrant occupied by the bear specimens are described in the corresponding corner. The key results here are: first, that bears are clearly differentiated from all other carnivores when it comes to shape of the radius and scapula, especially so for the scapula; and, second, that the main differentiating theme is captured by the term "robust." In other words, even after controlling for the effects of body size, bears have stout and strongly built front limb bones, most remarkably so in the case of the scapula. Which implies that these robust bones are built to support disproportionately strong front-limb muscles. Which further implies that the "bear niche" is typified by activities that require deployment of strong front legs anchored to a strong scapula. The graphic above (thanks to a 1949 paper by Dwight Davis) emphasizes the concluding point made at left, more specifically that the stout skeletal infrastructure of the front limbs in bears--most especially the scapula--is host to proportionately robust muscles. Each color in the bars above corresponds to the proportion of all muscle mass in the shoulder and arms of lions (Panthera), wolves (Canis), and bears (Ursus) consisting of the muscle group identified to the right of the three bars. The key point here is that scapular muscles (anchored to the scapula) account for a disproportional amount of muscle mass in bears. This is especially true for the brown or grizzly bear, which accounts, in part, for the "hump" that typifies this species.
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Post by brobear on Mar 22, 2017 11:33:31 GMT -5
Continued.... Dexterity The figures above illustrate the flexibility (or dexterousness) of the forelimb in bears. The bar-shaped summary values at upper left derive from the work of Andy Iwaniuk, who devised a scheme for scoring dexterity and then applied it to a number of carnivore species through direct observations of zoo animals. The result of his work is pretty obvious. Bears (i.e., Ursids) are by far the most dextrrous of large carnivores, whether reckoned in terms of "distal" or "total" forepaw dexterity--comparable to the dexterity of the much smaller procyonids (e.g., raccoons). This is remarkable given that the bears have to support much more weight on their front legs and paws than do raccoons and their relatives.
The results above right relate pretty directly to the idea of "dexterity." Here, based on the work of Ki Andersson, main themes in variation of the shape of the ulna (upper arm bone) where it intersects the elbow joint are plotted against body mass, again restricted to carnivores. Each dot represents a species, with bears denoted by brown dots, canids (dogs) by dark red dots, and felids (or cats) by the orange dots. "GB" denotes "grizzly bears" (Ursus arctos). The key distinction noted in this graph is between the "cursors" and "grapplers"--the cursors being those that are built to run long distances efficiently and the grapplers those built to grab and subdue large prey after a short rush. Or, perhaps, simply dig roots. Bears are clearly among the grapplers, which is wholly consistent with a dexterous forelimb and paw.
Olecranon process of the ulna Size and shape of the olecranon process have been singled out by morphologists as being especially indicative of how animals move and, related to that, the basic configuration of their posture. The olecranon process is at the end of the ulna, where it is an integral part of the elbow as well as the anchor for the triceps muscles (the ulna is paired with the radius in the lower arm). The relationship of the olecranon process to the "semilunar notch" and to the main axis of the ulnar bone are particularly signifying. The semilunar notch is the half-sphere hollow in the side of the ulna that hosts the end of the humerus. This is where the radius and ulna pivot around the end of the humerus, which is the upper arm bone.
A lesser capacity for sustained speed and a greater capacity for bearing the load of a heavy body are indicated by: (1) a relatively short olecranon process; and (2) a wide angle between the main axis of the ulna below the notch and the main axis of the olecranon process--which indicates a rearward facing process. The basic difference is characterized as either "cursorial" (built for sustained speed) or "ambulatory" (built to ambulate). The figure above again draws on the work of Martin-Serra, but focused here on the shape of the ulna. This shape-related variation is what remains after accounting for the effects of body size. As you can see, the bears (dark brown dots) are differentiated from all other carnivores primarily by the shape of the olecarnon process (along PC II). More specifically, bears have relatively the shortest and most rearward (or caudal)-facing process, which signifies a less important role for the triceps (versus the biceps)...which signifies, in turn, a design to hold up under various load stresses rather than to efficiently move about over long distances.
The figures above, left, show how the various bear species (each denoted by an orange dot) plot relative to other carnivores when comparing body mass to proportional length (top) and orientation (bottom) of the olecranon process. These data come mostly from Blaire van Valkenbugh, but also from other researchers. Orientation is measured in terms of degrees of inclination toward the tail-end (caudal end) of the body. These results contrast with the results shown in the figure to the immediately above, which depict variation left over after controlling for the dominant effect of body size.
Considering all of the carnivores, the basic theme is one of an increasing backward angle (but at a decreasing rate) and increasing relative length (at an increasing rate) as body mass increases. Generally speaking, the bears cluster with other carnivores of the same size. They tend to have a similar configuration of the olecranon process, roughly of the sort you would expect given how big they are.
The one exception, though, is the brown or grizzly bear (Urar, Ursus arctos). The grizzly bear has a lesser backward orientation to the olecranon relative to what you would expect by its body size, and the longest olecranon process relative to the ulnar shaft length of any carnivore. This last feature is noteworthy given that long olecrana have been associated with animals who spend a lot of time digging--which is consistent with the notion that adaptations for digging are a key feature of the grizzly bear niche. Which is consistent with the robust scapula and well-developed scapular muscles of most bears (see above), which also seem designed for activities such as digging. The results from Martin-Serra's work (above right) could be read as suggesting that all bears have a short olecranon--in contrast to the result just described above for grizzly bears. Reconciliation of this apparent discrepancy could involve the considerable statistical gymnastics that Martin-Serra went through to "control for" effects of body size and evolutionary relatedness. It could also have something to do with the brown dot in his graph denoting the bear species with the lowest score on his PC II axis. Although the species associated with this dot was not identified, it may represent brown/grizzly bears and, if so, be an outlier for this family, at least in the context of Martin-Serra's work.
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Post by brobear on Mar 22, 2017 11:33:59 GMT -5
Continued.... Running speed The comparatively slow running speed of bears lends additional support to the theme that bears have heavily built forelimbs and highly flexible paws that are "designed" to facilitate activities such as climbing or digging rather than moving long (or even short) distances efficiently and rapidly.
The slow speed of bears is illustrated graphically immediately above. Maximum running speed of numerous terrestrial mammals is plotted relative to body features that have previously been correlated with speed: total mass (or overall size) and length of the fore- and hind-limbs. The general relationship is one of increasing absolute speed with size, and then a decline. Increasing limb length would likely produce a longer stride--and, hence, speed--but only up to a point. That point being where increasing bulk would (1) mandate that bones accommodate load stresses rather than deliver speed, as well as (2) slow the gait because of the inertia of increasingly massive limbs.
But notice the bears (brown dots). They fall well below the trend line in all three relationships, suggesting that at least the three bear species represented here are slow given their size (roughly 50 kph [30 mph], maximum speed). This is not surprising in light of the information summarized above, all of which paints a picture of limbs that provide ample strength and flexibility for climbing, digging, and grappling, rather than ample stored and released energy for rapid and efficient movement. If you watch a bear run (albeit faster than most people), you will probably be struck by how inefficient the whole process seems, especially compared to a deer or an elk.
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Post by brobear on Mar 22, 2017 11:46:34 GMT -5
Continued.... Claws The final topic that I cover here under "forelimbs" is claws (or the ungal: highly-modified terminal phalanges). As a general point, bear claws are not retractable like those of cats--they are more like the non-retractable claws of dogs and the larger members of the weasel family. Looking at the graph immediately to the upper left (again, thanks to Blaire van Valkenburgh), bears fall out in a distinctive quadrant when you plot the characteristic depth of a claw against its curvature--which signifies how stout versus elongate it is. Considering all of the carnivores, there is a general trend from deep slightly curved claws, characteristic of the cats (the orange dots), to elongate claws (curved, but less deep) more common of canids (dark red dots) and bears (brown dots). The inset profiles of various claw shapes illustrate the extremes. So...notice, again, the brown or grizzly bear (Uar, U. arctos). Next to the badger (the gray dot farther to the right and below), grizzlies have the most elongate claws of all the carnivores shown here. The graph identifies the realm of "scratch diggers"--animals that live aboveground but dig for a significant part of their food, whether rodents, squirrels, or roots. Grizzly bears fall within this type. Added to the evidence presented above, this bit of data pretty conclusively establishes that Ursus arctos is well-built to excavate roots and rodents--that, in fact, the ability to excavate such foods probably defined a significant part of the evolutionary imperative that led to the emergence of this species (more on this under Foods and History). Parenthetically, relative claw length seems to systematically vary among brown bear genetic lineages (clades; see Evolution). The map above shows the length of claws relative to the length of skulls for various populations of brown bears in Eurasia. The claw profiles are scaled to this value, which is presented in numeric form immediately above. This information is overlain on a map of brown bear clades and skull sizes (see Body size) for reference. The data come from publications by Bjorn Kurten and Sergei Ognev that date to the middle part of the last century--when biologists were interested in compiling such arcana. Brown bears in Europe, the Caucasus, and among the salmon-eating bears along the Pacific coast had the smallest claws. Everywhere else the claws were relatively larger. But the key pattern in this map relates to the exceptionally large claws of the bears living on and near the Tibetan Plateau, part of the relatively limited Clades 5 and 6. These clades were apparently isolated at high elevations during the late Pleistocene, which may explain why they are so distinct in appearance--including claw size. But there is also a likely link to diet. Brown bears on the Tibetan Plateau rely to an exceptional degree on excavated foods, including the Plateau pika, the Tibetan marmot, as well as various roots (see Foods). On the other hand, brown bears living in Europe and among salmon along the Pacific coast don't dig much at all--other than into fairly soft sand (the coast) or ant hills (Europe). So...the importance of claws for digging is potentially evident even in variation of these appendages among brown bear populations in Eurasia.
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Post by brobear on Mar 22, 2017 11:48:05 GMT -5
jeb.biologists.org/content/218/19/3102
RESEARCH ARTICLE Grizzly bear (Ursus arctos horribilis) locomotion: gaits and ground reaction forces Catherine L. Shine, Skylar Penberthy, Charles T. Robbins, O. Lynne Nelson, Craig P. McGowan Journal of Experimental Biology 2015 218: 3102-3109; doi: 10.1242/jeb.121806
Locomotion of plantigrade generalists has been relatively little studied compared with more specialised postures even though plantigrady is ancestral among quadrupeds. Bears (Ursidae) are a representative family for plantigrade carnivorans, they have the majority of the morphological characteristics identified for plantigrade species, and they have the full range of generalist behaviours. This study compared the locomotion of adult grizzly bears (Ursus arctos horribilis Linnaeus 1758), including stride parameters, gaits and analysis of three-dimensional ground reaction forces, with that of previously studied quadrupeds. At slow to moderate speeds, grizzly bears use walks, running walks and canters. Vertical ground reaction forces demonstrated the typical M-shaped curve for walks; however, this was significantly more pronounced in the hindlimb. The rate of force development was also significantly higher for the hindlimbs than for the forelimbs at all speeds. Mediolateral forces were significantly higher than would be expected for a large erect mammal, almost to the extent of a sprawling crocodilian. There may be morphological or energetic explanations for the use of the running walk rather than the trot. The high medial forces (produced from a lateral push by the animal) could be caused by frontal plane movement of the carpus and elbow by bears. Overall, while grizzly bears share some similarities with large cursorial species, their locomotor kinetics have unique characteristics. Additional studies are needed to determine whether these characters are a feature of all bears or plantigrade species.
Within terrestrial animals a continuum of foot postures exists, from plantigrade species with their entire foot on the ground, to unguligrade animals that stand on the tips of their toes (Ginsburg, 1961; Carrano, 1997). The plantigrade posture is ancestral for mammals and it is generally agreed that digitigrade and unguligrade postures evolved as adaptations for speed and endurance. Because of this, numerous studies have examined the gait mechanics of digitigrade and unguligrade species (Budsberg et al., 1987; Hutchinson et al., 2006; Robilliard et al., 2007; Hudson et al., 2012). However, relatively few studies have examined the links between the plantigrade posture and locomotor mechanics. Plantigrade species are considered locomotor generalists, and because of the lack of cursorial specialisations, their limb movements are less restricted to the sagittal plane (Liem et al., 2001). Within mammals, plantigrade species include raccoons, badgers, weasels, as well as all rodents and primates. All of these animals are small compared with most digitigrade and especially unguligrade species; however, bears also retain the plantigrade stance. The goal of this study was to determine whether the locomotor mechanics of a stereotypical plantigrade quadruped, the grizzly bear (Ginsburg, 1961), differ from those of more extensively studied cursorial quadrupeds.
The selection of gaits used by plantigrade and cursorial species could represent some of the locomotor differences observed between these postures. Analysis of gaits, through footfall patterns, has been applied broadly to a wide range of terrestrial species (e.g. Gray, 1968; Hildebrand, 1976, 1977). Within quadrupedal animals, a lateral walk, in which the placement of the hindfoot is followed by the placement of the ipsilateral forefoot, is the gait used at slow speeds by the majority of species, including bears (Hildebrand, 1976). But, there is variation in terms of intermediate and faster gaits. The most common intermediate gait is the trot, defined by diagonal couplets, as this is seen in digitigrade (e.g. dogs and cats) and unguligrade (e.g. horses) animals, although these animals will also use a pace (ipsilateral couplets; Alexander, 1984). Interestingly, plantigrade carnivorans have not been shown to trot, but there have been a few observations of a pace (McClearn, 1992). Faster gaits include canters and gallops. Canters can be considered a slow gallop; however, they are characterised as being a three beat gait with one diagonal couplet (Hildebrand, 1976). Rotary gallops, as described above for the lateral walk, and transverse gallops, with the leading hindfoot placement being followed by the contralateral forefoot, can both be observed in the same species (Vilensky and Larson, 1989; Walter and Carrier, 2007), although there may be energetic differences between them (Bertram and Gutmann, 2009). Gallops are the fastest gait used by quadrupedal animals and studies have demonstrated that galloping occurs in species representing all three foot postures – unguligrade, digitigrade and bears within plantigrade species (Hildebrand, 1989; Renous et al., 1998; Robilliard et al., 2007; Walter and Carrier, 2007). Within carnivorans, bears are the most plantigrade along the posture continuum (Ginsburg, 1961). The specific morphological features defining plantigrady include: well-developed digits on both forefeet and hindfeet; different sizes of the metapodials, e.g. metapodials 3 and 4 are rarely the same length in plantigrade species; and a substantial angle produced between the ulna and the humerus during elbow extension (20 deg in bears; Ginsburg, 1961). Ursidae is considered a generalist family; yet, the individual species exhibit substantial differences in diet, habitat and ecology. Grizzly bears have the broadest range of behaviours in Ursidae and are able to climb (particularly as juveniles), swim and have been reported to run as fast as 13.3 m s−1 (Garland and Janis, 1993; Brown, 2009). There has been very limited research into the locomotion and biomechanics of Ursidae (Gambaryan, 1974; Inuzuka, 1996; Renous et al., 1998); however, it is likely that differences in limb morphology and locomotor behaviour may exist within Ursidae (Irschick and Garland, 2001), as well as between bears and other quadrupeds.
Previous studies have shown that locomotion by cursorial animals over a large size range can be described as dynamically similar across all speeds (Farley et al., 1993; Alexander, 2005). Locomotion is considered to be dynamically similar if, at a given dimensionless speed (Froude number), parameters can be made identical by multiplying forces, linear dimensions and time intervals by constant factors (Alexander and Jayes, 1983). In their seminal study, Alexander and Jayes (1983) characterised cursorial animals as those that stand with the humerus and femur closer to vertical than horizontal, which excludes other morphological characteristics that are considered cursorial in other studies (described above).
Relative to cursorial species, bears appear to have substantial movement in the frontal plane during locomotion. For example, bears have an unusual carpal movement, which manifests as a medial rotation during swing (Davis, 1949; Gray, 1968; Inuzuka, 1996). Further, grizzly bears have a medially directed forefoot position during stance, relative to the direction of travel. This differs from most cursorial species, which limit movement to the frontal plane to enhance efficiency and restrict forces to the direction of travel (Liem et al., 2001). Because of this, the mediolateral forces generated by cursorial animals are comparatively small and frequently ignored in the analysis of locomotion (Budsberg et al., 1987). However, some primates walking bipedally and animals with sprawling gaits have been shown to produce mediolateral ground reaction forces equal to or greater than the magnitude of their anterior–posterior forces (Willey et al., 2004). Currently, it is unclear to what extent the forces generated by bears during locomotion are similar to or differ from those of well-studied groups of terrestrial mammals, particularly considering the angle of the forefoot during stance.
In addition to terrestrial locomotion, the forelimbs may be involved in a wide range of other activities, especially in non-predatory carnivorans that may forage for food or exhibit escape behaviours such as climbing. The requirement of predators to chase down vertebrate prey overcomes the need for dexterity upon capture; therefore, forelimb dexterity in carnivores is negatively correlated with vertebrate predation. Bears and other plantigrade carnivores (i.e. generally omnivorous species) have higher dexterity scores than digitigrade carnivorans (Iwaniuk et al., 2000). Contributing to this dexterity is the morphology of the forelimbs, such that the ulna and radius are separate in plantigrade animals, resulting in the ability to supinate and pronate (rotate the forearm to point the palm up or down). In cursorial animals, the ulna and radius are fused to increase stability and therefore speed (Liem et al., 2001). Additionally, pentadactyly is only retained in plantigrade species as loss of digits is characteristic of digitigrade and unguligrade postures; this is associated with the reduction of distal limb mass that, along with elongation of the distal limbs, increases speed in cursorial animals (Garland and Janis, 1993). The difference in forelimb bone anatomy, as well as the differences in ecology, between cursorial and plantigrade species of the Carnivora is likely to have resulted in differences in locomotion.
The overall goal of this study was to determine whether locomotion by grizzly bears differs from that of other large quadrupedal animals, which tend to be digitigrade or unguligrade. We hypothesised that the gaits used by grizzly bears would be similar to those used by smaller plantigrade animals, as opposed to similarly sized cursorial animals, because of the differences in morphology of the distal limb. Further, we predicted that the mediolateral ground reaction forces would be higher in forelimbs of bears, compared with other species, as a result of their medially directed stance. These hypotheses were addressed by examining the footfall patterns and stride parameters of grizzly bears to identify gaits, and characterising the magnitude, time-varying shape and relative distribution of three-dimensional ground reaction forces generated by the forelimbs and hindlimbs over a range of speeds.
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Post by brobear on Mar 22, 2017 11:48:52 GMT -5
Mechafire debates grizzly strength vs tiger strength... "Even if bicep strength is the same. The bear is still stronger. The fact that bears don't walk on their toes gives it a bit of an advantage "The primary advantages of a plantigrade foot are stability and weight-bearing ability; plantigrade feet have the largest surface area." Not only that but brown bears have a high shoulder hump "Brown bears have a bulky muscle mass located above the shoulders. This hump is designed to power the forelimbs and makes them exceptionally powerful diggers. This is one of the features that distinguishes them from the more common North American black bear which lacks such a shoulder hump" www.pbs.org/wnet/nature/episodes/bears-of-the-last-frontier/hour-one-city-of-bears/brown-bear-fact-sheet/6522/Bears obviously have stronger abdominal and back muscles, which gives it greater postural support and strength in the midsection. This all indicates a more robust and more heavily built animal. " Shoulder hump, stiff rigid backbone, its shorter (length on all fours) build makes it the stronger animal at parity. Plantigrade feet, stronger more rotund midsection due to abdominal and back muscles, and more stable lower body (for standing upright) give it more stability which is important in a wrestling or grappling match (as one is trying to throw off the other's balance). Now the tiger can be said to be more powerful (strength+speed) at parity, but you are arguing about strength. Even in usage, the bear can be said to be stronger. Read more: theworldofanimals.proboards.com/thread/483/ussuri-brown-bear-siberian-tiger?page=3#ixzz4UawqzPiE
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Post by brobear on Mar 22, 2017 11:49:15 GMT -5
by Blaire Van Valkenburgh - First posted by grraahh - shaggygod.proboards.com/ If bears are not built for speed, then what does the combination of massive limbs, plantigrade hindfeet, cumbersome paws, and a short back provide? Strength and mobility of limb movement are the answers. The stout limbs of bears are capable of producing large forces over a much greater range of motion than those of dogs or even cats. Bears use these capabilities when digging for food or shelter, fishing for salmon, climbing to escape danger, and battling with members of their own species as well as other predators. Imagine a wolf trying to perform a bear hug or climb a tree. Dogs have forfeited these abilities in favor of speed. Cats are more like bears in their range of possible movements, but lack strength. Bears may not be able to outrun danger, but can successfully defend themselves through brute force.
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Post by brobear on Mar 22, 2017 11:49:45 GMT -5
Turning to distinguishing features of brown (or grizzly) bears (Ursus arctos):
Brown bears are perhaps self-evidently "brown," at least as a dominant theme when it comes to the color of their coat (or pelage). That said, "brown" bears can vary in color from near black to lighter shades that are best described as blonde, gray, or silver. The extreme in this regard is the "blue" bear of Tibet (considered by some to be a separate subspecies, Ursus arctos pruinosus). This variant can look downright variegated, with pronounced patches of near white in juxtapose with light brown to grayish hindquarters. Brown bears also tend to sport guard hairs with light nearly translucent tips, which has resulted in populations with lighter-colored hairs being described as "grizzled." Hence, the North American grizzly bear. Brown bears have especially robust muscles surmounting their scapula, which gives them a distinctive hump over their shoulders. This hump can readily differentiate them from the black bears (Ursus americanus and U. thibetanus) with which they often co-occur in North America and eastern Asia. Finally, brown bears have relatively long claws compared to other bears species. This is most pronounced among Tibetan brown bears, and least pronounced among European populations and the exceptionally large salmon-eating bears of the Pacific coast. Claw length seems to correlate with the extent to which brown bears are regionally dependent on excavated foods such as roots and underground-dwelling rodents, squirrels, and pikas.
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Post by brobear on Mar 22, 2017 11:50:04 GMT -5
I end this introductory section with a comment on what doesn't distinguish bears: most notably, their skull and jaw ( ), at least in the sense that bear skulls do not exhibit any extreme features when compared to other carnivores. They have the forward-looking eyes and stereoscopic vision of virtually all carnivores, which surmount or partly straddle the snout (or rostrum), much like other carnivores as well. Although their skulls tend to be robust, they are not more robust than those of many other carnivores species--with the notable exception of the strongly-built skull of the bamboo-eating (crushing and grinding) giant panda. Their snout is neither extremely long nor extremely short, even among the shorter-faced species. Their teeth are also not highly specialized, certainly not as much as the meat-shearing (carnassial) teeth of cats. A bear's teeth reflects adaptations to a generalized omnivorous diet, although variation among bear species ranges from better developed meat-shearing cusps on the molars of the carnivorous polar bear to robust flat-topped (or bunodont) molars deeply anchored in the jaws of the more herbivorous giant panda and South American spectacled bear (Tremarctos ornatus).
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Post by brobear on Mar 22, 2017 11:50:27 GMT -5
First, bears are large.--as a group, the largest of all the extant carnivores. The only rivals to this claim are the lion and tiger. Large size engenders many consequences, including demands placed on the skeleton to carry around a lot of weight. Second, bears are fat--among the most corpulent of all terrestrial mammals. The hibernating bear species of northern latitudes have 20 to in excess of 30% body fat, in contrast to a median tendency of roughly 5% (3-10% interquartile range) among other land-dwelling mammals. The obesity of bears gives them a round portly appearance. The body fat also imposes costs, including the energetic expense of carting around so much extra weight, plus losses of speed and endurance. But the payoff is an ability to survive extended food shortages, including the figurative famine of hibernation. Third, bears have exceedingly strong front quarters that manifest in a scapula and bones of the front limb that are robust even accounting for their size. These powerful front quarters facilitate a life of climbing, digging, or grappling with large prey. Fourth, bears are quite dexterous, especially given their size. Of large land mammals, probably only the gorilla and orangutan exceed them in this regard. This dexterity provides bears with considerable facility at grasping, manipulating, and extracting foods. Such facility is carried to its extremes in the giant panda (Ailuropoda melanoleuca), which sports a "thumb" (an enlarged and mobile radial sesamoid bone in its paw) that allows it to efficiently handle bamboo--it's primary food. Fifth, bears have large brains and small eyes, which correlate, in the first case, with considerable intelligence and, in the latter case, with comparatively underdeveloped eyesight. Parenthetically, anatomical evidence is ambiguous insofar as sense of smell is concerned. The majority of evidence suggests that bears are no more gifted in this regard than most carnivores--less so, perhaps, than dogs. The minority suggests otherwise--that bears do, in fact, have an acute sense of smell. Sixth, bears lack a tail, which clearly differentiates them from the dogs (canids) and cats (felids). The bears presumably lack this feature because their lack of dependence on high maneuverability at fast speeds negates the benefits of carrying around an elongate appendage on their hind end. Finally, bears are extremely small at birth relative to the size of their moms; among the smallest of all placental mammals. This correlates with being "altricial"--relatively helpless, and late in opening their eyes. As a result, bears (especially females) grow comparatively rapidly to reach adult weight during their first several years of life.
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Post by brobear on Mar 22, 2017 11:51:11 GMT -5
graphs:
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Post by brobear on Mar 22, 2017 11:54:26 GMT -5
Bears (also called ursids, in reference to their family Ursidae) are, as a group, the largest extant terrestrial carnivores. Many carnivorous marine mammals are larger, but the focus here is on land-dwelling species.
The graph at top left shows the median and range of body sizes for species within each of the families of terrestrial carnivores. This provides a visual sense of just how much larger bears are compared to other carnivorans. The only exceptions to this rule are the two largest cats, or felids, represented by the outlying dots above the Felid bar. These represent the tiger and the lion.
Another way to reckon size is by dimensions of the skeleton, represented in the bottom left graph by the size of the jaw (i.e., mandible). Here, again, bears are the largest, but not by as much of a margin--although this is visually distorted by the fact that the y-axis to the left is of log-transformed values, which tends to compress things. Even so, the median size of jaws among the bone-crushing hyaenas is nearly as large as the jaws of bears.
The bottom line: Ursids are exceptionally large, especially when compared to other contemporary land-dwelling carnivores.
Given that fact, there are many consequences that flow from being large. Species tend to live longer, reproduce at a slower rate, and metabolize at a slower rate (at least per unit body mass). The following link takes you to a page that elaborates on all of this, including how bears conform to the expectations of large body mass--and also how they deviate.
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Post by brobear on Mar 22, 2017 11:54:54 GMT -5
A second link, which follows, takes you to a page that offers a few more details on how the body size of brown/grizzly bears varies in space, among populations, and between the sexes--as well as how size has varied in recent epochs.
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Post by brobear on Mar 22, 2017 11:56:41 GMT -5
The four panels to the left, two top and two bottom, show the relationship between some key life parameters (the vertical or y-axis) and body mass, expressed in grams (the horizontal or x-axis). Each dot represents a parameter summarized for a given species, all of which are terrestrial mammals--ranging from the smallest (i.e., shrews) to the largest (i.e., elephants). The various bear species are denoted by a larger symbol colored brown. The central relationship between each life parameter and body mass is represented by a solid black line. Note that adult body mass is consistently log-transformed, which tends to compress values, especially at the high end of the range.
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Post by brobear on Mar 22, 2017 11:56:59 GMT -5
The four panels above and Panel D below emphasize relationships with body mass from which the bears do not deviate substantially--in other words where they conform to what you might expect given their size. Graphically, this shows up as dots representing bears clustering near the central trend line. This holds for the number of years between litters (Panel A), litter size (number of cubs per litter, Panel B), age at which females first reproduce (Panel D below), longevity (Panel C), and the number of offspring produced in a lifetime (Panel D above).
That said, because of their large size, bears first reproduce late in life, have long intervals between births, and have small litters, all of which is offset by the fact that they tend to live for a long time. This dependence of life-time reproductive success on longevity highlights how critical the annual survival of females is to maintenance of populations. In other, a high rate of female survival from one year to the next is critical to conserving virtually all bear populations.
Three of the four panels at left highlight relationships where some (if not all) bear species deviate substantially from what might be expected given their size. Panel B shows the numbers of days before newborns open their eyes, which is an indicator of how helpless these neonates are. Such helplessness is more common among smaller-bodied species; bears comprise the majority of large-bodied species giving birth to such altricial young. This, of course, because birth happens in a den where helpless newborns are relatively secure. In line with this, bears give birth to the smallest neonates--relative to the size of the mother--of just about any species with a placenta (in other words, barring marsupials). Again, this goes hand-in-hand with relative helplessness, and the opportunities afforded by denning (but more on all this under Life strategy).
Finally, Panel C at left requires some explanation. This shows length of gestation relative to body mass. All but one of the bear species represented here have gestation lengths that seem to fall in line with the dominant trend. However, this is deceptive. Three of six bear species are known to delay the implantation of fertilized eggs, presumably as a hedge against times of food shortage (again, see Life strategy for more information). The upshot is that, even though among brown bears fertilization typically occurs in June and birth the following January, actual gestation is only 90 or so days. So duration of sustenance from the placenta is actually quite short for bears, and belies the ostensible results in Panel C, which do not account for delayed implantation.
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Post by brobear on Mar 22, 2017 11:57:30 GMT -5
graph.
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Post by brobear on Mar 22, 2017 11:58:57 GMT -5
Given that bears, as a group, are large, and that brown/grizzly bears are amongst the largest of the lot, there is substantial within-species variation in body size. As with so many mammal species, males are consistently larger than females. Superimposed on this sex-related difference, body size varies substantially among populations as an apparent function of the average amount of meat consumed. More dietary meat translates into larger bears, which is probably also a driver of differences between the sexes; on average, males consistently eat more meat than do females in any given population (see Foods). Insofar as some details are concerned:
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Post by brobear on Mar 22, 2017 11:59:19 GMT -5
The graphs at left show the average size of adult females (A) and males (B; on the vertical axis) relative to the amount of meat in the diet (horizontal axis). Each dot represents the average for a population, all of which are North American. (Estimates of meat consumption come from an extensive data set compiled and generated by Garth Mowat, a researcher from British Columbia). The colored dots with a white center denote averages for the Yellowstone grizzly bear population.
I fitted a trend line to each of these data sets, differentiating coastal populations with access to substantial numbers of spawning salmon from interior populations without. A quick scan of the data clearly indicated that these two subpopulations exhibited different responses in body size depending on whether the meat consumed was salmon as opposed to terrestrial mammals. Although all of the relationships were positive, fish-eating populations were larger at any given level of meat consumption and, on average, increased in average size at a greater rate in response to each additional increment of dietary meat.
Why this difference in population-level response to consumption of salmon versus terrestrial meat? This question has not been directly addressed by researchers, but it is easy to imagine that the per kg energetics of eating fish are more favorable than those of eating meat from, say, caribou, moose, or elk. Terrestrial meat tends to come in larger packages that are often difficult to procure, and spread thinly and unpredictably on the landscape. By contrast, spawning salmon are highly concentrated, predictable, and digestible, and, under the right circumstances, easy to obtain.
The nutritional and energetic connection between dietary protein in the form of meat and growth and body size is straight-forward. Which means that these population-level responses are not surprising nor hard to explain (for details on nutritional and energetic aspects see Nutrition).
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Post by brobear on Mar 22, 2017 11:59:42 GMT -5
The link between dietary protein--especially from salmon--and body size of brown bears is reinforced by hemispheric patterns. The map below shows the historical distribution of brown bears (along with denotations of genetic clade; see Evolution), overlain with isopleths of skull size (specifically, length of the skull [condylobasal length]); with skull size indicating overall body size. The darker the green, the larger the size. Large size is clearly associated with coastal areas hosting spawning runs of salmon. . By contrast, the smallest bears are in Europe, the Tibetan Plateau, and much of interior North America. (Data on skull size were compiled and georeferenced by Masaaki Yoneda using measures made by Bjorn Kurten and Bob Rausch).
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Post by brobear on Mar 22, 2017 12:00:56 GMT -5
The shrinkage of brown bears
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