The gaits of marsupials and the evolution of diagonal-sequence walking in primates.

OBJECTIVES: Documenting the variety of quadrupedal walking gaits in a variety of marsupials (arboreal vs. terrestrial, with and without grasping hind feet), to aid in developing and refining a general theory of gait evolution in primates. MATERIALS AND METHODS: Video records of koalas, ringtail possums, tree kangaroos, sugar gliders, squirrel gliders, wombats, numbats, quolls, a thylacine, and an opossum walking on a variety of substrates were made and analyzed to derive duty factors and diagonalities for symmetrical walking gaits. The resulting distributions of data points were compared with published data and theories. RESULTS: Terrestrial marsupials' gaits overwhelmingly plot slightly below the theoretical "horse line" (Cartmill et al., Zoological Journal of the Linnean Society. 2002;136:401-420) typical of terrestrial mammals; arboreal marsupials' gaits overwhelmingly plot more decisively above it. Both distributions are roughly parallel to the horse line, but arboreal animals exhibit increased diagonality, so that their higher-speed walking gaits overlap with those of typical primates on the Hildebrand diagram of diagonality against duty factor. CONCLUSIONS: Quadrupeds avoid gaits lying exactly on the (theoretically optimum) horse line, to avoid fore/hind limb interference ("forging"). This can be accomplished by either a slight reduction in diagonality ("downshifting") or a more decisive increase ("upshifting"). Tree-dwellers adopt the second option to eliminate unilateral bipods of support from the gait cycle. The upshifted horse line represents an early phase in the evolution of primate-like diagonal-sequence gaits.

A sort of revolution: Systematics and physical anthropology in the 20th century.

During the first four decades of the 20th century, a system of ideas about the evolution and systematics of humans and other primates coalesced around the work of George Gaylord Simpson and W. E. Le Gros Clark. Buttressed by the "new physical anthropology" of the 1950s, that system provided an authoritative model-a disciplinary matrix or paradigm-for the practice of that aspect of biological anthropology. The Simpson-Le Gros Clark synthesis began to unravel in the 1960s and collapsed in the 1970s under the onslaught of cladistic systematics. The cladistic "revolution" resembles a paradigm shift of the sort proposed by Thomas Kuhn because it was driven, not by new biological discoveries or theories, but by a change in aesthetics.

Anxiety in first year medical students taking gross anatomy.

To study anxiety levels in first-year medical students taking gross anatomy. Thirty medical students per year, for 2 years, completed the Beck Anxiety Inventory (BAI) 10 times during a 13-week gross anatomy course. In addition, behavioral observations were made by a psychiatrist during gross anatomy for demonstrations of assertive, destructive, neutral, or passive behavior. Additional qualitative outcome measures were group exit interviews with the faculty and students. The mean BAI for all 60 students per year, for 2 years, was 2.19 ± 3.76, 93% of the scores indicated minimal anxiety, and 89% of BAI values were less than five which confirmed a minimal level of anxiety. The low level of reported BAI contrasted sharply with verbal reports by the same students and face-to-face exit interviews with the psychiatrist. Symptoms of stress and anxiety emerged as a result of these conversations. The high levels of subjective stress and anxiety revealed by the interviews were unknown to the gross anatomy faculty. The low scores of students on the BAI's stand in sharp contrast to the BAI's reported for medical students in other published reports. Although it is possible that our students were truthfully devoid of anxiety, it is more likely that our students were denying even minimal anxiety levels. There have been reports that medical students feel that admitting stress, depression, or anxiety put their competitiveness for a residency at risk. We conclude that students may be in frank denial of experiencing anxiety and, if so, this behavior is not conducive to good mental health.

Clavicle length and shoulder breadth in hominoid evolution.

For a given body mass, hominoids have longer clavicles than typical monkeys, reflecting the lateral reorientation of the hominoid glenoid. Relative length of the clavicle varies among hominoids, with orangutans having longer clavicles than expected for body mass and gorillas and chimpanzees having shorter clavicles than expected. Modern humans conform to the general hominoid distribution, but Neandertals and Upper Paleolithic Homo sapiens have longer clavicles than expected for their size and exhibit marked positive allometry in clavicle length. Relative to clavicle length, adult and newborn humans have broader shoulders (biacromial breadths) than comparable apes, because the reduced elevation of the human shoulder swings the acromion laterally downward away from the head. Since broadened shoulders yield an increased risk of maternal and neonatal injury and/or death from shoulder dystocia during birth, we might expect hominins to manifest trends toward reduction in shoulder breadth and clavicle length. They do not, presumably because of countering selection pressures favoring a long clavicle in the adults. The marked sexual dimorphism seen in patterns of clavicular growth and static adult allometry in humans suggests that those selection pressures have disproportionately affected the males.

Primate origins, human origins, and the end of higher taxa.

When people learn that I study human evolution and we start talking about it, they sometimes ask me, "How long ago did the first humans live?" My answer is usually another question: "What do you mean by 'humans'?" That response seems as baffling and wrong-headed to them as their question seems to me, and it usually takes us a while to straighten things out. © 2012 Wiley Periodicals, Inc.

Convergent? Minds? Some questions about mental evolution.

In investigating convergent minds, we need to be sure that the things we are looking at are both minds and convergent. In determining whether a shared character state represents a convergence between two organisms, we must know the wider distribution and primitive state of that character so that we can map that character and its state transitions onto a phylogenetic tree. When we do this, some apparently primitive shared traits may prove to represent convergent losses of cognitive capacities. To avoid having to talk about the minds of plants and paramecia, we need to go beyond assessments of behaviourally defined cognition to ask questions about mind in the primary sense of the word, defined by the presence of mental events and consciousness. These phenomena depend upon the possession of brains of adequate size and centralized ontogeny and organization. They are probably limited to vertebrates. Recent discoveries suggest that consciousness is adaptively valuable as a late error-detection mechanism in the initiation of action, and point to experimental techniques for assessing its presence or absence in non-human mammals.

The effect of substrate size on the locomotion and gait patterns of the kinkajou (Potos flavus).

Diagonal-sequence (DS) gaits, which are very rare among mammals, are common and well documented in primates and some arboreal marsupials. DS walking gaits have been reported in the kinkajou (Potos flavus), which shows ecological similarities with primates and arboreal opossums but lacks prehensile specializations of the hindfoot. Nevertheless, the actual frequency of DS gaits and the functional context in which these gaits occur in this highly arboreal mammal remain unknown. We examined the effect of substrate size on the locomotion and gait patterns of kinkajous by recording gaits in two individuals walking and running on poles of two different diameters and on a runway. Diagonality and limb duty factors were calculated for 534 gait cycles. Kinkajous relied mostly on DS gaits and trots during walking, and increased the diagonality of their gait patterns on thinner substrates. The proposed functional link between locomotion on thin branches and the presence of a grasping, primate-like hindfoot is not supported by these data. However, further analysis of kinkajou gait cycles showed that DS gaits may have advantages overlooked earlier. DS gaits, during walking, minimize the distance between two ipsilateral feet during short periods of unilateral bipedality, and per corollary maximize the distance between two contralateral feet during the much longer periods of diagonal bipedality. Such foot positioning during the gait cycle could be beneficial in walking on a relatively thin substrate and could explain why kinkajous adopt DS walking gaits, especially on thinner poles, despite lacking prehensile specializations of the hindfoot.

Adaptive value of ambling gaits in primates and other mammals.

At speeds between the walk and the gallop, most mammals trot. Primates almost never trot, and it has been claimed that they transition directly from a walk to a gallop without any distinctive mid-speed running gait. If true, this would be another characteristic difference between the locomotion of primates and that of most other quadrupedal mammals. Presently, however, few data exist concerning the actual presence or absence of intermediate-speed gaits (i.e. gaits that are used between a walk and a gallop) in primates. Video records of running in twelve primate species reveal that, unlike most other mammals, all the primates studied almost exclusively adopt an 'amble'--an intermediate-speed running gait with no whole-body aerial phase--rather than trot. Ambling is also common in elephants and some horses, raising the question of why ambling is preferred over trotting in these diverse groups of animals. Mathematical analyses presented here show that ambling ensures continuous contact of the body with the substrate while dramatically reducing vertical oscillations of the center of mass. This may explain why ambling appears to be preferable to trotting for extremely large terrestrial mammals such as elephants and for arboreal mammals like primates that move on unstable branches. These findings allow us to better understand the mechanics of these unusual running gaits and shed new light on primate locomotor evolution.