Both rapid, economical running and fighting are important in the life histories of most or all terrestrial species. Nevertheless, musculoskeletal "design" that allows rapid, economical running is likely to be incompatible with musculoskeletal design that is appropriate for fighting. This locomotor versus fighting tradeoff may explain many cases of divergence in body form among closely related species and may also help explain why males and females of the same species often differ in the size and proportions of their musculoskeletal system. This study will use comparative methods to test hypotheses of functional tradeoff between specialization for rapid, economical running versus specialization for fighting. The first aim of the investigation is to test a broad set of hypotheses of functional design of the skeletal system in two mammalian orders: Artiodactyla (e.g., deer and antelope) and Carnivora (e.g. wolves and lions). Using museum collections, limb bone proportions will be measured in a large number of species and compared with proxies for male-male aggression (size sexual dimorphism) and maximum running speed. The second and third aims of the study are to quantify sexual differences in the properties of limb bones and muscle architecture in 2 species exhibiting high levels of male-male competition (black bears and bisons) and 2 highly cursorial species (wolves and pronghorns). Hence, this investigation will test hypotheses of functional tradeoff between musculoskeletal specializations for running versus fighting and it will significantly expand our knowledge and understanding of the sexual dimorphism of the mammalian musculoskeletal system. The study will also provide research opportunities for 6 to 9 undergraduate students. We will also create an open access, online library of high resolution CT scans of anatomically aligned cross-sections of the whole limb bones from all of the subjects. Finally, although sexual dimorphism is poorly understood by the general public, it is of broad importance to physiology, evolutionary biology, and human evolution and behavior. We expect the research proposed here to be of general interest and to provide opportunities to educate the public about the biological significance of sexual dimorphism.
(0817782) supported 19 peer-reviewed papers that are in print or accepted and 4 manuscripts that are almost ready to submit for review. Seven undergraduate students are co-authors on these papers. Four of these publications received widespread attention from the scientific media. Examples of this media coverage are listed here. www.sciencedaily.com/releases/2010/02/100212092304.htm www.sciencedaily.com/releases/2011/05/110518171343.htm www.bbc.com/news/science-environment-20790294 www.bbc.com/news/science-environment-27720617 http://paleomagonline.com/episode-14-david-carrier-and-james-gregory/ www.bbc.com/news/science-environment-27763015 SIGNIFICANT RESULTS Sexual dimorphism in Grey Wolves - To test for morphological adaptation associated with aggression and prey capture performance, we assessed skeletal sexual dimorphism in three subspecies of grey wolves. Male wolves appear to exhibit a higher degree of morphological adaptation that is associated with aggression-based activities. Postcranial dimorphism varied by subspecies, likely reflecting differences in selective pressures based on prey size and the resultant differences in capture techniques. Sexual dimorphism in Carnivores - We collected a large comparative data set on skeletal sexual dimorphism in morphological characters associated with specialization for aggression. The results of this study reveal widespread skeletal shape dimorphism in terrestrial carnivorans that may be associated with improving aggressive performance when males compete. Are humans economical walkers or endurance runners? - Although humans are clearly highly specialized for terrestrial locomotion compared to other great apes, the results of this study indicate that our locomotor muscles are not tuned to specific walking or running speeds. This is inconsistent with the evolution of an optimal phenotype for economical walking or endurance running. Instead, the locomotor muscles of the human body appear to have evolved for a variety of functions, some of which clearly entailed limits on specialization for locomotor economy. Protective buttressing of the human fist - The proportions of the human hand provide a performance advantage when striking with a fist. We propose that the derived proportions of hominin hands reflect, in part, sexual selection to improve fighting performance. Protective buttressing of the hominin face - In this review, we suggested that many of the facial features that characterize early hominins evolved to protect the face from injury during fighting with fists. In this scenario, the evolution of reduced facial robusticity in Homo is associated with the evolution of reduced strength of the upper body and, therefore, with reduced striking power. The protective buttressing hypothesis provides a functional explanation for the puzzling observation in modern humans that the jaw adductor muscles of males are 42% stronger and have 8-times more fast fibers than those of females. If the protective buttressing hypothesis is correct, the primary differences in the face of robust versus gracile australopiths may be more a function of differences in mating system than differences in diet as is generally assumed. The protective buttressing hypothesis is also consistent with observations that modern humans can accurately assess a male’s strength and fighting ability from facial shape and voice quality. Male-male competition in house mice - Collectively, the results of this study suggest that social dominance is a complex phenotype with many factors interacting to determine its value. We found that this measure of social dominance ability was highly heritable within our population, even when the effect of body mass and litter sex-ratio were removed. Body mass had a moderate, positive correlation with social dominance ability. Functional consequences of plantigrade foot posture - The results of these studies suggest that plantigrade foot posture (1) improves the economy of walking, (2) increases the kinetic energy of side strikes and lateral pushes, and (3) the capacity of humans to apply free moments to the ground greatly exceed those used in normal maximal effort locomotor behavior. Evolution of habitual bipedal posture - Many quadrupedal species stand bipedally on their hindlimbs to fight. This posture may provide a performance advantage by allowing the forelimbs to strike an opponent with the range of motion that is intrinsic to high-speed running, jumping, rapid braking and turning; the range of motion over which peak force and power can be produced. The observations of this study suggest that sexual selection may have contributed to the evolution of habitual bipedalism in the lineage of apes that led to humans. BROADER IMPACT The work supported by the previous grant involved students at all levels. Nine undergraduates participated in research supported by the previous award. Three presented their research at annual meetings of Society of Integrative and Comparative Biology and five are authors on peer-review papers (bold print in the list of publications above). Graduate student research supported by the grant includes the work of Rebecca Walter, Jessamyn Markley, Christopher Cunningham, Jeremy Morris and Lindsey Reader. During the grant period, the PI conceived and cofounded The Utah Citizens’ Counsel; an independent, non-partisan group of senior community advocates dedicated to improving public policy on complex issues (including health, environmental and science education) through dialogue, creative problem solving, and consensus building (www.utahcitizenscounsel.org/).
