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Morphological Structure and Function in the Desert Heteromyid Rodents

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Great Basin Naturalist Memoirs Volume 7 Biology of Desert Rodents Article 4 8-1-1983 Morphological structure and function in the desert heteromyid rodents Joyce C. Nikolai Department of Biology, University of Utah, Salt Lake City 84112 Dennis M. Bramble Department of Biology, University of Utah, Salt Lake City 84112 Follow this and additional works at: https://scholarsarchive.byu.edu/gbnm Recommended Citation Nikolai, Joyce C. and Bramble, Dennis M. (1983) "Morphological structure and function in the desert heteromyid rodents," Great Basin Naturalist Memoirs: Vol. 7 , Article 4. Available at: https://scholarsarchive.byu.edu/gbnm/vol7/iss1/4 This Article is brought to you for free and open access by the Western North American Naturalist Publications at BYU ScholarsArchive. It has been accepted for inclusion in Great Basin Naturalist Memoirs by an authorized editor of BYU ScholarsArchive. For more information, please contact [email protected], [email protected]. MORPHOLOGICAL STRUCTURE AND FUNCTION IN DESERT HETEROMYID RODENTS' Joyce C. Nikolai- and Dennis M. Bramhle- .\bstract.— The functional morphology of desert heteromyid rodents (Perognathus, Dipodomys, Microdipodops) is reviewed with considerable new information provided. Specific attention is given to the interaction of anatomical structure and the behavioral and ecological patterns of these rodents. Inflation of the auditory bullae, although apparently related to improved hearing, is also shown to directly impact the structure and function of the feeding apparatus in desert heteromyids. The mechanics of high speed seed pouch- ing behavior in Dipodomys deserti as well as the rates of digging activities of various heteromyids are described using data from slow motion films. The biomechanical consequences of cheek pouch loading for body size and locomotor behavior yield theoretical predictive models concerning interspecific differences in foraging behavior, dietary prefer- ence, and microhabitat selection. Stnictura! modifications of the forelimb associated with use of external cheek pouches reduce the mechanical competence of these limbs for shock absorption during fast quadrupedal running. The relative size of various front and hind limb segments are correlated with quadrupedal, tripodal, and bipedal gaits in heteromyid rodents. The in- terdependence of body balance, gait, and speed are examined in Dipodomys merriami. Factors possibly contributing to the origin of bipedalism in rodents are reviewed and discussed. The heteromyid rodents of North America (1932) comparative study of vertebral archi- offer, potentially, a superb opportunity to ex- tecture in saltatorial rodents, and Wood's amine the importance of morphological de- (1935) important survey of the fossil and Re- sign as a determinant of behavioral and eco- cent Heteromyidae. Herman's (1979) recent logical patterns under natural conditions. multivariate statistical analysis of hind limb This follows from the considerable range of bone and muscle morphology in bipedal ro- morphologies found within this circum- dents constitutes a very significant extension scribed group as well as the impressive beyond the older comparative anatomical breadth of habitats that its living representa- works. tives presently occupy. Perhaps more signifi- Recent functional morphologic studies cant is the fact that heteromyids have recent- have been more analytical and experimental, ly become the focus of numerous but also of more limited scope. Thus, Pink- investigations aimed at gaining a better un- ham (1976) has investigated the gaits and me- derstanding of their biology on multiple lev- chanics of quadrupedal and bipedal running els (e.g., physiology, behavior, ecology, com- in Liomys and Dipodomys by combining high mimity level interactions). Such studies have speed cinematography with force platform begun to offer the kinds of information recordings. Using similar techniques, but also against which carefully framed hypotheses of including cineradiography, Biewener et al. a hmctional-morphologic nature might be (1981) have studied the mechanical behavior critically appraised. of the major hindlimb tendons in kangaroo Earlier morphologic studies on hetero- rats. Additional information on the ankle me- myids are primarily descriptive but also con- chanics of Dipodomys and the physiological tain comments on form-function relationships properties of its associated mu.sculature has that are, of necessity, relatively superficial been presented by Williamson and Frederick and speculative. Excellent works of this type (1977). Kaup (1975) has also commented on are Howell's (1932) monograph on the myol- the biomechanical and evolutionary signifi- ogy and osteology of Dipodotnys, Hatt's cance of hind limb anatomy in heteromyids. 'From the .symposium "Biology of Desert Rodent-s," presented at the annual meeting of the .\nierican Society of Mammalogists, hosted by Brigham Young University, 20-24 June 1982, at Snowbird, Utah. 'Department of Biology, University of Utah, Salt Lake City. Utah 84112. 44 1983 Biology of Desert Rodents 45 Of special note are the investigations of the Websters (see below) on the auditory appa- ratus of heteromyids. Not only have their studies provided a large body of comparative morphologic data, but they also offer one of the few examples wherein specific hypoth- eses concerning the adaptive value of a major morphological complex have been tested. In the present paper we briefly review the existing data on the functional morphology of heteromyid rodents and point out significant gaps in our knowledge. Considerable atten- tion is given the forelimbs and cheek pouches, two structures that have received httle attention in the past but whose struc- tural organization may place important con- straints on the behavior of these animals. Here and elsewhere we have tried to show how functional morphologic analyses can lead to predictive, testable models con- cerning the natural behavior and ecology of heteromyid rodents. Limitations of both time and materials have forced us to restrict the present dis- cussion to "desert heteromyids" of the genera Perognathiis, Dipodoniys, and Microdipodops. This is done with full knowledge that a better understanding of the fimctional anatomy and behavior of the modern heteromyines (Liomys, Heteromys) would imdoubtedly broaden our appreciation of form-fimction and evolutionary patterns within pe- rognathine and dipodomyine heteromyids and might well alter some of our conclusions. Skull and Neck Morphology Fig. 1. Influence of auditory specialization on the feeding apparatus of desert heteromyids. A generahzed desert rodent, Neotoma lepida (A) is compared to Pe- (B), Dipodomys merriami (C), and Skull rognathtts forinosus Microdipodops megacephalus (D). Inflation of the audi- tory bufla (stippled) reduces the area of origin of the The most striking cranial feature of desert temporalis musculature (hatched) and also restricts gape heteromyids and that which has received the by crowding the mandible from behind. The specialized most attention is the enlarged middle ear everted angle (ea) of the mandible reduces the impact of chambers or auditory bullae. The auditory bullar inflation in heteromyids. Maximum gape between cheek teeth (but not incisors) in Perognathiis (35°) is bullae are moderately inflated in Pe- the about equal to that in Neotoma (36°), but extreme rognathiis, but grossly so in both Dipodoniys middle ear hypertrophy has severely reduced gape in and Microdipodops (Fig. 1). Relative to over- Dipodomys and Microdipodops. All skulls drawn to same all head size, the middle ear chambers length. achieve their greatest volume in the latter genus (Webster 1961). Detailed comparative The innovative studies of the Websters and morphologic data on the auditory region of their collaborators have gradually revealed heteromyids have been provided by Webster the functional and probable adaptive signifi- and Webster (1975, 1977, 1980). cance of the modified ears of desert hetero- 46 Great Basin Naturalist Memoirs No. 7 myids. Auditory specializations in these ro- presumed that mortality was due chiefly to dents, and in certain Old World desert spe- higher rates of predation upon kangaroo rats cies (Lay 1972), improve the detection of rel- whose ability to detect low frequency sound atively low frequency sound, especially in the had been reduced. Laboratory recordings in- 1-3 KHz range. Selective sensitivity to these dicate that the predatory strikes of both owls frequencies has been established on the basis and rattlesnakes produce significant sound in of physiological (Ruppert and Moushegian the 1-3 KHz range (Webster 1962). In sum, 1970, Vernon et al. 1971, Webster and Stro- the available data strongly imply that the ther 1972, Webster and Webster 1972) and specialized auditory apparatus of desert het- experiments (Webster and Web- behavioral eromyids is indeed adaptive, and that it may suite of structural features ap- ster 1972). A confer its greatest advantage on individuals pear to be responsible for increased sensi- foraging under conditions of dim tivity to low frequency sound by lowering illumination. impedance and, hence, increasing the trans- There has been little functional analysis of mission of such sound from the external to in- the feeding mechanism of heteromyid ro- ner ear. Among these features are: a rela- (1) dents. Most studies have been concerned with tively large, compliant tympanic membrane; dental morphology as it relates to systematics a small, low-mass,
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