Animal biomechanics
From Biomch-W
Contents |
The Comparative Approach
Introduction
Under Construction
Phylogenetic Approach
The phylogenetic approach is a useful tool for organizing cross-species observations regarding the functional morphology of specialization and conservation. Closely related to systematics, the field of cladistics was founded by Willi Hennig (1966) in an attempt to explain biological diversity through logical data structures such as trees. This "tree of life" approach has fared quite well and provides logical consistency with Darwinian theory, although in some biomechanical circumstances other relational structures provide a more parsimonius explaination of the data. Cladistic theory will provide us with a number of rules and assumptions (quite independent of the relational structure used) for inferring how traits have evolved over time given their current distribution in related taxa.
Definitions
- Taxa
- Unit of analysis. This is usually a species, but can also be a morphologically specialized population within a species.
- Trait
- A single feature or function, such as a wing, a joint configuration, or a gait pattern. Traits can be further divided into states. States (at an interval or ordinal level of measurement) are particularly important for the inferrence of evolutionary hypotheses.
- Specialization
- A trait that arises de novo in a particular lineage and is not directly associated with other lineages. Can also be referred to as a derived trait.
- Conservation
- A trait that is retained across multiple lineages. A conserved trait is assumed to have a common ancestral origin, regardless of how many lineages it is associated with.
- Synapomorphy
- A shared and derived trait, found within a single lineage.
- Apomorphy
- A derived trait present in only a single taxa.
- Outgroup
- A taxa or group of taxa that are used to polarize the phylogeny. Typically, polarization is used to determine which state of the trait is ancestral. They are also used to order various states of a trait. Taxa that are far removed phylogenetically are typically used so as to ensure that any traits shared in common with the taxa being analyzed are ancestral forms.
- Clade
- A distinct lineage that shares common trait states among themselves and can be clustered into a subgroup based on this evidence.
- Sympleisiomorphy
- Technical name for a conserved trait. In this case, the trait has remained in an ancestral state in most, if not all, of the taxa under analysis.
- Homology
- The holy grail of comparative work. A homology is a trait of the same or closely related states distributed across related taxa. When a trait is homologous, it is usually evidence of modification through descent. Therefore, homologous traits point to evoluitionary divergence. The bony fish pectoral fin and the forelimbs of tetropods are considered to be homologous.
- Homoplasy
- The type II error of comparative work. Homoplasy can be equated with convergent evolution, or a lack of evolutionary divergence between two taxa even though they share traits with closely related states. Comparing the bat wing and the bird wing demonstrates this -- it is apparent that each structure is a wing and they both help the organism fly, even though each type of wing arises through different mechanisms and the patterns of flight are quite different.
Sources
Hennig, W. (1966). Phylogenetic Systematics. University of Illinois Press.
Developmental Approach
Another way to evaluate current biomechanical diversity is to probe the biomechanical performance of organisms during development. This is useful in two ways:
- Traits that appear at different stages in development can determine the future morphology of the organism. Koehl (1990) and Koehl et al (2000) argue that morphogenesis during development provides a template upon which further morphological and biomechanical change can take place. For example, the osmotic inflation of notochords in Xenopus laevis embryos results in an elongated and straightened vertebral axis. This was primarily due to both inflation at a particular stage in development, and the arrangement of helically-shaped fibers around the chord.
- This approach conforms to the old idea of "phylogeny recapitulating ontogeny" (see Gould, 1977) -- that is, forms of a trait that appear early in development are indiciative of an organism's phylogenetic history.
Sources
Gould, S.J. (1977). Ontogeny and Phylogeny. Cambridge, MA: Harvard.
Koehl, M. A. R. (1990). Biomechanical approaches to morphogenesis. Seminars in Developmental Biology 1, 367-378.
Koehl, M.A.R, Quillin, K.J., and Pell, C.A. (2000). Mechanical design of fiber-wound hydraulic skeletons -- the stiffening and straightening of embryonic notochords. American Zoologist, 40(1), 28-41.
Experimental Approach
Under Construction
Model Systems
Muscular Hydrostat
This model was proposed by Kier and Smith (1985) to approximate the behavior of elongated muscular structures such as the tentacles of an octopus or a lizard's tongue (Meyers et al, 2004). These structures are composed mainly of muscle, but need to be coordinated and controlled during movement much like the forelimbs and hindlimbs of tetropods.
Tetropods control their limbs using two sets of principles; the kinetic link principle and theories regarding motor control such as the point-equilibrium hypothesis and the unconstrained manifold hypothesis). Since the muscular structures in question do not have distinct anatomical segments, these principles do not apply directly. However, experimental investigations have demonstrated that both octopii and lizards can control the production of force and motor control without the standard tetropod control system.
Passive Dynamic Walking
This approach was proposed by McGeer (1990) using an inverted pendulum to demonstrate that dynamic regulation of the center of mass during gait can provide a organizing and stabilizing mechanism for movement. Bertram et al (2003) have demonstrated that gibbon brachiation can be explained using this framework.
Neuromechanics
Nishikawa (2002) have advanced this method using closely related fish species, observing their behaviors, and then probing the activity of related neural circuits. It has been found that closely related species with somewhat similar behaviors can exhibit quite different neural circuitries.
Biologically-Inspired Modeling
This approach examines the structure and function of specialized systems in a number of model organisms, and attempts to extrapolate their design and performance dynamics to technological systems. The work of MacIver and Nelson (2000, 2001) and Nelson et al (2002) on the electrosensory system of weakly electric fishes may prove to provide fertile ground for further investigation.
Sources
Bertram, J.E.A., Ruina, A., Cannon, C.E., Chang, Y-H., and Coleman, M. (1999). A Point Mass Model of Gibbon Locomotion. Journal of Experimental Biology, 202, 2609-2617.
Kier, W.M. and Smith, K.K. (1985). Tongues, tentacles, and trunks: the biomechanics of movement in muscular hydrostats. Zoological Journal of the Linnean Society, 83, 307-324.
MacIver, M.A. and Nelson, M.E. (2001). Towards a biorobotic electrosensory system. Autonomous Robots, 11, 263-266.
MacIver, M.A. and Nelson, M.E. (2000). Body modeling and model-based tracking for neuroethology. Journal of Neuroscience Methods, 95, 133-143.
Meyers, J.J., O'Reilly, J.C., Monroy, J.M., and K.C. Nishikawa 2004. Mechanism of tongue protrusion in Microhylid frogs. Journal of Experimental Biology, 207, 21-31.
Nelson, M.E., MacIver, M.A., and Coombs, S. (2002). Modeling electrosensory and mechanosensory images during the predatory behavior of weakly electric fish. Brain, Behavior, and Evolution, 59, 199-210.
Nishikawa, K.C. 2002. Evolutionary convergence in nervous systems: insights from comparative phylogenetic studies. Brain, Behavior and Evolution, 59(5-6), 240-249.
