Browsing by Subject "Body size"

To evaluate alternative hypotheses for the role of mass and muscle-induced forces in joint construction, the allometric scaling relationships of the articular facets of the talus were estimated with phylogenetic regressions. Many articular surfaces scale with significant positive allometry, suggesting that mass-induced forces are an important influence for the bony architecture of synovial joints.

Using a large sample of primates and their nearest living relatives, body mass prediction equations were generated from the articular facet areas of the talus and calcaneus. Those facets that scaled with positive allometry were both accurate and precise. Compared to previously published prediction equations, the novel equations developed for this study were substantially more reliable.

Several methodological debates for body mass prediction were also evaluated. Prediction equations had their highest correlations when species with greater than a 20% difference between sexes are represented by both males and females. Using dental measurements from cercopithecoids housed at the National Museum of Natural History, predictive accuracy was maximized when body mass was predicted using a mean value estimated from a robust sample. Even when only a single individual was represented, tests of predictive accuracy using primates with associated body masses from several localities (Hacienda La Pacifica, Costa Rica; Gombe Stream National Park, Tanzania; Amboseli Reserve, Kenya; and the Duke Lemur Center) demonstrated that prediction equations provide more accurate predictions of species mean values than individual-specific values.

The importance of longitudinal change in body mass was evaluated by comparing coefficients of variation for individual and mean body mass of the populations at La Pacifica, Gombe, and the Duke Lemur Center. Individual coefficients of variation were significantly greater than the population coefficients of variation, which suggests that mean body masses are more stable “targets” of prediction.

Finally, the novel prediction equations were applied to a sample of sympatric primates with associated dental and postcranial elements from the middle Eocene of Wyoming, including Notharctus tenebrosus, Smilodectes gracilis, Omomys carteri, and Hemiacodon gracilis. New body mass predictions suggest two pairs of similarly sized primates: N. tenebrosus and S. gracilis (~2500g), and O. carteri and H. gracilis (~400g). Thus, niche partitioning between closely related taxa was probably achieved through differences in diets, rather than differences in body mass.

Islands have been the inspiration for some of evolutionary biology's most important advances. This is largely due to the unique properties of islands that promote the differentiation of island species from their mainland counterparts. Rodents are widely distributed across even the most remote islands, a rarity among mammals, making them uniquely suited to study the factors leading to the divergence of insular species. In this dissertation, I use two case studies to examine the morphological and genetic divergences that take place in an insular environment.

In chapters one and two, I examine how different factors influence insular body size change in rodents. In chapter one, I examine factors influencing the direction of island body size change using classification tree and random forest (CART) analyses. I observe strong consistency in the direction of size change within islands and within species, but little consistency at broader taxonomic scales. Including island and species traits in the CART analyses, I find mainland body mass to be the most important factor influencing size change. Other variables are significant, though their roles seem to be context-dependent.

In chapter two, I use the distributions of mainland rodent population body sizes to identify `extreme' insular rodent populations and compare traits associated with those populations and their islands with those island populations of a more typical size. I find that althought there is no trend among all insular rodents towards a larger or smaller size, `extreme' populations are more likely to increase in size. Using CART methods, I develop a predictive model for insular size change that identifies resource limitations as the main driver when insular rodent populations become `extremely small'.

Chapters three and four shift their focus to a single rodent species, the deer mouse Peromyscus maniculatus, as they examine the genetic differentiation of deer mice across the California Channel Islands and the nearby mainland. In chapter three, I sequence a region of the mitochondrial control region for individuals from 8 populations across the northern Channel Islands and two mainland sites, and I analyze these sequences by calculating population genetics parameters and creating a Bayesian inference tree and a statistical parsimony haplotype network. All of these analyses reveal significant divergences between island and mainland populations. Among the islands, Santa Barbara and Anacapa islands both display unique genetic signatures, but the other northern islands remain relatively undifferentiated.

In chapter four, I genotype individuals from the previous chapter at 5 microsatellite loci, I calculate additional population genetics parameters and I utilize a Bayesian clustering algorithm to examine the similarities and differences between nuclear and mitochondrial analyses. I find the nuclear data to be largely congruent with the mitochondrial analyses; there are significant differences between island and mainland populations, and Anacapa Island is significantly differentiated from the other islands. Unlike the previous analyses, Santa Barbara Island is not significantly different from the northern islands, yet San Miguel Island has a unique genetic signature.

These studies underscore the importance of ecological processes and historical biogeography in the generation of diversity, and they highlight the role of islands as drivers of evolutionary divergence.

Positron emission tomography (PET) is a nuclear medicine diagnostic imaging exam of metabolic processes in the body. Radiotracers, which consist of positron emitting radioisotopes and a molecular probe, are introduced into the body, emitted radiation is detected, and tomographic images are reconstructed. The primary clinical PET application is in oncology using a glucose analogue radiotracer, which is avidly taken up by some cancers.

It is well known that PET performance and image quality degrade as body size increases, and epidemiological studies over the past two decades show that the adult US population's body size has increased dramatically and continues to increase. Larger patients have more attenuating material that increases the number of emitted photons that are scattered or absorbed within the body. Thus, for a fixed amount of injected radioactivity and acquisition duration, the number of measured true coincidence events will decrease, and the background fractions will increase. Another size-related factor, independent of attenuation, is the volume throughout which the measured coincidence counts are distributed: for a fixed acquisition duration, as the body size increases, the counts are distributed over a larger area. This is true for both a fixed amount of radioactivity, where the concentration decreases as size increases, and a fixed concentration, where the amount radioactivity increases with size.

Time-of-flight (TOF) PET is a recently commercialized technology that allows the localization, with a certain degree of error, of a positron annihilation using timing differences in the detection of coincidence photons. Both heuristic and analytical evaluations predict that TOF PET will have improved performance and image quality compared to non-TOF PET, and this improvement increases as body size increases. The goal of this dissertation is to parameterize the image quality improvement of TOF PET compared to non-TOF PET as a function of body size. Currently, no standard for comparison exists.

Previous evaluations of TOF PET's improvement have been made with either computer-simulated data or acquired data using a few discrete phantom sizes. A phantom that represents a range of attenuating dimensions, that can have a varying radioactivity distribution, and that can have radioactive inserts positioned throughout its volume would facilitate characterizing PET system performance and image quality as a function of body size. A fillable, tapered phantom, was designed, simulated, and constructed. The phantom has an oval cross-section ranging from 38.5 × 49.5 cm to 6.8 × 17.8 cm, a length of 51.1 cm, a mass of 6 kg (empty), a mass of 42 kg (water filled), and 1.25-cm acrylic walls.

For this dissertation research, PET image quality was measured using multiple, small spheres with diameters near the spatial resolution of clinical whole-body PET systems. Measurements made on a small sphere, which typically include a small number of image voxels, are susceptible to fluctuations over the few voxels, so using multiple spheres improves the statistical power of the measurements that, in turn, reduces the influence of these fluctuations. These spheres were arranged in an array and mounted throughout the tapered phantom's volume to objectively measure image quality as a function of body size. Image quality is measured by placing regions of interest on images and calculating contrast recovery, background variability, and signal to noise ratio.

Image quality as a function of body size was parameterized for TOF compared to non-TOF PET using 46 1.0-cm spheres positioned in six different body sizes in a fillable, tapered phantom. When the TOF and non-TOF PET images were reconstructed for matched contrast, the square of the ratio of the images' signal-to-noise ratios for TOF to non-TOF PET was plotted as a function, f(D), of the radioactivity distribution size, D, in cm. A linear regression was fit to the data: f(D) = 0.108D - 1.36. This was compared to the ratio of D and the localization error, σ_d, based on the system timing resolution, which is approximately 650 ps for the TOF PET system used for this research. With the image quality metrics used in this work, the ratio of TOF to non-TOF PET fits well to a linear relationship and is parallel to D/σ_d. For D < 20 cm, there is no image quality improvement, but for radioactivity distributions D > 20 cm, TOF PET improves image quality over non-TOF PET. PET imaging's clinical use has increased over the past decade, and TOF PET's image quality improvement for large patients makes TOF an important new technology because the occurrence of obesity in the US adult population continues to increase.

The initiation of metamorphosis causes the cessation of the larval growth period which determines the final body size of adult insects. Because larval growth is roughly exponential, differences in timing the initiation of metamorphosis can cause large differences body size. Although many of the processes involved in metamorphosis have been well characterized, little is known about how the timing of the initiation of metamorphosis is determined.

Using different strains from Tribolium castaneum, Tribolium freemani, and Manduca sexta and varied nutritional conditions, I was able to document the existence of a threshold size, which determines when the larva becomes competent to metamorphose. Threshold size, however, does not dictate the exact timing of initiation. The exact timing for the initiation of metamorphosis is determined by a pulse of the molting hormone, ecdysone, but only after threshold size has been reached. Ecdysone pulses before the larva attains threshold size only cause the larva to molt to another larval instar. These results indicate the timing of metamorphosis initiation is controlled by two factors: (1) attainment of threshold size, at which the larva becomes competent to initiate metamorphosis and (2) the timing of an ecdysone pulse after attaining threshold size.

I hypothesize the attainment of threshold size, and therefore competence to metamorphose, is mediated by the effect of changing juvenile hormone concentrations caused by the increase in size of the larva. While the larval body grows nearly exponentially, the corpora allata, which secretes juvenile hormone, grows very little if at all. The difference in relative growth causes juvenile hormone concentrations to gradually become diluted. When juvenile hormone concentrations fall below a threshold, changes in protein-protein binding occur that can cause changes in signaling networks and ultimately gene expression. These changes make the larva competent for metamorphosis.

I have demonstrated that only threshold size is consistently correlated with body size; other growth parameters such as growth rate, duration of instars, or number of instars do not consistently correlate with variation in body size. Using the black mutant strain of M. sexta I have shown that lower juvenile hormone titers correlate with lower threshold sizes. My hypothesis is consistent with the large body of literature indicating the involvement of juvenile hormone. I also hypothesize that the diversity of metamorphosis types in holometabolous insects can be explained by heterochronic shifts in the timing of threshold size and other developmental events related to metamorphosis. The heterochronic shifts affect not only the morphology of organs, but can also affect the overall phenotypic response of the larva to changes in the environment. The different phenotypic responses among species may make the more or less suited for certain types of niches.