Differential growth of the adductor muscles, eyeball, and brain in the chick Gallus gallus with comments on the fossil record of stem-group birds.

The avian head is unique among living reptiles in its combination of relatively large brain and eyes, coupled with relatively small adductor jaw muscles. These derived proportions lend themselves to a trade-off hypothesis, wherein adductor size was reduced over evolutionary time as a means (or as a consequence) of neurosensory expansion. In this study, we examine this evolutionary hypothesis through the lens of development by describing the jaw-adductor anatomy of developing chickens, Gallus gallus, and comparing the volumetric expansion of these developing muscles with growth trajectories of the brain and eye. Under the trade-off hypothesis, we predicted that the jaw muscles would grow with negative allometry relative to brain and eyes, and that osteological signatures of a relatively large adductor system, as found in most nonavian dinosaurs, would be differentially expressed in younger chicks. Results did not meet these expectations, at least not generally, with muscle growth exhibiting positive allometry relative to that of brain and eye. We propose three, nonmutually exclusive explanations: (1) these systems do not compete for space, (2) these systems competed for space in the evolutionary past, and growth of the jaw muscles was truncated early in development (paedomorphosis), and (3) trade-offs in developmental investment in these systems are limited temporally to the perinatal period. These explanations are considered in light of the fossil record, and most notably the skull of the stem bird Ichthyornis, which exhibits an interesting combination of plesiomorphically large adductor chamber and apomorphically large brain.

Modeling visual fields using virtual ophthalmoscopy: Incorporating geometrical optics, morphometrics, and 3D visualization to validate an interdisciplinary technique.

Visual fields are of particular interest to comparative biologists studying the complex interplay between anatomy, physiology, ecology, and optics. Visual fields have been measured in nearly 100 bird species, and investigators have uncovered associations between quantitative aspects of visual fields and foraging behavior, skull dimensions, and even brain morphology. However, limiting factors including time, access to living subjects, and experimental constraints complicate study of the visual apparatus of rare, endangered, or extinct species. We introduce a modeling technique called virtual ophthalmoscopy (VO) for estimating visual fields of vertebrates. We compare this in-silico technique, which draws on geometrical optics, morphometrics of eyes, and 3D visualization, against experimental data from 12 bird species from behavioral literature. Known values of optical properties, including axial length, lens curvatures, and refractive index, are used to construct and test virtual, schematic eyes in ray-tracing software. Resulting visual fields are measured in 3D-visualization software. These measurements are compared qualitatively and quantitatively with visual fields from the literature. Schematic eyeballs and in-silico visual fields, after iterative improvements using anatomical information from cadaveric specimens, approximate experimental data to the extent of falling within the range of intraspecific variation, suggesting VO is a viable technique for modeling visual fields. Virtual ophthalmoscopy creates an opportunity to expand the sample of species for which visual fields can be quantified and allows new hypotheses regarding the evolution of visual systems to be tested.

Intraspecific variation and symmetry of the inner-ear labyrinth in a population of wild turkeys: implications for paleontological reconstructions.

The cochlea and semicircular canals (SCCs) of the inner ear are vital neurosensory devices. There are associations between the anatomy of these sensorineural structures, their function, and the function of related biological systems, for example, hearing ability, gaze stabilization, locomotor agility, and posture. The endosseous labyrinth is frequently used as a proxy to infer the performance of the hearing and vestibular systems, locomotor abilities, and ecology of extinct species. Such fossil inferences are often based on single specimens or even a single ear, representing an entire species. To address whether a single ear is representative of a population, we used geometric morphometrics to quantitatively assess the variation in shape and symmetry in a sample of endosseous labyrinths of wild turkeys Meleagris gallopavo of southern Ohio. We predicted that ears would be symmetrical both within individuals and across the sample; that labyrinth shape and size would covary; that labyrinth shape would vary with the size of the brain, measured as width of the endocranium at the cerebellum; and that labyrinths would be morphologically integrated. To test these predictions, we microCT-scanned the heads of 26 cadaveric turkeys, digitally segmented their endosseous labyrinths in Avizo, and assigned 15 manual landmarks and 20 sliding semilandmarks to each digital model. Following Procrustes alignment, we conducted an analysis of bilateral symmetry, a Procrustes regression analysis for allometry and other covariates including side and replicate, and analyses of global integration and modularity. Based on Procrustes distances, no individual's left and right ears were clearly different from each other. When comparing the ears of different specimens, statistically clear differences in shape were found in only 66 of more than 1,300 contrasts. Moreover, effects of both directional and fluctuating asymmetry were very small-generally, two orders of magnitude smaller than the variance explained by individual variation. Statistical tests disagreed on whether these asymmetric effects crossed the threshold of significance, possibly due to non-isotropic variation among landmarks. Regardless, labyrinths appeared to primarily vary in shape symmetrically. Neither labyrinth size nor endocranial width was correlated with labyrinth shape, contrary to our expectations. Finally, labyrinths were found to be moderately integrated in a global sense, but four weakly separated modules-the three SCCs and cochlea-were recovered using a maximum-likelihood analysis. The results show that both fluctuating and directional asymmetry play a larger role in shape variation than expected-but nonetheless, endosseous labyrinths are symmetrical within individuals and at the level of the population, and their shape varies symmetrically. Thus, inferences about populations, and very possibly species, may be confidently made when only a single specimen, or even a single ear, is available for study.