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The purpose of this study was to examine the pattern of postnatal brain growth in two wild canid species: the coyote (Canis latrans) and gray wolf (Canis lupus). Adult regional and total brain volume differences were also compared between the two species as well as within each species by sex. Three-dimensional virtual endocasts of endocranial airspace were created from computed tomography scans of 52 coyote skulls (28 female, 24 male; 1 day to 13.4 years) and 46 gray wolf skulls (25 female, 21 male; 1 day to 7.9 years). Age was known in coyotes or estimated from dentition patterns in wolves. The 95% asymptotic growth of the endocranium is completed by 21 weeks in male and 17.5 weeks in female coyotes and by 27 weeks in male and 18.5 weeks in female wolves. These ages are well before age at first reproduction (coyote – 40.4 weeks; wolf – 91.25 weeks). Skull growth as measured by centroid size lags behind endocranial growth but is also completed before sexual maturity. Intra- and interspecific comparisons of brain volumes in the adult wolves and coyotes revealed that relative anterior cerebrum (AC) volume was greater in males than females in both species. Relative brain size was greater in the coyote than in the wolf as was relative cerebrum volume. However, relative AC volume and relative cerebellum and brainstem volume was greater in the wolf than coyote. One explanation for the increased AC volume in males compared to females may be related to the role of social information processing. However, additional data are needed to determine the correspondence between regional volumes and functional differences either between or within these species. Nonetheless, these findings provide important baseline data for further studies on wild canid brain variations and development. [ABSTRACT FROM AUTHOR]

Canalization may play a critical role in molding patterns of integration when variability is regulated by the balance between processes that generate and remove variation. Under these conditions, the interaction among those processes may produce a dynamic structure of integration even when the level of variability is constant. To determine whether the constancy of variance in skull shape throughout most of postnatal growth results from a balance between processes generating and removing variation, we compare covariance structures from age to age in two rodent species, cotton rats ( Sigmodon fulviventer) and house mice ( Mus musculus domesticus). We assess the overall similarity of covariance matrices by the matrix correlation, and compare the structures of covariance matrices using common subspace analysis, a method related to common principal components (PCs) analysis but suited to cases in which variation is so nearly spherical that PCs are ambiguous. We find significant differences from age to age in covariance structure and the more effectively canalized ones tend to be least stable in covariance structure. We find no evidence that canalization gradually and preferentially removes deviations arising early in development as we might expect if canalization results from compensatory differential growth. Our results suggest that (co)variation patterns are continually restructured by processes that equilibrate variance, and thus that canalization plays a critical role in molding patterns of integration. [ABSTRACT FROM AUTHOR]

In the absence of processes regulating morphogenesis and growth, phenotypic variance of a population experiencing no selective mortality should increase throughout ontogeny. To determine whether it does, we measure variance of skull shape using geometric morphometrics and examine its ontogenetic dynamics in the precocial cotton rat (Sigmodon fulviventer) and the altricial house mouse (Mus musculus domesticus). In both species, variance of shape halves between the two youngest samples measured (between 1 and 10 days postnatal and 10 and 15 days postnatal, respectively) and thereafter is nearly constant. The reduction in variance did not appear to result from a general regulation of skull size or developmental timing, although skull size may also be regulated and developmental timing is an important component of the variation in skull shape of young house mice. The ontogenetic dynamics of variance suggest two possible scenarios. First, variation generated during fetal or early postnatal growth is not immediately compensated and therefore accumulates, whereas later in growth, variation is continually generated and rapidly compensated. Second, variation generated during fetal and early postnatal growth is rapidly compensated, after which no new variance is produced. Based on a general model for bone growth, we hypothesize that variance is generated when bone grows under the direction of disorganized muscular movements and decreases with increasing neuromuscular control. Additionally, increasing coherence of signals transmitted by the growing brain and sensory organs, which exert tensile forces on bone, may also canalize skull shape. [ABSTRACT FROM AUTHOR]

Abstract Variation in neonatal maturity among mammals is often explained by variation in gestation length, but species may also differ in developmental rate, a quantity that is difficult to measure because the conventional formalism makes two important and potentially unrealistic assumptions: (1) ontogeny of form can be described by a single line, and (2) species have the same ontogeny of form. We examine two species, one precocial (Sigmodon fulviventer ), the other altricial (Mus musculus domesticus ), and find that neither assumption is met. Therefore, we introduce an alternative metric, the rate of shape differentiation away from the average neonate. We find that S. fulviventer has a lower developmental rate than M. m. domesticus ; consequently, while more mature at birth, S. fulviventer loses ground to M. m. domesticus over time. Surprisingly, despite differences in gestation length and developmental rate, these species reach developmental and life-history milestones at nearly identical degrees of skull shape maturity. [ABSTRACT FROM AUTHOR]