Weight support patterns vary widely among mammals. Differences in how much of the body weight is supported by the fore- versus the hind-limbs are well documented among and within species. Intraindividual variation due to ontogenetic processes has been studied in several hindlimb-dominated species and consistently showed a caudal shift in the limbs' support roles. We hypothesized that forelimb-dominated species exhibit a cranial shift in their support pattern and tested this hypothesis by examining the vertical ground reaction forces in growing dogs. Six male Beagle siblings were studied from 9 to 51 postnatal weeks (PW) of age while they trotted on an instrumented treadmill. Ontogenetic shifting in fore-to-hind support was evaluated using vertical force ratios (i.e., peak and impulse) as well as the stance time ratio of the fore- and the hind-limbs. Because morphological and kinematic characteristics influence weight support patterns, changes in body shape (i.e., trunk shape), and average limb position were determined. As in adult dogs, the forelimbs carried a greater proportion of the body weight than the hindlimbs at all ages. When the dogs were younger, peak vertical force occurred earlier during stance in the hindlimbs but not the forelimbs. Both the increasing ratio of the vertical force and the increasing ratio of the stance times indicate an increasing weight support by the forelimbs (i.e., 59% at PW9 vs. 63% at PW51). The observed ontogenetic changes in trunk shape and average limb angle were consistent with this cranial shift in weight support.

BackgroundMammalian juveniles undergo dramatic changes in body conformation during development. As one of the most common companion animals, the time line and trajectory of a dog’s development and its body’s re-proportioning is of particular scientific interest. Several ontogenetic studies have investigated the skeletal development in dogs, but none has paid heed to the scapula as a critical part of the mammalian forelimb. Its functional integration into the forelimb changed the correspondence between fore- and hindlimb segments and previous ontogenetic studies observed more similar growth patterns for functionally than serially homologous elements. In this study, the ontogenetic development of six Beagle siblings was monitored between 9 and 51 weeks of age to investigate their skeletal allometry and compare this with data from other lines, breeds and species.ResultsBody mass increased exponentially with time; log linear increase was observed up to the age of 15 weeks. Compared with body mass, withers and pelvic height as well as the lengths of the trunk, scapula, brachium and antebrachium, femur and crus exhibited positive allometry. Trunk circumference and pes showed negative allometry in all, pelvis and manus in most dogs. Thus, the typical mammalian intralimb re-proportioning with the proximal limb elements exhibiting positive allometry and the very distal ones showing negative allometry was observed. Relative lengths of the antebrachium, femur and crus increased, while those of the distal elements decreased.ConclusionsBeagles are fully-grown regarding body height but not body mass at about one year of age. Particular attention should be paid to feeding and physical exertion during the first 15 weeks when they grow more intensively. Compared with its siblings, a puppy’s size at 9 weeks is a good indicator for its final size. Among siblings, growth duration may vary substantially and appears not to be related to the adult size. Within breeds, a longer time to physically mature is hypothesized for larger-bodied breeding lines. Similar to other mammals, the Beagle displayed nearly optimal intralimb proportions throughout development. Neither the forelimbs nor the hindlimbs conformed with the previously observed proximo-distal order of the limb segment’s growth gradients. Potential factors responsible for variations in the ontogenetic allometry of mammals need further evaluation.