Boxfish (Ostraciidae) have peculiar body shapes, with conspicuous keels formed by their bony carapaces. Previous studies have proposed various hydrodynamic roles for these keels, including reducing drag during swimming, contributing to passive stabilization of the swimming course, or providing resistance against roll rotations. Here, we tested these hypotheses using computational fluid dynamics simulations of five species of Ostraciidae with a range of carapace shapes. The hydrodynamic performance of the original carapace surface models, obtained from laser scanning of museum specimens, was compared with models where the keels had been digitally reduced. The original carapaces showed no reduced drag or increased passive stability against pitch and yaw compared to the reduced-keel carapaces. However, consistently for all studied species, a strong increase in roll drag and roll-added mass was observed for the original carapaces compared to the reduced-keel carapaces, despite the relatively small differences in keel height. In particular, the damping of roll movement by resistive drag torques increased considerably by the presence of keels. Our results suggest that the shape of the boxfish carapace is important in enabling the observed roll-free forward swimming of boxfish and may facilitate the control of manoeuvres.
Intra-uterine undernutrition in humans typically results in low birth weight (small for gestational age; SGA) and delayed post-natal neuromotor maturation. Since SGA and intra-uterine growth retardation are also common in domestic pigs, piglets are premised as models to study delayed motor development. Applied to the locomotor paradigm, however, questions emerge: (i) how to map the developmental time scale of the precocial model onto the altricial target species, and (ii) how to distinguish size from maturation effects? Gait data were collected at self-selected voluntary walking speed during early development (0 hours–96 hours post-partum; pp) for SGA- and normal (appropriate for gestational age; AGA) piglets. Dimensionless spatiotemporal gait characteristics (according to dynamic similarity) become invariant already after 4 hours pp, suggesting rapid post-natal neuromotor maturation. Moreover, dimensionless gait data are largely identical for SGA- and AGA-siblings, indicating that primarily size effects explain absolute locomotor differences. This is further supported by (i) normalized force-generating capacity of limb muscles, (ii) joint kinematics (< 10 hours pp), and (iii) normalized ground reaction forces (< 5 days pp) being indifferent between SGA- and AGA-piglets. Furthermore, predictive modelling based on limb joint kinematics is unable to discern the majority of SGA- from AGA-piglets (<10 hours PP). All this leads to the conclusion that, although smaller than the AGA-piglets in absolute terms, SGA-piglets mature (neuromechanically speaking) just like, and equally fast as their AGA-littermates. Yet, it remains a fact that early SGA-piglets are reported to be less mobile, less vital, and less competitive than their AGA-siblings (even often die before day 3 pp). This conspicuous difference likely results from the energy level (blood glucose and glycogen) and its mobilisation being considerably different between the piglet categories during early development.
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