Daffodil flowers extend laterally from the long axes of their stems; as a result, wind on a flower exerts torsional as well as flexural stress on the stem. Stems respond by twisting, and thus flowers reorient to face downwind in moderate winds, in the process reducing their drag by ϳ30%. This repositioning is facilitated by the stems' relatively low torsional stiffness. Daffodil stems have a ratio of flexural to torsional stiffness of 13.27 Ϯ 0.96 (SD), compared with 8.33 Ϯ 3.20 (SD) for tulip stems, which bear flowers as symmetrical extensions of their long axes, and compared with 1.5 for isotropic, incompressible, circular cylinders.

Most biological beams bend and twist relatively easily compared to human-made structures. This paper investigates flexibility in 57 diverse biological beams in an effort to identify common patterns in the relationship between flexural stiffness and torsional stiffness. The patterns are investigated by mapping both ideal and biological beams into a mechanospace defined by flexural and torsional stiffness. The distribution of biological beams is not random, but is generally limited to particular regions of the mechanospace. Biological beams that are stiff in bending are stiff in torsion, while those that bend easily also twist easily. Unoccupied regions of the mechanospace represent rare combinations of mechanical properties, without proving that they are impossible. The mechanical properties of biological beams closely resemble theoretical expectations for ideal beams. Both distributions are potentially being driven by the interdependence of the material and structural properties determining stiffness. The mechanospace can be used as a broadly comparative tool to highlight systems that fall outside the general pattern observed in this study. These outlying beams may be of particular interest to both biologists and engineers due to either material or structural innovations.

Foetal dolphins (Tursiops truncatus) are bent ventrolaterally, such that the tailflukes and lower jaw are juxtaposed. The lateral flexibility required en utero may compromise the efficiency of the dorsoventral oscillations required of the swimming neonate. The m. intertransversarius caudae dorsalis (IT) is the most laterally placed epaxial muscle. Bilateral contractions of the IT could limit lateral deformations of the flexible tailstock of the early neonate. We test the hypothesis that the IT is functioning as a postural muscle in neonates by examining its morphological, histological and biochemical properties. The neonatal IT has a relatively large cross-sectional area and bending moment, as well as a large proportion of slow-twitch fibres and elevated myoglobin concentrations. Our results demonstrate that the IT is functionally capable of performing this specific postural function in neonatal dolphins. In later life-history stages, when postural control is no longer needed, the IT serves to fine-tune the position of the tailstock during locomotion. The changing function of the adult IT is concomitant with changes in morphology and biochemistry, and most notably, with an increase in the proportion of fast-twitch fibres. We suggest that these changes reflect strong selective pressure to improve locomotor abilities by limiting lateral deformations during this critical life-history stage.

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Lily pads (Nymphaea odorata) exhibit heterophylly where a single plant may have leaves that are submerged, floating, or above (aerial) the surface of the water. Lily pads are placed in a unique situation because each leaf form is exposed to a distinctly different set of mechanical demands. While surface petioles may be loaded in tension under conditions of wind or waves, aerial petioles are loaded in compression because they must support the weight of the lamina. Using standard techniques, we compared the mechanical and morphological properties of both surface and aerial leaf petioles. Structural stiffness (EI) and the second moment of area (I) were higher in aerial petioles, although we detected no differences in other mechanical values (elastic modulus [E], extension ratio, and breaking strength). Morphologically, aerial petioles had a thicker rind, with increased collenchyma tissue and sclereid cell frequency. Aerial petioles also had a larger cross-sectional area and were more elliptical. Thus, subtle changes in the distribution of materials, rather than differences in their makeup, differentiate petiole forms. We suggest that the growth of aerial petioles may be an adaptive response to shading, allowing aerial leaves to rise above a crowded water surface.

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SUMMARYLate-term fetal bottlenose dolphins (Tursiops truncatus) are bent ventrolaterally en utero, requiring extreme flexibility of the axial skeleton and associated soft tissues. At birth, neonatal dolphins must immediately swim to the surface to breath, yet the dorsoventral oscillations used during locomotion may be compromised by the lateral flexibility evident in the fetus. The unique fetal position of dolphins, coupled with their need to swim at birth, places conflicting mechanical demands on the tailstock. Our previous research demonstrated that neonatal dolphins possess laterally placed, axial muscles that are functionally specialized to actively maintain the straightened posture of the tailstock. Here, we investigated the development of passive lateral stability in the tailstock of bottlenose dolphins by performing whole-body bending tests on an ontogenetic series of stranded dolphin specimens (N=15), including fetuses, neonates and juveniles (total length 58-171 cm). Structural stiffness increased, while overall body curvature decreased, with increasing body length. Scaling analyses suggest that increased structural stiffness is due to increases in size and probably changes in the passive material properties of the tailstock through ontogeny. The neutral zone was approximately constant with increasing size, while the relative neutral zone (neutral zone/total length) decreased. The lateral stability of the tailstock appears to be controlled by a combination of active and passive systems and the role of these systems varies through ontogeny. While neonates use active, muscular mechanisms to limit lateral deformations of the tailstock, the stability of the maturing tailstock is due primarily to its passive tissue properties.

Heterophylly, the production of different leaf forms on the same plant, is a widespread phenomenon in terrestrial and aquatic plants and provides an opportunity to study how sessile organisms sense and respond to changes in environmental factors. Nymphaea odorata subsp. tuberosa (American White Water Lily) produces 2 distinct leaf forms: a floating surface leaf and an aerial form in which the lamina is held above the water. Previous research suggests that changes in the light environment may be a critical determinant of heterophylly in Nymphaea. In this study, we tested the hypothesis that changes in light quantity and light quality stimulate the production of aerial leaf forms in water lilies. Specifically, shade cloth was used to reduce light intensity (quantity), and varying plant densities were used to increase leaf cover (affecting light quality) in artificial ponds. Aerial leaf production was not stimulated by reduction in light quantity alone but was when leaf cover exceeded 30-40%. We suggest that as the surface of a pond becomes covered with a canopy of leaves, American White Water Lily responds with the production of aerial leaves that rise above the surface of the water to gain access to light. Interestingly, water lilies exhibit an atypical shade response in that aerial leaves have short, thick petioles that allow them to rise above the surface of the water, rather than displaying the elongated phenotype associated with etiolation, which is the typical shade response of other flowering plants.