Design of A Highly Sensitive, Low-cost Underwater Force Plate to Record Substrate Reaction Forces.

The study of underwater walking presents major challenges because the small forces applied during underwater walking are difficult to measure due to the lack of a sufficiently sensitive force plate that functions underwater. Understanding the force interaction between the underwater walker and the substrate may lead to better understanding of the evolution, ecology, and biomechanics of underwater walking. The shift from aquatic to terrestrial life was a crucial transition in animal evolution where, underwater walking preceded the invasion of land and combines mechanics from terrestrial locomotion (substrate reaction forces) and aquatic swimming (buoyancy and drag). In this work, we describe our design of a low-cost underwater force plate made using 3D printed multi axis load cells equipped with commercial strain gauges amplified with a custom circuit board, and custom code to gather force data. The use of 3D printed sensors allows customization of the material and thickness of the shear beam load cell to accommodate the loads for a wide range of study species. We show that our design can detect loads as small as 1 mN (filtered) with minimal noise and present sample live animal trials of several species. The 3D multiaxial load cells, circuit design, and custom code are open-source and available online.

Ground Reaction Forces and Energy Exchange During Underwater Walking.

Underwater walking was a crucial step in the evolutionary transition from water to land. Underwater walkers use fins and/or limbs to interact with the benthic substrate and produce propulsive forces. The dynamics of underwater walking remain poorly understood due to the lack of a sufficiently sensitive and waterproof system to measure substrate reaction forces (SRFs). Using an underwater force plate (described in our companion paper), we quantify SRFs during underwater walking in axolotls (Ambystoma mexicanum) and Spot prawn (Pandalus platyceros), synchronized with videography. The horizontal propulsive forces were greater than the braking forces in both species to overcome hydrodynamic drag. In axolotls, potential energy (PE) fluctuations were far smaller than kinetic energy (KE) fluctuations due to high buoyant support (97%), whereas the magnitudes were similar in the prawn due to lower buoyant support (93%). However, both species show minimal evidence of exchange between KE and PE, which, along with the effects of hydrodynamic drag, is incompatible with inverted pendulum dynamics. Our results show that, despite their evolutionary links, underwater walking has fundamentally different dynamics compared with terrestrial walking and emphasize the substantial consequences of differences in body plan in underwater walking.

External power amplification drives prey capture in a spider web.

Power amplification allows animals to produce movements that exceed the physiological limits of muscle power and speed, such as the mantis shrimp's ultrafast predatory strike and the flea's jump. However, all known examples of nonhuman, muscle-driven power amplification involve anatomical structures that store energy from a single cycle of muscular contraction. Here, we describe a nonhuman example of external power amplification using a constructed device: the web of the triangle-weaver spider, Hyptiotes cavatus, which uses energy stored in the silk threads to actively tangle prey from afar. Hyptiotes stretches its web by tightening a separate anchor line over multiple cycles of limb motion, and then releases its hold on the anchor line when insects strike the web. Both spider and web spring forward 2 to 3 cm with a peak acceleration of up to 772.85 m/s2 so that up to four additional adhesive capture threads contact the prey while jerking caused by the spider's sudden stop subsequently wraps silk around the prey from all directions. Using webs as external "tools" to store energy offers substantial mechanical advantages over internal tissue-based power amplification due to the ability of Hyptiotes to load the web over multiple cycles of muscular contraction and thus release more stored energy during prey capture than would be possible with muscle-driven anatomical elastic-energy systems. Elastic power amplification is an underappreciated component of silk's function in webs and shows remarkable convergence to the fundamental mechanical advantages that led humans to engineer power-amplifying devices such as catapults and ballistae.

Chasing maximal performance: a cautionary tale from the celebrated jumping frogs of Calaveras County.

Maximal performance is an essential metric for understanding many aspects of an organism's biology, but it can be difficult to determine because a measured maximum may reflect only a peak level of effort, not a physiological limit. We used a unique opportunity provided by a frog jumping contest to evaluate the validity of existing laboratory estimates of maximum jumping performance in bullfrogs (Rana catesbeiana). We recorded video of 3124 bullfrog jumps over the course of the 4-day contest at the Calaveras County Jumping Frog Jubilee, and determined jump distance from these images and a calibration of the jump arena. Frogs were divided into two groups: 'rental' frogs collected by fair organizers and jumped by the general public, and frogs collected and jumped by experienced, 'professional' teams. A total of 58% of recorded jumps surpassed the maximum jump distance in the literature (1.295 m), and the longest jump was 2.2 m. Compared with rental frogs, professionally jumped frogs jumped farther, and the distribution of jump distances for this group was skewed towards long jumps. Calculated muscular work, historical records and the skewed distribution of jump distances all suggest that the longest jumps represent the true performance limit for this species. Using resampling, we estimated the probability of observing a given jump distance for various sample sizes, showing that large sample sizes are required to detect rare maximal jumps. These results show the importance of sample size, animal motivation and physiological conditions for accurate maximal performance estimates.