Bridging the gap: Biomechanics of vertical gap-crossing in snakes and cicadas

Abstract

The ability to locomote through arboreal environments is essential for the animals which make those environments their home. An animal's ability to move among branches can depend on both environmental features, such as branch diameter or discontinuities in branch support, as well as the animals' features such as claws, sticky toe pads, or the presence or absence of limbs. Periodical cicadas (Magicada septendecim) are an example of animals which must overcome both discontinuities and changes in branch diameter. After spending 13-17 years of their lives underground, periodical cicadas burrow their way to the surface, climb trees to molt into adults, and then climb to the edges of branches, mate, and oviposit their eggs. Both nymphs and adults must overcome the same surfaces, yet how the environmental factors of these surfaces interact with the age of the subject to influence climbing performance remains unknown. Additionally, the influence of arboreal surface structure on animal locomotion is not limited to insects, or even limbed animals. Limbless animals, such as arboreal snakes, must contend with these same issues. While perch diameter's effects on horizontal locomotor speed and balance have been well explored, its influence on limbless animal's abilities to cross vertical gaps between branches remains relatively understudied. Despite perch diameter's known influence on an animal's ability to prevent toppling, no study has recorded how this influence translates to its influence on vertical gap crossing ability for limbless animals. This dissertation seeks to answer how arboreal surface structures shape locomotor performance across diverse taxa. Chapter 2 presents a study asking how increasing perch diameter and varying surface friction influences adult periodical cicadas ability to navigate cylindrical surfaces, showing that, at smaller diameters, cicadas are able to cross distances which proved difficult to impossible at larger diameters. Chapter 3 expands on this, exploring gap size's relationships with successful climbs but also compares the performance of adult cicadas to those of nymphs. Additionally, a series of trials are introduced within this chapter to explore surface roughness's influence on climbing capability for both age groups, revealing relationships between surface roughness and success for both age groups, as well as age groups and how well they succeed, defined by how quickly they completed the cross. Finally, Chapter 4 explores diameter's influence on vertical gap crossing in arboreal snakes, exploring factors such as the heights they reach, the speed at which they reach these heights, how well they maintain balance in doing so, and the torque the portion of the bodies within the gap experience. My results have found that perch diameter does influence several performance variables in arboreal locomotion for cicadas and snakes. These variables include, but are not limited to, maximum size of gaps possible to cross for the former, and heights reached for the latter. Additionally, I found that age groups influence the performance of periodical cicadas on climbing structures, while the surface conditions of these structures influence their ability to successfully climb.