Take-off and landing forces and the evolution of controlled gliding in northern flying squirrelsGlaucomys sabrinus

Abstract

SUMMARY Flying squirrels are well known for their ability to glide between trees at the top of a forest canopy. We present experimental performance and behavioural evidence that flight in flying squirrels may have evolved out of a need to control landing forces. Northern flying squirrels were filmed jumping from a horizontal branch to a much larger vertical pole. These were both slightly compliant (less than 1.9 mm N–1), and instrumented using strain gauges so that forces could be measured. Take-off and landing forces were both positively correlated with horizontal range between 0.5 and 2.5 m (r=0.355 and r=0.811, respectively, P<0.05), but not significantly different to each other at each range tested. Take-off forces ranged from 1 to 10 bodyweights, and landing forces were between 3 and 10 bodyweights. Glide angles increased rapidly with horizontal range, approaching 45° at 3 m, above which they gradually decreased, suggesting that northern flying squirrels are optimised for long distance travel. We show that northern flying squirrels initiate full gliding posture at ranges of less than 1 m, without landing any higher than an equivalent ballistic projectile. However, this gliding posture enables them to pitch upwards, potentially stalling the wing, and spreads the landing reaction force over all four extended limbs. At steeper approach angles of close to 45°, flying squirrels were unable to pitch up sufficiently and landed forelimbs first, consequently sustaining higher impact forces. We investigate four hypotheses to explain the origin of flight in these animals and conclude that the need to reduce landing impact forces was most likely to have stimulated the development of aerial control in flying squirrels.

Touchdown to take-off: at the interface of flight and surface locomotion

“…Exploiting the principles of claws, van der Waals forces and wet adhesion, animals have evolved to generate the required attachment forces (figure 2) [60][61][62][63][64][65][66][67][68]. This enables animals to negotiate and exploit complex surfaces with a combination of effective aerial approaches, contact strategies, surface locomotion techniques and take-off manoeuvres of which the dynamics are not well understood [1,[69][70][71][72][73][74][75][76][77][78][79][80][81][82][83][84][85]. By contrast, aerial robots are just starting to implement some of these successful perching and locomotion strategies.…”

“…Gliding vertebrates, including flying squirrels, colugos, snakes, lizards and frogs, use their extended aerodynamic surfaces to navigate in the air [71,[74][75][76][77][78][79]. Flying squirrels and colugos employ their patagium, the skin connecting the feet to the hands, for manoeuvring [71,74,75]. Snakes, as well as lizards of the genus Draco, extend their ribs to achieve a higher glide ratio [76,79].…”

“…Some finches and doves, for example, have been shown to use their wingbeats to absorb 3-10 times the energy absorbed by the legs while landing [100]. Many of these animals pitch upwards to reduce their speed before landing on vertical surfaces, such as tree trunks [1,71,74,[78][79][80][81]85]. This technique is found in a wide range of animals from insects to mammals, though it is more quantified in larger animals.…”

“…The definition of wingspan is as follows: for flapping and fixed wing flyers it is the tip to tip distance, for rotorcraft it is the distance between the outer rotors, for lizards, snakes and amphibians it is the width between the outermost positions of their appendages. Data were listed or estimated from [4,74,77,79,95,[181][182][183][184][185] Scaling principles also put constraints on body structure [186]. Assuming isometric scaling, 'tiny' exoskeletons can withstand much larger loadings than 'big' animal skeletons or robot structures can when measured as multiples of body weight [23].…”

Scansorial Landing and Perching

“…An example of this phenomenon can be found in the flying squirrel, with its low aspect ratio wing providing aerodynamic stability and lift at angles of attack up to 40 degrees. Furthermore, squirrels deliberately stall themselves prior to landing, allowing them to reduce by 60% their horizontal velocity, while spreading the impact over all four limbs (Byrnes et al, 2008;Paskins et al, 2007). Animals such as geckos also exploit aerodynamics for gliding and landing on vertical surfaces (Jusufi et al, 2008).…”