![]() ![]() The outcome of these measurements will be directly affected by substrate compliance. By measuring take-off and landing forces, we propose to quantify the advantage flying squirrels can achieve as a result of their unique morphology. This paper will investigate these hypotheses through experimental determination of the performance and behaviour of these animals. Of course, none of these four hypotheses are mutually exclusive. There is published evidence to support this( Caple et al., 1983), where it was calculated that increasing the amount of lift available to a body from 0 to 5% would not noticeably lengthen the jump nor allow much turning, but would give the animal significantly improved control around the pitch and roll axes. We propose a fourth hypothesis: that the patagia, the flexible membranes that squirrels stretch by fully extending their forelimbs and hindlimbs, evolved to reduce or control landing forces. However, this behaviour may increase susceptibility to attack from their most likely predator, owls( Scheibe and Robins, 1998). Tree squirrels react to disturbances by moving to the opposite side of their tree, whereas flying squirrels climb upwards and then glide to another tree. The third hypothesis is that gliding evolved primarily as an escape mechanism ( Scheibe et al.,1990 Keith et al.,2000). An alternative suggestion was that, rather than reducing cost of transport, gliding may offer a means of foraging over a larger area in a certain time, making flying squirrels better able to exploit a patchy food resource than non-gliders of similar size ( Scheibe et al.,2006). The squirrel can jump and glide from one tree to the next, enabling it to cover greater distances within the canopy more quickly than would be possible by climbing down and moving across the forest floor. The first suggestion( Norberg, 1985) is that gliding may have evolved as a means of reducing the energetic cost of foraging. There are three principal hypotheses to explain the divergence of flying squirrels from other types of squirrel. 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. ![]() 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. However, this gliding posture enables them to pitch upwards, potentially stalling the wing, and spreads the landing reaction force over all four extended limbs. 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. 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. Take-off forces ranged from 1 to 10 bodyweights, and landing forces were between 3 and 10 bodyweights. 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. These were both slightly compliant (less than 1.9 mm N –1), and instrumented using strain gauges so that forces could be measured. Northern flying squirrels were filmed jumping from a horizontal branch to a much larger vertical pole. We present experimental performance and behavioural evidence that flight in flying squirrels may have evolved out of a need to control landing forces. Flying squirrels are well known for their ability to glide between trees at the top of a forest canopy.
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