Intro: Gliding distance and body mass

The distance an animal glides appears to determine the impact on the tree upon which it lands. Longer glides allow the animal to re-orient the aerodynamic forces on its body before landing, allowing it to reduce its speed and thus landing forces, although there are contradictory studies on the landing forces of gliding mammals. A study of captive Northern Flying Squirrels discovered that they exert between one and 10 times their body weight during take-off and between three and 10 times their body weight during landing. This study also found increasing forces with increasing glide distance, although these were over relatively short distances so the flying squirrels may not have been able to perform a complete braking phase in preparation for landing. By contrast, other research on the wild Malayan Colugo over longer glide distances discovered that landing forces are greatest for the shorter glides and less for longer glides. The ability of an animal to reduce its velocity before landing allows gliding mammals to travel long distances between trees with reduced risk of injury.

The limited data available suggest that among the gliders there is a remarkably consistent angle of descent, which is reflected in the consistent relationship between the body mass and patagium surface area. There should also be a most efficient body mass for a species to use gliding and, as the body mass increases or decreases from the optimum, it should become increasingly less efficient until it reaches the upper or lower bounds. As the body mass exceeds these thresholds it becomes too large or small so gliding is no longer advantageous over climbing between destinations.

A comparison of body mass of every species of gliding mammal suggests that the most common body mass range is approximately 100–500 grams, with the next most common being less than 100 grams. The difference between climbing horizontally and gliding energy may explain, in small part, the body mass distributions of gliding mammals. Above approximately 400 grams, the requirement for gliding has been predicted to decrease until the upper mass limit is reached, which is approximately 2500 grams. This prediction is supported by research that compared the energetic costs of climbing horizontally to the energetic costs of climbing vertically (required to obtain enough height to glide), in order to achieve the same horizontal distance. This study calculated that gliding mammals weighing 400 grams achieve the greatest energetic savings over climbing horizontally, whereas gliding mammals substantially smaller or larger than 400 grams should expend roughly similar energy climbing vertically to glide as their non-arboreal counterparts do climbing horizontally. These observations are supported by the body masses of almost all known species of gliding mammals, which are typically below 2000 grams, with the exception of the Bhutan Giant Flying Squirrel which has been recorded up to 2700 grams.

The relationship between glide distance and body mass suggests that larger gliders must glide further than smaller gliders to save energy over climbing horizontally. Several glider studies have shown that in order to achieve this, larger gliders launch higher and glide longer in order to minimise the angle of descent. Therefore short distances are more likely to be reached using quadrupedal motion by larger gliders than small ones. In contrast, small gliders can be energetically cost effective with steeper glides than larger gliders so glide short distances more often. These observations are supported by the limited number of studies that show an increase in the average glide distance with increasing body mass.

Calculations of the relationship between angle of descent and body mass suggest that gliders weighing less than 19 grams need not have an angle of descent less than 45° to be cost effective, and can therefore use parachuting and still expend less energy than required to climb between two points. The implication of this is that gliding membranes are unnecessary for very small mammals. Despite this proposal, several species of gliders do exist that weigh less than 19 grams, though typically with less developed patagia and uniquely feather-like tails.

Marsupial gliders have been observed to glide from less than 10 metres to more than 100 metres, while the flying squirrels have been recorded to glide more than 150 metres. There are few records of glide distances in scaly-tailed flying squirrels, although there is one anecdotal record of an individual that glided an amazing 250 metres over the entire length of an open valley. The colugos appear to be able to consistently glide further than all other gliding mammals with the help of their extremely well-developed gliding membranes. A colugo has been observed to glide 136 metres with a loss in altitude of only 12 metres.

A detailed view of the different species of Australian gliding marsupials reveals that heavier species, such as the Yellow-bellied Glider and Mahogany Glider, make longer glides and launch higher in the tree canopy than smaller species, such as Sugar Gliders, that typically glide between the mid to lower canopy. Sugar Gliders in southern Australia spend most of their time in the mid-stratum of the forest in vegetation 10–20 metres high that contains Acacia trees, while the larger Squirrel Glider spends more time in the upper strata at 25–30 metres in height.

Although all gliding mammals make short glides, it appears that heavier species prefer to climb short distances when there are interconnecting canopies rather than glide; they launch from a higher elevation in more open habitat where longer, faster glides can be made. In contrast, smaller species seem to prefer making shorter glides from the mid to lower storey with a higher tree density and where the turbulence caused by wind is less. These observations suggest that as body mass increases above the optimal body mass for gliding, the need for longer glides increases as they are more cost-effective (and maintain glide efficiency), and short distances are more likely to be traversed by climbing.