Author: Jenni

Figure 1: comparison of bone morphology and shape of a bat wing and a bird wing. Image courtesy of Mika
Grondahl.c

Torpor
Torpor is a state entered
by bats and other mammals where oxygen intake lessens, body temperature drops to the ambient and breathing rate slows. Hibernation is often described as a sequence of multiple bouts of torpor. This short-term adaptation (normally lasting a number of days) starts when the animal enters a state of decreased physiological activity meaning heart rate and metabolism slows. It is an adaptation frequently used by bats for energy conservation in times of environmental stress such as periods of cold temperatures.
Torpor is especially useful in bats because they are small, expend large volumes of energy when active and only have a small capacity for fat storage (Geiser and Stawski, 2011). It is not clear why bats come in and out of hibernation and torpor, recent studies suggest it may be for excretion, mating or feeding/drinking purposes.

The Energetics of Flying Mammals

Introduction
Bats are unique in that they are the only mammal capable of flight. During flight they are able to reach and maintain high metabolic rates similar to those of birds. These rates are significantly higher than land mammals of a comparable size. This elevated metabolic rate allows bats to reach high flight speeds which optimize chances of prey capture. Bats have a wide ranging diet- from fruit and nectar to insects, fish and blood. Herbivorous and carnivorous bats actively regulate their temperate (thermoregulators) whereas insectivorous bats do not regulate their temperature (thermoconformers). Before flight, bats show a characteristic rapid increase in heart and respiratory rate and their metabolic rate increases to about 34 times their standard. This then remains relatively constant throughout the flight. Bats have adapted the ability to recover rapidly after flights so as to conserve energy.

Butler J., Elsdon A., Graves B., Jolly W., Joyce P., Novak E.

Bat anatomy
Bats processes many anatomical adaption’s to aid in flying efficiency. The thin and complaint skin membrane that connects the bones of the wings and feet is highly elastic and stretches during flight (figure 3) this generates more lift and less drag at low speeds reducing the energy cost of for taking off and landing (Norberg 1995)
Bats processes high agility and when flying , can easily exploit the smallest of wind vortices to aid in changing altitude or direction. They can do this due to the short fine hair growing on the wings.
Also within the skin on their
wings, lies tiny muscles that
can contract and control
Skin tension that helps to
Adept to current wind speeds
( Norberg, Brooke and
Trewhella 2000).

Difficulties in measuring
respiration levels in bats
Although the variation between flight
energetics recorded for birds and bats may be
biological, many differences could be due to problems
interpreting data collected by techniques not suitable for
small animals. Obtaining accurate respiration measurements
without disturbing the flight performance of small bats can be
problematic. Bats can be difficult to train to fly in controlled
conditions such as a wind tunnel, and echolocating bats are
more reluctant to wear a mask needed to measure gas exchange
(McGuire et al 2009). Winter and von Helversen (1998) developed
an indirect method of measuring flight energy expenditure based on
time budget analysis whereby nectar feeding (Glossophaginae) bats
could complete a natural flying/resting cycle in an automated
‘flying room’.

Figure 2. The body mass of bats and birds and their associated cost of flight. Bats are solid and open symbols and birds are dashed lines. Taken from Winter and Helversen (1998)

Contact details: School Office, School of Marine Science and Technology, Ridley Building, University of Newcastle.

Figure 3. A computer Simulation of the unsteady
aerodynamics
of a bat
flying at
3.4m/s
(image c
ourtesy
of Willis and
Kosandov,
Brown university).

Are bats more energetically efficient than birds?
When comparing the metabolic cost of flight in small bats and birds it can be seen that bats have metabolic flight cost of about 20-25% lower than birds (Winter and Helversen, 1998). This may be partially attributed to wing morphology in bats.(figure 1) .Short, broad wings (low aspect ratio) and a larger wing area (low wing loading) (Norberg and Rayner, 1987).
Figure 2 shows that the cost of flight in bats is lower to the cost of flight in birds at the same body mass (g). Bats convert sugars into fats as their main energy reserve, this results in a large reduction in mass compared to if they used glycogen storage. This mechanism allows for more efficient reduction in body mass resulting in more energy efficient flight (Norgerg and Rayner, 1987). There is a lot of variation in records of flight energetics in birds and bats, it is also hard to compare the two animals as they both have a large variety of morphologies that are adapted to different flight strategies.

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References
Geiser, F. and Stawski, C. (2011) Integr. Comp. Biol. 51, 337-348. McGuire L.P. and Guglielmo C.G. (2009) Journal of Mammalogy 90(6): 1290-1297. Norberg and Rayner (1987) Proc. R. Soc. B. 316, 335-427. Salcedo, H et al (1995) J. Exp. Biol. 198, 2245-2251. Winter Y. and von Helversen O. (1998) J. Comp. Physiol. B. 168: 105-111. Norberg (1995) Vol. 9, No. 1 (Feb., 1995), pp. 48-54 . Norberg, Brooke and Trewhella (2000) J Exp Biol vol 203, pages 651-664.