Flight in bats

Flight in bats

• Wings– Evolution– Adaptations for flight

• Aerodynamics– Powered flight– Energetic cost of flight– Wing shape and foraging ecology

Discussion readings

Speakman, J . R. 2001. The evolution of flight and echolocation in bats:another leap in the dark. Mammal Review 31:111-130.

Fenton, M. B., D. Audet, M. K. Obrist, and J . Rydell. 1995. Signal Strength,Timing, and Self-Deafening - the Evolution of Echolocation in Bats.Paleobiology 21:229-242.

Schnitzler, H. U., C. F. Moss, and A. Denzinger. 2003. From spatialorientation to food acquisition in echolocating bats. Trends in Ecology &Evolution 18:386-394.

Thewissen, J . G. M., and S. K. Babcock. 1992. The Origin of Flight in Bats -to Go Where No Mammal Has Gone Before. Bioscience 42:340-345.

Hypothetical evolution of a bat wing

Colugo

Bat

Flying lemur (colugo)

Current phylogeny

Teeling et al. PNAS 2001

Wing evolution

Powered flight has evolved 3 times in vertebrates.

Bats have much greater control over wing shape and curvature than birds or pterosaurs

Aerofoil and lift

• Convex curvature of aerofoil causes air to pass faster over than under the wing.

• This creates negative pressure above the wing, causing the wing to lift.

• Greater angle of attack permits greater lift at low speed, until a stall.

lift

Bat wing is an aerofoil

Digits prevent turbulence

• Greater camber causes turbulence

• Turbulence erodes lift, causes drag

• Digits reduce turbulence and drag

Bats can manipulate camber

Adjusting angle of propatagium and membrane alters camberwhich can increase lift and greatly increase maneuverability

Bat wing properties

Wing extension mechanism

Triceps connects to scapula and carpi radialis connects to humerus. Consequently, when suprapinatus contracts, arm and fingers automatically extend. Only shoulder muscles need to be large.

Wing pivot differs in birds and bats

Bats: clavicle pivots on sternum, while scapula is pulled up and down. Allows for equal power on up and down strokes.

Birds: humerus pivots on scapula. Most power is in downstroke.

Downstroke Upstroke

Vampire bat wing motion

Bat flight aerodynamics

L = liftD = dragV = velocityR = aerodyamic power

Wing flapping creates vortices

Wing flapping and flight power

At a flight speed of 2.35 m/s, Plecotus auritusgenerates thrust mainly on the upstroke.

Wing flapping in horizontal flight of Rhinolophus ferrumequinum

Thrust is generated on upstroke only at low speeds.

Hovering flight

Glossophaga soricina hovering

Vampire bat jumping

Optimal flight speed

Hovering is costly, as ishigh speed flight.

Physiological adaptations for flight

• Bats flap their wings 10 times per second• Most bats take a breath on every wing beat• Exhalation occurs late in the upstroke.

Echolocation coincides with wing beats, except during attack.

• Heart rate increases 2-6 times resting rate during flight

• Metabolic rate (oxygen consumption) during flight is about 14 times resting rate

Flight is more expensive than running in birds and bats

Bats differ in wing shape

• Wing loading = mass/wing area

• Aspect ratio = wingspan2/wing area– Low AR =

short, broad wings– High AR =

long, narrow wings

Wing shape and flight

style

Call design fits foraging strategy