Energetic cost of hovering flight in nectar-feeding bats (Phyllostomidae: Glossophaginae) and its scaling in moths, birds and bats |
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Authors: | C C Voigt Y Winter |
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Institution: | Institut für Zoologie II, Universit?t Erlangen, Staudtstrasse 5, D-91058 Erlangen, Germany e-mail: chvoigt@biologie.uni-erlangen.de, ywinter@biologie.uni-erlangen.de, DE
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Abstract: | Three groups of specialist nectar-feeders covering a continuous size range from insects, birds and bats have evolved the
ability for hovering flight. Among birds and bats these groups generally comprise small species, suggesting a relationship
between hovering ability and size. In this study we established the scaling relationship of hovering power with body mass
for nectar-feeding glossophagine bats (Phyllostomidae). Employing both standard and fast-response respirometry, we determined
rates of gas exchange in Hylonycteris underwoodi (7 g) and Choeronycteris mexicana (13–18 g) during hover-feeding flights at an artificial flower that served as a respirometric mask to estimate metabolic
power input. The O2 uptake rate (V˙
o2) in ml g−1 h−1 (and derived power input) was 27.3 (1.12 W or 160 W kg−1) in 7-g Hylonycteris and 27.3 (2.63 W or 160 W kg−1) in 16.5-g Choeronycteris and thus consistent with measurements in 11.9-g Glossophagasoricina (158 W kg−1, Winter 1998). V˙
o2 at the onset of hovering was also used to estimate power during forward flight, because after a transition from level forward
to hovering flight gas exchange rates initially still reflect forward flight rates. V˙
o2 during short hovering events (<1.5 s) was 19.0 ml g−1 h−1 (1.8 W) in 16-g Choeronycteris, which was not significantly different from a previous, indirect estimate of the cost of level forward flight (2.1 W, Winter
and von Helversen 1998). Our estimates suggest that power input during hovering flight P
h
(W) increased with body mass M (kg) within 13–18-g Choeronycteris (n = 4) as P
h
= 3544 (±2057 SE) M
1.76 (±0.21 SE) and between different glossophagine bat species (n = 3) as P
h
= 128 (±2.4 SE) M
0.95 (±0.034 SE). The slopes of three scaling functions for flight power (hovering, level forward flight at intermediate speed and submaximal
flight power) indicate that: 1. The relationship between flight power to flight speed may change with body mass in the 6–30-g
bats from a J- towards a U-shaped curve. 2. A metabolic constraint (hovering flight power equal maximal flight power) may
influence the upper size limit of 30–35 g for this group of flower specialists.
Mass-specific power input (W kg−1) during hovering flight appeared constant with regard to body size (for the mass ranges considered), but differed significantly
(P < 0.001) between groups. Group means were 393 W kg−1 (sphingid moths), 261 W kg−1 (hummingbirds) and 159 W kg−1 (glossophagine bats). Thus, glossophagine bats expend the least metabolic power per unit of body mass supported during hovering
flight. At a metabolic power input of 1.1 W a glossophagine bat can generate the lift forces necessary for balancing 7 g against
gravitation, whereas a hummingbird can support 4 g and a sphingid moth only 3 g of body mass with the same amount of metabolic
energy. These differences in power input were not fully explained by differences in induced power output estimated from Rankine-Froude
momentum-jet theory.
Accepted: 10 November 1998 |
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Keywords: | Hovering flight Glossophagine bats Hummingbirds Sphingid moths Allometry |
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