Energetic cost of hovering flight in nectar-feeding bats (Phyllostomidae: Glossophaginae) and its scaling in moths, birds and bats.

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 (VO2) 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 Glossophaga soricina (158 W kg-1, Winter 1998). VO2 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. VO2 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 Ph(W) increased with body mass M (kg) within 13-18-g Choeronycteris (n = 4) as Ph = 3544 (+/- 2057 SE) M1.76 (+/- 0.21 SE) and between different glossophagine bat species (n = 3) as Ph = 128 (+/- 2.4 SE) M0.95 (+/- 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.