Benefits of being small? The scaling of flight performance in stingless bees and size‐dependent scaling of insect flight metabolism

Abstract

Intra‐specific evolutionary studies usually report major advantages to large body size, yet most species in all animal clades are small. Smaller animals have some fundamental disadvantages such as reduced force production and movement speed that inhibit their abilities to effectively compete with larger species. In addition, smaller animals generally have higher mass specific metabolic rates at rest, leading to higher costs of living and per gram food requirements. What are the evolutionary advantages of being small? Across insects, birds and mammals, the cost of flight has been reported to scale hypometrically, with larger species having lower mass‐specific energy costs during hovering flight. We studied the flight physiology of stingless bee species ranging from 1–115 mg in mass using flow‐through respirometry to assess metabolic rate, high speed video and sound analysis to measure wingbeat frequency, and “grad and stab” thermocouple measurements of body temperature. We also measured the masses of head, thorax, and abdomen, wing size, and voluntary load‐lifting abilities. Thorax masses scaled isometrically, and all bees lifted similar fractions of their body mass. Bees less than 7 mg had body segment temperatures 1–3°C above air temperature, independent of mass, while larger bees had substantially elevated body temperatures. Metabolic rate during hovering scaled hypermetrically (scaling slope = 1.26) and hypermetry could not be explained by thermal effects. Wingbeat frequency was constant across size, but larger bees had relatively smaller wings. Combining our data with prior literature, we show that the scaling of flight metabolic rate changes from hypermetric to hypometric at approximately 40 mg body mass in insects. Thus a reduced absolute and per‐gram cost of flight likely provides selective advantages for the evolution of small body size in insects. This research was partially supported by NSF IOS 1122157 and a short‐term fellowship from the Smithsonian Tropical Research Institute.