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. 2013 Jun 4;110(23):9380-4.
doi: 10.1073/pnas.1304838110. Epub 2013 May 20.

High flight costs, but low dive costs, in auks support the biomechanical hypothesis for flightlessness in penguins

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Free PMC article

High flight costs, but low dive costs, in auks support the biomechanical hypothesis for flightlessness in penguins

Kyle H Elliott et al. Proc Natl Acad Sci U S A. .
Free PMC article

Abstract

Flight is a key adaptive trait. Despite its advantages, flight has been lost in several groups of birds, notably among seabirds, where flightlessness has evolved independently in at least five lineages. One hypothesis for the loss of flight among seabirds is that animals moving between different media face tradeoffs between maximizing function in one medium relative to the other. In particular, biomechanical models of energy costs during flying and diving suggest that a wing designed for optimal diving performance should lead to enormous energy costs when flying in air. Costs of flying and diving have been measured in free-living animals that use their wings to fly or to propel their dives, but not both. Animals that both fly and dive might approach the functional boundary between flight and nonflight. We show that flight costs for thick-billed murres (Uria lomvia), which are wing-propelled divers, and pelagic cormorants (Phalacrocorax pelagicus) (foot-propelled divers), are the highest recorded for vertebrates. Dive costs are high for cormorants and low for murres, but the latter are still higher than for flightless wing-propelled diving birds (penguins). For murres, flight costs were higher than predicted from biomechanical modeling, and the oxygen consumption rate during dives decreased with depth at a faster rate than estimated biomechanical costs. These results strongly support the hypothesis that function constrains form in diving birds, and that optimizing wing shape and form for wing-propelled diving leads to such high flight costs that flying ceases to be an option in larger wing-propelled diving seabirds, including penguins.

Keywords: adaptive landscape; energetics; flight performance; morphology.

Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.
Fig. 1.
A comparison of flight costs and morphology across flying vertebrates, primarily birds. (A) A comparison of power output during flight across different bird species. The thick-billed murre value is indicated. The sustained maximum output limit proposed by Videler (16) is shown by the solid line. (B) Dive costs as a function of body mass for bird species. Thick-billed murre diving costs averaged across dive times. Average dive costs during deep diving of murres and emperor penguins also shown. Values were corrected to 13 °C to remove variation associated with temperature (26). Trend line for foot-propelled divers is significantly different from the trend line for penguins (analysis of covariance: F1,12 = 20.85, P = 0.0008). (C) Activity costs, as a multiple of basal metabolic rate, for sustained activity across bird and bat species. Flying thick-billed murre is indicated. Running metabolic rate represents maximal metabolic during sustained running, whereas swimming and flight metabolic rates represent levels that approximate minimal cost of transport. (D) Discriminant analysis of avian locomotory traits. Wing area is heavily loaded on the first axis (RD1), and body mass is heavily inversely loaded on the second axis (RD2). Murres (average of both Uria species) and the great auk are shown.

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