3009 INTRODUCTION Efficient underwater locomotion should include the ability of the swimmer to control its trajectory and maneuver when necessary. For cormorants swimming underwater in search and pursuit of their motile prey (fish), the ability to make abrupt maneuvers while swimming is equally as important as the ability to swim faster than their prey. Maneuverability is broadly defined as the opposite of stability, i.e. the responsiveness of the body to deviate from a steady trajectory (Fish, 2002; Webb, 2002; Weihs, 1993; Weihs, 2002). The term maneuver describes unsteady aspects of motion and can also be used to describe changes in swimming speed without a change in swimming direction (i.e. accelerating and braking). However, here, we refer exclusively to the ability and limits of making a controlled change to the swimming direction. Turning involves accelerating normal to the instantaneous direction of swimming. Since this centripetal acceleration is a function of tangential speed and trajectory radius these two parameters have been widely used to characterize the maneuvering performance of various swimmers (e.g. Blake et al., 1995; Fish, 2002; Fish and Nicastro, 2003; Fish et al., 2003a; Rivera et al., 2006; Walker, 2000). However, these two parameters per se cannot explain how animals control their maneuvers or highlight the factors limiting maneuverability. To obtain such insights, one must explore how the organism maneuvers (e.g. Fish and Nicastro, 2003; Webb and Weihs, 1983). In the present study, we report on the hydrodynamic mechanism used by cormorants to maneuver in the vertical plane. Cormorants are foot-propelled aquatic predators that rely exclusively on submerged swimming for capturing fish (Johnsgard, 1993). They are extremely efficient aquatic predators, reportedly yielding some of the highest catch per unit effort recorded for avian divers (Grémillet et al., 2001). They achieve this remarkable foraging performance despite a limited adaptation for a pelagic life style. Like several other avian divers, cormorants utilize the aquatic media while retaining full flight capabilities. The primary adaptation of the avian body for flight results in low specific density. Underwater this translates into avian divers being among the most buoyant pelagic swimmers (Lovvorn and Jones, 2004; Wilson et al., 1992). The high buoyancy is due to the light skeletons of birds and the large air volumes carried underwater inside the body (air sacs) and trapped in the plumage. Enstipp et al. showed that the energy expenditure of swimming cormorants changes with dive depth (Enstipp et al., 2006). Water depth provides relief from buoyancy to deep divers by compressing the air volumes in the body. However, great cormorants seldom dive to depths exceeding 10 m (Grémillet et al., 1999). As a result, they forage in the part of the water column where their swimming is affected by their positive buoyancy. Understanding how cormorants cope with their buoyancy to maneuver efficiently underwater is an important step for a better understanding of the foraging behavior and habitat selection (e.g. preferred foraging depth) in these aquatic predators. Cormorants use foot-propulsion for swimming underwater. During swimming, the wings are folded next to the body and do not participate in swimming (Johnsgard, 1993). As a result, the body of cormorants is deprived of median control surfaces that are used for trim-control in many fish and marine mammals (Fish, 2002; Fish and Shannahan, 2000; Webb and Weihs, 1983). While swimming horizontally in a straight line, cormorants do so with their body inclined (pitch) at a negative angle-of-attack (AoA) to the The Journal of Experimental Biology 211, 3009-3019 Published by The Company of Biologists 2008 doi:10.1242/jeb.018895 Consequences of buoyancy to the maneuvering capabilities of a foot-propelled aquatic predator, the great cormorant (Phalcrocorax carbo sinensis) Gal Ribak 1, *, Daniel Weihs 2 and Zeev Arad 1 1 Department of Biology and 2 Faculty of Aerospace engineering, Technion, Haifa 32000, Israel *Author for correspondence at present address: Department of Biology, University of South Dakota, Vermillion, SD 57069, USA (e-mail: gal.ribak@gmail.com) Accepted 20 June 2008 SUMMARY Great cormorants are foot-propelled aquatic divers utilizing a region of the water column where their underwater foraging behavior is affected by their buoyancy. While swimming horizontally underwater, cormorants use downward lift forces generated by their body and tail to overcome their buoyancy. Here we assess the potential of this swimming strategy for controlling maneuvers in the vertical plane. We recorded the birds swimming through a submerged obstacle course and analyzed their maneuvers. The birds reduced swimming speed by only 12% to maneuver and were able to turn upward and then downward in the sagittal plane at a minimal turning radius of 32±4 cm (40% body length). Using a quasi-steady approach, we estimated the time- line for hydrodynamic forces and the force-moments produced while maneuvering. We found that the tail is responsible for the pitch of the body while motions of the body, tail, neck and feet generate forces normal (vertically) to the swimming direction that interact with buoyancy to change the birdsʼ trajectory. Vertical maneuvers in cormorants are asymmetric in energy cost. When turning upward, the birds use their buoyancy but they must work harder to turn downward. Lift forces generated by the body were always directed ventrally. Propulsion improves the ability to make tight turns when the center of the turn is ventral to the birds. The neck produced only a small portion (10%) of the normal vertical forces but its length may allow prey capture at the end of pursuit, within the minimum turning radius. Key words: maneuverability, locomotion, swimming, diving, torque, pitch, trim-control. THE฀JOURNAL฀OF฀EXPERIMENTAL฀BIOLOGY