Abstract
BackgroundDrafting in cetaceans is defined as the transfer of forces between individuals without actual physical contact between them. This behavior has long been surmised to explain how young dolphin calves keep up with their rapidly moving mothers. It has recently been observed that a significant number of calves become permanently separated from their mothers during chases by tuna vessels. A study of the hydrodynamics of drafting, initiated in the hope of understanding the mechanisms causing the separation of mothers and calves during fishing-related activities, is reported here.ResultsQuantitative results are shown for the forces and moments around a pair of unequally sized dolphin-like slender bodies. These include two major effects. First, the so-called Bernoulli suction, which stems from the fact that the local pressure drops in areas of high speed, results in an attractive force between mother and calf. Second is the displacement effect, in which the motion of the mother causes the water in front to move forwards and radially outwards, and water behind the body to move forwards to replace the animal's mass. Thus, the calf can gain a 'free ride' in the forward-moving areas. Utilizing these effects, the neonate can gain up to 90% of the thrust needed to move alongside the mother at speeds of up to 2.4 m/sec. A comparison with observations of eastern spinner dolphins (Stenella longirostris) is presented, showing savings of up to 60% in the thrust that calves require if they are to keep up with their mothers.ConclusionsA theoretical analysis, backed by observations of free-swimming dolphin schools, indicates that hydrodynamic interactions with mothers play an important role in enabling dolphin calves to keep up with rapidly moving adult school members.
Highlights
Drafting in cetaceans is defined as the transfer of forces between individuals without actual physical contact between them
The problem of separation of mother-calf pairs in chase situations has become a serious concern in fishing-related cetacean mortality, in particular in the eastern tropical Pacific Ocean where tuna are fished with a purse-seine method, in which schools of dolphins are encircled with a fishing net to capture the tuna concentrated below [1]
The hydrodynamics of the drafting situation is extremely complex, as it deals with unsteady motions of two flexible bodies of different size, moving, while changing shape, at varying speeds and distances from the water surface and from each other, and periodically piercing the surface
Summary
Drafting in cetaceans is defined as the transfer of forces between individuals without actual physical contact between them This behavior has long been surmised to explain how young dolphin calves keep up with their rapidly moving mothers. A study of the hydrodynamics of drafting, initiated in the hope of understanding the mechanisms causing the separation of mothers and calves during fishing-related activities, is reported here. There are several different preferred drafting positions for the calf [2], which appear at different ages and different modes of motion These include the newborn being supported high on the mother’s flank, within a few centimeters of her body, immediately following birth; this is sometimes called the ‘neonate position’. The echelon position is characterized by the infant making relatively few tail fluke movements as it ‘drafts’ alongside its mother, indicating a hydrodynamic advantage
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