Animal Orientation Strategies for Movement in Flows

Abstract

For organisms that fly or swim, movement results from the combined effects of the moving medium - air or water - and the organism's own locomotion. For larger organisms, propulsion contributes significantly to progress but the flow usually still provides significant opposition or assistance, or produces lateral displacement ('drift'). Animals show a range of responses to flows, depending on the direction of the flow relative to their preferred direction, the speed of the flow relative to their own self-propelled speed, the incidence of flows in different directions and the proportion of the journey remaining. We here present a classification of responses based on the direction of the resulting movement relative to flow and preferred direction, which is applicable to a range of taxa and environments. The responses adopted in particular circumstances are related to the organisms' locomotory and sensory capacities and the environmental cues available. Advances in biologging technologies and particle tracking models are now providing a wealth of data, which often demonstrate a striking level of convergence in the strategies that very different animals living in very different environments employ when moving in a flow.

Navigating the flow: individual and continuum models for homing in flowing environments

“…Whether an organism corrects or can actively compensate (by detecting flow direction) is a significant question [6]: a capacity to determine flow direction has rsif.royalsocietypublishing.org J. R. Soc. Interface 12: 20150647 been suggested for a variety of airborne (moths and songbirds [45]) and aquatic species ( jellyfish [9] and juvenile turtles [46]).…”

“…A complicating factor is the flow in the surrounding medium, from ocean and river currents for aquatic organisms to wind for airborne populations [4][5][6]. Flow can be benevolent or malevolent: on the one hand, movement is energetically demanding and currents provide a useful conveyor belt; on the other, a powerful current or storm could transport population members into inhospitable or unfamiliar environments.…”

“…Flow can be benevolent or malevolent: on the one hand, movement is energetically demanding and currents provide a useful conveyor belt; on the other, a powerful current or storm could transport population members into inhospitable or unfamiliar environments. Whether they assist or hinder, effective navigation clearly demands a finely tuned navigational system capable of correcting or compensating for currents [6].…”

Commuting fruit bats beneficially modulate their flight in relation to wind

“…These developments have provided important insights for different movement phenomena, including long-distance migration, foraging and homing behaviour [10,11]. In addition to determining the animal's position, in the case of flying animals it is essential to concomitantly estimate the wind vector encountered aloft, so that the influence of the ambient flow of air can be separated from the animal's own movement [4,12]. The ability to correctly determine wind direction and speed is critical for inferring animal flight performance, because optimal movement in relation to wind may involve changes in airspeed (speed relative to air), and modulation of this response when the wind is blowing from different directions relative to the heading [13].…”

“…In order to arrive at their destination under crosswind conditions, flying animals must therefore overcome induced drift by modifying their behaviour in response to wind. A common solution to this problem is to compensate for lateral drift by changing flight heading [3][4][5] (electronic supplementary material, figure S1). Because the cost of no compensation and consequent sideways drift may be exceedingly high, fitness-relevant behavioural compensatory mechanisms to minimize wind drift have convergently evolved in both birds and insects [4,6,7].…”