The understanding of the interplay of movement, behaviour and physiology that biologging offers has applied relevance for a range of fields, including evolutionary ecology, wildlife conservation and behavioural ecology. In recognition of this, the Journal of Animal Ecology has an upcoming Special Feature on Biologging (submissions due 20th September).
Bio-telemetry devices are used ubiquitously across vertebrates in studies of movement and behavioural ecology and can provide scientists with unparalleled opportunities to collect large amounts of data on wild animals. However, while bio-logging offers great promise in answering crucial questions in ecology, conservation and management, there are many challenges that still exist.
Here, William Kay from Swansea University discusses the impacts of increased drag on animals tagged with bio-logging devices and the need to consider all forms of device-induced detriment.
In recent decades the use of bio-logging devices to collect information on the behaviour, movement and physiology of animals, and simultaneously the conditions of the environments that they inhabit, has increased dramatically. Bio-logging devices can collect unparalleled datasets, particularly for cryptic species, and this has inevitably led to their widespread and rapid uptake in ecological research worldwide.
Researchers deploy bio-logging devices on animals for a wealth of well-justified reasons, such as to understand the transmission of disease, monitor the impacts of anthropogenic disturbance, or improve conservation strategies for species at risk of extinction. These studies often result in substantial, positive impact thanks to increased understanding of the behaviour of individuals and populations, or developments towards appropriate management strategies.
However, while bio-logging studies offer a plethora of opportunity, it has long-been acknowledged – for at least three decades – that they are plagued by a “catch-22” effect; the attachment of devices will inevitably impact on the very behaviours and measures of performance of an animal that are being quantified (Wilson, Grant, & Duffy, 1986). For example, attaching devices adds mass, may change buoyancy, and could affect aero- or hydrodynamic drag, and these effects may give rise to changes in behaviour or power requirements for the animal bearing the device.
Recent reviews, for example Bodey et al., 2017, have highlighted the pervasiveness of deleterious effects and these findings have understandably reinforced longstanding concerns about the ecological relevance and usefulness of the data collected by animal-attached devices, and the potential implications for animal welfare.
In the last couple of decades there has been a phenomenal increase in the sophistication and miniaturization of bio-logging technology, so why do some detrimental impacts persist?
One possible reason, highlighted in a recent commentary from Portugal and White (2018), is that the miniaturisation of bio-logging devices has not necessarily led to animals bearing smaller devices. Instead, smaller devices are now just being deployed on even smaller animals. Indeed, the last 48 years has seen no reduction in device mass relative to the mass of the animals being tagged (Portugal & White, 2018).
Additionally, guidelines routinely adopted for bio-logging studies only tend to consider device weight – e.g. “the 5 % rule” – but ignore several other important impacts such as drag (cf. Kenward, 2001). These factors have been shown to be crucial in determining tag influence for aerial and aquatic organisms and in many cases may be the more important metrics of concern. This is particularly pertinent to large, marine organisms such as pinnipeds and cetaceans that are frequently tagged with relatively large devices because mass is only considered of minor concern – yet hydrodynamic drag is critical. Rosen, Gerlinsky, & Trites, 2017 recently showed that tags attached to fur seals significantly affected their diving behaviour and energetics, even despite the tags being less than 1 % of the study animals’ mass.
Limiting device mass alone is perhaps then not always the best strategy for reducing animal-detriment in bio-logging studies, particularly in marine applications. Instead, other parameters that should be considered include device size and shape, and the positioning of the device on the animal.
One tool that can be used to understand the relative impacts of factors such as these is Computational Fluid Dynamics (CFD), and part of my PhD project at Swansea University is doing just that.
CFD is the primary tool for aerodynamic design and can efficiently model drag; for example having been used extensively in the design of commercial aircraft and Formula One cars. CFD analysis is implemented quickly and efficiently in a simulated computational environment and can gather repeated measures with the accuracy of results comparable to physical experiments.
Despite its utility and accuracy, the use of CFD features in few publications examining bio-logging devices and these tools are not currently being employed routinely into device design processes.
My research aims to address this gap by highlighting how we can maximise the use of CFD to quickly and efficiently evaluate device-induced drag. By modelling the drag of tags in a computational environment I was able to test the drag loading of different tags. I discovered that improving tag shape was far more important for reducing drag than reducing tag size per se, and that the positioning of devices is essential. The latter result here is a non-trivial one because the position of the device on an animal can determine the quantity and quality of data that are attainable; consider for example the placement of devices requiring satellite fixes on to marine animals where only part of the animal breaks the surface – such as the fins of sharks.
My final aim for this project was to provide the scientific community with a simple guide for undertaking CFD analysis in the hope that this will encourage ecologists using biologging devices to consider quantifying tag impact in this way. This project was undertaken in collaboration with aerospace engineers at Swansea University and was an excellent opportunity to combine their knowledge with our biology and undertake interdisciplinary research, and I would strongly encourage others to do the same!
There are several pleas in existing literature asking for improved standards of data in bio-logging (Bodey et al., 2017; Campbell, Urbano, Davidson, Dettki, & Cagnacci, 2016), or better ways to measure and quantify device-induced effects (Wilson & McMahon, 2006). In addition, I think one more request should be made: for more appropriate guidelines for tags. For example, it would be fantastic to one day see that guidelines for devices in aerial and aquatic applications had changed to include a “maximum drag loading”, as opposed to just a “maximum weight”.
Bodey, T. W., Cleasby, I. R., Bell, F., Parr, N., Schultz, A., Votier, S. C., & Bearhop, S. (2017). A Phylogenetically Controlled Meta-Analysis of Biologging Device Effects on Birds: Deleterious effects and a call for more standardized reporting of study data. Methods in Ecology and Evolution, 12(10), 3218–3221. doi: 10.1111/2041-210X.12934
Campbell, H. A., Urbano, F., Davidson, S., Dettki, H., & Cagnacci, F. (2016). A plea for standards in reporting data collected by animal-borne electronic devices. Animal Biotelemetry, 4(1), 1. doi: 10.1186/s40317-015-0096-x
Kenward, R. (2001). A manual for wildlife radio tagging. Academic Press.
Portugal, S. J., & White, C. R. (2018). Miniaturisation of biologgers is not alleviating the 5% rule. Methods in Ecology and Evolution, 1(1), 1–2. doi: 10.1111/2041-210X.13013
Rosen, D. A. S., Gerlinsky, C. G., & Trites, A. W. (2017). Telemetry tags increase the costs of swimming in northern fur seals, Callorhinus ursinus. Marine Mammal Science, 1–18. doi: 10.1111/mms.12460
Wilson, R. P., Grant, W. S., & Duffy, D. C. (1986). Recording Devices on Free-Ranging Marine Animals: Does Measurement Affect Foraging Performance? Ecology, 67(4), 1091–1093. doi: 10.2307/1939832
Wilson, R. P., & McMahon, C. R. (2006). Measuring devices on wild animals: What constitutes acceptable practice? Frontiers in Ecology and the Environment, 4(3), 147–154. doi: 10.1890/1540-9295(2006)004[0147:MDOWAW]2.0.CO;2