Biosensing: Advantages, constraints, drawbacks

While tracking is the common technique used for monitoring relatively accurate location and movements of animals, biotelemetry transducers (i.e. devices that convert physical quantities such as pressure into an electrical signal, and vice versa) are used to measure their physiological status as well as various behavioural, energetics and environmental parameters for a range of purposes within medical research or conservation practices (Güler and Übeyli 2002).

Miniature biosensors such as temperature recorders or pressure loggers are widely used in biological research to accurately detect physiological parameters such as heart rate, respiration rate, blood flow and pressure, blood oxygen levels, and body temperature both in laboratory animals and wildlife. Bio-loggers such as tri-axial accelerometers allow users to measure changes in movement (e.g. from stationery to walking) or orientation of body

parts (e.g. head tilting) by reacting to the earth’s gravitational field (Wilson et al. 2008). In particular, accelerometers and gyroscopes are the sensors employed in pet health and activity monitors currently in vogue among pet carers (Ahn et al. 2016). UHF-RFID (Ultra High Frequency-Radio Frequency Identification) proximity loggers are used for studying social and interactive behaviour among conspecifics (e.g. Prange et al. (2006) studied their application on free-ranging raccoons). Neurologgers are EEG (Electroencephalography) recorders able to detect changes in neuronal activity and are used, for example, with homing pigeons to study how they recognise landmarks during flight (Vyssotski et al. 2009). Particularly on marine birds and mammals, accelerometers and Time Depth Recorders (TDRs) are used to collect a wide range of energetics parameters such as muscle activity, swim speed, diving depth and duration, flipper stroke frequency, jaw or beak movements, and other rare behavioural events (such as prey captures). For example, TDRs were used for recording the diving behaviour and time budgets7 _{of seabirds in order to estimate their}

prey requirements and submerged catch rates (Harding et al. 2009). Multi-sensor archival devices have been developed for use in a wide range of wild animals. For example, the Daily Diary (DD) device incorporates tracking technologies and biosensors able to log data for up to 14 parameters (Wilson et al. 2008).

As an alternative to tracking (which aims to map movement paths of individuals with accuracy), the coarser localisation of an animal in an area of interest (for example, if and how many times some individuals visit a certain spot) is possible thanks to less obtrusive technologies. These are typically designed for particular species or habitat conditions. For example, individuals marked by a microchip can be detected in certain sites through RFID systems or camera traps. This is particularly applicable with territorial animals who have defined home range and habits (e.g. badgers dwell in stable burrows and visit regularly fixed spots such as latrines (Dyo et al. 2010)). Approximate tracing of flyways by means of miniaturised light-based geo-locators (i.e. archival trackers that record solar irradiance to determine location) is also employed for studying the diurnal flight of migratory animals, where the use of satellite methods is not advisable, as is the case with small migratory birds (Lisovski et al. 2012). Finally, animals also wear bio-loggers for purposes other than the monitoring of their biological parameters. Oceanographic data-logger applications use wildlife as sampling mobile stations for the remote monitoring of environmental conditions around the animals (for example, humidity, ambient temperature and water salinity (Wilmers et al. 2015)).

Figure 2.4 shows a selection of these bio-sensors: a) Daily Diary logger, b) neurologgers, c) UHF-RFID, and d) TDR loggers.

7 _{Time budget is about recording the amount of time devoted by the animal for their usual activities (e.g.} sleeping, resting, foraging, pecking, etc.)

Figure 2.4: A sampling of bio-sensors: a) external design and description of the direction axes of a Daily Diary (DD) logger (© www.wildlifecomputers.com); b) neurologger partially implanted in a homing pigeon’s skull for studying spatial cognition of the species (Vyssotsky et al., 2006); c) UHF RFID proximity tags attached through perforation on cattle ears for identifying them (© www.rfidreaderuhf.com); d) Time-Depth Recorder (TDR) logger attached on the back of a beaked whale by means of a suction-cup (© www.savethewhales.org).

The logging of a wide range of biological data is enabled by relatively cheap, miniaturised, and lightweight transducers, which are highly desirable qualities both from the perspective of usability and wearability. Such tag properties have encouraged the usage of body- attached devices on several fauna for studying the intimate life of individuals and have broadened the purposes of tracking, while reducing the load on wearers. However, particularly for free-ranging animals, bio-sensing tags have been increasingly integrated into tracking devices (Kays et al. 2015) or have been clustered together in multi-sensor units, such as the DD (size 55 ´ 30 ´ 15 mm; 42 g – (Wilson et al. 2008)). This has provided the advantage of maximising the quantity and quality of data collected through single devices or interventions (Rutz and Hays 2009), but has also taken away the benefit of using small light-weight devices employing single small-scale sensors (Matthews et al. 2013). Moreover, typically, biosensing data is stored locally in the memory of the devices, which therefore need to be retrieved. While this is relatively easy to do with confined animals, thanks to the use of equally small short-range radio transceivers, data-retrieval in wide- roaming animals remains an issue, requiring the use of more expensive and obtrusive transmitting technologies, such as ARGOS, GSM and GPRS modules, or stress-inducing practices, such as physical re-capturing. Hence, although miniaturisation is again considered the feature most capable of enabling a ubiquitous use of animal biotelemetry, in practice animal-attached devices are still bulky, since multiple smaller components are combined in single devices to maximise the gathering of various data, or to monitor the life of ever smaller species instead of decreasing the relative mass of attached tags (Portugal and White 2018).

Overall, a main concern in biotelemetry design has been that of shrinking the device weight and size for both the advantages of monitoring small animals and of reducing biotelemetry- induced impacts (Kays et al. 2015). With regards to impacts, device mass has been deemed to cause effects ranging from energetic extra-expenditure (Wilson et al. 1986) to decreased

flight manoeuvrability (Aldridge and Brigham 1988); therefore, miniaturisation has become a crucial goal. However, although miniaturising tags is certainly a primary way of reducing the burden that animals carry on their bodies, this is not the only design aspect that should be taken into consideration during design. To understand which other factors impinge on wearers and, therefore, need to be taken into account when designing, the next section discusses the welfare implications of wearing biotelemetry devices, exposing the impacts derived from a wider range of device features than just their weight.