UNIDAD DE ABASTECIMIENTO

The most commonly reported and debated design guideline is the so called ‘5% rule’ (or 2%, or 3%), according to which the weight of the tag should not exceed a certain percentage (2% or 3% or 5%) of the animal’s body weight. Such standards originated precisely to employ lighter tags in order to reduce the impact associated with too heavy a load. However, beyond the fact that there is not even agreement on the percentage itself, various studies have challenged this rule and showed that it is too coarse a measure. For example, Aldridge and Brigham (1988) tested it by studying the flight manoeuvrability of bats fitted with tags. Since the ratio of 5% between animal body and tag mass is a fixed value, heavier individuals were fitted with heavier tags. The authors found that, in comparison with lighter bats, heavier tags correlated with a higher decrease in the heavier bats’ ability to make sharp turns. The assumption implied by the rule is that a heavier body can carry a heavier load. However, the muscular power that allows a volant animal to fly does not get enhanced by the tag. Since flying movements necessitate higher muscular power to move heavier bodies, the author concluded that a heavier extra load impinges more on flight behaviour and thus suggested that the rule ought to be applied on a case basis (Aldridge and Brigham 1988). From this, it could be argued that the rule needs to be refined, for example, by accounting for other variables in addition to body mass, such the kind of movement made by the wearer (flight) and the proportion of muscles available to perform that movement.

After the work of Aldridge and Brigham, various other authors have criticised the rule. For example, Brown et al. (1999) endorsed the idea of replacing it with an index based on more scientific scales considering the weight, buoyancy and volume of the tag in relation to the mass and activity of the wearer. Likewise, Jepsen et al. (2005) critiqued that a ‘credible’ tag- body/mass-ratios recommendation must consider other aspects such as the tag attachment method, in relation to the wearer’s life stage, size, species, sex and habitat. The authors claim that “it is insufficient to assume that a tag/bm [body mass] ratio is appropriate” and “few studies have systematically investigated the effects of different tag/bm ratios, and recommendations on maximum ratios often seem to be unfounded statements”. Despite these proposals to discard the rule, this is still followed and considered good practice by many. In fact, in a study conducted by Smircich and Kelly (2014) on the safety of the 2% rule in brook trout, the authors suggest that the guideline can be safely extended up to a 7% ratio. In the case of EcoLocate - a network composed of six different weight categories of tags that communicate with each other wirelessly in an open landscape (mentioned in section 2.1.1, p. 17) - the 5% rule is exploited to justify the mounting of heavier tags on big animals (such as elephants or rhinos) so that they can serve as receiving and transmitting stations. As they carry many extra batteries, their function is to carry out most of the transmitting work of the network (Markham and Wilkinson 2008). Although this system enables the use of very light tags on small fauna, this is done to the potential detriment of large animals and in contrast with another guideline which states that, from an individual’s

perspective, a device should be as light as possible (Morton et al. 2003). EcoLocate exemplifies concerns recently expressed by Portugal and White (2018), who conducted a meta-analysis on 48 years of biotelemetry literature about the topic of device miniaturisation. The authors found that, although technology has advanced towards miniature tags, this progress has not translated into a reduction of the rule’s 5% value, which could benefit the wearer. On the contrary, technological progress has made possible and incentivised monitoring of ever smaller species, for which the 5% rule has continued to be implemented (Portugal and White 2018).

The inadequacy of the 5% rule can be demonstrated by performing a rough calculation on familiar animals. For example, for a 5-kilogram cat, 5% is equal to 250 grams, comparable to the weight of two smartphones; for a 60-kilogram human, 5% corresponds to 3 kg; and in the case of a 720 kg cow, the device could weigh up to 36 kilograms. Even when the 2% rule is considered, the load is still substantial: 100 g for the cat, 1.2 kg for the human, and 14.4 kg for the cow respectively. It has to be stressed that biotelemetry devices are in many cases (especially in the case of wildlife) constantly fastened to the wearer’s body. This is analogous to a human carrying a 1.2-3 kg backpack for months without being able to remove it. The analogy highlights how inadequate the 5% rule is, particularly in its generalised terms, and raises questions such as: would a 36 (or even 14.4) kilogram load on a cow be experienced as snugly and be comfortably borne by her? If not, how would one decide what is the actual burden that she can comfortably carry?

In contrast to the oversimplification of the 5% (or 2% or 3%) rule, animal welfare researchers such as Morton and colleagues (2003), Hawkins (2004), Wilson and McMahon (2006), and Casper (2009) highlighted the importance, on both welfare and scientific grounds, of considering the needs of individual animals in more detail. With respect to equipment, welfarists have offered a more inclusive set of recommendations, in which they argued that, in addition to mass (which must be kept at a minimum), designers and researchers should carefully consider the physical aspects of shape, material, colour, location and method of attachment in relation to the biological and behavioural characteristics of the animal. Their key considerations are summarised in Table 2.1 and include:

the shape and orientation of the device should be such that drag and abrasion on the

animal’s body are minimised, and that movement and performance of vital functions are not impaired (Morton et al. 2003; Wilson and McMahon 2006; Casper 2009);

the materials used for the implementation and attachment of the devices should wherever

possible be temporary so that the device does not have to remain attached to the animal longer than necessary (Morton et al. 2003; Casper 2009), and should not lacerate animals’ tissues, disrupt thermal mechanisms and waterproofing, or be buoyant (Casper 2009);

the colour of the external components including harnesses, cases and markers should

ensure that the appearance of the device does not affect the animal’s social status or attract the attention of predators, prey or ill-intentioned humans (Morton et al. 2003; Casper 2009), that the device blends in with the animal colour (Wilson and McMahon 2006), and that the dye used is not toxic (Casper 2009);

the length and size of the device should be considered in relation to the animal’s sleeping

habits, to avoid pressure on the bladder, liver or diaphragm whilst in the sleeping position (Morton et al. 2003; Hawkins 2004);

the position of the device in relation to the animal’s barycentre should be such that it does

not compromise their posture and equilibrium (Morton et al. 2003; Casper 2009);

the method of attachment should be tailored to the species such that it causes the least

discomfort or distress possible (Morton et al. 2003) and it should address the risk of trapping wearers (Casper 2009).

Additionally, these authors provide specific directions as to how their recommendations could be applied in practice in relation to the biological and behavioural requirements of animals involved in biotelemetry studies. For example, they discourage the use of red components (Hawkins 2004), recommend the streamlining of the tag shape following the aero- or hydrodynamic form of the animal (Wilson and McMahon 2006), and advocate the use of biocompatible, non-buoyant, dissolvable or time-releasable materials (Casper 2009).

Physical aspect Recommendations

Shape / Orientation

Minimise drag Minimise abrasion

Not to impair movements Not to impair vital functions

Material Be temporary

Not to lacerate tissues

Not to disrupt thermal and waterproofing Not to be buoyant

Colour Not to affect animals’ social status

Not to attract attention of predators, prey or ill-intentioned humans

Be blend in with animal coat Not to use toxic dye

Length / Size

Consider animal’s sleeping habits Avoid pressure on internal organs

Position Consider the animal barycentre to not compromise equilibrium Method of

attachment

Tailored to the species to not cause discomfort Avoid the risk of trapping animals