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Table 2 Advantages and disadvantages of different methods used to measure body temperature and activity in free-ranging sheep

From: Technologies for the automated collection of heat stress data in sheep

Method of deployment Invasiveness Difference from CT Disadvantages Advantages
Indwelling rectal logger Moderate 0 ℃ Expulsion risk, constrains biological function, affected by faecal temperature [46], deployment difficulties [47], duration limited by battery power, not real-time Core temperature [51], high collection frequency, remote measurement, detailed and accurate temperature profile, indwelling without surgery, secure data storage
Indwelling vaginal logger Minimal  ± 0.01–0.2 ℃ [17, 50] Infection risk, affected by changes in blood flow during oestrus [52, 53], female animals only, data logger size limitations, duration limited by battery power, not real-time Close correlation to core [51], high collection frequency, detailed and accurate temperature profile, quick response time ~ 8 s, simple to implant, indwelling without surgery, no biological function constraints
Tympanic (ear canal) probe Minimal −0.8–2.5 ℃ [96, 99] Infection risk, prone to displacement/dislodgement, influenced by environmental factors, high activation threshold [19] Simple to implant, devices commercially available, visual signal to identify pre-determined temperature breach [19], real-time wireless transmission
Reticulorumen bolus (rumen) Minimal  + 0.45–0.75 ℃ [66] Irretrievable before slaughter, temperature influenced by; water volume [60, 61], feed intake levels and rumen microorganisms [65] Sensitive to reading body temperature [58, 59], no infection risk, suitable for long-term use, real-time wireless transmission [56], simple administration [55],
Infrared thermography; ocular region, udder, muzzle, vulva, forehead, flank Non-invasive −1.7–2.1 ℃ [130] Influenced by; ambient radiations [130], dust, temperature, humidity and obstacles [46], affected by wool coverage [15] and age, stress, gender and reproductive state [86] Contactless/non-obstructive, > 3000 m range, quick response time 1 ms to 250 ms [30], measure changes in blood flow related to heat load [25], potential to measure respiration
Subcutaneous microchips: abdomen, flank, tail-base, shoulders High (surgical intervention) Site dependent ± 0.2–3.0 ℃ [20, 86] Possibly irretrievable, risks safety of product for human consumption, requires surgery, accuracy is site-dependent, data collected by handheld receiver [86], not real-time Highly secure, allows permanent placement, accurate, high resolution of data
GPS collars Non-invasive N/A Large data output, extensive analysis required, not real-time Measures spatial movement, more effective than direct flock observation [115], non-obstructive [116]
Accelerometers: neck, leg, chest, ear Non-invasive N/A Large data output, difficult to determine new behavioural algorithms, data can be misinterpreted Potential to determine respiration associated with heat stress [29, 112], can measure mutually exclusive behaviours and activity