Abstract
Conservation biologists can use hormone measurements to assess animals’ welfare, reproductive state, susceptibility to stressors, as well as energy expenditure. Quantifying hormone concentrations from faecal samples is particularly advantageous as samples can be collected without disturbing animals’ behaviour. In order for an endocrine marker to be useful for wildlife managers, we need to understand how extrinsic and intrinsic factors affect hormone concentrations in free-ranging animal populations. Thyroid hormones are linked to basal metabolic rate and energy expenditure. Previous research demonstrated that triiodothyronine (T3) can be measured successfully in faecal matter of African elephants, Loxodonta africana. However, to our knowledge, research into factors affecting changes in elephant T3 levels has only been carried out in captive elephants so far. Thus, we present the first study of faecal T3 metabolite (mT3) concentrations of a large population of free-ranging African elephants. Over 15 months, we collected faecal samples from identified (n = 43 samples) and unidentified (n = 145 samples) individuals in Madikwe Game Reserve, South Africa. We investigated whether vegetative productivity [normalized difference vegetation index (NDVI)] in interaction with mean monthly temperature, age and sex affected mT3 concentrations. We found a significant negative interaction effect of NDVI and temperature. Increasing NDVI was related to higher concentrations of mT3, but increasing temperature was related to a decrease in mT3 concentrations in individually identified and unidentified elephants. In unidentified individuals, juvenile elephants had significantly higher mT3 concentrations compared to adult elephants. Faecal T3 can successfully be quantified in samples from free-ranging elephant populations and thus provides insight into energy expenditure in large herbivores.
Highlights
Monitoring of hormones of wild and captive animals has long been used by conservation biologists to investigate areas of interest, including animal health and welfare and how individuals cope with environmental challenges (e.g. Millspaugh and Washburn, 2004; Millspaugh et al, 2007; Teixeira et al, 2007; Ganswindt et al, 2010a, b; Palme, 2012; Scheun et al, 2015)
Through the assessment of energetic condition conservation biologists can infer the effects of changing environmental conditions and anthropogenic disturbance on an animal‘s energy budget, behaviour and life history (Ellenberg et al, 2007; Schaebs et al, 2016; Gesquiere et al, 2018)
Reduced caloric intake is linked to a decrease in thyroid hormone concentrations and reduced basal metabolic rate (BMR), whilst greater energy expenditure requires an increase in BMR, which is linked to increased thyroid hormone concentrations (e.g. howler monkeys, Alouatta palliata (Wasser et al, 2010; Dias et al, 2017), Barbary macaques, Macaca sylvanus (Cristóbal-Azkarate et al, 2016); yellow-breasted capuchins, Sapajus xanthosternos (Schaebs et al, 2016), baboons, Papio spp. (Gesquiere et al, 2018); Eales, 1988; Behringer et al, 2018)
Summary
Monitoring of hormones of wild and captive animals has long been used by conservation biologists to investigate areas of interest, including animal health and welfare and how individuals cope with environmental challenges (e.g. Millspaugh and Washburn, 2004; Millspaugh et al, 2007; Teixeira et al, 2007; Ganswindt et al, 2010a, b; Palme, 2012; Scheun et al, 2015). Through the assessment of energetic condition conservation biologists can infer the effects of changing environmental conditions and anthropogenic disturbance on an animal‘s energy budget, behaviour and life history (Ellenberg et al, 2007; Schaebs et al, 2016; Gesquiere et al, 2018). This is especially important in species with slow growth rates and reproduction (Schaebs et al, 2016). Behringer and colleagues (2018) provided a comprehensive review of the role of thyroid hormones in mammalian growth, ecology, and maintenance
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