Bomb calorimetry: Gross energy and behavioural analysis
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1)In semio-chemical research, the production of glandular scent-secretions is generally considered energetically costly. Measuring these costs, however, has hitherto been difficult. Whereas most glandular secretions contain volatile organic compounds (VOCs), non-volatile organic compounds (non-VOCs), and proteins, current analytical techniques are time-consuming, and specific to one or two chemical component classes, necessitating several analyses to be run in parallel. In addition, most techniques measure relative rather than absolute compound amounts, thus requiring further data processing. Bomb calorimetry bypasses these limitations by oxidising all chemical bonds within a sample simultaneously. Our proof-of-concept study shows that bomb calorimetry can quantify the energetic content (‘Calorific Value’: CV) of freeze-dried subcaudal gland secretions (CV<sub>dry</sub>), which European badgers (<em>Meles meles</em>) use for fitness advertisement. From this measure, it is possible to calculate additional biologically-meaningful metrics: The calorific value of natural (non-dried) secretion (CV<sub>natural</sub>), energy content of individual scent-marks (CV<sub>scentmark</sub>), and the (minimum) energy an individual allocates to scent production (CV<sub>pouch</sub>). Female CV<sub>dry</sub>, CV<sub>natural</sub> and CV<sub>scentmark</sub> were highest in autumn, while equivalent male values showed no significant variation. In contrast, male – but not female - CV<sub>pouch</sub> was significantly higher in spring than in other seasons. Sexual maturity had a significant impact on female - but not male - CV<sub>natural </sub>with sexually mature females having higher CV<sub>natural</sub> than immature females. These results accord with scent-marking behaviour where males increase marking activity to advertise, while females vary olfactory information content. Here, our biological validation with scent-provisioning experiments evidenced that badgers can detect these differences in calorific value of secretions and modify their response behaviour accordingly. We conclude that bomb calorimetry provides a valuable tool for semio-chemical research, although the biological interpretation of results must take additional measures to CV<sub>dry</sub> into account to be comprehensive.
