Refined methodology for identification of bitterness aversion in mature rams through quantification of fluid intake and behavioral response to phenylthiocarbamide

Authors: SOUTHERLAND, CLAIRE - University Of Idaho; Taylor, Joshua - Bret; YELICH, JOEL - University Of Idaho; ELLISON, MELINDA - University Of Idaho

Submitted to: Applied Animal Behaviour Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/22/2022
Publication Date: 7/16/2022
Citation: Southerland, C.N., Taylor, J.B., Yelich, J.V., Ellison, M.J. 2022. Refined methodology for identification of bitterness aversion in mature rams through quantification of fluid intake and behavioral response to phenylthiocarbamide. Applied Animal Behaviour Science. 254. Article 105706. https://doi.org/10.1016/j.applanim.2022.105706.
DOI: https://doi.org/10.1016/j.applanim.2022.105706

Interpretive Summary: Some humans avoid bitter foods, but others tolerate and even seek out bitter-tasting foods and drinks, like broccoli and coffee. But, what about other animals such as sheep? Like humans, do sheep vary in their ability to taste bitter flavor? If so, does ability to sense bitter flavor guide their acceptance or rejection of certain plants? We have set out to answer these questions in hopes to better understand what heritable traits influence sheep foraging behavior. Unlike humans, sheep cannot answer the question “Can you taste bitter?”. Therefore, complex tests must be constructed to best estimate a sheep’s ability to taste or not taste bitter flavor. We developed a semi-rapid test using water spiked with a bitter flavor, where individual sheep are allowed free choice of standard drinking water or water spiked with the bitter flavor. Intake of each water type (bitter or not bitter) was measured, and behavioral responses to water type was evaluated via video surveillance. We found that drinking duration, head bobbing, lip smacking, lip movement, and sniffing are all behaviors related to the amount of bitter-flavored water a sheep avoided or consumed. Compared with previously published methods, we were able to test more sheep in less time with greater certainty that sheep were accurately classified as “bitter or non-bitter tasters.” These findings are useful in developing groups of sheep that can be used to further study how sensitivity towards bitter taste affects sheep herbivory.

Technical Abstract: Sheep have a sensitive threshold for bitterness, which may affect feed selection. Previous research attempted to classify sheep according by bitter sensitivity by measuring fluid intake of the compound phenylthiocarbamide (PTC) at various concentrations over a 14-d period. Although successful, the study was laborious, lengthy, required multiple layers of acclimation and testing, and accommodated a limited number of animals. Accordingly, a more efficient test is needed to classify bitter-taster status of sheep. The objective of this experiment was to develop a more rapid and refined method to assist in identification of PTC-taster status in rams utilizing only two concentrations of PTC-mixture over a 2-day test period. Mature Targhee, Rambouillet, Polypay, and composite-breed rams (n = 44), were equally stratified into three barns and subjected to 4-d acclimation, 2-d sham, and 2-d testing phases. The sham phase was conducted to determine if ethanol (delivery vehicle for PTC) influenced voluntary fluid intake or behavior; no effect (P = 0.13) was observed. The 2-d test phase was a side-by-side preference study between water and one of two concentrations (0.20 and 2.03 mM) of PTC-mixture delivered on alternate days. Video surveillance was conducted to evaluate the number and duration of approaches to the PTC-mixture and subsequent sniffing, head bobbing, jerking, shaking, and lip movements, smacking, and licking behaviors. Overall, intake of PTC-mixture was less (P < 0.001) than water intake. The PTC-taster classifications were determined based on average PTC intake relative to ±1.0 standard deviation of the study population mean and used to sort rams into super- (n = 9), intermediate (n = 29), or non- (n = 6) PTC-taster classifications. The amount of time spent drinking either PTC-mixture or water was different (P < 0.001) amongst PTC-taster classifications and correlated with both water (r = 0.23, P = 0.028) and PTC-mixture (r = 0.46, P < 0.001) intake. Head bobbing (P = 0.035) and lip smacking (P < 0.001) were associated with PTC intake across all classes. Super-tasters appeared to utilize sense of smell (sniffing) more frequently to identify PTC-mixture than the intermediate or non-tasters (P = 0.012). Non-tasters did not exhibit behavioral differences (P = 0.12) regardless of solution type. The streamlined methodology outlined in this study will help facilitate future studies aimed at better understanding forage selection and consumption in sheep.