DISCRIMINATION BETWEEN THE ODORS OF FRESH ESTROUS AND NON-ESTROUS EWE URINE BY RAMS

Four sexually experienced adult rams were trained using operant conditioning techniques to discriminate between oestrous and non-oestrous ewe urine odours. Fresh oestrous and non-oestrous urine was collected from different ewes within 1 h prior to each experiment. Oestrous urine odour was the positive discriminative stimulus (S+) and non-oestrous urine odour was the negative discriminative stimulus (S-). The rams performed 22 olfactory discrimination experiments and on each test day urine from various pairs of ewes and from various oestrous cycles was used. The only consistent feature of S+ therefore was the oestrous state of the donor ewe and the only consistent feature of S- was the non-oestrous state of the donor ewe. All rams could discriminate between fresh oestrous and non-oestrous urine odours on every occasion. During the first 12 experiments, the rams learnt to respond to the consistent difference in odour between the two urine types. In the last 10 experiments, they were able to choose the new oestrous urine odour consistently in the initial trial. These results demonstrate that rams are able to differentiate between sexually receptive and non-receptive ewes on the basis of differences in urine odour. The implications of these findings and suggestions for further experimental work are discussed. © 1990.