Long-term effects of tryptophan on behavioural response and growing-furring performance in silver fox ( Vulpes vulpes)

Abstract

The effects of dietary tryptophan (TRP) supplementation on behavioural response, body weight, feed consumption, and winter fur development was assessed on silver fox pups from July 28 until December 5. Ten males and ten females received a commercial fox ration (control) and 10 males and 10 females the same ration supplemented with TRP (1.2 g/MJ ME). Dietary TRP supplementation increased the consumption of protein and gross energy in September and November and total DM in September. The male foxes also consumed more feed and gained more weight than the females throughout the trial. Dietary TRP supplementation did not affect body weight gain, initiation of winter fur growth or fur quality in the test groups. There was a trend toward later priming of fur in the TRP supplemented group. The number of contacts with the novel object increased and the latency time until contact with the tidbit and the novel object reduced towards the end of the experiment. In the tidbit test, dietary TRP supplementation reduced the latency time of the females (40.4 s) compared with the non-supplemented females (58.0 s), the TRP supplemented males (51.7 s), and the non-supplemented males (47.6 s, P=0.001). In the novel object test, the latency time of the TRP females (32.5 s) was likewise reduced compared with the control group females (46.9 s) and TRP group males (44.0 s) being comparable to the control group males (38.5 s, P=0.029). It appears that dietary TRP supplement reduces fear and enhances exploratory behaviour in the female silver fox. This is likely due to the female being more sensitive to the imbalance between TRP and other large neutral amino acids, the supplement leading to increased brain serotonin synthesis. Further research needs to elucidate the effects of dietary TRP on the pineal function due to potential interference with seasonal breeding and furring controlled by the photoperiod.