Rumination in the goat (Capra hircus) is a circadian rhythm synchronized by feeding time

Rumination is usually considered to be a daily rhythmic process that is not dependent on food intake. Although this rhythm is described as circadian in the literature, to our knowledge, no experimental study has yet demonstrated its circadian origin. The objective of the present study was to explore the circadian control of the rumination rhythm in the goat (Capra hircus) and its possible entrainment by the timing of food availability.

Rumination was continuously recorded in bucks housed under constant darkness (DD) and constant ambient temperature (CTa). During Experiment 1 (n=6), food was provided once daily at 10h in Stage 1, four times a day (00h, 06h, 12h, and 18h) in Stage 2, ad libitum in Stage 3, and finally in Stage 4, bucks were food-deprived for four consecutive days. Experiment 2 was designed to investigate the effect of time-shifting of food distribution on the rumination rhythm under constant conditions. Food was given once a day, at 10h in stage 1, then shifted to 22h in stage 2 and returned to 10h in stage 3.

In Stage 1 and 2 of Experiment 1, rumination exhibited a clear rhythmic profile with a period not significant from 24.0 h. In Stage 3, the rhythm exhibited a progressive daily phase shift typical of a free-running state with a circadian period of 24.9 ± 0.1 h. During Stage 4, food deprivation did not abolish rumination or its rhythmicity, though it did alter the free-running period. Experiment 2 showed that the inversion time of the food distribution immediately induced a shift in rumination rhythm, the acrophases were delayed from 02h56 ± 28min (stage 1) to 14h55 ± 8min (stage 2). The return to amorning feeding in stage 3 has led to a phase advance in rumination rhythm, the acrophases being advanced from 14h55 ± 8min (stage 2) to 02h07 ± 13min (stage 3).

This study provides the first evidence supporting a circadian origin of the rumination rhythm in goat. The results also clearly showed that feeding time was able to synchronize the rumination rhythm. Nevertheless, additional research is required in order to elucidate the complete neurobiological mechanisms underlying the circadian control of rumination in the goat.