Major role of suckling stimulation for inhibition of estrous behaviors in lactating rabbits: acute and chronic effects.

Lactation in rabbits induces anestrus: sexual receptivity and scent-marking (chinning) are reduced despite the brevity of suckling (one daily nursing bout, lasting around 3 min). The mechanisms underlying this effect are unknown but, as chinning, lordosis, and ambulation in an open field are immediately inhibited by the peripheral stimulation received during mating we hypothesized that, across lactation, suckling stimulation would provoke a similar effect. To test this possibility we provided litters of 1, 3, 5, or 10 pups across lactation days 1-15 and quantified chinning and ambulation frequencies, the lordosis quotient, and milk output. Baseline chinning frequency, determined before the daily nursing bout, was low across lactation days 1-15 in does nursing 3, 5 or 10 pups but it increased steadily across days 1-10 in rabbits suckling one pup. Yet, a single young was sufficient to abolish chinning for about 1h, after which this behavior rose again. Suckling litters of all sizes reduced (but did not abolish) ambulation frequency, both chronically (baseline levels declined across days 1-5) and acutely. Sexual receptivity was significantly reduced on lactation day 15 only in does that had nursed 10 pups. Large litter size promoted a larger milk output and a normal duration of nursing episodes. Results support a major role of suckling stimulation for the suppression of estrous behaviors and ambulation through as yet unidentified mechanisms.