The maintenance and termination of maternal behavior in rabbits: Involvement of suckling and progesterone

- Weaning occurred on lactation day 29, but only in concurrently pregnant rabbits.
- Nipple-removed not pregnant does showed maternal-like behavior for 40 days.
- Concurrence with pregnancy reduced nursing behavior to 26 days.
- Progesterone (P) injections did not induce weaning in lactating-only does.
- Daily P injections reduced milk output in late lactation.

Rabbits mated at postpartum estrus become concurrently pregnant and lactating. They wean the first litter shortly before delivering the second one. Lactating-only rabbits do not spontaneously wean their young. These differences suggest that: a) suckling contributes to the long-term maintenance of maternal responsiveness and b) pregnancy-associated factors are essential to promote weaning. To explore if suckling stimulation interacts with the pregnancy condition we compared the behavior of intact and thelectomized (thelx; nipple-removed) rabbits, mated or not at postpartum estrus (Experiment 1). All lactating-only rabbits still showed nursing behavior by postpartum day 44; only 71% of thelx not pregnant does displayed maternal responsiveness for 31 days and none by postpartum day 44. If mated at postpartum estrus maternal responsiveness was observed only in 25% of pregnant-lactating does on postpartum day 29 and in 8% of thelx-pregnant rabbits on postpartum day 26. As in pregnant-lactating does progesterone (P) is present across ca. 3 weeks and then declines, but it is absent in lactating-only rabbits, in Experiment 2 we explored the effect of injecting P to lactating-only animals on their maternal responsiveness and milk output. P injections (20 mg/day) were given across lactation days 1-30 or 1-23. Neither treatment modified maternal behavior: nearly all females entered the nest box, crouched over the litter and suckled it for ca. 3 min, as did oil-injected nursing rabbits. In contrast, both P treatments accentuated the decline in milk output, with respect to oil-treated does. Results suggest that suckling promotes the long-term maintenance of maternal behavior while pregnancy-associated factors (not P) are essential to trigger weaning.