Age-related stimulatory and inhibitory effects of suckling regulate lactation in rabbits

Milk production and the duration of lactation were compared between control rabbits nursing their own pups and those whose litters were replaced by younger, i.e., 4-6 days old, or older pups (20-23 days old) starting at pp days 4, 10 or 15. In rabbits whose pups were exchanged on pp day 4 by older pups, an early decline of milk secretion was induced and similar yields to those of late-lactating controls were produced during mid lactation. Conversely, yields similar to those of mid-lactating controls were produced in late-lactating mothers nursing young pups continuously since pp day 10 and high yields were maintained in these animals until pp day 45. When litter exchanges, by either older or younger pups, were delayed until pp day 15, yields were not significantly different from controls. These results indicate that qualitative changes in suckling stimulation develop as the litter grows older, i.e., young pups stimulate milk secretion, whereas old pups inhibit it. Thus, suckling itself may determine the overall pattern of milk secretion in these species.