Behavioral interactions rather than milk availability determine decline in milk intake of weanling rats.

We examined the relation between milk availability and milk intake during the period in which rat pups gradually abandon milk as a food source. The amount of milk produced by rat dams does not change from postpartum Day 15 to Day 20, but decreases thereafter and completely disappears around Day 30. In contrast, the amount of milk actually obtained by pups does begin to decline between Days 15 and 20. This decline in milk intake can be attenuated by integrating 20-day-old pups into 15-day-old litters. We concluded that pups do not begin to ingest less milk because of diminishing milk supplies. Rather, the decreased tendency of mothers to nurse older pups and the diminished tendency of older pups to extract available milk, together appear to underlie the decline in pups' milk consumption. Milk supplies decline after changes in behavioral interactions and may play an instrumental role in the eventual abandonment of suckling.