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A total of 96 lactating mice and their pups from each of three replications of lines selected 21 generations for increased litter size (LS) followed by 23–25 generations of random selection and contemporary unselected control lines (LC) were characterized for maternal behavior. Sixteen dams and their pups from each replication of LS and LC lines were sampled. Litter sizes at birth for LS versus LC in the first, second, and third replicates were 17.3 versus 13.2, 15.9 versus 13.1, and 12.3 versus 10.6 pups, respectively, with LS dams averaging 2.87 ± .70 pups more. One-half of the litters were standardized to either 12 pups (Replications 1 and 2) or 8 pups (Replication 3) on day 1 of lactation; the others were not standardized. Behavioral categories included time dams spent: (1) nursing pups, (2) licking pups, (3) retrieving pups, (4) nest building, (5) resting with pups. (6) resting alone, (7) eating and drinking, (8) grooming, and (9) in other activities. Categories one through five were summed as an index of maternal behavior. Activities during 30 min observation periods from days 2 to 22 of lactation were recorded. Interactions of line by stage of lactation (days 2–4, 5–8, 9–12, 13–17, and 18–22) and line by litter standardization were not significant for any trail except time spent resting with pups (line × stage, P = 0.03).
The maternal behavior index was 7.0 ± 2.2% greater (P = 0.08) for LS than LC dams. The difference was due mostly to increased time LS dams spent nursing pups (LS – LC = 9.7 ± 2.1%, P = 0.04). Averaged across lactation stages, LS dams tended to spend less time resting with their pups (–2.32 ± 1.04%, P = 0.15) than LC dams, but the difference was greatest and significant only in the last stage (–10.45 ± 2.20, P < 0.01). Selection for larger litters altered maternal behavior by increasing the time dams spent nursing pups. Interaction of line by litter size standardization existed for pup survival to weaning (P = 0.03). Survival of pups in standardized litters raised by LS dams was 10.4 ± 4.9% greater (P = 0.04) than in standardized LC litters whereas line difference in non-standardized litters was not significant (P > 0.30). Because interactions of line by litter size standardization for maternal behavior traits were not significant, correlated responses in maternal behavior were considered to be independent of size of litter being nursed.