Milk yield, suckling behaviour and milk ejection in the lactating rat nursing litters of different sizes.

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Abstract
The quantitative relationship between the suckling stimulus and several suckling-dependent variables was studied in lactating rats. The intensity of the suckling stimulus, measured as the product of duration and number of pups suckling was in proportion to litter size for litters of 1, 10 or 22 pups. Pups in the litters of 10 grew the fastest. Single-pup litters grew most slowly, some not at all. Litter weight gain, corrected for inevitable losses was used to estimate milk yield. Total milk yield was greater for litters of 22-24 pups than for litters of 10 pups only in the 1st week. Milk yield for single pups was up to 7% of the yield for 10 pups. Milk yield per mammary gland sucked was similar for the larger litters but less for single pups. The number of milk ejections, measured as pup stretch reactions, in observation periods of 225 min, was similar for litters of 10 and 22-24 pups, but less for single-pup litters. The suckling stimulus applied by single-pup litters inhibited ovarian growth less effectively than did the stimulus applied by larger litters. There was no change in serum osmolality during lactation. Litters of 10 or 22-24 pups applied suckling stimuli of different intensities, but these had similar effects on the daily oxytocin secretion rate, milk production and suppression of ovarian growth. Single-pup litters were less effective.