= Communal breeding
<Communal breeding> combines reproduction by multiple individuals with shared care of their offspring. In communally nursing <house mice>, females pool their litters in a common nest and care for the resulting group. Establishing this arrangement requires evidence about both parentage and care; the presence of several adults in one area is insufficient.<Maternal investment of communally nursing female house mice (Mus musculus domesticus) by B. König>[Primary Abstract: communal rearing within a breeding unit]{ref}
A 1993 experiment compared singly breeding females with pairs of familiar sisters and pairs of genetically unrelated females that were initially unfamiliar. The study measured offspring numbers and mass at weaning. Females sharing with a sister weaned more young than the solitary comparison, while the number of litters and average litter size did not differ significantly across the groups. The distinction connects reproductive output with successful rearing, rather than merely counting pregnancies.<Maternal investment of communally nursing female house mice (Mus musculus domesticus) by B. König>[Abstract: comparison groups, litter measures, weaned offspring and mass]{ref}
An original accounting example shows why the unit of comparison matters. Suppose one solitary female weans10 young, while a communal nest containing two breeding females weans18. The nest total is larger, but its mean is $18/2=9$ young per breeding female. This does not establish that either female individually produced nine young; parentage observations would be needed for that assignment. These invented counts are not the study's measurements.
Relatedness, familiarity, access to nest sites and the distribution of care can all be questions for a controlled comparison. In the reported laboratory groups, sisters were familiar whereas the unrelated partners began unfamiliar. As an inference from that design description, the contrast alone does not independently separate kinship from every effect of familiarity.
The interesting scientific question is how sharing changes each participant's reproductive outcome under stated conditions. A larger communal brood, more care and greater per-female success are different measurements. <Communal breeding> identifies an arrangement to investigate; it does not guarantee the same benefit for every participant or in every environment.
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