# Gilbert, ‘Mammary Number and Litter Size in Rodentia’, 1986 > [!cite] > Gilbert, Avery Nelson. ‘Mammary Number and Litter Size in Rodentia: The “One-Half Rule”’. *Proceedings of the National Academy of Sciences of the United States of America, vol. 83, no. 13 (1986), pp. 4828–4830. https://www.jstor.org/stable/27601?origin=JSTOR-pdf. > [!abstract] > Litter size and mammary number in the mammalian order Rodentia show a significant positive correlation. Mean litter size is typically one-half the number of available mammaries, while maximum litter size approximates mammary number. Similar relationships are found in the families Muridae, Cricetidae, and Sciuridae. The relationship of litter size to mammary number is significantly different between the arboreal and terrestrial squirrels, and between the hystricomorph and nonhystricomorph rodents. Mammary number may have operated as a selective constraint on litter size over evolutionary time. --- Baby rodents are called *pups*. [[Gilbert, ‘Mammary Number and Litter Size in Rodentia’, 1986|(Gilbert 1986, 4828)]] ‘Mammals are distinguished by the ability to nurse their young. Lactation has been a crucial factor in the evolution of mammalian parental investment and reproductive strategies …’ [[Gilbert, ‘Mammary Number and Litter Size in Rodentia’, 1986|(Gilbert 1986, 4828)]] ^09e8cb ’The one-half rule can be interpreted as a statistical consequence of the frequency distribution of litter sizes. These distributions typically have a lower boundary of one and an upper boundary equal to the mammary number. Because the modal litter size of most species is of an intermediate number, the result is that the mean litter size falls halfway between one and the mammary number. One interpretation of these results is that natural selection has favored mammary numbers adequate for the occasional extreme rather than the more frequent typical litter size.’ [[Gilbert, ‘Mammary Number and Litter Size in Rodentia’, 1986|(Gilbert 1986, 4829)]] ^e6922d