Litter Sizes and Mammary Numbers of Naked Mole-Rats: Breaking the One-Half Rule

Among mammals generally and rodents particularly mean litter sizes usually are about one-half the number of mammae, and maximum litter sizes approximate mammary numbers. Naked mole-rats (Heterocephalus glaber: Bathyergidae) are exceptions to both generalizations. Field-caught litters averaged 11.3 young ± 6.2 SD (n = 82), and captive-born litters averaged 11.4 ± 5.6 young (n = 190). Similarly, numbers of mammae on breeding females averaged 11.6 ± 1.1 (n = 43) in the field and 11.5 ± 2.0 (n = 29) in captivity. Maximum litter sizes were 28 in the field and 27 in captivity, whereas the maximum number of mammae was 15. More than one-half of field-caught and captive males and females had different numbers of mammae on the two sides of their body. Neither total numbers of mammae nor fluctuating asymmetries in mammary numbers differed significantly between males and females, nor between breeders and nonbreeders. There was no relationship between litter sizes and numbers of mammae or fluctuating asymmetries in mammary numbers. Breeding female naked mole-rats can bear and successfully rear litters that are far more numerous than their mammae because, on a proximate level, young take turns nursing from the same mammary and, on an ultimate level, breeding females are fed and protected by colony mates, enabling them to concentrate their reproductive efforts on gestation and lactation.