Milk composition and lactation strategy of a eusocial mammal, the naked mole‐rat

Abstract

The reproductive female, or queen, in a eusocial colony must allocate sufficient nutrients to reproduction to maintain a high rate of reproductive output. In mammals, the energetic costs of lactation greatly exceed those of pregnancy, and thus, lactation should be exceptionally costly for a eusocial queen, such as the naked mole-rat Heterocephalus glaber. We predicted that naked mole-rat milk would be energy- and nutrient-dense. Naked mole-rat milk averaged 17.2% dry matter, 4.5% fat, 4.8% protein, 5.7% sugar and 1.1% ash; and per gram contained 3.0 mg calcium, 1.1 mg phosphorus, 0.44 mg magnesium and 0.54 mg potassium. Other than elevated protein and low sugar in colostrum, the composition of milk did not change over the course of lactation. Naked mole-rats not only had the lowest energy content of milk (3.9 kJ g −1 ) reported for any rodent but also appeared to be an outlier from a trend for milk dry matter, fat and energy concentrations to be inversely related to body mass in rodents. The dilute nature of naked mole-rat milk indicates that an unusually large amount of milk (equivalent to about half of body mass) must be produced daily to sustain the energy needs of an average litter (12 young). Sustaining high water throughput during lactation may be necessary to meet expected water needs of the offspring but may limit the queen to foods that are high in moisture. The concentrations of macrominerals in milk were within the range described for other rodents, except that the Ca : P ratio of milk (2.8:1) was unusually high. Given a lifespan that can exceed 30 years, large average litter size and several litters per year, the lifetime lactation output of a mole-rat queen must be phenomenal and warrants further study.