Bats almost universally have exactly two mammary glands, positioned in the axillary (armpit) region of the chest — the same pectoral placement seen in primates. This reflects the typical singleton birth of most bat species. The pup nurses while clinging to the mother at the roost, hanging inverted and gripping the teat with its milk teeth.
Many bat species possess one or two non-functional "false nipples" (pubic teats or inguinal nipples) in the groin region that produce no milk. Pups grip these during flight or when the mother roosts, acting as a safety anchor. This arrangement frees the functional pectoral teats for undisturbed nursing during rest periods.
Bat milk undergoes a dramatic compositional shift across the 3–4 week lactation period. Early milk is relatively protein-rich and dilute; by the second and third weeks, fat content rises steeply — from ~5% to over 20% in some species — matching the explosive growth of the pup's muscle and wing tissue leading up to first flight.
In clustered maternity colonies of hundreds or thousands of bats, mothers and pups locate each other using distinct vocalization signatures. Each pup produces a unique isolation call; the mother responds with a matching call. This prevents misdirected nursing, although allosuckling (nursing another female's pup) does occur at low rates in some species.
Early bat milk is thin and watery with a slightly sweet, protein-forward character due to elevated whey proteins. By peak lactation it thickens markedly as fat globule concentration rises. The very high late-lactation fat content gives it an almost cream-like consistency in species like the little brown bat (Myotis lucifugus).
The rapid fat ramp-up is considered an adaptation to the energetic demands of flight development. Wing membrane growth and the rapid ossification of wing bones require both structural protein and high-energy lipids. Lactose concentration inversely mirrors fat — a common pattern in mammals where higher fat milk tends to lower osmotic lactose to balance total solute load.
Calcium and phosphorus are elevated relative to body size, supporting the rapid skeletal growth needed for flight by 28–35 days of age. Fat-soluble vitamins A and D, acquired through the mother's insect diet, are passed through milk. Taurine levels are high — consistent with the carnivorous (insectivorous) diet of most microbat species.
The magnitude of compositional change in bat milk is among the largest recorded in any mammal — fat content can increase fivefold between day one and day 21. This dynamic profile has attracted interest in lactation biology research as a model for understanding how gene expression in mammary gland cells is temporally regulated across a single lactation period.