A baby bat is correctly termed a pup, and bats are the only mammals capable of sustained flight. The variety across the order Chiroptera is immense, ranging from the tiny Bumblebee Bat, the world’s smallest mammal, to the massive Flying Fox species. This diversity means that the size of a newborn pup varies dramatically between microbat and megabat species. Despite this range, newborn bats are born at a relatively large size compared to their mothers, presenting unique biological challenges immediately after birth.
Initial Size and Weight at Birth
The absolute size of a newborn pup is surprisingly small for most common species, often measuring only one to two inches in length. A typical North American microbat, the Little Brown Bat, provides a clear example of this initial size. At birth, a Little Brown Bat pup weighs approximately 2.2 grams, comparable to the weight of a few paperclips. The forearm length, a common measurement in bat studies, is less than 17.2 millimeters at this stage. A slightly larger species, the Big Brown Bat, averages between 3.45 and 3.65 grams when first born. These newborns are born altricial, meaning they are naked, blind, and entirely dependent on the mother for warmth and nutrition, relying on the consistent care provided within the maternity roost.
Relative Size Compared to the Mother
While the raw birth weight seems small, the size of a bat pup is extraordinary in proportion to the mother’s body mass. Bat pups exhibit the highest values of relative neonatal body mass across the entire mammalian class. This ratio often falls between 25% and 30% of the female’s postpartum weight, and in some species, it can be as high as 43%. For context, a human baby is typically only about 5% of the mother’s body weight at birth, underscoring the exceptional size of the bat pup. The Big Brown Bat pup, for instance, represents about 22% of its mother’s mass at birth, placing an immense physiological demand on the female during pregnancy. This large relative size is attributed to the mother’s unique challenge of needing to fly while carrying the developing fetus. Evolutionary pressure favors a single, large pup that can develop quickly, reflecting a strategy for rapid postnatal development.
Growth and Development Milestones
Following birth, bat pups undergo one of the fastest growth rates observed in mammals, necessary to achieve flight capability quickly. For the Big Brown Bat, a pup’s mass increases rapidly at approximately 0.40 grams per day. The forearm length, which determines flight capability, also grows quickly, expanding by about 0.844 millimeters daily in early life. Physical milestones are met within a compressed timeline. The eyes open around Postnatal Day two or three, and pups begin to grow fur by the end of the first week, covering the entire body by Postnatal Day seven or eight. For the Little Brown Bat, this rapid development allows the pup to nurse for only about two weeks before attempting flight. True powered flight is typically achieved around four weeks after birth; Big Brown Bat pups reach this milestone at approximately 27 to 28 days. This progression ensures the young can leave the maternity colony and forage independently before the end of the summer season.