Which animal is born lighter than a rat? - briefly
The common house mouse (Mus musculus) is born at roughly 1–2 g, far lighter than a newborn rat. Several other tiny mammals, such as shrews, also have birth weights well below that of a rat.
Which animal is born lighter than a rat? - in detail
A newborn rat typically weighs between 5 and 6 grams. Numerous vertebrate and invertebrate species are born with a lower mass.
- House mouse (Mus musculus) – newborns weigh 0.5–2 g.
- Common shrew (Sorex araneus) – newborns weigh 0.1–0.3 g.
- Little brown bat (Myotis lucifugus) – newborns weigh 1–2 g.
- European robin (Erithacus rubecula) – hatchlings weigh about 1 g.
- American robin (Turdus migratorius) – hatchlings weigh 1.5–2 g.
- Tree frog (Hyla arborea) – tadpoles at hatching weigh less than 0.01 g.
- Honeybee (Apis mellifera) – emergent adult (often considered “born”) weighs roughly 0.1 mg.
These examples illustrate that many small mammals, birds, amphibians, and insects emerge with a birth weight well below that of a typical rat pup. The disparity arises from differences in gestation strategies, metabolic rates, and developmental patterns across taxa.