The Collaborative International Dictionary of English v.0.48:

Mammalogy \Mam*mal"o*gy\, n. [Mamma breast + -logy: cf. f.
   mammalogie.]
   The science which relates to mammals or the Mammalia. See
   Mammalia.
   [1913 Webster]

WordNet (r) 3.0 (2006):

mammalogy
    n 1: the branch of zoology that studies mammals