Such was Billy's popularity in the 1930s that at the end of the decade he traveled to Queens, New York, for the 1939 New York World's Fair, where he appeared in the Firestone exhibit.
 Billy and Hannah successfully had more calves, and because of Billy's worth as a breeding stud, Mann traveled with the Smithsonian-Firestone Expedition to Liberia in 1940 and acquired a second mate for Billy, another female who came to be known as Matilda.

Between 1931 and 1954, Hannah gave birth to 15 of Billy's calves, 7 of whom were reared or lived at least one year.
 Between 1943 and 1956, Matilda gave birth to 8 of Billy's calves, 6 of which were reared.
 At some point during the 1940s, the press started to refer to Billy as William Johnson Hippopotamus.
 The zoo developed the tradition of naming all his calves Gumdrop, using Roman numerals to distinguish them.
 By the birth of Gumdrop XVI, the zoo had noticed something curious: of all Billy's calves, only one was male.
 Research would later confirm that pygmy hippos in captivity are far more likely to give birth to females, though not by such an extreme ratio as Billy.
  Twenty-three years after Coolidge's death (January 5, 1933) Billy himself died on October 11, 1955, five months before the birth of Gumdrop XVIII.
 "He carried his work on to the end", said a zoo official.
 Hannah followed Billy in death on March 6, 1958.
Such was Billy's popularity in the 1930s that at the end of the decade he traveled to Queens, New York, for the 1939 New York World's Fair, where he appeared in the Firestone exhibit.  Billy and Hannah successfully had more calves, and because of Billy's worth as a breeding stud, Mann traveled with the Smithsonian-Firestone Expedition to Liberia in 1940 and acquired a second mate for Billy, another female who came to be known as Matilda.
Between 1931 and 1954, Hannah gave birth to 15 of Billy's calves, 7 of whom were reared or lived at least one year.  Between 1943 and 1956, Matilda gave birth to 8 of Billy's calves, 6 of which were reared.  At some point during the 1940s, the press started to refer to Billy as William Johnson Hippopotamus.  The zoo developed the tradition of naming all his calves Gumdrop, using Roman numerals to distinguish them.  By the birth of Gumdrop XVI, the zoo had noticed something curious: of all Billy's calves, only one was male.  Research would later confirm that pygmy hippos in captivity are far more likely to give birth to females, though not by such an extreme ratio as Billy.   Twenty-three years after Coolidge's death (January 5, 1933) Billy himself died on October 11, 1955, five months before the birth of Gumdrop XVIII.  "He carried his work on to the end", said a zoo official.  Hannah followed Billy in death on March 6, 1958.
Billy (pygmy hippo)