It's interesting that no further hippos seem to have been born following Millie in 1969. As the video shows, the trio were kept together.Nice to see the Common hippopotami too. I saw three, which must be 0.1 Genevieve (1955), 0.1 Henrietta (1961) and the male acquired in 1966; with Rangi having died in 1968 (tennis ball of course).
It's been speculated the unknown male sired Millie; but considering Rangi died the previous year, it's also possible he sired Millie shortly before his death.