They were believed, in Chinese tradition, to appear always in affectionate pairs; indeed, their name yuanyang means literally male (yuan) and female (yang) mandarin duck.
1889, Congressional Serial Set, page 460:
The yuanyang are the male and female repectively of Anas galericulata, commonly called by Europeans "Mandarin duck." These beautiful water-fowl manifest when mated a singular degree of attachment for each other, and they have hence been elevated into the emblems of connubial affection and fidelity.
2006, Pi-Ching Hsu, Beyond Eroticism: A Historian's Reading of Humor in Feng Menglong's Child's Folly:
Yuanyang, the Mandarin duck, mates for life, and is hence a symbol of conjugal felicity.
It is even enjoyed as a unique half tea, half coffee milky mix, yuanyang, a conjugal pairing as disparate as male and female Mandarin ducks (yes, it gets its name from the ducks).
2013, Matthew Gerwitz, Disposable Catheters and Other Musings, ISBN1300980044, page 7:
After just thirty minutes and very limited math skills, my research indicated there are far too many people buying Double-Espresso Macchiato Yuanyangs just prior to entering the expressway for the morning's "rush hour", if you know what I mean.
2015, T Turner, Hong Kong 2016 - : Have an Adventure!:
Yuanyang is also a popular drink mixed with milk tea and coffee.