Giraffes and the pollination ecology of knobthorns (Acacia nigrescens)

Giraffes (Giraffa camelopardalis) consume large quantities of knobthorn (Acacia nigrescens) flowers every year, and may be acting as pollinators. Because knobthorns flower in the late dry season, nutritionally a critical time of year for ungulates, the flowers are an important source of food for giraffes, especially since they are not physically protected against herbivory. Giraffes visit the flowering trees reliably year after year, carry pollen on their heads and necks, and cover large distances between knobthorns. In this study, conducted in the Kruger National Park during September 2003, I addressed four questions. (1) Are knobthorn flowers a nutritional reward for giraffes compared with other browse available at that time of year? Knobthorn flowers were of higher nutritional quality than knobthorn leaves and a composite browse type in that they contained approximately 50 % more water, almost twice as much protein and about 33 % less acid detergent fibre. However, knobthorn flowers were of lower quality than knobthorn leaves in that they contained almost three times as much condensed tannin. (2) Does giraffe foraging behaviour conform to predictions about pollinators? Although giraffes consumed the majority of knobthorn flowers within their reach, they also moved directly between flowering knobthorns when browsing, and transported knobthorn florets on their faces. (3) Does knobthorn flowering biology differ from that of insect-pollinated Acacias? Knobthorn flowers produce tiny volumes of highly concentrated nectar, suggesting melittophily. However, flowers release pollen over several days rather than a few hours, do not show any synchronised daily peak in pollen release, take longer to open than other African Acacia flowers (apparently remaining sexually active for longer), and do not show the typical high proportion of non-hermaphrodite florets. These differences imply that a different pollination process may be operating in knobthorns. (4) What other pollination processes might operate for knobthorns? A wide diversity of insects visits knobthorn flowers, but only about 20 % are potentially effective pollinators. Birds, vervet monkeys and wind may also play a minor role in knobthorn pollination. I conclude that giraffes may be important pollinators of knobthorns, since the benefits of pollen transfer over large distances may outweigh the drawbacks of flower consumption.

Last Updated
January 30, 2021
1.07 MB