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Some trees succeed in life by offering giraffes a meal of flowers.
Bats are the most common mammalian pollinators, but within the last decade, a number of small, flightless mammals have also been found to transfer pollen. For instance, Delbert Wiens, a biologist at the University of Utah, found that the flowers of the South African protea are pollinated by rodents (see "Secrets of a Cryptic Flower," Natural History, May 1985). Mammals are covered with fur or hair, which is good for collecting and carrying pollen grains, and because they are so much larger than insects, they can carry more. Their biggest disadvantage is that most mammals cannot fly, making them poor cross-pollinators. Efficient cross-pollinators must have easy access to flowers in the tree canopy and the ability to travel directly between widely distributed plants of the same species. Giraffes meet all these qualifications. While browsing fifteen feet above the ground, their heads and necks become covered with pollen as they shake branches and brush through clusters of blossoms. (While inspecting trees on which the giraffes had fed, I inadvertently collected pollen on my forearms.) Giraffes are capable of traveling more than ten miles a day between stands of flowering trees.
All plants pollinated by animals have to pay a price for the service, usually in the form of pollen and nectar, which are rich in energy and protein. Because giraffes operate on a larger scale, their reward is whole flowers—the one disadvantage to the tree of giraffe pollination. Their potential as pollen vectors remains academic unless the benefits of pollen transfer override the drawbacks of flower loss.
Browsing by giraffes is less of a threat to the reproductive fitness of flowering knobthorns than might be imagined, however. Despite swarms of flying insects around the flowers of some species, African acacias are noted for a particularly low ratio of seed pods to flowers. Many of the flowers may never get fertilized, but another reason for the small number of pods may be that most, and possibly all, African acacia flower clusters are mixtures of bisexual, male only, and sterile flowers. Some botanists have suggested that large numbers of male and sterile flowers could function as petals and are simply there to attract pollinators. The loss of sterile flowers to giraffe browsing is of no consequence to the reproductive fitness of the acacia trees. Nor is the consumption of male flowers if their pollen is transferred in the process. Furthermore, since all the knobthorns produce flowers simultaneously, the giraffe population (which is limited by the scarcity of food during the rest of the dry season) cannot consume a significant fraction of the female flowers in the short time that the trees bloom.
Even though an animal may be capable of transferring pollen, it is not necessarily a reliable pollinator. The animal must visit the plants regularly, as part of its usual feeding pattern. In central Kruger, this is precisely the case. Knobthorns provide giraffes with much more than flowers. During the wet season, knobthorn foliage is the staple diet of giraffes, making up more than 40 percent of their annual diet. The intimate link between the two species is reflected in the distribution of giraffes and knobthorns in the park, which can be neatly superimposed upon a map.
Perhaps the strongest support for my suggestion that giraffes pollinate knobthorns is the trees’ lack of defenses. If giraffes browsing on flowers were harmful to the reproductive success of knobthorns, there would be selective pressure for the evolution of an effective defense. Witness the well-defended pompon flowers of the umbrella thorn, a species that is pollinated by insects. Borne on short stalks, the flower clusters lie close to the branches, between long and formidable thorns. In contrast, the knobthorn’s bottle-brush flowers extend beyond the protection of the trees’ prickles. Nor are the knobthorn flowers defended by noxious chemicals. In contrast, I never saw a giraffe even nibble at the spectacular bright yellow flowers of a Cassia abbreviata, a tree related to acacias, even though they are easily accessible, suggesting that they may contain a particularly unpleasant toxin.
While I still lack definitive proof that giraffes are pollinating knobthorns, there is solid evidence for the coevolution of some acacias and large African herbivores. The seedpods of the umbrella thorn are aromatic and nutritious, so when they are shed in the dry season they are avidly sought out by impalas, kudus, steenbok, and other herbivores. Their hard, thick coats protect the seeds from damage as they pass through the herbivore’s gut, and the seeds are ready for germination when they are eventually deposited in dung some distance from the parent plant. If coevolution between large herbivores and African acacias could result in an efficient system of seed dispersal, I see no reason why it could not lead to efficient pollen dispersal as well.
Traditionally, pollination by animals has been regarded as the exclusive domain of small species that fly. Only within the last decade have some small flightless mammals proved to be pollen vectors, and these discoveries were initially considered bizarre. Now I am really bucking tradition by suggesting that some acacias may be pollinated by the tallest living animal. I must point out, however, that this story represents only the first step of scientific inquiry: the formulation of a testable hypothesis. My next step will be to test whether making knobthorns inaccessible to giraffes will significantly reduce production of knobthorn seeds. This will not be easy, however, because any barriers I erect to exclude giraffes must be strong enough to avoid destruction by elephants, which can wreck just about anything--especially fences. But the effort will be worthwhile if it confirms that the knobthorn has evolved a pollination system that relies on the world’s tallest animal.
