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Some trees succeed in life by offering giraffes a meal of flowers.
A person has time to think when sitting in a parked car, waiting for someone. I often found myself in this situation while studying the feeding patterns of giraffes, kudus, impalas, and steenbok in South Africa’s Kruger National Park. On a typical day, lunchtime music was playing softly on my radio, and cicadas shrilled in the grass. The landscape was dry and dusty, except for the rivers in the distance, which were visible as bands of green vegetation that stood in bold contrast to the brown hills. I might have been watching one of my study animals, perhaps a giraffe cow ruminating in the shade of a knobthorn acacia. Beneath a veil of creamy white flowers, the gray limbs of the tree shimmered in the midday heat. The giraffe, her ears laid back and her eyes half shut, chewed steadily on acacia flowers.
This scene is typical of the late dry season in the lowveld, South Africa’s expanse of semiarid savanna. The lowveld receives only twenty-four inches of rainfall each year, mostly in the austral midsummer from November to February. During this short wet season, plants sprout new leaves and many burst into flower. Antelopes and many other medium-sized herbivores calve at this time to take advantage of the plentiful food. With the onset of the dry season, however, conditions begin to deteriorate as soil moisture evaporates and seeps into the folds of the landscape. By August, most trees are dormant, standing bare over the brown earth and brittle grass, and most large herbivores have moved downslope to riverbanks where they can still find some green leaves to browse on. The animals must survive several more hot, dry months before purple-gray rain clouds arrive. Ribs and pelvic bones will protrude farther before the green flush of the next rainy season.
During my three years in Kruger National Park, I spent my days tracking the movements of my study animals and recording their choices of habitats and food. Many interesting results emerged as my work proceeded, but perhaps the most remarkable was the relationship between giraffes and the knobthorn acacias (Acacia nigrescens), common trees of the dry plains and ridges. Toward the end of the dry season, giraffes are lured away from the riverine vegetation by the knobthorn flowers. Borne in clusters on the ends of branches, knobthorn blossoms are easily cropped by giraffes as they roam from tree to tree, and provide them with up to a quarter of their food during August and September, when the trees are in full bloom. Other browsers lack the giraffes’ height advantage and are unable to reach flowers on most knobthorn trees, although I often saw impalas scavenging dead flowers that had fallen to the ground.
At various times of the year, I observed giraffes feeding on the flowers of other tree species, but these provide little food compared with the knobthorns. I found it odd that only the knobthorns produce such an abundance of flowers at a time when the giraffes need them most as food. In the dry season, the survival of plants depends on adaptations that protect them from browsers. So why hadn’t the knobthorns evolved defenses to protect their flowers? Wouldn’t the loss of so many flowers reduce the trees’ ability to reproduce?
As my research progressed, I began to wonder if the relationship between giraffes and flowering knobthorns might not be as one-sided as it first appeared. Perhaps the giraffes were paying for a palatable meal by carrying pollen between patches of flowering knobthorns. This possibility became increasingly plausible as I learned more about the reproductive biology of African acacias.
Although the pollinators for only a few species of African acacia have been identified, the general assumption has been that insects perform the task. (The pollen grains of acacia trees are too large and heavy to be effectively dispersed on the wind. Birds and bats are unlikely pollinators, because the knobthorns do not provide the nectar they seek.) Therefore, on the plains of a semiarid savanna, it makes sense for a tree to flower in the wet season, especially after each rainstorm, when flying insects that are likely to transfer pollen are most active. The umbrella thorn (Acacia tortilis), one of the most common trees in my study area, follows this pattern, flowering sporadically in the wet season. Each flowering is triggered by recent rain so that the same tree may flower several times a season.
During the hot dry season, however, bees are rarely found far from water; I often saw swarms of them along the edges of stagnant pools or on damp sand in drying riverbeds. It makes sense for riverine trees to flower in the dry season when competition for insect pollinators is low and bee activity is restricted mainly to the same habitat as the flowering plants. Indeed, Kipling’s fabled fever trees (A. xanthophloea) occur only along rivers and bloom in the late dry season, when their golden "pompon" flowers sweeten the air and the riverine canopy hums with bees.
The distribution of flowering knobthorns, however, does not appear to make much sense. Although they also flower late in the dry season, they are rarely found along rivers and are often miles from surface water—and bees. On the hot, arid plains, their flowering canopies are striking to the eye, but they are neither fragrant nor humming.
Based on the general appearance of their flowers, taxonomists divide the African acacias into two different groups. One, which includes the umbrella thorn and the fever tree, generally has bright golden or orange-yellow pompon flower clusters and is probably pollinated by insects. These acacias flower at times when flying insects are active in their habitats, their fragrance and colors attract insects, and their spiny thorns protect their flowers from browsing herbivores. The other group, which includes the knobthorn, has flower clusters that resemble slender bottle brushes and do not seem to be as well adapted for insect pollination. In fact, the knobthorn’s pale flowers fit the tendency for flowers pollinated by mammals to have dull colors.