The Bullhorn Acacia
By: Mike Boston
Mike is an honor’s biology graduate of the University of Portsmouth and is the owner / operator of Osa Aventura, the premier guiding outfitter for Corcovado National Park, and has led groups into the Park for the past eighteen years. Contact him directly at firstname.lastname@example.org.
When I’m guiding I often point to a small, unremarkable looking tree called the bullhorn acacia (Acacia collinsii) cornizuelo in Spanish, and describe some of the interesting biology associated with it. Its name comes from the paired, swollen thorns on its branches that resemble horns of a bull. Within these thorns, the acacia accommodates aggressive, stinging ants.
This acacia recruits ants as body guards to protect itself from browsing mammals, leaf-eating insects and smothering vines. In return, the acacia houses and feeds the ants – perfect symbiosis by all appearances!
From sugar-rich nectaries on its stems and from small, protein-rich Beltian bodies on the tips of its leaves, the bullhorn acacia provides a banquet for the acacia ant, Pseudomyrmex ferrugineus. Not having to forage far afield for its energy and nutritional needs is advantageous to the ants. And having a full-time defense force brings obvious benefits to the acacia. Matters couldn’t be fairer!
Recently, however, a researcher in Mexico has revealed a sinister side to the acacia’s part in this cosy relationship.
Ants, indeed all animals, acquire their energy needs primarily from carbohydrates or sugars. In order to extract that energy, ants must break down (digest) large-molecular carbohydrates into their fundamental units of glucose and fructose. The enzyme invertase in their digestive track catalyses this process. As it happens, the most common carbohydrate found in nature, including nectar, is the relatively large-molecule sugar, sucrose. Consequently, the digestive enzyme, invertase, is present in all animals, fungi and some plants too, in order to harvest energy from sucrose. Strangely, though, the nectar produced by the bullhorn acacia contains no sucrose, only glucose and fructose.
Curious to know why, this researcher analysed the contents of the digestive tracks of acacia ants and found that they lacked invertase. Initially, he assumed that over time the ants had lost their ability to produce invertase through redundancy, because the acacia fed them pre-digested glucose and fructose. However, when he analysed the contents of the digestive tracks of the ants’ larvae he found invertase. So, what was going on?
Looking again at the acacia’s nectar, he found that it contained invertase, which explained its pre-digested sugar content of glucose and fructose. But what the researcher also found surprised him: the acacia nectar contained another enzyme which inhibits invertase production in the ants. So, when an adult P. ferrugineus ant emerges from its pupa and takes its first meal of nectar from the acacia its ability to produce invertase is irreversibly disabled. The ant can no longer feed its energy needs from any other source other than from the acacia – the ant is trapped, enslaved to the acacia for life! This often cited example of harmonious symbiosis in nature is biased, it seems, in favour of the bullhorn acacia.
While on the subject of the bullhorn acacia I often mention another animal that has tapped into the acacia’s largess to its ants. It’s a jumping spider, named Bagheera kiplingi in the late 1800s after a panther in a well-known children’s book of that era “Jungle Book”, by Rudyard Kipling. But, it was not until 2001, in Costa Rica, that the spider’s remarkable diet came to light: it dines almost exclusively on the Beltian bodies of the bullhorn acacia. Of the over 40 thousand spider species found on our planet, the vegetarian jumping spider is the only known vegetarian among their ranks.
Another animal that takes advantage of the acacia’s association with its ants is a bird: the scarlet-rumped cacique (Cacicus uropygialis). Often I see its long, pendulous nests hanging from the upper branches of some of the taller bullhorn acacias. I explain that this bird often uses aggressive hymenopterans for protection against predators that would invade its nest. For instance, it is known to build its nest close to occupied wasps’ nests. In the acacia, the scarlet-rumped cacique is harnessing the tree’s coveted ant bodyguards to keep its eggs and chicks safe from nest raiders.