You may have heard about or seen acacia plants before – they’re the ones you don’t touch because they’ve got big, spiky thorns growing out of their branches at all angles. If you don’t mind the thorns on acacias, you’ll definitely mind the colonies of aggressive ants that live on them.
Some species of acacias have evolved a mutualistic relationship with Pseudomyrmex ants, where the ants defend the acacias from herbivores in return for shelter inside of their hollow thorns and food in the form of nectar produced from glands on the leaf stems as well as protein and lipid-rich food bodies growing on the tips of leaves. The glands are called extrafloral nectaries because they produce nectar outside of the flower.
A study conducted by Stefanie Kautz from the University of Duisberg-Essen in Germany and colleagues from the US and Mexico demonstrated that acacias take charge of this relationship by secreting nectar that is lacking a specific molecule: sucrose.
Pseudomyrmex ant species prefer this sucrose-free nectar because their colonies spend their entire lives on the acacias and it’s what they’re raised on. These ants couldn’t feed on nectar that contains sucrose anyway because they’ve lost their ability to digest it. Digestion of sucrose requires an enzyme called invertase that breaks it down, and mutualistic acacia ants just don’t have it. The ants have essentially become dependent on the sucrose-free diet their acacia hosts provide. Smart, right?
Babies with a Sweet Tooth
This dependence has to be imposed on the ants at an early stage, it seems, in order to make them stick to their restricted diet. Kautz et al. found that every mutualistic ant does actually have the ability to digest sucrose when they are larvae, probably because larvae are fed a special diet of food bodies that do contain sucrose. But as the ants grow older they’re weaned instead on extrafloral nectar and thus become sucrose-sensitive. The acacias don’t run a risk from this switch-up in diet throughout their ants’ lives because the larvae aren’t going anywhere–they’re stuck inside thorns until they grow legs.
There are other, non-mutualistic ants that can digest sucrose, and actually prefer it. These ants can be founding residing within the thorns of acacias and feeding from their extrafloral nectaries, but they’re also capable of feeding on sucrose-containing nectar from other sources. They’re not considered to be mutualistic because they do no protect their plant host from getting eaten.
These unwelcome ants can be cheaters, opportunists or straight up parasites. Cheaters are ants that steal nectar from acacias but have evolved from ants that are mutualistic, opportunists just try to get a meal wherever they can but don’t stick around long enough to cause harm to the acacia plant, while parasites will stick around, but they sure as hell don’t pay for their room and board. Parasites have been parasites throughout their evolutionary history–they’ve stuck to their strategy since the time of their ancestors. Mutualistic acacia-ants depend on their hosts, but the acacias depend on their miniature guards as well. It costs an acacia a lot to feed an ant colony that doesn’t protect it.
Of the five Pseudomyrmex species included in the study, three were known mutualists: P. ferrugineus, P. mixtecus and P. peperi. The other two, P. nigropilosus and P. gracilis, are the moochers to watch out for. Pseudomyrmex gracilis appears to be just your typical opportunist–it can take its business elsewhere if necessary–while P. nigropilosus is a bona fide parasite that only lives on Acacias.
No Picky Eaters Allowed
So we’ve got a parasite and we’ve got an opportunist, but where’s the cheater? As it turns out, cheaters are actually hard to find, which proves that the acacias have a strategy that does work to some degree. Ants that have a give-and-take history with acacias tend not to stray from this mutually-beneficial relationship. However, opportunists with a more general diet and a tolerance of sucrose-free nectar could take advantage of acacias, and so can parasites that will put up with sucrose-free nectar. Acacias can take solace in that their mutualistic ants won’t change their stripes, but greed always finds a way for those insistent on getting a free meal.
Kautz, S., Lumbsch, H. T., Ward, P. S. & M. Heil. 2009. How to prevent cheating: a digestive specialization ties mutualistic plant-ants to their ant-plant partners. Evolution 63(4):839-853