Plant Incentive

Vachellia collinsii, a species of acacia tree, is famous for its symbiotic relationship with ants. Managing different ant species with distinct work ethics presents a challenge to these trees.

V. collinsii employs ants for protection. The ants attack herbivorous insects which eat the tree’s leaves, remove encroaching vegetation, and also protect acacia from disease by distributing antibiotics synthesized by bacteria living on the ants’ legs.

In return, the tree rewards ants with food: protein-rich Beltian and sugar-rich nectaries. The tree also provides secure housing, inside hollow thorns evolved specifically for that purpose.

This cozy arrangement is subject to negotiation. One of the best known ant symbionts of acacias is Pseudomyrmex spinicola. These ants do everything expected of them to help the plants thrive.

By contrast, Crematogaster crinosa are less desirable tenants. They are lazy defenders against herbivores, fail to clear encroaching vegetation, and are not known to spread antibiotics for the trees.

So how does an acacia incentivize C. crinosa ants? The quality and quantity of housing accommodations does not vary between the ants. The food rewards on offer, however, vary by quite a bit.

Trees with ants sported 75% more nectaries than those without. But the plants also treated the 2 types of tenant differently.

Though the distribution of Beltian bodies is the same for both ants, acacias supporting colonies of P. spinicola only produced nectaries along the bases of their leaves. Those supporting C. crinosa did this too, but also sported sugar dispensaries at the tips of their leaves, encouraging otherwise recalcitrant workers of that species to traverse the leaves to reach extra reward. That brings these ants into contact with pests they might not otherwise have encountered, whereupon the ants drive those pests away.

That plants provide extra reward rather than punishment to indolent ants runs counter to human ethics. To get their needs met, plants eschew morality for practicality. Sugar is a mere trifle. And, after all, ants are just creatures, lacking the wisdom of trees.


Acacia and ants photo courtesy of Sabrina Amador.

Ants, acacias and shameless bribery,” The Economist (10 July 2021).

Finote Gijsman et al, “Short-term plasticity and variation in acacia ant-rewards under different conditions of ant occupancy and herbivory,” The Science of Nature (1 July 2021).