Pollen consumption in ants: a widespread phenomenon?

Background/Question/Methods

Pollen, for those organisms able to utilize it, is an exceptional food source. Depending on the plant species, it can contain high amounts of protein and fat and various micronutrients. Pollen-eating (palynivory) is widespread among some taxa, but is best known in hymenopterans, particularly bees. Most bees use pollen as the main source of nitrogen for developing larvae, while adults are largely restricted to feeding on plant exudates. Similarly, in ants, adults are thought to fuel their activities with carbohydrates while larvae require high amounts of protein for development. Unlike bees, though, ants are thought to derive protein mostly from insect prey. However, scattered observations have suggested that ants may also feed on pollen, but the extent to which this occurs is unknown. Here, we conducted a survey of Paleo- and Neo-tropical ants using acetolysis, a process in which organic material is dissolved leaving behind pollen exines. Samples, consisting of larvae or adult worker ants, were collected opportunistically and via baiting in Papua New Guinea and Peru in 2010-2011. After dissolving samples in an acetic anhydride/sulfuric acid mixture, we used light microscopy to look for pollen grain exines.

Results/Conclusions

We examined more than 200 samples, representing over 60 species, and found pollen in more than 20% of the samples. Our results corroborate previous observations of palynivory in some genera (e.g. Cephalotes, Pseudomyrmex), and we also found pollen in several genera in which pollen consumption was previously unknown (e.g. Solenopsis, Monomorium). We typically found small numbers of pollen grains (<10) in an ant. This may reflect how ants forage; ants probably do not often actively collect pollen from flowers, but will opportunistically consume pollen that has fallen onto leaves as they forage for other substances. Pollen may be used as a protein source for developing larvae in ants, particularly among arboreal ant species where ant abundance is much higher than potential prey sources. Our results also suggest an additional reason for the existence of ant-repellent scents present in some plant species’ pollen: not only would repellent chemicals limit ant access to flowers and decrease their consumption of nectar and reduce interference competition between pollinators and ants, it would also reduce the loss of plant gametes to a non-pollinating visitor.