Standing about 10 m tall, Humboldtia brunonis trees look like most other understorey trees. Also called hasige mara in Kannada and kaadu Ashoka (forest Ashoka) in Malayalam, they have brown branches, green leaves, and pretty pink and white flowers that give off a pleasant fragrance. Sounds about average, right? But these trees that grow only in the wet evergreen forests of the lower Western Ghats, lead rather strange lives. They house mini zoos in their stems.
Free housing and cafeterias on leaves
Kaadu Ashoka are ant-plants or myrmecophytes (Latin myrme: of ants; phyte: plant) and have mutually beneficial relationships with ants. The trees provide food and shelter to the ants, and the ants patrol the trees and protect them from leaf-eaters or herbivores such as caterpillars.
The shelters that the kaadu Ashoka provides ants with are hollow stem structures called “domatia” (Latin domum: home). The domatia of this tree are typically formed in small side branches, where the portion between two leaves, an area known as the “internode”, forms a bulging, hollow structure with a small natural slit that serves as an entry/exit point.
In addition to these shelters, the kaadu Ashoka also provides ants with a sweet liquid produced by special glands called extrafloral nectaries. Usually, nectaries are only found in flowers where they secrete nectar to attract pollinators; however, this tree grows nectaries on its young leaves and flower buds to attract ants. The ants, in return, protect these vulnerable parts of the tree from caterpillars and other insects that would eat the tender leaves and buds.
An interesting detail about this housing-and-food-for-protection partnership is that this tree uses its extrafloral nectar to also protect itself from sap-sucking insects such as aphids. Many ant species care for herds of aphids on plants, for the sugar-rich honeydew that the aphids produce as a waste product when they feed on plant sap. To protect itself from this exploitation, the wily kaadu Ashoka produces extrafloral nectar that is not only rich in sugars, but also in essential amino acids, which are useful for protein production. Thus, the kaadu Ashoka rewards its ant partners with a food source that is much more valuable than honeydew!
Good and bad tenants
While some myrmecophytes, like bullhorn acacias, partner exclusively with one species of ant (Pseudomyrmex ferruginea), other myrmecophytes may form relationships with several ant species.
The kaadu Ashoka falls under the latter category and has been observed to have up to 17 ant species associated with it, with seven of these species nesting in the domatia while the others only patrolled the plants for nectar. What is even more interesting is that the anti-herbivore protection the ants provide varies geographically from north to south across this plant’s distribution in the Western Ghats.
It turns out that the only ant species that protects the kaadu Ashoka effectively is the white-footed ant (Technomyrmex albipes), which rarely occurs in the northern Western Ghats; however, in the southern regions, where these ants are more abundant, they have a protective partnership with the plant. This is a unique case of shifting evolutionary relationships where one can observe evolution occurring in a system across habitats rather than over time — a phenomenon termed “evolution across geography” instead of the more common concept of “evolution across time”.
In contrast to this mutually beneficial alliance, the liaison between the kaadu Ashoka and the ant Crematogaster dohrni is markedly different. These ants exploit the plant by nesting in the domatia and feeding on the extrafloral nectar while not really providing protection from herbivory, and go one nefarious step further. To ensure that the plant keeps up its production of nectar and does not divert these resources to fruit production, Crematogaster ants actively castrate the flowers and scare away pollinators.
Not the good, the bad, and the ugly – just rather strange tenants
In addition to ants, the domatia on the kaadu Ashoka also house other invertebrates, including bees, wasps, roaches, beetles, millipedes, spiders, centipedes, and perhaps the most paradoxical of all, an arboreal (tree-living) earthworm! Currently, the two most well-studied non-ant domatia tenants are the Puang reed bee (Braunsapis puangensis) and the arboreal earthworm (Perionyx pullus).
The Puang reed bee is a small stingless bee that nests within domatia, where it raises its brood. This little bee also provides pollination services to the kaadu Ashoka, meaning these organisms have a very unique brood-site pollination mutualism. In typical brood-site pollination mutualisms, the plants usually host their pollinators in flowers; for example, in the fig–fig wasp system, the figs house the developing brood of pollinator fig wasps within highly specialised collections of flowers called syconia. However, like most of its other interactions, this relationship between the kaadu Ashoka and its pollinator partner still seems to be in transition.
The connection between the kaadu Ashoka and its strangest tenant, the arboreal earthworm, is shrouded in mystery. The only thing known is that juvenile worms of this species use domatia during the dry season and vacate them during the monsoons. No one yet knows whether this tree provides food to these worms (or what these worms eat since their anatomy suggests that they cannot process soil like usual earthworms) or whether the plant is necessary for their reproduction. The only thing that is known is that this worm can affect the relationship between the kaadu Ashoka and its ant partners. Most species of ants avoid domatia which contain the worms and areas of the plant that are exposed to the worms’ mucous. It is perhaps no surprise that the northern Western Ghats, where the worms are most abundant in domatia, is also the region where ant occupancy of domatia is the least, and where these plants receive the least protection services from their ant partners.
Hosting with a twist
The range of relationships that the kaadu Ashoka has with its myriad partners is not just restricted to food-and-shelter-for-protection or pollination. In a weirdly circular way, these trees that provide food to a host of invertebrates may also depend on these invertebrates for nourishment.
Research has shown that the inner walls of the kaadu Ashoka domatia have a layer of cells with small grooves, channels, and numerous connections with the inner plant cells. This structure is reminiscent of those found in another myrmecophyte, Myrmecodia, which hosts ants not just for protection but also for a supply of absorbable nitrogen from the ants’ wastes.
Both the kaadu Ashoka and Myrmecodia also use their ant partners as fertiliser production units! Although its production of free housing and food with no mechanisms to attract “right” partners or sanctions to discourage “wrong” partners has led to some exploitation, the kaadu Ashoka is a truly remarkable host. Its unspecialised, free-for-all approach to partnership has allowed it to ally with a broad variety of multi-legged and non-legged partners, which can set an ecologist’s heart racing with excitement.
About the contributor
Anusha Krishnan
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