It was a misty tropical winter morning – the sun still turning sleepily beneath the leaves, and the birds waking up and greeting each other. It was my first day of fieldwork, and I was in a forest in Agumbe, in the Karnataka Western Ghats with my companions, Yathiraj and Srinivasan. We had walked three kilometres from where we had left our field jeep at the head of the trail, me leading the single file, when I froze suddenly. It seemed as if the morning breeze had brought us the sweetest fragrance from a heavenly garden. I took a deep breath and walked in search of the source. Twenty metres ahead, the trail opened onto a beautiful little meadow, surrounded by hundreds of trees of the same kind, all laden with flowers as if in a celebration – thousands and thousands of inflorescences, with florets of white petals and pink sepals, standing upright, filling the air with that heavenly smell. We had reached a patch of Humboldtia brunonis, Haasige Mara in local parlance, our reason for being in Agumbe. It was as if the trees were welcoming us, wearing perfume and their best attire in a seasonal flush of flowers and young leaves.

humboltia_1

Humboldtia brunonis, or Haasige Mara

The young leaves, unlike the erect and proud inflorescences, hang limply as if shy! Each leaf comprises two pairs of leaflets. But this is no ordinary leaf. Each young leaflet bears about five to ten nectaries on its surface, each nectary glistening with a tiny drop of the nectar that I have heard and read so much about. I touched my fingertip to one nectary and tasted it. Sweet!  This was my first meeting was with Humboldtia brunonis, whose mysteries I had come to unravel.

Image 2

Dried up extrafloral nectary on a mature leaf of Humboldtia brunonis

  
Nectaries

Leaflet with active extrafloral nectaries

H. brunonis is an understorey tree, endemic to the low-elevation moist evergreen forests in the Western Ghats. It grows in dense clusters or patches of less than a kilometre on its widest axis, which are scattered across its distribution range – from near the Someshwara Wildlife Sanctuary in the north, down to the Brahmagiri Wildlife Sanctuary in the south, spanning a north-south range more than 300 kilometres. Most of this area lies within south Karnataka, with a small part jutting into northern Kerala. H. brunonis is an ant-plant, or in technical terms a myrmecophyte (myrmeco – ants, phyta – plant) – a plant that has a mutualistic relationship with ants. All individuals of H. brunonis bear extrafloral nectaries (nectaries that are outside the flower, and hence not meant to attract pollinators), on the leaves, and bracts of flowers. The extrafloral nectar is rich in sugars, and hence very attractive to ants. Additionally, some individuals of this species also bear ant shelters called domatia. These are swollen hollow chambers, formed by the modification of branches, and each with a spontaneously formed slit that serves as an entry point for the ants.

Image 1

Self-opening slit on the domatium of H. brunonis. This slit is an opening that is formed spontaneously at the distal end of every domatium as it develops. It serves as a gate and allows ants and other invertebrates gain access to the shelter of the domatium.

Research has shown that in most other myrmecophytic systems, in return for similar rewards, the ants act as security guards, patrolling the plant, and chasing away herbivores such as caterpillars that come to eat the plant’s leaves and flowers. However, in the case of H. brunonis, only one ant species – Technomyrmex albipes – of the 16 that occupy its domatia is known to protect the plant. Besides the ants, numerous other invertebrates also take shelter in the domatia, of which the most peculiar one is an arboreal earthworm (yes, the name is an oxymoron!)

Image 3

Vombisidris humboldticola queen with eggs inside a domatium of H. brunonis. Wing stubs are visible on the thorax. So far, this ant species has been reported only in the domatia of H. brunonis and H. decurrens, both in the Western Ghats.

 
Image 4

Crematogaster sp. near the self-opening slit of its domatium.

Producing extrafloral nectar and domatia is an expensive affair for the plant. Why does it continue to do so when it does not get any return from most of these domatia inhabitants? Does the arboreal earthworm that lives in the branch domatia benefit the plant in any way? Why did the plant develop these mutualistic traits?  Over the course of five years that followed, I spent six months – from November to April – each year visiting many H. brunonis patches in many sites spanning the entire distribution range, looking for the answers; Agumbe and Kudremukh in the north, Uppangala, Sampaje and Solaikolli in the south. At each site, we recorded the proportion of trees that bore domatia and counted the number of nectaries per leaf. We measured the volume of nectar produced per leaf on a day and took back samples to the lab to analyse its chemical composition. We measured herbivory pressure at each site, and also whether the presence of ants reduced the herbivory. We conducted experiments to see whether the organic waste from the domatia inhabitants gets absorbed by the plant through the domatia; then we pollinated flowers to see if plants with domatia had greater fruit sets. Finally, we measured the growth rate of leaves as they grew from buds to see if they grow fast enough to escape herbivory.

We found that young leaves of H. brunonis also grow at a very rapid pace to escape excessive herbivory. When the magnitude of herbivory pressure increases, H. brunonis plants attract the protective ant with costly rewards such as richer extrafloral nectar. We also found that even non-protective ants and the arboreal earthworm in the domatia which were earlier thought to be mere interlopers actually provide the host plant with nutrition through their excreta, suggesting that nutrients alone or along with protection benefits could lead to the evolution of mutualism in ant-plant systems. These findings are now published (References 1, 2 and 3).

Image 7

The arboreal earthworm, Perionyx pullus, a frequent inhabitant of the domatia of H. brunonis. This earthworm species has not been reported in soil!

Being an ecologist has many unmatched perks. And in my case, besides the opportunity to understand nature’s wondrous ways, it has also given me the opportunity to live a life I would never have otherwise – to wake up to the mellifluous notes of the Malabar Whistling Thrush at dawn, and to rest atop a huge rock in the middle of a bubbling stream after a hard day’s work, surrounded by swarms of tiny fish and an occasional lobster. I made friends with a few crabs with whom I often shared wild mangoes, and made daily acquaintance with lazy Malabar Pit Vipers who took weeklong siestas on the plants I was working on! Besides close brushes with nature and wildlife, continuous months of fieldwork in remote places provided lessons in human nature that I would not have learnt otherwise. Far from the comforts and familiarity of urban life, one learns to live with the most meagre of requirements – meals become just a means of survival, and a bucketful of hot bath water becomes a rare luxury. Every day, our work started at dawn and ended with the setting sun; whether weekday or weekend made no difference. The camaraderie and mutual trust that come when three people live together for months, far away in a remote forest guest house with no running water and electricity, is beyond description. The project is over now and my degree awarded. I have now moved on to a new project. But each time when my mind seeks peace and rest, I remember that wonderful slice of my life that began with that winter morning many years ago.

 

Karthikeyan S. would like to thank Mr Prashanth, Mr Ramachandra and Mr Nayak for their help in photographing some of the above images.

References:
1. Chanam, J., Sheshshayee, M. S., Srinivasan, K., Jagdeesh, A., Joshi, K., and Borges, R. M. 2014.  Nutritional benefits from domatia inhabitants in an ant–plant interaction: interlopers do pay the rent. Functional Ecology doi: 10.1111/1365-2435.12251
This article has also been highlighted in the New Scientist magazine in February 2014. Link: http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12251/full
2. Chanam, J., Srinivasan, K., Pramanik, G. K., Jagdeesh, A.,  Joshi, K., and Borges, R. M. 2014. Context dependency of rewards and services in an Indian ant–plant interaction: southern sites favour the mutualism between plants and ants. Journal of Tropical Ecology 30: 219–229.
Link:http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=9242365&fileId=S026646741400011X
3. Chanam, J., Srinivasan, K., Pramanik, G. K., Jagdeesh, A.,  Joshi, K., and Borges, R. M. 2014. Foliar extrafloral nectar of Humboldtia brunonis (Fabaceae), a paleotropical ant-plant, is richer than phloem sap and more attractive than honeydew (accepted in Biotropica)