After almost 5 months in the lockdown, the operations at the Centre for Urban Ecology and Sustainability (CUES) resumed, albeit slowly. Recently, while working at Dheerpur Wetland Project Site (DWPS), two of my colleagues found out that the flowers of Oroxylum indicum tree had started blooming. The news got us pretty excited because the species Oroxylum indicum is specifically pollinated by bats during night. To observe and record this fascinating (plant-pollinator) interaction, a team was devised. The idea was to install camera traps around the tree to record the night activity and to do some still photography/videography at certain intervals during the night.
The operation was supposed to be carried out in two days. On 3rd September 2020, post noon, I issued the camera traps, tripod stand and headlamps from CUES and headed towards Dheerpur Wetland Project Site (DWPS). My teammates and fellow researchers, Fizala Tayebulla and Shiwani joined me at DWPS by evening. After locating the Oroxylum tree, we quickly identified the sites where camera traps could be installed. Thanks to my teammate Ajay Immanuel Gonji, the camera traps were already configured and ready to use. With the help of staff (Vijay and Baba) working at DWPS, we secured two camera traps on separate poles. The poles were then affixed to the ground at an angle of around 45 degrees each and a distance of 4-5 m from the subject tree.
A medium-sized semi-deciduous tree with a campanulate (bell-shaped) and bisexual flower [having both male (androecium) and female (gynoecium) reproductive structures], Oroxylum can grow 12-20 m tall. It is native to Indo-Malayan region and occurs in many parts of India, mainly as an ornamental species. The tree has many vernacular names, the most common being Indian Trumpet Flower, Broken Bones plant and Bhoot-vriksha.
In India, it has been observed that the flowering of Oroxylum happens during summer up until the rainy season (May-August) and the fruit appears by November. The flowers have reddish-purple petal lobes and pale, cream coloured corolla tubes. There are 56-66 flower buds that are arranged axially on a 1-1.5m flagelliflorous infloroscence (Sharma and Jain, 2016). One to four flowers bloom each night and emit a stinky odour which attracts bats (Gould, 1978). The anthesis (flowering period of a plant) occurs at night and lasts for a few hours only. The nocturnal blooming flowers open between 8-9 p.m. which also marks the onset of stigma receptivity (between 9:30-10:30 p.m.) and dehiscence of anthers. (Vikas et al., 2009). The flowers drop before dawn.
Being self-incompatible, Oroxylum largely depends on animals for pollination. In his pioneering study, Gould (1978) demonstrated that the flowers of Oroxylum in Malaysia were only visited by the bat species E. spelaea. On the basis of morphological fit between the flower and pollinator, Gould (1978) suggested that only E. spelaea were heavy enough to bend corolla tubes to an appropriate angle in order to obtain nectar. This hypothesis was confirmed empirically by Srithongchuay et al. (2008). The unique 1-1.5 m long iridescence on top of the Oroxylum means that the tree can present its flowers to the bats who would have a clear flight path to access flowers.
In ecosystems consisting of various pollinators and where numerous animal-pollinated plants flower simultaneously, the pollination of a given plant by specific animals is likely to be beneficial (Richards in Srithongchuay et al., 2008). In other words, specialized plant-pollinator relationships should ideally enhance the efficiency of pollination. In the case of India, Gould predicted the bat species Cynopterus sphinx to be the likely pollinator of Oroxylum (based on morphological fit model). The study conducted by Vikas et al. (2009) confirmed that it was indeed C. sphinx which was solely responsible for the pollination of Oroxylum.
After securing the camera traps, we took a little break. We visited the site again at around 9 p.m. with our cameras. Initially, we didn’t observe much apart from the large fruit bats in the sky and slugs on the ground. The chirping sound of crickets filled the surrounding as we waited patiently for the arrival of bats. And then a bat passed within a metre of the flower. Taking a curved flight, it came back, only to hover before the flower. Its departure was followed by a gymnastic flight pattern, looping the canopy of the tree. More bats arrived but none landed on the flower. Every time they manoeuvred the same flight pattern. Passing the flower and then coming back to hover around it. Interestingly, these flight manoeuvres are also observed by Gould (1978). We visited the location thrice after small intervals (till 12:30 a.m.) but didn’t observe the bat landing on the flower.
The next morning, we checked the footage of the camera traps. To our delight, we had a number of videos which documented the landing of bats on the flowers. On average, a bat spent no more than 1.5 seconds on the flower. While foraging, the bat would perch on the flower by clutching on to it by using its claws. The weight of the bat would bend the corolla tube and then the bat would completely insert its snout into the corolla tube to draw nectar.
After an initial analysis of the videos, what I found the most interesting was an emergent temporal pattern of bat visitation. For instance, between the time interval of 11:43 and 11:48 p.m., the camera trap triggered 5 times and 4 cases were recorded where bats hanged on to the flower to forage nectar. Similarly, between 1:39 and 1:58 a.m., camera trap triggered 7 times and a total of 10 cases of bats foraging the nectar were recorded. Between this two hour window, the camera trap triggered only four times and bats foraged only twice. The two peaks in foraging activity are also observed by Gould (1978) in case of Eonycteris spelaea in Malaysia.
Our initial observation is different from Gould (1978) and Vikas et al. (2008) in a few ways. The two peaks observed by Gould also have an interval of 2 hours but they occur at 9:15 and 11:15 p.m. Vikas et.al. (2008) found a single peak (between 10-11 p.m.) in case of Cynopterus sphinx in Rishikesh, India. Both Vikas et al. (2008) and Gould (1978) have demonstrated the correlation between bat visitations and nectar volume. At the peak(s), the nectar volume is highest, after which a constant decline in observed in both nectar volume and bat visitation. For both, the bat visitations taper off close to zero post 1 a.m. Interestingly, we observed that the second peak (10 foraging incidents between 1:39-1:58 a.m.) was higher than the first (4 foraging incidents between 11:43-11:48 p.m.). Although we haven’t studied the temporal change in nectar volume, it seems that the nectar volume must have been the highest between 1:30-2:00 a.m. when most foraging activity occurred. Interestingly, we couldn’t document any activity of bats via camera trapping on the second night of field work. However, we were lucky to observe a flurry of bat foraging incidents at around 9 in the night and managed to get some photos as well.
What could be the reasons for such differences? Bat visitations can be influenced by co-variates such as temperature, humidity, light (artificial/moon phase), human-presence/activity, and so on. A long-term study needs to be carried out in order to say something conclusive or to draw comparisons. Our initial observations, however, have gotten us excited to take this further. In the coming days, we plan to carry out more observations and cover more Oroxylum trees in Dheerpur Wetland Project Site.
We’ll keep updating this blog every time we come up with more data.
- Gould, E. (1978). Foraging Behavior of Malaysian Nectar-Feeding Bats. Biotropica, 10(3), 184-193. doi:10.2307/2387904
- Sritongchuay, T., Bumrungsri, S., & Sripao-raya, E. (2008). The pollination ecology of the late-successional tree, Oroxylum indicum (Bignoniaceae) in Thailand. Journal of Tropical Ecology. 24. 477 – 484. 10.1017/S026646740800521X.
- Vikas, Gautam, M., Tandon, R., & Ram, H. (2009). Pollination Ecology and Breeding System of Oroxylum indicum (Bignoniaceae) in the Foothills of the Western Himalaya. Journal of Tropical Ecology, 25(1), 93-96. Retrieved September 9, 2020, from http://www.jstor.org/stable/25562591