Happy #arachtober everyone! A whole month to celebrate the wonderful world of spiders and other arachnids. Arachnid lovers on Twitter are busy posting photos of their favourite beasties, spreading their passion for these often-feared animals.
Given this, it seemed auspicious timing for the publication of our natural history note on the bizarre feeding behaviour by the jumping spider, Orsima ichnueumon.
My favourite part of being a biologist are the exploratory trips to new places to find creatures you’ve never worked on before. There’s always an element of self-doubt that you’ll not find what you’re looking for, but the effort can be rewarded with a rush of euphoria when you first spot your animal. During these first observations in the wild you start to build a mental picture of the animal’s subtle habitat requirements and the way they behave. Often it’s during these first observations that you realise several of the ideas that you came up with while sitting at your desk are total rubbish, but it’s an exciting time to start formulating new questions based on what you see happening in front of you.
The story we just published in Frontiers in Ecology & Enviroment is based on one of these exciting first observations of a species doing something unexpected.
I became fascinated by O. ichnuemon during my postdoc in Singapore, where I was working on the evolution of colour in jumping spiders. This mesmerizing little species is closely related to several South East Asian jumping spiders that have been the focus of Daiqin Li’s behavioural ecology group. Much of the work has been done on Cosmophasis umbratica, the males of which use elaborate dances to display to potential mates and foes, flashing their iridescent and ultra-violet colours.
Unlike their sister species C. umbratica, O. ichnuemon is rarely observed and almost nothing is known about its natural history. Not only does O. ichnuemon display an impressive array of colours, but they also look rather ant-like in their movement and body shape. Elongated spinnerets (silk-laying structures) extending from their abdomen look like antennae and mouthparts, and the deep constriction in the male’s abdomen gives the appearance of an extra body part to look like an ant with head, thorax and abdomen. These adaptations, in combination with the way they move their body while walking, fool the observer (of the human-kind at least) into thinking they are watching an ant move backwards.
While many invertebrates make easy pickings for predators, ants are often avoided because of their nasty tempers and ability to inflict damage when under attack. For this reason hundreds of insects and spiders mimic ants using an impressive spectrum of ant-like body shapes, structures and behaviour. However, unlike O. ichnuemon, most ant mimics are typically dark brown, red or black, matching the colour of most ants (although there are colourful ants!).
So began my obsession with this little spider!
I began plotting experiments and got excited about working on a relatively unknown species. I was interested in trying to figure out whether Orsima are really fooling potential predators into perceiving them to be dangerous ant prey and what elements of their mimicry were helping them avoid being eaten. At the same time, I also wanted to figure out the role of the complex colours displayed by Orsima, and whether the colours help them to attract mates or repel male competitors, ward off predators as warning colours, or perhaps some combination of both.
Of course, the first challenge to kicking off a project like this is to find your species in the wild. Fortunately a student in the Li lab had spotted them before in the hills above Kuala Lumpur in Malaysia, and so I was able to use his locations to begin my search.
With luck, on the first trip to the University of Malaya Gombak Field Station we were able to find a surprising abundance of Orsima wandering around on forest-edge shrubs. From this my very productive Honour’s student, Renee, was able to do a series of experiments and observations to start teasing apart the possible reasons why Orsima are so colourful.
In the lab Renee staged interactions between either two males or a male and a female to observe the types of behaviours they perform. Jumping spiders have incredible visual acuity, so they tend to be highly responsive to the presence of another spider.
Renee was able to record 28 major elements of behaviour that Orsima perform during courtship or male-male fighting interactions. She found that males had a more complex repertoire of dance and fight moves than females, and that during these displays males would show off their colourful patches by waving their abdomen, palps and legs. Her work has just been published in a really nice paper that painstakingly describes each behavioural element, coupled with lovely photos. Renee went on to do some neat experiments manipulating the colour of the male’s abdomen to test if this changed the response of female spiders and predators. More on that in coming months!
While Renee was busy in the lab, Caleb and I headed off on a second trip to the Gombak Field Station to find more Orsima, with plans for further observations and experiments. We picked up our hire car from KL airport and attempted to navigate our way up the winding roads leading up to the field station on the route to the Genting Highlands.
Unfortunately we managed to time our visit during a very rainy November, and on the night before our arrival several landslips had blocked off the road into the field station. We scratched our heads at the roadblock above the field station trying to decide what to do. As any field biologist knows, a lot of money and time is spent planning these trips, and we didn’t want to have to go back to Singapore without seeing our spiders.
Luckily we had found other locations nearby with very similar vegetation in our previous trip, so we headed over in hope of finding Orsima there. In the tropics, ants are the most dominant insect seen foraging on trees, so it is quite tricky to spot Orsima spiders among them, despite their bright colours. It wasn’t long, however, before we found several Orsima scurrying around on the leaves of a common roadside shrub.
It was here that we spotted the behaviour that we wrote about in our short note for Frontiers in Ecology & Environment. We watched a female Orsima patrolling around on a single tree leaf, where she appeared to be walking between a bunch of little raised discs scattered over the leaf surface, and dipping her head down to feed. At the time, I didn’t know that we were looking at extra-floral nectaries (EFN’s), tiny nectar-filled structures produced by the plant to attract ants. EFN’s are considered a mutualism between the plant and ants, because the plant receives the benefit of reduced herbivory by insect pests (like munching grasshoppers), while the ants receive a reliable source of food.
Not only did we notice that Orsima were feeding on these nectaries, we also watched as they performed a little ceremonial dance after each feeding bout. The spider would walk over the depleted nectary and then waggle it’s abdomen from side to side, leaving behind a patch of silk next to the nectary.
Over the next few hours, we watched several Orsima doing the same behaviour. The spider would remain on the same leaf surface, appearing to be guarding that particular patch of nectaries. The many ants also found on the same tree were also feeding on the nectaries, but they wandered from leaf to leaf, sampling from each nectary on their path. We noticed that the Orsima would sometimes chase away an ant competitor that arrived onto their leaf, and once we even spotted an ant trip over in the little silk patch that the spider had laid around the nectary. We started to wonder if these silk fences were acting as mini barricades to repel ants from foraging on the nectaries.
We thought we’d witnessed something unusual, but it wasn’t until we were back in the lab chatting to our colleagues that we started to figure out how unique the guarding behaviour was. We don’t know of any other spiders that do this strange silk-laying dance after foraging on a nectary.
It’s special because spiders are almost always considered to be carnivores that may occasionally supplement their diet with plant nectar. We’ve seen Orsima hunting small insects in the wild so we know they aren’t strict vegetarians, but it was still hard for us to explain why they would spend time and energy guarding the nectaries and using up silk if this was a small part of their diet.
To be successful mimics Orsima may need to hang out in close vicinity to the ants they resemble so that they can fool potential predators and go under the radar. However, a large number of ants foraging on these trees are also likely to reduce the numbers of potential insect prey for Orsima, meaning less food availability. Although speculation at this stage, we propose that Orsima may offset the costs of fewer prey by guarding the energy-rich nectaries from ant competitors.
We’re looking forward to working further on Orsima to keep teasing apart their fascinating natural history.
To get the full run down of our work on Orsima so far, check out these free to access publications:
Painting, C. J., C. C. Nicholson, M. W. Bulbert, Y. Norma-Rashid, D. Li (2017) Nectary feeding and guarding behaviour by a tropical jumping spider. Frontiers in Ecology & Environment 15: 469-470. Link
Huixin, R. W.*, Y. Norma-Rashid, D. Li, C. J. Painting (2017) Courtship and male-male interaction behaviour of Orsima ichneumon, an ant-mimicking jumper spider (Arachnida: Salticidae). Raffles Bulletin of Zoology 65: 426-439. Link