The wet lands stretched out before us, flat and still. Squadrons of wild fowl, Jabiru storks, and scarlet ibis flew across the sky, in search of a safe retreat for the night. We pulled over as a family of capybara (giant marsh dwelling rodents) basked in the last rays of sun. For a moment, the land, stretching to the horizon, glowed a heavy, deep red. Then the sun was gone, the marshland cool and grey.

We had now been travelling for eight hours, driving across the Venezuelan Llanos, through the lattice of road bridges dividing these vast marsh plains. A strong wind blew steadily from the east, forming dust devils that covered our equipment in a film of fine sand. It had been a long day, with no evidence of the green anaconda, the snake we had come so far to see.

Snake experts Dr Jesus Rivas and Dr Ramon Gill had told us that it would not be until the height of the dry season in April that anacondas would be visible from land. Even though a mature female anaconda grows more than 7.5 metres in length and can weigh over 90 kilos, the marshes were waterlogged, restricting visibility. We would need to get much closer if we were to find them, venturing on foot, deep into the marshes.

That night we talked with our hosts at the biological research station of Hato El Frio. Mauricio and Ramon, the resident zoologists, agreed that the best option would be to search the marshes closely with their snake tracker, Jesus. We had some surprises in store. First, we discovered that anacondas shared their muddy waters with red piranha, electric rays, caiman alligators and giant Orinoko crocodiles. Our second alarming lesson was in the best way of locating anacondas; by standing on them in our bare feet. We would be leaving this practice to Jesus. We talked of our numerous experiences with wild animals, but in truth we were all a little daunted by the prospect of wading barefoot into the unknown. It was almost dawn when we turned in, and despite our tiredness we slept lightly, anacondas and large biting fish in our dreams.

'Helpless' and 'out of our depth' best describe the aquatic experience of a human-predator encounter. It is undeniably exciting tracking predators in their natural habitat: finding spoor (evidence of animals passing) gives you both an amazing sense of 'oneness' with the environment, and a heady shot of adrenaline, keeping you alert to the possibility that there is a wild animal close by. On land, this feeling is tempered with a tangible understanding of solid ground, a knowledge of how to react with respectful, non-threatening postures and movements, and, of course an ability to run.

In an aquatic environment, the rules are different. Large or small marine mammals, predatory fish, reptiles and snakes, appear unannounced at extremely close quarters, the only indication of their imminent arrival a subtle ripple or fleeting glance of a fast surfacing fin, snout, tail or claw. The arrival of a waterborne predator is sometimes indicated by a sudden splash, but more regularly by a slight muddying of water and an occasional bubble, which are often spied too late for the unsuspecting visitor to take adequate evasive action. So it was with trepidation that we waded into the cool brown water.

Jesus took the lead, probing the mud slowly and methodically as we followed in single file, each clutching a long stick. Water lilies, marsh wort, tangled roots and vines form the spongy, chaotic infrastructure of the flats, giving food and protection to a large array of life. Over 450 species of birds survive in the Llanos, with the sustainable food base provided by the diverse wildlife living in the mineral-rich waterways. They have a symbiotic relationship with the fish, mites, water nymphs, mosquitoes, nesting flies, frogs and toads. Crested cara-cara, fish eagles and Llanos vultures watch from low branches for signs of life and death. Ever vigilant 'bubba' caiman and Orinoko crocodile wallow silently by day. Ocelot, puma and the elusive jaguar stalk the riverbanks and estuaries by night. We had been prodding and wading for less than an hour when we were startled by a loud splash. Greg had disturbed a large crocodile that had been lurking in a clump of water lilies. As it crashed excitedly into the deeper water, Greg froze, terrified by the size of the creature that had been lying so close to him only seconds ago. This had been a sharp reminder of our untrained eyes and of our vulnerability in this unforgiving alien environment. There was laughter at Greg's shock, but it was uncomfortable laughter, as we each reconsidered what we were letting ourselves in for.

We walked on, slower now and rather tentatively. After no more than twenty yards, Jesus stopped dead in his tracks. We cautiously approached the mud bank he was pushing. Each time Jesus probed the bank, the mud oscillated gently in response. He turned to us and handed us his stick. Surely he was not expecting us to take control of whatever he had found? We took the stick, as Jesus ran to a tree where he cut a v-shaped branch with his panga. With a deft, well-practised movement he thrust the v-shaped stick into the mud, attempting to pin down the snake. He missed. The anaconda retaliated with a fast strike from the mud, then recoiled beneath the cloudy surface. Jesus tried again and this time the snake was held fast.

He indicated for us both to hold the stick as he plunged his hand deep into the mud and took a firm grip of the anaconda's fist-sized head. As the snake lashed left and right, we struggled to keep control of her writhing body. Greg took shots of the capture. We quickly walked to land and gently lowered the snake into the hessian sack that Jesus had brought.

For the scientists this was all in a day's work, as their research includes the marking and tagging of anacondas in the Hato el Frio sanctuary. The snakes are released once they have been tagged and sampled for genetic analysis. Valuable range and breeding information can be gained if the researchers are lucky enough to capture individual snakes more than once.

For the past six years, our art-based fieldwork has been made possible by kind hosting and collaboration with research teams around the world. Once we have located an animal, scientists conduct their tests and studies and we are permitted to paint and take prints from the animals. What most interests us is this relationship between humans and animals, between the civilized world and the wilderness, between art and science.

We carried the snake back to the ranch and immediately prepared our natural, non-toxic paints, brushes and paper. We filled a large drum with cool water for the anaconda to lie in, and we would use this later to wash the paint from her body. We mixed light green paint and carefully covered the underside of the snake. As with previous work with salt-water crocodiles, we were keen first to take relief prints of the anaconda and then to leave her to move freely across the paper. The serendipity of the patterns that the anaconda would make would direct the artwork that would result. This is the essence of our collaborative art; some structure and some chance, 'clarity and ambiguity in the same painting', as Mark Rothko neatly expresses.

The anaconda slithered across two pieces of Japanese paper leaving beautiful and subtle marks, a trace of her scaly skin. After each painting, we washed the snake in the water. She was now calm, seemingly a willing partner in this collaboration. We outlined her coils and delineated her silhouette. After no more than an hour our work with the snake was complete. In his book The Postmodern Animal Steve Baker suggests that this mode of working is characterised by 'intensity, immediacy, intimacy and urgency', which is a crucial part of our work, freeing us from the rigidity of preconceptions.

The New York abstract expressionist painter Willem De Kooning said that painting should 'take you on a journey', and this concept is integral to our own painting process. We start with an idea and then travel with the work, unable to control the animal's path. The result can be a happy accident, but not all of our paintings are equal in beauty, or successful as works of art. Some artists choose to work and labour over a single painting for days, weeks, or even months or years, honing and polishing the finished product. We work differently. We paint and draw quickly, producing many pictures that we leave exactly as they were finished in the field. We then edit, on our return, with the benefit of a more objective eye. This time we were lucky. The prints, patterns and shapes made by the writhing snake were the most beautiful we had gathered. The anaconda had made this a great day.

The next day all the snake tests had been completed and necessary data had been gathered. Again, we walked out across the swamp with one of our paintings and the anaconda. At the spot were we had first encountered the snake, we laid out our work. Greg prepared the cameras for our final interaction: the anaconda's release.

For the past five years Greg has accompanied us into the field, to record events and set our work in context. His photographs tell the story of our site-specific collaborations and he creates images that hang next to the paintings. His photographs of the anaconda passing over the artwork and into the Llanos act as evidence of the snake's presence.

Images such as 'Shark Bite' and 'Stingrays' show this beautiful, raw and unique relationship with the wild. In some photographs there is a deep level of interaction, whilst others serve more as a record. Steve Baker alludes to the fact that although photographic representation of our art-making is documentary in its approach, some images are more involved with the creation of art. These images seem to add something to the slithering, bitten or trodden marks found on the works themselves, not just as a detached witness, but as active participants in the 'signature and passing' of an animal that is here now but may not be for much longer.

The anaconda waited, forked tongue flicking in and out of her mouth, tasting and recognising the familiar surroundings of the marsh. She slid gracefully towards the painting at the water's edge and in a single movement passed over the work, slipping into the shallow water. Mud traces on the paper verified her recent presence. The water became still and the anaconda was gone.

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