The Hungry Tide - Amitav Ghosh [58]
Through the early hours of the day the pace of Piya’s work had been dictated by the belief that this was a school of migrating dolphins that might depart at any minute. But now she began to wonder: these animals hadn’t given her the impression of being headed anywhere in particular. On the contrary, she had gotten the feeling that they had gathered here to wait out the ebb tide until the water rose again. But that made no sense either, she told herself; it just didn’t fit with what she knew about these animals.
Orcaella were of two kinds: one tribe liked the salt water of the coast while the other preferred rivers and fresh water. The difference between these communities was not anatomical — it had only to do with their choice of habitat. Of the two populations, the coastal was by far the more numerous. The waters of southern Asia and northern Australia were reliably believed to contain several thousand of them. Fresh-water Orcaella, on the other hand, were a rare and dwindling breed. Only a few hundred now remained in Asia’s rivers. Coastal Orcaella were not known to linger for hours in one place and were more likely to range freely along the shore. Their fresh-water cousins were more territorial and not nearly so gregarious. In times of heavy rainfall, when the rivers rose, they would roam far afield, chasing their prey into minor tributaries and even into flooded rice fields. But in dry periods, when the rivers began to drop, they would make their way back to certain spots. These were usually deep-water pools, created by quirks of geology in the riverbed or by the water’s patterns of flow. In Cambodia Piya had tracked populations of Orcaella in several pools along the Mekong, from Phnom Penh to the Laos border. She had found the same individuals returning to the same pools year after year. But when the seasons changed these dolphins traveled hundreds of miles downriver; in one unfortunate instance an animal had swum all the way down from the Laos border only to drown in a gill net near Phnom Penh.
Piya had come to the Sundarbans believing that any Orcaella she found there would be of the coastal variety: this seemed only logical, considering how salty the waters were in this region. But what she had seen today made her wonder if she hadn’t made a mistake. If these were coastal Orcaella what were they doing congregating in a pool? That was out of character for them — only their river-dwelling kin did that. But these could not be river dolphins either. The water was too salty. And anyway, riverine Orcaella didn’t leave their pools in the middle of the day; they spent a whole season in them. So what kind of animal was this and what did this odd behavior mean?
As she mulled over these questions a thought came into Piya’s mind. Was it possible that these Sundarbans Orcaella did twice each day what their Mekong cousins did once every year? Had they found a novel way of adapting their behavior to this tidal ecology? Could it be that they had compressed the annual seasonal rhythms of their Mekong relatives so as to fit them into the daily cycle of tides?
Piya knew that if she could establish any of this she would have a hypothesis of stunning elegance and economy — a thing of beauty rarely found in the messy domain of mammalian behavior. What was more, the idea might well have profound implications for the conservation of this endangered species: protective measures would be much more effective if they could be focused on particular pools and specific movement corridors. But the hypothesis begged as many questions as it answered. What, for instance, were the physiological mechanisms that attuned the animals to the flow of the tides? Obviously, it could not be their circadian rhythms since the timing of the tides changed from day to day. What happened in the monsoon, when the flow of fresh water increased and the balance of salinity changed? Was the daily cycle of migration inscribed on the palimpsest of a longer seasonal rhythm?
Piya remembered a study that had shown there were more species