You might not be that impressed to receive a clump of grass or branches on a first date, but a boto dolphin might think differently. A new study suggests that these Amazonian dolphins wave bits of flotsam to attract mates.
The boto is a freshwater river dolphin that swims through the currents of the Amazon and the Orinoco. They are elusive creatures that are difficult to study, so very little is known about their social lives.
Tony Martin from the University of St Andrews spent three years in the Amazonian Mamiraua reserve studying the behaviour of botos. During this time, he spotted over 200 groups of dolphins playing with objects, a behaviour that other scientists have noted throughout the animals' range and was recently filmed in the "Fresh Water" episode of Planet Earth.
The botos pick up branches, sticks, vegetations and lumps of clay in their mouths, often thrashing them against the water surface or throwing them with jerks of the head. None of the objects are edible and the carriers often swim in a ritualised way, spinning slowly with their head above the water.
Play or courtship
Dolphins are naturally playful creatures and marine species are known to play games such as Pass the Seaweed. It seems natural to guess that the botos' behaviour might have similar intentions but Martin first started to question that when he spotted three botos carrying objects in a single day.
Many of the local individuals have been marked on their dorsal fins using a cold metal block that leaves a light patch of skin behind. Based on this 'freeze-branding' Martin noted that the three carriers he saw were all adult males, and began to wonder if the behaviour was a form of courtship. He spent the next three years recording data on object-carrying to test his idea.
He recorded the behaviour in about 4% of the groups he observed and found that it is usually the province of adult males, and performed in large groups in the presence of adult females. All the dolphins who were seen carrying objects repeatedly were adult males, as were the vast majority of marked and unmarked individuals seen performing the behaviour at least once. Juvenile botos only rarely carried objects, which seems unusual if the behaviour was really about playing.
Martin found that groups which practiced object-carrying were on average about three times as large as those where the behaviour wasn't seen and noted that the practice became more common as the number of adult females increased. He also detected 40 times more instances of aggressive behaviour in the object-carrying groups, even after accounting for the different number of males present. To him, these figures suggest that females are the target audience of the strange displays, which trigger aggressive responses from other males.
The displays were most common in March, and between June and August, even though the objects used are available throughout the year. Martin notes that pregnancy for botos lasts about 11-15 months and they tend to give birth most frequently in September. That fits with the idea that most conceptions happen in the summer although it isn't clear why high levels of object-carrying were also seen in March.
Martin is aware of the limitations of his data. The turbid waters of the dolphin's waterways prevented Martin's team from seeing anything that happened under the surface and even after three years and 13,000 hours of dolphin-watching, he has never managed to see botos actually mating. As he says:
"Observing river dolphins is analogous to peeping through an aquatic keyhole. Our inability to see below the water surface masks most of their behaviour from view."
Only further research will confirm Martin's theory, and conservation efforts are needed to ensure that these elusive creatures will still be around for it. The boto is doing well by river dolphin standards and is certainly faring better than the Chinese baiji, now believed to be functionally extinct, or the Indian or Ganges river dolphins, which are down to a few thousand survivors. Even so, the IUCN lists the species as 'vulnerable' and they are threatened by entangling fishing nets, the prospect of damming projects that will fragment their habitats, and pollution from mercury and oil leaks.
More distressingly, Martin has uncovered evidence that the dolphins are being increasingly killed by fishermen to be used as bait for a profitable scavenging catfish, the piracatinga. He has found calves that were harpooned, roped and mutilated and eyewitness accounts of local killings. Thankfully, the hunts appear to be the work of a minority of local people, and most are strongly opposed to it. Martin remains optimistic that the hunters can be caught and the dolphins protected.
Reference: Martin, A., da Silva, V., Rothery, P. (2008). Object carrying as socio-sexual display in an aquatic mammal. Biology Letters, -1(-1), -1--1. DOI: 10.1098/rsbl.2008.0067
Images by Tony Martin and Dennis Otten