By Natasha Gilbert
Crocodiles are bad long-distance swimmers. Instead, their talents lie in surfing, according to a study published June 7 in the Journal of Animal Ecology.
Estuarine crocodiles (Crocodylus porosus) have the largest geographical range of any crocodile species, spanning more than 10,000 square kilometers of the southeast Pacific Ocean.
That wide distribution suggests that they can cross the ocean to reach distant locales, but until now only three estuarine crocodiles had been tracked on transoceanic voyages. Zoologists didn't know how the reptiles traveled such long distances given the sustained level of swimming required.
The answer is that the crocodiles ride surface currents, says a group led by Craig Franklin, a zoologist at the University of Queensland in Australia. The reptiles only travel when the current flows in the direction of their desired journey, the researchers report. When the tide turns, the crocodiles either climb onto the riverbank or dive to the bottom of the river to wait for the current to reverse.
"Crocodiles ride the currents to cut the energy costs of traveling. They get a free ride," says Franklin.
The group, which included the late Steve Irwin, better known as "The Crocodile Hunter," spent a year studying 20 adult crocodiles in the Kennedy River in North Queensland, Australia. Implanted with acoustic devices that emit pulses through the water, the reptiles' movements were tracked by 20 receivers placed along a 63-kilometre stretch of the tidal river. The signals allowed the team to identify the crocodile, and determine its body temperature.
The researchers compared their data with estimates of surface water currents from the Commonwealth Scientific and Industrial Research Organisation, Australia's national science agency. They found that eight crocodiles undertook a total of 42 long-distance journeys of more than 10 kilometers per day. In 96 percent of these trips, the reptiles traveled with the current flow. In contrast, the crocodiles were equally likely to travel with and against the current flow when making short journeys.
When the tide was against the crocodiles' direction of travel, their recorded body temperatures rose to around 32 degrees Celsius, suggesting they were basking in the sun on the riverbank. When the tide turned in their favor, their body temperatures dropped to 25 degrees C, indicating that they were back in the water.
"They know when the current is flowing in the direction they want to travel," says Franklin. "It is like they are purposeful. They seem to be making a decision prior to the journey that they will travel with the current."
It is not clear whether this behavior is learned or inherited, says Franklin. He says that correlations can be drawn between the migratory behavior and cognitive abilities of crocodiles and birds, because the former are more closely related to the latter than to other reptiles. Previous studies have shown that both animals use magnetic cues to navigate.
The latest study indicates that surfing the ocean currents is an effective migration method for estuarine crocodiles. Surfing also provides a way for individuals from distant populations to cross ocean barriers and breed, helping to explain why estuarine crocodiles have not diversified into different species.
But James Perran Ross, a wildlife ecologist at the University of Florida in Gainesville, is not convinced that the crocodiles' ocean travels are intentional, but instead says that they are more likely to be "occasional mishaps." "Just heading off blindly downstream isn't much of a strategy," he says.
"That would be quite some mishap," counters Franklin. "And if it is a mishap, why have other crocodile species not also made the same mistake?" His team plans to track the crocodiles over the next 10 years or so, to shed light on why the reptiles travel long distances and how this behavior arose.