[naturenews] from [nature.com]
[nature > Nature News]
Published online 7 June 2010 | Nature | doi:10.1038/news.2010.282
News
Crocodiles go with the flow
｛クロコダイル（ワニの総称）、流れに乗る｝
Surfing currents allows crocodiles to travel long distances.
By Natasha Gilbert
｛クロコダイル、流れに乗って長距離の移動ができる｝
{Crocodiles may surf ocean currents to reach distant shores.}
｛クロコダイル、潮流に乗って遠く離れた沿岸まで到着する｝
Crocodiles are bad long-distance swimmers. Instead, their talents lie in surfing, according to a study published today in the Journal of Animal Ecology１.
｛今日発刊のＪＯＡＥ（動物生態学ジャーナル）によると、クロコダイルは長い距離を泳ぐのは苦手である。その代わりに、その能力は波乗りの技術にある。｝
Estuarine crocodiles (Crocodylus porosus) have the largest geographical range of any crocodile species, spanning more than 10,000 square kilometres 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 travelled 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 travelling. They get a free ride," says Franklin.
Surf's up
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.
{{Transmitters attached to the crocodiles recorded their location and temperature.}
Australian Zoo}
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 kilometres per day. In 96% of these trips, the reptiles travelled 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 ºC, suggesting they were basking in the sun on the riverbank. When the tide turned in their favour, their body temperatures dropped to 25 ºC, 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."
Magnetic attraction
It is not clear whether this behaviour is learned or inherited, says Franklin. He says that correlations can be drawn between the migratory behaviour 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 ten years or so, to shed light on why the reptiles travel long distances and how this behaviour arose.
References
1. Campbell, H. A. et al. Journal of Animal Ecology advance online publication doi:10.1111/j.1365-2656.2010.01709.x (2010).
[One-Minute World News] from [BBC NEWS]
[BBC > Science & Environment]
Page last updated at 22:10 GMT, Monday, 7 June 2010 23:10 UK
By Katia Moskvitch
Science reporter, BBC News
Crocodiles 'surf' long distance on ocean currents
{Estuarine crocodiles are poor swimmers}
Saltwater crocodiles enjoy catching a wave and can travel hundreds of kilometres by "surfing" on ocean currents, a study suggests.
Australian researchers used sonar sensors and satellite transmitters to monitor 20 reptiles' movements.
They found the crocodiles undertook numerous trips of over 10km (6.2 miles), but only when a current flowed in their direction of travel.
The results of the research appear in the Journal of Animal Zoology.
The TV personality Steve Irwin, who was nicknamed "The Crocodile Hunter", but died in 2006, took part in the study.
Estuarine or saltwater crocodiles are the world's largest reptiles and can grow up to five-and-a-half metres in length.
They are poor swimmers and mainly live in salt water - but their "home" spans over 10,000sq km of the South-East Pacific, from Sri Lanka to Fiji and from Thailand to northern Australia.
Researchers have long been puzzled by how crocodiles managed to spread themselves so widely.
"Of all the amazing things animals can do, the ability of certain species to migrate significant distances across formidable geographical barriers is one of the most remarkable," write the authors of the recent study.
Although the crocodiles spend most of their life in salt water, they are not considered marine animals as they rely on land for food and water.
The open sea
During the research, a team led by Dr Hamish Campbell, from the University of Queensland, captured 20 crocodiles living in the North Kennedy tidal river in Queensland, northern Australia, and tagged them with satellite transmitters.
They found that during the period of study, eight of them ventured out into the open ocean. One travelled from the river mouth all the way to the west coast of the Cape York Peninsula, in Queensland's far north. That amounts to a total of 590km covered over 25 days.
To do that, the ocean-trotter hitched a ride on a current within the Gulf of Carpentaria (that separates Cape York from Arnhem Land, to the west). This current occurs seasonally, during the summer monsoon.
{20 crocodiles were tagged with satellite transmitters}
"[These crocodiles] can survive for long periods in saltwater without eating or drinking, so by only travelling when surface currents are favourable, they would be able to move long distances by sea," commented Dr Campbell.
It took another adventurer - a 4.84m-long male - just 20 days to go more than 411km from from the east coast of Australia's Cape York Peninsula through the Torres Strait (which divides Australia from New Guinea) to the Wenlock River on the west coast of Cape York.
When the crocodile arrived in the Torres Strait, strong currents were flowing in the opposite direction to where it was headed.
So the animal waited in a sheltered bay for four days and continued its trip when the currents changed direction.
Important clues
The scientists also tagged 27 crocodiles with sonar transmitters and spent a year tracking their every move inside the North Kennedy River with underwater receivers.
They found that both male and female crocodiles regularly travelled more than 50km from home, swimming to the river mouth and back.
But the team discovered that crocodiles would only set out on a long journey within an hour of the tide changing. This allowed them to "catch a wave".
They put their trips on hold when the tides reversed, moving out of the river and on to the banks.
Dr Campbell said that the results of the study gave important clues to understanding the evolution of the world's largest reptiles.
"This not only helps to explains how estuarine crocodiles move between oceanic islands, but also contributes to the theory that crocodilians have crossed major marine barriers during their evolutionary past," he said.
[nature > Nature News]
Published online 7 June 2010 | Nature | doi:10.1038/news.2010.282
News
Crocodiles go with the flow
｛クロコダイル（ワニの総称）、流れに乗る｝
Surfing currents allows crocodiles to travel long distances.
By Natasha Gilbert
｛クロコダイル、流れに乗って長距離の移動ができる｝
{Crocodiles may surf ocean currents to reach distant shores.}
｛クロコダイル、潮流に乗って遠く離れた沿岸まで到着する｝
Crocodiles are bad long-distance swimmers. Instead, their talents lie in surfing, according to a study published today in the Journal of Animal Ecology１.
｛今日発刊のＪＯＡＥ（動物生態学ジャーナル）によると、クロコダイルは長い距離を泳ぐのは苦手である。その代わりに、その能力は波乗りの技術にある。｝
Estuarine crocodiles (Crocodylus porosus) have the largest geographical range of any crocodile species, spanning more than 10,000 square kilometres 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 travelled 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 travelling. They get a free ride," says Franklin.
Surf's up
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.
{{Transmitters attached to the crocodiles recorded their location and temperature.}
Australian Zoo}
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 kilometres per day. In 96% of these trips, the reptiles travelled 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 ºC, suggesting they were basking in the sun on the riverbank. When the tide turned in their favour, their body temperatures dropped to 25 ºC, 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."
Magnetic attraction
It is not clear whether this behaviour is learned or inherited, says Franklin. He says that correlations can be drawn between the migratory behaviour 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 ten years or so, to shed light on why the reptiles travel long distances and how this behaviour arose.
References
1. Campbell, H. A. et al. Journal of Animal Ecology advance online publication doi:10.1111/j.1365-2656.2010.01709.x (2010).
[One-Minute World News] from [BBC NEWS]
[BBC > Science & Environment]
Page last updated at 22:10 GMT, Monday, 7 June 2010 23:10 UK
By Katia Moskvitch
Science reporter, BBC News
Crocodiles 'surf' long distance on ocean currents
{Estuarine crocodiles are poor swimmers}
Saltwater crocodiles enjoy catching a wave and can travel hundreds of kilometres by "surfing" on ocean currents, a study suggests.
Australian researchers used sonar sensors and satellite transmitters to monitor 20 reptiles' movements.
They found the crocodiles undertook numerous trips of over 10km (6.2 miles), but only when a current flowed in their direction of travel.
The results of the research appear in the Journal of Animal Zoology.
The TV personality Steve Irwin, who was nicknamed "The Crocodile Hunter", but died in 2006, took part in the study.
Estuarine or saltwater crocodiles are the world's largest reptiles and can grow up to five-and-a-half metres in length.
They are poor swimmers and mainly live in salt water - but their "home" spans over 10,000sq km of the South-East Pacific, from Sri Lanka to Fiji and from Thailand to northern Australia.
Researchers have long been puzzled by how crocodiles managed to spread themselves so widely.
"Of all the amazing things animals can do, the ability of certain species to migrate significant distances across formidable geographical barriers is one of the most remarkable," write the authors of the recent study.
Although the crocodiles spend most of their life in salt water, they are not considered marine animals as they rely on land for food and water.
The open sea
During the research, a team led by Dr Hamish Campbell, from the University of Queensland, captured 20 crocodiles living in the North Kennedy tidal river in Queensland, northern Australia, and tagged them with satellite transmitters.
They found that during the period of study, eight of them ventured out into the open ocean. One travelled from the river mouth all the way to the west coast of the Cape York Peninsula, in Queensland's far north. That amounts to a total of 590km covered over 25 days.
To do that, the ocean-trotter hitched a ride on a current within the Gulf of Carpentaria (that separates Cape York from Arnhem Land, to the west). This current occurs seasonally, during the summer monsoon.
{20 crocodiles were tagged with satellite transmitters}
"[These crocodiles] can survive for long periods in saltwater without eating or drinking, so by only travelling when surface currents are favourable, they would be able to move long distances by sea," commented Dr Campbell.
It took another adventurer - a 4.84m-long male - just 20 days to go more than 411km from from the east coast of Australia's Cape York Peninsula through the Torres Strait (which divides Australia from New Guinea) to the Wenlock River on the west coast of Cape York.
When the crocodile arrived in the Torres Strait, strong currents were flowing in the opposite direction to where it was headed.
So the animal waited in a sheltered bay for four days and continued its trip when the currents changed direction.
Important clues
The scientists also tagged 27 crocodiles with sonar transmitters and spent a year tracking their every move inside the North Kennedy River with underwater receivers.
They found that both male and female crocodiles regularly travelled more than 50km from home, swimming to the river mouth and back.
But the team discovered that crocodiles would only set out on a long journey within an hour of the tide changing. This allowed them to "catch a wave".
They put their trips on hold when the tides reversed, moving out of the river and on to the banks.
Dr Campbell said that the results of the study gave important clues to understanding the evolution of the world's largest reptiles.
"This not only helps to explains how estuarine crocodiles move between oceanic islands, but also contributes to the theory that crocodilians have crossed major marine barriers during their evolutionary past," he said.
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