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Posts Tagged ‘crocodilian’

The estuarine, or saltwater, crocodile (Crocodylus porosus) is the largest reptile in the world at around 4.5m in length, and with individuals over 7m long spotted.  They are found in freshwater and estuarine habitats such as mangroves, rivers and estuaries, from India through SE Asia and along the island chains to Australia and the South-east Pacific. This wide distribution of one species, with considerable tracts of open ocean in-between populations that should be un-navigable to the crocodiles, set Australian scientists wondering why this saltwater barrier has not lead to the separated populations becoming different species (‘speciation’). Their recent work has demonstrated that estuarine crocodiles can travel vast distances across the oceans by using water currents to speed up their journeys, enabling the seemingly unconnected populations to intermingle and breed. Yes, surfing crocodiles exist!

The team captured and radio-tagged 27 estuarine crocodiles in Australia then followed their satellite-tracked movements and compared their travels to the water speed and direction in that section of the river or ocean at that time. Their data showed that the crocodiles had two distinct travel modes:

(1) a short-range movement of around 1-3km per day in one direction; this was their typical daily pattern and is likely to have been day-to-day travel within their home range, and

(2) a less frequent long-range movement of more than 25km per day in a constant direction (although for the analysis all journeys of over 10km per day were included as long-range).

Intriguingly, while short-distance travel did not follow a set tidal pattern, long journeys were always started within one hour of the tide changing direction, giving the crocodile 6-8 hours travelling with a ‘tailwind’ current helping them along (less than 4% of long-distance travel was against the current and this dramatically reduced their travel speed). When the tide turned against them again, the crocodiles would stop their journey and climb out or dive to the bottom of the river, resting while the tide was not in their favour.

Some of the crocodiles travelled incredible distances – one taking an ocean voyage that coincided with a strong sea current that helped him travel 590km in 25 days, entering a different river along the Australian coastline. Another moved more than 411km in 19 days, later returning all the way back to the exact location within the river where it was originally captured – so they are excellent navigators as well.

The researchers suggest that using the ocean and river currents for migratory travel allows the crocodiles to travel far further than they would be able to do under their own power as crocodiles, despite being aquatic, are not really well-built for swimming – either in speed or efficiency. Both males and females made these long-distance journeys but the purpose of the travel is not yet known although it may be to take advantage of fish migrations. These travels do mean, however, that throughout their 10,000sq km range the ocean does not act as a barrier to gene flow between the various populations of estuarine crocodiles, maintaining the population as one species despite the ocean barriers in between.

Reference

Campbell HA, Watts ME, Sullivan S, Read MA, Choukroun S, Irwin SR & Franklin CE. 2010. Estuarine crocodiles ride surface currents to facilitate long-distance travel. Journal of Animal Ecology 79: 955-964

Further info

Ecology Asia

Unique Australian animals

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