Graeme Hornby
An Indo-Pacific humpback dolphin from the coastal waters of Oman.
OurAmazingPlanet
updated 3/29/2011 3:25:35 PM ET 2011-03-29T19:25:35

Dolphins in the western Indian Ocean stick with their own kind — not that they can help it. Blame the ocean's currents, a new study suggests.

Researchers found that genetically distinct populations of the Indo-Pacific humpback dolphin may have been created by currents, surface temperature differences and other environmental barriers in the ocean waters.

The Indo-Pacific humpback dolphin is a distant relative of the more familiar bottlenose dolphin. It is listed as "near threatened" by the International Union for Conservation of Nature.

Martin Mendez
The Indo-Pacific humpback dolphin gets its name from the distinctive hump just below its dorsal fin.

Using genetic data and satellites, the study team, which included members of the Wildlife Conservation Society and the American Museum of Natural History in New York, found correlations between regional ocean differences and genetic breaks between populations of dolphins from Mozambique and Tanzania in Africa, and Oman on the Arabian Peninsula.

Currents played a large role in separating these populations, according to the study. The South Equatorial Current — which runs west across the Indian Ocean before diverging north and south as it meets the African continent — seems to represent a barrier between genetically distinct populations of Mozambique and Tanzania; the current may play a role in creating them.

Seasonal monsoons also potentially contribute to what researchers found was a lack of southbound migration (or detectable gene mixing) along the African coast.

The researchers also found that differences in temperature, chlorophyll, turbidity and dissolved organic matter coincided with genetic differences between dolphin populations in Mozambique, Tanzania and Oman.

The two coastal regions without detectable genetic distinction between dolphin populations — Mozambique and South Africa — lacked significant environmental differences, the study said. The study team wanted to enhance its understanding about how the marine environment drives evolution of its creatures.

"Unlike studies of terrestrial species in easily observable environments, marine species are difficult to follow, and the barriers they encounter are often invisible to us," team member Martin Mendez of the American Museum of Natural History said in a statement. "Molecular technologies and remote sensing data can be combined to shed light on these mysteries."

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Howard Rosenbaum, study team member and director of the Wildlife Conservation Society's Ocean Giants Program, noted that coastal habitats are being threatened by development, so "understanding the population structure of the Indo-Pacific humpback dolphin in conjunction with environmental factors is an important step" in figuring out how to protect the species.

The study appears in the advance online version of the journal Heredity.

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