For Dolphins, Chemistry Is in the Water

March 25, 2011

‘Invisible’ barriers within the western Indian Ocean are keeping populations of Indo-Pacific humpback dolphins from intermingling. New research advises conservation plans to take environmental conditions such as currents into consideration.

Deciphering dolphin chemistry isn’t always easy. But new research has gotten to the bottom of some of the mating choices Indo-Pacific humpback dolphins make. And the dolphins don’t have as much choice in the matter as previously assumed.

As it turns out, there are barriers in the water—hidden to us, not to them—that prevent certain groups of dolphins from intermingling.

What makes some dolphin relationships star-crossed? Ocean currents and differences in water temperature, turbidity, chlorophyll levels, and the amount of dissolved organic matter, to name a few. WCS scientists and partners learned this by following the genetic patterns of dolphins interspersed throughout the western Indian Ocean and matching them up with the region’s water quality, based on satellite data collected over a period of 13 years.

“Examining how environmental factors affect the population structure of marine species is a complex task. Doing this over entire regions is a challenge,” said lead author Dr. Martin Mendez of the American Museum of Natural History. “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. Molecular technologies and remote sensing data can be combined to shed light on these mysteries.”

The researchers examined the mitochondrial DNA of more than 90 Indo-Pacific dolphins, tracing the passage of this genetic material between mothers and their offspring at different locations. The cetaceans they studied were swimming off the coasts of Oman, Mozambique, Madagascar, Tanzania, and South Africa.

The genetic data suggests that certain ocean currents are keeping the dolphin populations apart. For instance, the South Equatorial Current runs west across the Indian Ocean before diverging north and south as it meets the African continent. It divides the dolphins of Mozambique and Tanzania. Seasonal monsoons might also prevent dolphins from migrating south along the eastern African coast.

Such high-tech research gives conservationists a better idea of how best to protect these marine mammals and their habitats. The International Union for Conservation of Nature (IUCN) deems Indo-Pacific dolphins near-threatened due to habitat loss, disruption, and fishing activity. 

The study’s findings could help parse together the factors driving the evolution of these mammals. A distant relative of the bottle-nosed dolphin, Indo-Pacific dolphins can grow to 10 feet, with skin color that ranges from dark gray to a pink or white. WCS scientists discovered a similar phenomenon last year in Franciscana dolphins off the coast of Argentina.

“With increasing development and potential threats to coastal habitats,” said Howard Rosenbaum, director of WCS-Ocean Giants, “understanding the population structure of the Indo-Pacific humpback dolphin in conjunction with environmental factors is an important step in formulating management recommendations and protection measures for the species.”
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