False killer whales (Pseudorca Crassidens)
False killer whales and bottlenose dolphins in New Zealand form long-term partnerships that might help them fend off predators or find food, researchers suggest.
Masters student Jochen Zaeschmar, and colleagues, from Massey University's Coastal-Marine Research Group, report their findings in a recent issue of Mammal Review.
"There is a long-term association between, not just the two species, but between actual individuals," says Zaeschmar.
False killer whales (Pseudorca crassidens) are actually a rare type of dolphin that are sometimes found together with bottlenose dolphins (Tursiops truncatus).
But, until now there has been little research to investigate whether this association is just a matter of coincidence, or whether there issomething more to it.
"The first time I ever saw them together I was intrigued straight away because it seemed to be not random," says Zaeschmar. "They were so happy in each others' company. It was almost like it was one species."
False killer whales are three times the volume and two times the length of bottle nose dolphins and are jet black rather than grey.
Fin Identification System
In a study spanning 17 years and 700 kilometres of territory in New Zealand waters, Zaeschmar and colleagues studied the movements of 61 false killer whales and 200 bottle nose dolphins, identifying individual animals by unique markings on their fins.
"There are nicks and notches and cuts in the backs of their fin that they accumulate over time - and they're permanent," says Zaeschmar. "We produced an identification catalogue for each species."
The study was challenging, with researchers having to get up close to take photos of the fins while the animals were moving in the open ocean.
But the findings have been worth the trouble.
"Not only are we seeing the same whales over and over again, but also the same dolphins," says Zaeschmar.
"They basically do everything together. They feed together, they travel together, they rest together. We have not seen any physical state where they have not been together."
The animals also have physical contact and have been known to produce viable hybrids in captivity.
"It's a pretty exciting thing that these relationships last much longer than we thought," says Zaeschmar.
Safety In Numbers
Zaeschmar offers a number of theories as to why this long-term association exists.
One idea is that there is safety in numbers - the more individuals there are in a group, the more eyes there are looking out for predators, and if a predator does come, the less chance there is of any one individual being chosen.
"It's a win-win," says Zaeschmar.
Another idea is that joining forces makes it easier to spot food sources. In the open ocean food can be tricky to find, but is usually in great abundance once it is found, he says.
"There may also be just a strong social component," adds Zaeschmar.
He says the animals have similar vocalisations so it stands to reason they can communicate with each other, and the two have also been known to hybridise in captivity.
But, says Zaeschmar, it is very difficult to test the different hypotheses because the animals are always together.
The study was only able to observe the false killer whales and bottle nose dolphins during the period of December to May, when the animals are assumed to be following warmer currents in search of food.
Zaeschmar says his team has no idea where the animals go during the other months of the year but future studies could use satellite tagging to investigate whether the animals continue to associate with each other during this time.
False killer whales were originally confused with real killer whales in the 1800s because of similarities in their skull structure. The animals also have big teeth like killer whales.
And it is a curious fact that in some parts of the world they sometimes eat other dolphins or whales, says Zaeschmar.
"In New Zealand they obviously have a very different relationship with dolphins, which all adds to the mystery," he says.
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