Bisexual dolphins or media fail?

Total. Media. Fail... Again...

While there is some evidence that dolphins display homosexual and bisexual behaviours, this, this, this, this and many other places are examples of the media completely failing to communicate the results of a scientific study¹. The results of the paper in question do not provide any evidence, at all, that would suggest that male dolphins are bisexual. The media has jumped on the word bisexual, which is used just once in the introduction (i.e. before the results of the study are described), and failed to understand it in the context that it was used; perhaps willfully in order to create a more sensational story.

The sentence in the paper reads:

The social system of Indo-Pacific bottlenose dolphins (Tursiops sp.) in Shark Bay, Western Australia, features a fission–fusion grouping pattern with stronger associations between adult males than adult females and bisexual philopatry.

The word 'philopatry' provides the context in which the word 'bisexual' should be interpreted. Here it does not refer to the sexual behaviour of the dolphins, but the fact that both sexes are philopatric. That is, both sexes have the tendency to stay in the area that they were born.


Although I can't be sure, I think the churnalism all started with a story on Discovery News. The story does do a pretty good job of discussing the results of the study, but includes the sentence:

Male bottlenose dolphins also were found to engage in extensive bisexuality, combined with periods of exclusive homosexuality.

This gets the situation completely wrong. An earlier paper² showed that male dolphins in Shark Bay form alliances with one or two other males. These first order male alliances cooperate to  aggressively herd females to gain mating access*. The first order alliances also prevent other males from mating with the females that they're herding. 


The first order male alliances can also cooperate with other first order groupd to produce a second order alliance, which in turn may join to form a third order alliance. The higher level alliances cooperate to 'steal' females from smaller groups. But, lower level alliances can also recruit other first order groups to defend the female they are herding. If a female is successfully stolen, only one of the joined first level alliances will herd her afterwards.


The higher level alliances are much less stable than first order alliances. For example, groups A and B may cooperate to steal a female from group C one day, then groups B and C may cooperate to steal a female from group A on another day. However, some high level alliances may persist for well over a decade. The reasons for such fluid, reversible alliances at higher levels are unclear, but it may have to do with the context of the encounters.


Despite the findings being less 'juicy' than the popular science articles make it seem, the paper is really interesting. The purpose of the paper was to test hypotheses about the structure of dolphin societies. Evidence from previous studies, such as the one described above, is strongly suggestive that males form their strong alliances in otherwise open social networks. This type of social structure has never been previously documented in mammals.


In mammals, male alliances form to acquire or defend valuable resources from other males, such as territories or female groups. Although it did not seem like this was occurring in the Shark Bay dolphins, two hypotheses had not been properly tested. The 'community defence model', which argues that there are dominant alliances that range over the entire population, and the 'mating season defence model', which argues that males defend territories or females only during the breeding season.


The study found little evidence to support either of the two hypotheses. No male alliances ranged over the entire site as the 'community defence model' predicted. Alliances also overlapped during the mating season and females were not consistently associated with particular alliances as the 'mating season defence model' predicted. So, now we have strong evidence that dolphin societies are unlike any other mammal societies.


The authors of the study point out that a society organised in such a way would put unusual pressures on social cognition. Not only must they remember previous encounters with other dolphins, but they must cope with uncertainty that the relationships are still the same. Individuals that were friends in earlier encounters may have formed new alliances since you last interacted with them and become foes. 


It's little wonder that social complexity correlates quite well with brain size, but it's hard to tell which came first. Larger brains may allow more complex social interactions to occur, but more complex social interactions may place selection pressure on large brain size. The authors argue that the complexity of dolphin society is probably a driver of brain size evolution in dolphins, and I tend to agree.


The authors also make the interesting argument that increased locomotion efficiency and low movements speeds have played a role in producing the social complexity and, therefore, brain size evolution. They argue that more efficient travel allows individuals to exploit larger areas and this would increase the chance that competitive encounters occur. A greater number of competitive encounters would, in turn, favour the formation of alliances with other individuals. I'm not so sure about this argument. Brains are energetically expensive to maintain, and probably favour more efficient use of resources elsewhere. Evolution always involves trade-offs.


So, it was a very interesting article with exciting results and the media did not communicate it at all well because they couldn't get past the word bisexual. They didn't even check to see whether it was used to mean what they thought it meant. Again I am disappointed with the media's coverage of science. 

*It's been called 'gang rape' by some articles, but this is not at all what it actually is. Although, by human standards, it's definitely not romantic. The herding males threaten, bite, hit and crash into the females they are herding.

Further reading:

1 Randic, S., Connor, R. C., Sherwin, W. B., and Krutzen, M. (2012) A novel mammalian social structure in Indo-Pacific bottlenose dolphins (Tursiops sp.): complex male alliances in an open social network. Proceedings of the Royal Society B. doi: 10.1098/rspb.2012.0264

2 Connor, R. C., Smolker, R. A., and Richards, A. F. (1992) Two levels of alliance formation among male bottlenose dolphins (Tursiops sp.). Proceedings of the National Academy of Sciences 89: 987 - 990

Diving gannets

Many years ago I stayed on an island off Wilsons Promontory in Victoria. Nesting on the island were a huge number of mutton birds. Every evening they would all come back from feeding and crisscross the island looking for their nests. It was amazing that so many birds could fly in so many directions at such high speed without colliding. Just as I was pointing this out to the people I was with, two birds clipped wings. We hadn't even stopped laughing before another two birds had a far more serious head-on collision. Both birds must have survived because we couldn't find them on the ground, but they could have been seriously injured.

It turns out that it's not uncommon for other seabirds to have collisions, some of them fatal. Gannets are an interesting seabird that plunge into the water at high speed order to catch fish from close to the surface (although I've heard of divers seeing them 40 meters down!). Gannets can form large flocks above schools of fish. At high densities collisions become more likely as one or more gannets target the same fish. Although collisions are not uncommon, deaths seem to be.

Now there is some cool underwater footage of gannets feeding with sharks and dolphins in Hauraki Gulf, New Zealand. It was taken by Gabriel Capuska, a PhD student at Massey University. It shows a collision between gannets and some interesting other behaviours. For more detail, go to the Massey University story.

Carnival of Evolution

The Carnival of Evolution collects the best blog posts on evolution from the last month. It's always a good read. It's hosted by a different blogger each month. This month it's on the blog 'The Atavism'.

If you're wondering what an atavism is, it's an ancestral trait that appears in an individual of a species that normally does not display that trait. For instance, whales evolved from four legged terrestrial ancestors. Their forelegs have become highly modified to flippers, while their hindlimbs have almost completely disappeared (only present as a remnant of the hindlimb girdle). But, from time to time examples of whales and dolphins with rudimentary hindlimbs are found. Mostly is it only the main bones of the leg (the femur, tibia and fibula) that are present, but examples with a complete set of foot bones have been documented too.

Atavistic hindlimbs on a dolphin captured by Japanese 'researchers' off the coast of the infamous Taiji, the town featured in the movie 'The Cove'.

An ocean of plastic

There are five major oceans in the world. There's the Arctic Ocean, the Atlantic Ocean, the Pacific Ocean, the Indian Ocean and the Southern Ocean. In the Pacific, the Atlantic and Indian oceans there are huge circular currents called gyres. The Indian Ocean has a single gyre, while the Atlantic and the Pacific have two, one in the northern hemisphere and one in the southern hemisphere.

The five great oceanic gyres showing the direction of rotation

The northern hemisphere gyres rotate in a clockwise direction, while the southern hemisphere gyres rotate in an anti-clockwise direction. The direction of rotation has to do with the Coriolis effect, which is what people joke about when the say that water goes down plug-holes in different directions in Europe compared to Australia. The Coriolis effect doesn't matter too much for water going down plug-holes (other forces are far more important), but operating over long time periods and over large distances it produces gyres.

Because the gyres rotate they are good at accumulating floating items in their centres. Waste material is drawn into the gyres from the countries that surround the gyre. When the waste reaches concentrations that are significantly higher than the rest of the World's oceans that area of ocean is termed a garbage patch. So far surveys have found garbage patches in the North Pacific, North Atlantic and Indian Ocean gyres. Garbage patches also form in other places, but the oceanic gyres form the biggest patches.

Of all the garbage patches the North Pacific gyre is the largest by a considerable margin. Mainland Australia has an area of 7.69 million square kilometres and estimates of the size of the North Pacific Garbage Patch are as high as 15 million square kilometres. So, basically there's a patch of garbage that could cover an area almost twice the size of Australia floating in the North Pacific. It should be noted, however, that other estimates are considerably smaller. Estimates vary largely because different studies use different densities of debris to define what a garbage patch is.

Plastic particles hanging underwater in the North Pacific garbage patch (photo Scripps Oceanography).

The garbage patches collect a huge array of debris and chemical waste. A lot of it, about 80%, comes from land-based sources. Natural disasters, such as a tsunami or a hurricane can lead to large amounts of waste entering the sea. However, the most common route is through storm water and waste water inputs. The other 20% of waste is lost or deliberately dumped from ships at sea. Although it has been illegal to dump waste at sea for the last 20 or so years, the law is almost impossible to enforce.

By far the most common thing found in the garbage patches is plastic. Mostly it's small particles of plastic, but sometimes very large items like fishing nets that are kilometers long can be found. The fact that it is mostly plastic is pretty amazing seeing as plastic has only become common since the Second World War. But the plastic is able to accumulate because, unlike many other type of rubbish that finds its way into the sea, there are very few organisms that can break it down.

A ghost net floating in the North Pacific garbage patch (photo Scripps Oceanography).

Plastic has a number of negative effects on marine animals. Probably the effect that most people would be familiar with is that large items of plastic, like ropes, fishing line and fishing nets can entangle marine animals. This can cause them to drown, if they breath air, it can inhibit their movements making them more vulnerable to predators and it can cause them injuries as they try to struggle free.

A beached whale's tail entangled with ropes (photo Mike Baird).

Another effect is that marine animals can consume the plastic because it looks to them like a tasty piece of food. At its most minor the animal has simply wasted its time and effort catching the plastic. But, if an animal eats enough plastic it can clog their digestive tracts making it hard for them to eat and digest real food. And it is not just the larger animals like whales, turtles and sea birds that are at risk from ingesting plastic. We know that there are some very small, even microscopic animals that are eating plastics.

Plastic bag fragments found in the contents of a turtle's stomach (photo Victoria González Carman).

Plastics have also been reported to accumulate toxic chemicals on their surface in high concentrations. And if marine animals eat the plastics the chemicals can be released during digestion and become incorporated into their tissues. So even if an animal eats plastic rarely, it can acquire a toxic dose of some chemicals that enter its system via the plastic. The research on toxic plastic is controversial and not yet widely accepted.

So plastic waste is a huge problem for life in the ocean. In fact, one researcher looking at plastics in the ocean has argued in a recent book that the biggest effect on the marine environment this century won't be climate change, it'll be plastic waste.

Dolphins, more intelligent than people

"...Man had always assumed that he was more intelligent than dolphins because he had achieved so much – the wheel, New York, wars and so on – whilst all the dolphins had ever done was muck about in the water having a good time. But conversely, the dolphins had always believed that they were far more intelligent than man – for precisely the same reasons." 
- From the Hitchhiker's Guide to the Galaxy, by Douglas Adams 


As a card carrying marine biologist who doesn't study dolphins or whales, I am obliged to say that they are some of the most uninteresting animals in the sea. But, over at Why Evolution is True, there is a post about dolphins playing with whales that other people might like.