Monday, April 2, 2012

Bisexual dolphins or media fail?

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

While there is some evidence that dolphins display homosexual and bisexual behavioursthis, 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

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