Tuesday, 4 October 2011

Knowledge is power for the sun spot owners

ResearchBlogging.orgEven as the butterfly season comes to an end, Speckled Wood butterflies, Pararge aegeria, are still going strong in my local overgrown cemetery. The hundred year old trees, shade the ground and sunny clearings are highly prized by Speckled Wood males. Yesterday, I watched them squabbling incessantly in a patch of grassy area amongst the trees. At some point, the resident male chased two other individuals, the three butterflies flying around in tight circles away from the prized spot. Another individual comes by oblivious to the ownership contest and takes possession of a sunny dock leaf in prime position (above). By the time the initial "owner" comes back victorious from the skirmish, the newly arrived one believes this is his spot and the subsequent squabble ensues. It is really never ending in this warm autumnal morning with the sunspot owners.
 What determines how is the fight settled? In the late 1970s Davies work suggested that prior ownership is a very strong predictor of success in a fight: residents always won. Later work by Christer Wiklund and collaborators have expanded and refined Davies' findings, indicating that although a good predictor, ownership is not the only factor involved in territorial fights. Age and size are factors that are known to be predictors of fight outcome, but in the Speckled Wood these have little importance. Martin Bergman, Martin Olofsson and Christer Wiklund investigated the effect of motivation on territorial fight outcome. Residents might be more motivated to fight because they have more information on the value of the resource they are fighting for: the resident male might have prior mating experience in that spot, or knows how prized the spot is for other males. The intruder, most of the time lacks such information. This creates an asymmetry in which the resident is more determined to fight than an intruder.
 To test this hypothesis, the researchers set up experiments in large outdoor cages with a prime large sun-spot, and smaller, suboptimal sunspots. They released two males in each cage and waited until there was a clear dominant male that took possession of the large sunspot. They they removed the dominant male and divided the subordinate males in two groups, half of the males repeatedly encountered a female during a 30 min residency period (female encounter group), while the other half (the control group) did not have any female encounters. Then they tested the effect of this prior experience - motivated vs unmotivated males - on contest outcome when they introduced the original dominant individual back in the cage. The results were very clear.
Outcome of contests between Speckled Wood males during the second contest period when the original winner had been reintroduced, and after the original losers had either interacted with a female during 30 min (female encounter group, n = 30) or been alone for 30 min (control group, n = 30); ‘reversal’ (open bars) denotes that the male that lost the contest during the first contest period reversed the outcome and won the contest against the original winner in the second contest period, and ‘no reversal’ (filled bars) denotes that the same male won in both contest periods (from Bergman et al 2010).

Half of the originally subordinate males won their fight with the original dominant in the "female encounter group". In contrast, most of the dominant males won their fight for the prime sunspot when no females had been present. Also, the fights in the treatment group were more intense and took longer to resolve, while the control group the fights were settled quickly. In conclusion, these experiments showed that male experience in a sun spot increase his persistence in a fight, and therefore increase the chances of winning it, even if he was initially a subordinate male, indicating that motivation is an important factor for contest resolution in the Speckled Wood.

Bergman, M., Olofsson, M. & Wiklund, C. (2010). Contest outcome in a territorial butterfly: the role of motivation Proceedings of the Royal Society B: Biological Sciences, 277 (1696), 3027-3033 DOI: 10.1098/rspb.2010.0646
Davies, N. (1978). Territorial defence in the speckled wood butterfly (Pararge aegeria): The resident always wins Animal Behaviour, 26, 138-147 DOI: 10.1016/0003-3472(78)90013-1


Ray said...

Beautifully written and fascinating.
There is a sunny little woodland clearing some 100 metres from home where I can find Speckled Woods basking for much of the year. I have always wondered about the dynamics of their relationships; they certainly didn't seem to be co-operating on anything... .

I've always find their feeding habits make them pretty special.

(I've downloaded both of your references.)

Blackbird said...

Thank you Ray. Research into everyday bugs never ceases to fascinate me. Both papers are really wonderful reading and explore other aspects I had to left out. This particular male was very fresh, it made a big contrast with the first "owner". I also found uplifting that researchers studying "motivation" in butterflies are taken seriously these days.