Wednesday, May 27, 2009

Two’s a Company, Three’s a Sexual Conflict Imposition of an established pair-bond by a combative third party leads to an antagonistic love triangle and homosexuality in one vulture population.

Weighing as much as fifteen pounds and sporting a wingspan greater than nine-feet in length, the Bearded Vulture (Gypaetus barbatus) is a massive bird. First described by Linnaeus in his Systema Naturae, it’s found throughout Eurasia and Africa where it nests at mountainous elevations beyond 1300 feet. Although becoming rarer within most of its natural European range, the huge bird is experiencing population growth and overcrowding in the Pyrenees and as a result has opted for co-occupancy with rival males.

Within un-crowded habitats, single male Bearded Vultures stake claim to territories and take-up with female members of the species; however with the population growth currently experienced in the Pyrenees, there just isn’t enough available habitat for free roaming males to settle down and raise families of their own. Having little other option, the roving males are beginning to invade the established territories of attached rivals where they move-in with the conventional male-female pair. The resulting trio of vultures, consisting of two males and one female, is leading to disharmony between the established pair-bond and is diminishing their reproductive success.

Typical Bearded Vulture male-female pairs breed between December and February and produce one or two eggs, but with the addition of the second male in the territory, the frequency of copulations is decreasing. This is happening for a couple of reasons. Firstly, the males are constantly fighting each other for access to the female. With time and energy being invested in combat and deterrence of rivals, the males simply don’t have the time or energy to get romantic. Exasperating this situation even further, when one male does find an opportunity to copulate with the female, the rival male will often physically interrupt the act and stop them mid-coitus!

In addition to the obvious and expected interlude brought on by the intrusion of a combative third party in sexual congress, the female is often put-off by the mere presence of a second male and will terminate copulation independently of any physical altercation.

Fortunately, there is hope for the Pyrenees populations. As with the cliché, “time heals all wounds,” it turns-out that over long enough periods, the polyandrous model can work for the Bearded Vulture. Key to success in the multi-male regime is a habitat of sufficient quality to sustain additional numbers and the willingness of the beta bird – usually the previously roving male – to demonstrate submission to the resident alpha through male-male copulations, which appear to curtail the aggression of the alpha.

Bertran, J., Margalida, A., & Arroyo, B. (2009). Agonistic Behaviour and Sexual Conflict in Atypical Reproductive Groups: The Case of Bearded Vulture Polyandrous Trios
Ethology, 115 (5), 429-438 DOI: 10.1111/j.1439-0310.2009.01628.x

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