Monday, January 3, 2011

Two is a company...

Anyone who has ever shared a residence can attest to the fact that cohabitating with others can be an extremely stressful state of affairs. Disputes resulting from a lack of privacy, the unequal sharing of common resources, and poor communication become routine occurrences. Even worse is a cohabiting situation in which those sharing the living space develop a mutual love interest – how does that get resolved? Luckily, a recent study published in an animal behavior journal may offer a glimpse of hope for those truly desperate for resolution.

Weighing as much as fifteen pounds and often sporting wingspans greater than nine-feet, bearded vultures (Gypaetus barbatus) are truly massive birds. First described by the father of modern taxonomy ( Carl Linnaeus) in his Systema Naturae, bearded vultures can be found competing for habitat throughout the mountainous regions of Europe, Asia and Africa. Even though a pining for expansive views has lead these bulky buzzards to prefer out-of-the-way nesting sites in hight rocky crags, pressure from human encroachment has caused their numbers to decline in recent years. The huge birds are experiencing population growth in a few isolated locales in the Pyrenees Mountains, but, unfortunately, the increase in the Pyrenees groups has contributed to overcrowding and a lack of housing options for many of the birds.

Under normal circumstances, in un-congested habitats, a bachelor bearded vulture will stake claim to a territory and take-up with a female member of the species; however, with the current surge in population in the Pyrenees, there simply isn’t enough available precipitous homesteads for all of the free-roaming males to settle down and raise families of their own. Lacking options, the roving males have developed a new strategy: they have begun to invade the established territories of their rivals – their already attached rivals. The itinerant bachelor invades the home of another male and claims residence with him and his already courted female companion. As might be predicted, the addition of an interloping male into the love nest of an established male-female pair has proven to adversely affect the reproductive success of the mate-pair.

Typically, a bearded vulture male-female pair will breed between the months of December and February and produce one or two eggs annually; but, the addition of the second male in the territory decreases the frequency and duration of the pair’s copulations. This occurs for a couple of reasons; firstly, the two males constantly fight each other for access to the female. The time and energy the males expend in combating and deterring one another leaves both far too exhausted to apply any romantic effort towards the female. Exasperating this situation even further, when one male does find the rare opportunity to copulate with the female, the rival male will often physically interrupt the act – he’ll stop them mid-coitus!

In addition to the mood-ruining intrusion of a combative third party during attempted sexual congress, the female can even be put-off by the mere presence of a second male — she’ll terminate copulation if she even spots a voyeur.

Fortunately, there is hope for the Pyrenees populations. As with the cliché, “time heals all wounds,” it turns-out that over long periods of time, the polyandrous model can work for the bearded vulture. Apparently the key to success in the multi-male regime is a willingness of the beta bird to demonstrate submission to the alpha – male on male copulations appear to curtail the aggression of the frustrated vultures.

Journal Reference:
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