There’s a shortage of single males on Oahu! (For the Laysan albatross, that is)

The Laysan albatross is a gull-like seabird with populations on several Hawaiian islands. For the birds living on Oahu, a recent immigration of female birds to the island has skewed the sex ratio of the population. The ratio of female birds to male birds is nearly 2 to 1! This shortage of males makes it really difficult for a female albatross to find a life partner – which is crucial, since incubating a single egg always requires two parents taking turns. Because of this obligate parental care, the albatross is a socially monogamous species (see this interesting blog post about social monogamy here) and they mate for life.

[1] An example of a Laysan albatross pair, and their nest and chick.

As a result of this skewed ratio, female albatrosses have changed their pairing behavior. Females who are not paired with males will “cheat” with an already-paired male in the colony to produce the egg. Then, they will pair up with another female to incubate that egg together. If they both produce eggs, only one egg can be incubated.

It’s kind of like when two high school girls go to prom together because they couldn’t find dates – after all, going to prom with your best girl friend is more fun than not going to prom at all. Just like human teenagers looking for prom dates, some female albatrosses choose to enter a same-sex pair rather than not pair at all. Since pairing is required in order to incubate a single egg, perhaps forming same-sex pairs confers some advantage to these female birds, allowing them to have offspring when they otherwise would be childless.

[2] While the lucky students in this photo were are able to find dates to the prom, often fewer male students are interested in dancing. This skews the sex ratio and causes many female students to attend the dance with their girl friends.

Researchers Young and VanderWerf are the experts in same-sex pairing in female Laysan albatrosses. In their recent publication, they wanted to quantify exactly what advantage the birds get from same-sex pairing. They compare if “cheating” and incubating the egg with another female was better than skipping mating altogether. To do this, they watched the birds during mating season and kept track of which birds made nests together. The study lasted for ten years.

The researchers found that nearly one-third of all paired birds were female-female pairs. These pairings could last for multiple mating seasons, and both females in the pair had chances to lay the egg each year. Since pairing up with another female sometimes allowed the female birds to have offspring, it was a better strategy than not mating at all. (Although it was still far less successful than if they were in a male-female pair, especially since the females had to take turns laying the egg each year, reducing their number of biological offspring by 50%.)

However, a surprising result was that there were some unpaired males in the colony. Sometimes, even when a male is available to pair, he doesn’t pair with a female who has been in a female-female pair the year before. This is especially true if the female-female pair did not produce a chick. This suggests that the male albatrosses may be choosing which females to pair with based on how successful those females had been the previous year. This is a form of sexual selection, and it’s interesting because usually males are the ones competing for female choice, but in the albatross population, the females must compete for male choice.

Another piece of evidence that sexual selection is happening is, that for the female-female pairs, if they successfully incubated the egg together in the first year, then on average 4% of them could gain a male partner the following year. But if a female-female pair failed to incubate their egg in the first year, then their chances of finding a male partner the next year were basically 0.

[3] A figure from the publication that shows the likelihood that a female albatross will transition between male-female and female-female pairings. A successful MF pair (had an offspring) is likely to stay together, and a successful FF pair is likely to gain a male partner the next year. But unsuccessful females are likely to end up being stuck with a female partner the next year.

Thus, if a female cannot get a male life partner, it makes sense for her to try to pair with another female. If she successfully has offspring while in a same-sex pair, she may be able to get a male partner the next year. However, if she fails to have offspring that first year, another better option is to skip breeding the next season, which still doesn’t produce a chick but is less detrimental to her health. This would explain why females in same-sex pairs skipped breeding more often than females in male-female pairs.

Conversely, if a female already has a male partner, she skips breeding less often, even when skipping might provide a health advantage. Why? Well, a female with a male partner who skips breeding is more likely to lose her partner the following year, so there is pressure on the females with in male-female pairings to keep breeding, no matter the physical cost. This sexual selection by male albatrosses on the females could be making the females breed more often, as they try to reserve their status as a high-quality female.

The story of the Laysan albatrosses shows that an imbalance in the sex ratio can lead to an unusual instance of sexual selection - where the females must compete for the attention of males, instead of the other way around. This kind of sexual selection can then cause radical behavior changes in the population, in a span of only a decade!

Journal Citation:

Young, L. C. and VanderWerf, E. A. 2014. Adaptive value of same-sex pairing in Laysan albatross. Proc R. Soc. B 281: 20132473. URLS: http://dx.doi.org/10.1098/rspb.2013.2473 and http://rspb.royalsocietypublishing.org/content/281/1775/20132473

Media credits:

[1] James Lloyd, Laysan pair and chick. Wikimedia Commons upload, March 2008. CC-BY-SA-3.0; Released under the GNU Free Documentation License. http://en.wikipedia.org/wiki/Laysan_albatross#mediaviewer/File:Laysan_pair_and_chick.jpg

[2] Ken Stokes, Preprom. Wikimedia Commons upload, May 2005. Released under the GNU Free Documentation License. http://commons.wikimedia.org/wiki/File:Preprom.jpg

[3] Young, L. C. and VanderWerf, E. A. 2014. Adaptive value of same-sex pairing in Laysan albatross. Proc R. Soc. B 281: 20132473. URLS: http://dx.doi.org/10.1098/rspb.2013.2473 and http://rspb.royalsocietypublishing.org/content/281/1775/20132473