The Hidden Glow

Okay, this is probably one of the coolest things I’ve learned about any bird mating in the past three years.

Now that I’ve raised the bar far to high for myself, enter: the Eurasian Blue Tit. A bird easily identifiable due to their bright yellow and blue coloration. The sexes of this species have always thought to be indistinguishable from one another, much like the Albatross, without genetic sexing. Only the most experienced handles are able to tell the difference, and even then only with the bird in hand.

A Eurasian Blue Tit

However, recent work done by a group of researchers in Sweden decided to look outside the box, and from the bird’s point of view. To be specific: in the UV scale. There are several groups of bird which view light within the UV spectrum, one of which is the passerines (which includes the Eurasian Blue Tit). Andersson, Örnborg, and Anderson (1998) thought to examine the markings of this species at a UV scale, which is a while new spectrum of color and light that humans cannot see without the assistance of special cameras, lenses, and other technological aid.

Turns out, the Blue Tit is in fact sexually dimorphic, simply on a wavelength that we are unable to see. They found that the males have a marked difference of the chroma on their brilliant crown patches. And oh how I wish they had taken pictures of the marking! It was even proven to stand out against both brown-grey and bright green backgrounds, translating into contrast with either tree foliage or even the dead leaf litter on the forest floor.

This find could potentially be the explanation for several behaviors and displays these birds are known to exhibit. For instance, this species is known to be polygamous, with females seeking extra-pair copulations with healthier males. The scientific community were unsure as to how the females decided which males were healthiest, with theories including certain vocalizations in song and even leg length, as it was the only determinable trait they could find between mate choices. Now it seems that the UV coloration is what is selected for, which is supported by the fact that during mating and territorial displays the males display their crown patches to females and other males.

Just goes to show there is more than meets the eye when it comes to bird behavior. 

Bonus: This is a direct quote from the journal article.

“Although hardly perceivable in the field (except when directly compared in the hand) to the UV-blind and yellow-biased primates that write ornithological handbooks…”

References

Andersson, S., Örnborg, J., Andersson, M., 1998. ‘Ultraviolet sexual dimorphism and assortative mating in blue tits’, Proc. R. Soc. Lond. B., Vol 265, p. 445-450.

Picture: http://www.raywilsonbirdphotography.co.uk/Diary/2010/December_2010.html

Flower Petals for Romance (We Think)

Last week I talked about the highly monogamous albatross, who will form long term pairs even when mates of the opposite sex aren’t available. This week I decided I’d address a species who are socially very monogamous, yet cheat on one another relentlessly.

Fairy Wrens are a group of birds that are quite common in Australia, and are well known for the iridescently bright plumage on the males. All species of the group are socially monogamous, pairing together for years on end with a very low divorce rate, and raising offspring together. However, not all those offspring are biologically the father’s. Sexually the Fairy Wrens are quite promiscuous, and from this a rather odd behavior has arisen.

Males of several species of fairy wren, notably the Red-Backed Fairy Wren, have been observed carrying colored petals of either pink or red in their beaks when pursuing females. The petals either help enhance their nuptial colors, or as the case of the Splendid Fairy Wren who is a bright blue act as contrast to their coloration.

Splendid Fairy Wren with a light pink petal for display.

There was a question at one point as to whether the petal displays were for courtship, or for as a territorial display towards other males in their territories. This was tested by Karubian and Alvarado (2003), but they were unable to come to any true conclusion on the matter.

Their results showed that while the males with petals only presented them to other males about 10% of the time, the amount of time females spent with males was not any different with the petal or without. This is interesting because it seems to be an arbitrary behavior the males have evolved which do not provide any direct advantage in increasing their mating chances with their mate or for extra-pair copulations, nor in keeping other males away from their territories.

There is always a chance that this behavior is a relic of a past mating strategy that required more displays and potentially gifts. It could also be a remnant of a time when these birds were more sexually monogamous, and using petals to show off their own fitness was a way to ward off competitors. Or, fairy wrens just like petals! This is one of those interesting cases when a notable behavior is seemingly unexplainable, though quite obviously tied to romance. As all flower petals should be, really.

References

Karubian, J & Alvarado, A 2003, ‘Testing the function of petal-carrying in the Red-backed Fairy-wren (Malurus melanocephalus)’, Emu, vol. 103, no. 1, pp. 87–92.

Picture: http://dailyorganism.blogspot.com.au/2011/08/splendid-fairy-wren.html

Mateless in Hawaii

             Albatross are almost synonymous with monogamy in the bird world, they are the poster child for long term breeding pairs and obligate dual parenting of chicks. Both genders also happen to look the exact same. So it came as a bit of a surprise to some researchers when they went out and decided to genetically sex a population of Laysan Albatross and discovered that there was a serious gender skew on the island, and even MORE shock when the realized what exactly that gender skew had wrought in terms of social behaviors. 

            I’m talking about the population of Laysan Albatross on Oahu Island in Hawaii that is 59% female, of which 31% of the breeding pairs are female-female. This is big, and the reason this is big is because every single female-female “breeding” pair were completely unrelated to one another.

            Cooparative breeding does occur in nature, wherein others of the species pitch in to help rear offspring which are not directly their own. However, in essentially every case of this type of behavior the offspring being reared or the parents being helped were in some way related to the helpers. But that is not the case on Oahu!

            Monogamy SHATTERED right?! Wrong. It turns out that in this male-barren land the unpaired females have managed to get the monogamous male albatross to have extra-pair copulations (Okay, monogamy a little bit shattered) in order to breed themselves. In one study done on the success of the female pairs it was noted that almost half the female-female pairs were together for the full 4 years of the study’s duration (Young, Zaun & VanderWerf 2008). They even found from another researcher that one pair of ladies had been together for a full 19 years, indicating that they chose the homosexual rearing strategy over the traditional one.  

Female-Female breeding pair of Laysan Albatross

 

 

            While the female-female pairs had less success in hatching the eggs than the male-female pairs, of the chicks that hatched the survival rate was essentially equal. Those pairs staying together for more than one year had at least one offspring from each parent hatch and fledge. That alone means that both females agreed to this continued cooperation. A sort of you hatch my egg I’ll hatch yours system that is essentially based upon a mutual agreement. 

            All in all, these same-sex pairs of Albatross have adapted to their new “environment” by recognizing that some reproductive success with another female is better than NO reproductive success by not breeding at all. It was decided by Young et al that while the Albatross’ social behavior has shifted to include same sex pairs, they are still operating under their monogamous instinct. Which is working for them. It also goes to show how dramatically gender ratios can affect the behavior and reproductive strategies of a species in a relatively short amount of time. 

Personal hypothesis as to why there are so few male albatross on Oahu.

 

References

Young, LC, Zaun, BJ & VanderWerf, EA 2008, ‘Successful same-sex pairing in Laysan albatross’, Biology Letters, vol. 4, no. 4, pp. 323–325.

Pictures:

http://esciencenews.com/articles/2009/06/16/same.sex.behavior.seen.nearly.all.animals.review.finds

http://www.timdrussell.com/pythonstills.htm

Feet Don't Lie

          I have a personal love for the foot coloration and courtship displays of the Blue Footed Booby. As the name suggests, the species is most notable for the brightly colored feet found on both males and females in addition to the infamous “Booby mating dance” in which the males lift their feet as high as possible to display their color to the female. This added to sky pointing and presentation of nesting materials (an artifact of ancestral behaviors as these birds don’t actually make nests any longer) can net a male Booby a mate for the breeding season. But the foot color is key in these displays, and this is because it is an incredibly honest indicator of both how well fed they are as well as their immunological state.

Male Blue Footed Booby lifting feet during courtship display.

The colors of feet range from blue-violet to bright green-blue, with the greener feet belonging to those individuals who are healthiest. One study was able to document the drastic reduction in vibrancy of the pigment in the feet from bright green to a dull blue-violet over the course of 48 hours when deprived of food (Velando, Beamonte-Barrientos, & Torres. 2006). Such a noticeable change makes it quite apparent to females whether their suitors are healthy, and vice versa as the female foot coloration is just an honest an indicator of health as the males. This is where it starts to get interesting in terms of reproduction.

Example of the difference in foot coloration amongst Blue Footed Boobies

Blue Footed Boobies are socially monogamous birds, and actually continue to undergo courtship displays after breeding has occurred. This isn’t the result of sentimentality, or of male blue-footed boobies being overly romantic. It is actually each member of the breeding pair checking up on the health of their partner and deciding whether or not it’s worthwhile to maintain the pairing. Should one of the pair find that their partner is “letting themselves go” in any way the results can actually be pretty dramatic.

Female boobies lay anywhere from 2-3 eggs a season, with the average being 2 laid several weeks apart. The same study on foot color by Velando, Beamonte-Barrientos, & Torres (2006) found that during that intermittent time frame between laying, if she discovers her mate is starting to drop in health she will actually adjust the amount of energy and nutrients she puts into producing offspring to result in an egg that is greatly reduced in size.  Essentially, the female puts in so little effort to egg formation that the offspring is almost assured not to hatch. If the female does not believe that the male can adequately help rear the offspring she simply ceases having them. All based on the color of his feet.

It’s not unheard of that females control their clutch sized based on environmental factors or mate quality, but in this case all the same requirements put upon males by females apply directly to them as well. In a study done by Torres and Velando (2005) it was found that females with duller foot coloration received less post-copulation courtship displays as well as fewer extra-pair copulations. Their own mates were also more likely to seek extra pair copulations with females who have brighter foot coloration.

             Essentially, the blue feet were selected for through sexual selection on behalf of both the male and the female of the species. Which makes sense, as their chicks require a long period of biparental care after hatching to rear them to independence. In many cases with birds only the males are pushed by female preference into large showy plumage, so I find it interesting when in nature you a find a sort of equality in the expectations of both males and females for brood rearing.

References:

Torres, R., Velando, A. 2005, ‘Male preference for female foot colour in the socially monogamous blue-footed booby, Sula nebouxii’. Animal Behavior, vol 69, pp. 59-65.

Velando, A., Beamonte-Barrientos, R., Torres, R. 2006, ‘Pigment-based skin colour in the blue-footed booby: an honest signal of current condition used by females to adjust reproductive investment’, Oecologia, vol 149, pp. 535-542.

Picture Credit

Dance: Personal Picture

Darker feet: http://photography.nationalgeographic.com/photography/photos/life-color-blue/#/blue-

                     feet-laman_1331_600x450.jpg

Lighter Feet: http://www.galapagos-islands.com/what-to-see/blue-footed-booby.html

The Funny Thing About Leks

There is a huge question still in evolutionary biology, which concerns the absurdly high degree of sexual traits (such as the tail of a peacock) of males who form leks. Now, a lek is when a large group of males all get together in a huge group and display for females, who then come and choose who they wish to mate with, and then promptly leave.

Females who visit leks almost always choose the same male to mate with, and that male is generally the one with the largest display, usually of plumage. The larger and more extreme these display traits are however, the lower the chances are of male survival. So the big, baffling question is why would females prefer these large displays, which increase the chances of death to the male, when they receive no other benefit than simply sperm?

A Sage Grouse lek, with displaying males.

In a review article written by Mark Kirkpatrick and Micheal J. Ryan (Kirkpatrick & Ryan 1991)on this very subject, they discussed three of the larger hypothesis about this behaviour. Spoiler Alert: We still don’t know which is the driving force behind extreme sexual displays and lekking! Which is why I personally think it’s pretty cool.

The first hypothesis essentially a direct selection by females on males, where the males with the largest displays get the most copulations wholly for selfish reasons on the female part. This means that the preference of larger displays by females is because, even though those traits increase the chances for the males of dying, choosing those mates will increase the females chances for survival or to reproduce themselves. For example, mating with larger, flashier males may reduce the pressures of searching for and choosing a male for the female. In addition, there is another very important factor called pleiotropy, which is when a gene associated with higher survival of the individual also affects that individual’s preference for mates. This means that a certain gene that may make a female bird of paradise REALLY like males with huge plumage also helps boost its immune system, or something similar. Since that gene is going to be kept, because it’s very helpful, that means the preference stays and the females will select for the males that meet that preference whether it’s good for the males or not.

Another hypothesis that was discussed in the paper was termed the “Runaway Process”. In this case it is thought that a sexual trait is linked to a gene, meaning that quality of environment or amount of food obtained doesn’t make the feathers brighter or longer. Essentially, the sexual trait appears and females select a bit for it and after a few generations almost the entire male population has developed this trait due to selection. The ever-present preference for the largest, most stunning trait will cause that trait to evolve to be bigger, brighter, more magnificent than any other male in order to meet that demand of the females no matter the consequences! Which is, as always, a higher mortality rate for the males. But now that the preference for the largest trait has been established, it will just continue to increase in speed like a runaway train. Smaller populations usually provide results that support this hypothesis, and the affects of genetic drift in those smaller groups most likely play a large role in establishing the immediate increase in the presence of a sexual trait by males.

The final hypothesis is based on a concept that is pretty familiar with most people, in that the brighter and bigger the bird the healthier he his and the stronger his genes for survival are. Also known as the parasite hypothesis, which is confusing until you think that the better a bird is at fighting of parasites that cause disease the healthier they are.  So those birds that look the best are probably free of parasites and have stronger immune systems. So by congregating in large numbers, they are better able to show off that they are brighter than other males, and increase their chance of mating.

Now, there are many other hypothesis about why certain displays have evolved over time, but I personally thought this article did a fabulous job explaining them. It goes on to explain why we are so unsure as to which one is true, and ways they think we should test it. I personally feel as though the runaway hypothesis is fairly week, and would be most likely to back the first one in which pleiotropy is correlating preference to female survival, but then I have never been one to immediately agree with parsimony.

 Original Review Article:  

Kirkpatrick, M & Ryan, MJ 1991, ‘The evolution of mating preferences and the paradox of the lek’, , Published online: 07 March 1991; | doi:10.1038/350033a0, vol. 350, no. 6313, pp. 33–38.

Picture Credit:

‘Eco-Vocab: Lek’ The Olive Tree, accessed February 25, 2014, from <http://oleaeuropea.wordpress.com/2011/01/25/eco-vocab-lek/>.