Sexual dimorphism in birds: a complexity of sexual selection.

Why is it that in some species of birds males and females look so different you could easily mistake them for different species, while in others they look exactly the same? Evolutionary biologists have been looking for answers to this question since Darwin published The Descent of Man and Selection in Relation to Sex (1871) (Dunn et al. 2001).

Extreme sexual dimorphism is displayed in the Golden Pheasant (Chrysolophus pictus) - the male (rear) being spectacularly ornamented, while the female (front) displays drab colouration and a smaller body size. Photo by Kimball Chen http://keaphotography.photoshelter.com/image/I0000F1Owdy4Zd9U. Viewed on 18th March 2017.

Mandarin Ducks (Aix galericulata) are another great example of sexual dimorphism in birds - the male (left) has striking ornamentation compared to the female (right). Image sourced from https://en.wikipedia.org/wiki/Mandarin_duck. Viewed on 19th March 2017.

The general trend when it comes to sexual dimorphism is that males often display much brighter plumage and/or are larger in size than their female counterparts. Darwin (1871) and Wallace (1889) initially put this down to differences in social mating systems and paternal care, with polygamy causing the competitive sex to become larger and more ornate than the choosy sex and the sex that provides the parental care to develop more cryptic plumage (Owens & Hartley 1998). But if this is strictly true, why do we see socially monogamous species exhibiting biparental care that are highly dimorphic?


While size dimorphism displayed in some species of birds can be largely accounted for by the traditional idea of intrasexual competition, polygamy and differences in parental care (Darwin 1871, Owens & Hartley 1998, Wallace 1889), plumage dimorphism in socially monogamous birds is a more contentious issue. It has now been suggested that plumage dimorphism in socially monogamous birds may be due to extra-bond copulations (this is when despite having one partner whom each bird rears their young with, both males and females copulate with other individuals) (Owens & Hartley 1998, Webster et al. 2007). Molecular techniques have shown that up to 65% of socially monogamous birds display extra-bond fertilisation (Owens & Hartley 1998). If the extra-bond copulations that result in fertilisations are not random, which is almost certainly the case, then sexual selection should occur and favour traits displayed by males that are preferred by the promiscuous females (Owens & Hartley 1998, Webster et al. 2007).

The Superb Fairy Wren (Malurus cyaneus) is a great example of sexual dimorphism - the male's (left) vibrant blue plumage is strikingly different to the dull grey/brown plumage of the female (right). They are a socially monogamous bird but are known to be extremely promiscuous!. Image sourced from https://en.wikipedia.org/wiki/File:Male_and_female_superb_fairy_wren_edit21.jpg. Viewed on 19th March 2017.

 Just to make things more complicated, support has been growing for the idea that Wallace (1889) originally favoured - that plumage colour dimorphism is driven by the parent who primarily cares for the young, in most cases the female Owens & Hartley 1998) - but not how you think! Research into how hormones control sexual dimorphism in birds has shown that in many species, the default plumage is actually the bright, showy plumage we expect to see in males (Owens & Hartley 1998). This means that plumage colour dimorphism, in at least some cases, is due to females actively trying to become more cryptic by suppressing the default bright, showy plumage (Owens & Hartley 1998). I never knew this and I think it is incredible!

The Eclectus Parrot (Eclectus roratus) displays a unique form of sexual dimorphism in that the female (right) is actually brighter and more conspicuous than the male (left). This is due to different selection pressures acting on each of the sexes (Heinsohn et al. 2005). Image sourced from https://en.wikipedia.org/wiki/Eclectus_parrot. Viewed on 20th March 2017.

References

Darwin, C. 1871, The descent of man and selection in relation to sex. London: Murray.

Dunn, P.O., Whittingham, L.A. & Pitcher, T.E. 2001, "Mating systems, sperm competition, and the evolution of sexual dimorphism in birds", Evolution, vol. 55, no. 1, pp. 161-17.

Heinsohn, R., Legge, S. & Endler, J.A. 2005, "Extreme Reversed Sexual Dichromatism in a Bird without Sex Role Reversal", Science, vol. 309, no. 5734, pp. 617-619.  

Owens, I.P.F. & Hartley, I.R. 1998, "Sexual dimorphism in birds: why are there so many different forms of dimorphism?", Proceedings of the Royal Society of London. Series B: Biological Sciences, vol. 265, no. 1394, pp. 397-407. 

Wallace, A.R. 1889, Darwinism: An Exposition of the Theory of Natural Selection, with Some of the Applications. Macmillan.
 
Webster, M.S., Tarvin, K.A., Tuttle, E.M. & Pruett-Jones, S. 2007, "Promiscuity drives sexual selection in a socially monogamous bird", Evolution, vol. 61, no. 9, pp. 2205-2211.

Bowerbirds: The greatest display of sexual selection in the world?????

I cannot complete this blog without having a post dedicated entirely to bowerbirds. Bowerbirds are exceptional in that they display a multifaceted and extremely entertaining approach to sexual display. Bower building and ornamentation is what they are most famous for, but dancing and strange vocal displays that often incorporate mimicry are also utilised to tempt females into reproduction (Marshall 1954). 

Bowers

The first and most important part of the sexual display exhibited by male bowerbirds, is the ability to build a bower (where courtship and copulation take place) of sufficient quality to attract a prospective mate (Borgia 1985, Marshall 1954). Males are highly territorial and frequently destroy and steal decorations from other bowers, so the ability of a male to build, defend and display in a highly decorated bower is thought to provide the female information as to the overall quality of the male (Borgia 1985, Robson et al. 2005). It has been shown that  females are more attracted to males who have high quality bowers, and it is these males who account for the majority of matings (Borgia 1985).

The male Fawn-breasted Bowerbird (Chlamydera cerviniventris) creates his 1-2 foot long tunnel with an open top, known as an avenue-style bower, decorated with bright green leaves. Image sourced from http://animals.sandiegozoo.org/animals/bowerbird. Viewed on 11th March 2017.

The bower of a Great Bowerbird (Chlamydera nuchalis) (the largest of the bowerbirds) from Northern Australia is decorated primarily with white and grey objects such as snail shells and pebbles. Photo by Peter Rowland http://australiaswildlife.com/2016/05/bowerbirds-of-australia/. Viewed on 12th March 2017.

The incredible bower of the male Satin Bowerbird (Ptilonorhynchus violaceus). Image sourced from: http://www.viralforest.com/bower-bird/. Viewed on 12th March 2017.

The 'totem' bower of the Golden Bowerbird (Prionodura newtoniana) of Far North Queensland can be up to 2m high and is decorated with lichen and freshly plucked flowers. Photo by Tim Laman, http://timlaman.photoshelter.com/image/I00004AVpNp1wFWk. Viewed on 11th March 2017.

up to 2 m high, decorated with lichen and freshly plucked flower - See more at: http://www.australianwildlife.org/wildlife/golden-bowerbird.aspx#sthash.jTZwvNR5.dpuf

up to 2 m high, decorated with lichen and freshly plucked flower - See more at: http://www.australianwildlife.org/wildlife/golden-bowerbird.aspx#sthash.jTZwvNR5.dpuf

up to 2 m high, decorated with lichen and freshly plucked flower - See more at: http://www.australianwildlife.org/wildlife/golden-bowerbird.aspx#sthash.jTZwvNR5.dpuf

up to 2 m high, decorated with lichen and freshly plucked flower - See more at: http://www.australianwildlife.org/wildlife/golden-bowerbird.aspx#sthash.jTZwvNR5.dpuf

Perhaps the most impressive of all bowers, the giant bower of the Vogelkop Bowerbird (Amblyornis inornata) is a true masterpiece! Make sure you watch the video to find out more and have your mind blown. Photo by Ingo Arndt, http://www.audubon.org/news/see-rare-extraordinary-and-natural-photos-natgeos-new-book. Viewed on 10th March 2017.

Display
If the male manages to get past the first stage of impressing the female with his architectural and adornment skills, he then gets the chance to perform his display to 'seal the deal' so to speak, and copulate. Research has shown that the intensity and aggression of the male Satin Bowerbird's (Ptilonorhynchus violaceus) display preferred by the female, varies with age and experience and that a successful courtship may be the result of the male reading the female's reaction to his display and adjusting it accordingly (Coleman et al. 2004, Patricelli 2004, Walsh et al. 2002) . This theory of variable female preference has been implicated as a possible driver of the evolution of multifaceted male displays (Coleman et al. 2004, Patricelli 2004, Walsh et al. 2002).

Bowerbirds and the behaviour they display are extremely diverse and what I have shown you here is just a glimpse into their complexity. As Darwin predicted, all evidence points towards a combination of female choosiness and male competition resulting in the exceptional behaviours and traits we see in bowerbirds. I have added some videos below that sum up why I love bowerbirds so much. Where I grew up there are lots of Satin Bowerbirds and the first time I found one of their bowers I was absolutely amazed. I can describe all the incredible things they do, but it is only fully appreciated when you see what they do. Enjoy!

       The male satin bowerbird's (Ptilonorhynchus violaceus) display https://www.youtube.com/watch?v=_H9TyXiXM2k. Viewed on 11th March 2017.

The male vogelkop bowerbird (Amblyornis inornata) shows us why he is so incredible! https://www.youtube.com/watch?v=E1zmfTr2d4c. Viewed on 10th March 2017.

 

     The flame bowerbird's (Sericulus aureus) bizarre and mesmerising display https://www.youtube.com/watch?v=1XkPeN3AWIE. Viewed on 11th March 2017.

References

Borgia, G. 1985, "Bower quality, number of decorations and mating success of male satin bowerbirds (Ptilonorhynchus violaceus): an experimental analysis", Animal Behaviour, vol. 33, no. 1, pp. 266-271.

Coleman, S.W., Patricelli, G.L. & Borgia, G. 2004, "Variable female preferences drive complex male displays", Nature, vol. 428, no. 6984, pp. 742-745.

Marshall, A.J. 1954, Bower-birds: their displays and breeding cycles, Clarendon Press, Oxford.

Robson, T.E., Goldizen, A.W. & Green, D.J. 2005, "The multiple signals assessed by female satin bowerbirds: could they be used to narrow down females' choices of mates?", Biology Letters, vol. 1, no. 3, pp. 264-267

Patricelli, G.L. 2004, "Female signals enhance the efficiency of mate assessment in satin bowerbirds (Ptilonorhynchus violaceus)", Behavioral Ecology, vol. 15, no. 2, pp. 297-304.

Walsh, G., Borgia, G., Patricelli, G.L. & Uy, J.A.C. 2002, "Sexual selection Male displays adjusted to female's response", Nature, vol. 415, no. 6869, pp. 279-280.

A Fascination With Birds

Have you ever wondered why there are so many strikingly beautiful male birds? Or maybe why birds produce such a large array of complex, often melodic sounds? What about why some birds have developed such elaborate ornamentation and courtship displays that you would almost need to see to believe?

The male Resplendent Quetzel has an extremely large and beautiful tail streamer up to 65 cm long. Image by Phoo Chan (https://www.flickr.com/photos/mallardg500/13950167241)

Being a bird lover, these are just a few of the questions I have pondered over the years. While there are many factors to consider when determining why birds have evolved such a staggering amount of exceptional and often extreme behavioural and morphological traits, it is now widely accepted that sexual selection plays a major role in much of what we see today in the bird world.

Extreme ornamentation displayed by a male tufted coquette hummingbird. Sourced from https://au.pinterest.com/pin/205195326749461523/

 Charles Darwin (1906) first developed the theory of sexual selection which he described as:

“the advantage which certain individuals have over other individuals of the same sex and species solely in respect of reproduction”.  

In other words, sexual selection can be thought of as intra-specific reproductive competition (Hosken & House 2011). While it was suggested by some that it was not needed to distinguish sexual selection from natural selection, Darwin wanted to explain how characters could exist in organisms whose functions were often deleterious to the individual's survival and apparently not favoured by natural selection (Darwin 1906, Hosken & House 2011). He stated that sexual selection often: 

“depends not on the struggle for existence, but on the struggle between males for possession of females.” 

Darwin also recognised that it was not only competition, primarily between males, that acted as a mechanism of sexual selection, but also the choosiness of females when selecting a mate (Darwin 1906, Hosken & House 2011).

A peacock displaying his brilliant plumage to what looks like a very uninterested and possibly choosy female. Image sourced from http://www.buzzle.com/articles/difference-between-male-and-female-peacocks.html

Over the coming weeks I will be sharing with you specific examples of incredible and diverse sexually selected characters including song, plumage, scent, cognitive skills and elaborate displays that have fueled my continuing fascination with birds. Hopefully over the next 9 weeks I can show you why I think birds and the incredible diversity they display are an endless source of wonder and provide an insight into their complex and often mysterious world.

The male Satin Bowerbird uses blue objects to adorn his bower, luring in the dull coloured female before performing his courtship display. Image by Gerhard Koertner (http://www.bbc.com/earth/story/20141119-the-barmy-courtship-of-bowerbirds)

 


References
Darwin, C 1906, The descent of man and selection in relation to sex, 2nd edn, Murray, London
Hosken, DJ & House, CM 2011, "Sexual selection", Current Biology, vol. 21, no. 2, pp. R62-R65