Sexual dimorphism in some raptors have been switched? how can this be explained?

In birds, sexual dimorphism is the morphological difference observed in appearance and size between males and females as well as in many species of vertebrae (Owens & Hartley 1998). This dimorphism is explained as being associated towards the social interactions in mating, the males tending to have distinct morphological differences in appearance (eg. colourful plumage and bigger in size) in most species being the competitive sex (Owens& Hartley 1998).

Although in the case of raptors like Falconidae as well as some other families, females show the morphological difference in bigger sizes, referred to as a reversed sexual dimorphism (RSD) (Olsen & Olsen 1987, Olsen 2013).  Even though it is not certain as to which sex changed, or when they changed in size, there are explanations in the attempts to distinguish why females and males show reversed dimorphism in their size (Montgomerie & Lundberg 1989).

 It is suggested that males being smaller allow efficient energetic output, longer hunting ranges as well as agility and speed when hunting prey items such as mammals and birds (Sonerud et. al 2013, Olsen 2013, Slagsvold & Sonerud 2007). This is because the male will dominantly provide and collect food for the female and chicks (Olsen 2013). Also, reversed sexual dimorphism is said to aid in flight, compensating for the increase in weight of the female prior to egg laying (Wheeler & Greenwood 1983). Meaning RSD is prominent in high flight performance dependent raptors (Wheeler & Greenwood 1983).

Another explanation is that intrasexual competition is the reasoning behind RSD as females will compete and command nest sites with a mate and that being bigger aids in bluffing and avoiding potential conflicts (Olsen & Olsen 1987)

Lastly, another explanation is that RSD allow mates to utilise different ranges of prey resources avoiding inter-sexual competition (Sonerud et. al 2013, Olsen & Olsen 1987). Although as mentioned by Wheeler & Greenwood (1983 p.148), this highlights the size relation to the diet where larger females and smaller males may predate large and small prey respectively, it doesn't address why the RSD occurs. 

These are some of the explanations attempting to illustrate why some species of raptors show this reversed sexual dimorphism. 

Reference:

Montgomerie, R. & Lundberg, A. 1989, "Reversed Sexual Dimorphism in Raptors: Which Sex Changed Size?", Oikos, vol. 56, no. 2, pp. 283-286.

Olsen, J. 2013, "Reversed Sexual Dimorphism and Prey Size Taken by Male and Female Raptors: A Comment on Pande and Dahanukar (2012)", Journal of Raptor Research, vol. 47, no. 1, pp. 79-81.

Olsen, P. & Olsen, J. 1987, "Sexual Size Dimorphism in Raptors: Intrasexual Competition in the Larger Sex for a Scarce Breeding Resource, the Smaller Sex", Emu, vol. 87, no. 1, pp. 59-60.

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.

Slagsvold, T. and A Sonerud, G., 2007. “Prey size and ingestion rate in raptors: importance for sex roles and reversed sexual size dimorphism.” Journal of Avian Biology, vol. 38, no. 6, pp.650-655.

Sonerud, G.A., Steen, R., Løw, L.M., Røed, L.T., Skar, K., Selås, V. & Slagsvold, T. 2013, "Size-biased allocation of prey from male to offspring via female: family conflicts, prey selection, and evolution of sexual size dimorphism in raptors",Oecologia, vol. 172, no. 1, pp. 93-103.

Wheeler, P. & Greenwood, P.J. 1983, "The Evolution of Reversed Sexual Dimorphism in Birds of Prey", Oikos, vol. 40, no. 1, pp. 145-148.

Owls: silent flight is it all the same?

Many owls have adapted features for silent flight mainly due to the serrated leading edges on their wings; other reasons are thought to be the soft downs on their wings as well as trial feathers (Weger & Wagner 2016, Bachmann et. al 2007, Kun et. al 2012).

These serrated edges are located on a few primaries and alula feathers (Fig1.) and reduce noise frequency below 2kHz (Weger & Wagner 2016, Kun et. al 2012, Bachmann et. al 2007). Although differences in these serrated comb-like edges are present between species, due to different activity times meaning diurnal and nocturnal owls are bound to predate different organisms (Weger & Wagner 2016).

Fig1. Labeled diagram indication location of serrations (from Weger & Wagner 2016 p.3)

This difference in prey would indicate different hunting methods. Such as nocturnal owls locate prey by bi-aural acoustics and hunt silently as well as slowly due to increased drag by the serrations allowing undetected approach (Weger & Wagner 2016). Whereas diurnal hunting owls that do not hunt in the cover of dark, the reduced flight speed from serrations would impede their hunting fitness thus you see lesser developed serrations (Weger & Wagner 2016).

Therefore, the silent flight is not an adaption that is observed in all Strigiformes species as serration formations vary in occurrence to the time of activity in the owls, but is seemingly advantageous in nocturnal species.



Reference:

Kun, C., Qingping, L., Genghua, L., Ying, Y., Luquan, R., Hongxiu, Y., Xin, C. 2012, "The Sound Suppression Characteristics of Wing Feather of Owl （Bubo bubo）", 吉林大学仿生工程学报：英文版, vol. 9, no. 2, pp. 192-195.

Jaworski, J.W. & Peake, N. 2013, "Aerodynamic noise from a poroelastic edge with implications for the silent flight of owls",Journal of Fluid Mechanics, vol. 723, pp. 456-457.

Weger, M. & Wagner, H. 2016, "Morphological Variations of Leading-Edge Serrations in Owls (Strigiformes): e0149236", PLoS One, vol. 11, no. 3, pp. 1-18.

Bachmann, T., Klän, S., Baumgartner, W., Klaas, M., Schröder, W. & Wagner, H. 2007, "Morphometric characterisation of wing feathers of the barn owl Tyto alba pratincola and the pigeon Columba livia", Frontiers in zoology, vol. 4, no. 1, pp. 1-2.

An insight to raptorial vision

In comparison to humans, birds generally have larger eyes considering their body and head size as the eyes can take up to 50% of the skulls volume (Jones, Pierce & Ward 2007). This means they depend on a more visually active lifestyle and that they use their eyesight to not only find food but also to avoid being predated, as well as observing their environmental surroundings (Jones, Pierce & Ward 2007, Garamszegi, Møller & Erritzøe 2002).

Fig 1. Orientation and visual field in avians from (Martin 2009)

With a focus towards raptorial adaptations, the orientation of the eyes in diurnal raptors is relatively laterally orientated, having a better monocular vision but narrow binocular vision, whereas forward orientated owls have a greater binocular visual field but lack posterior vision (Fig.1) (Iwaniuk, Hall & Wylie 2008).Also depending on what raptor is being observed there will be differences in the eyes size and shape (Jones, Pierce & Ward 2007). 

The eye shapes seen in birds of prey mainly consist of round and tubular shaped eyes (Fig 2.) (Sturkie 1987). These eye shapes are characteristic of diurnal and nocturnal raptors respectively, as diurnal raptors require the ability to see long distances in higher definition, in contrast to owls and with keener eyesight from their concave ciliary region (Jones, Pierce & Ward 2007, Sturkie 1987).

Fig.2 attained from Sturkie (1987 p.39)

But aside from orientation and exterior morphology, internal structures such as the retina and fovea will be discussed.
The retina (Fig3.) contains rods and cones, which allows diurnal and nocturnal raptors to see in their environment (Jones, Pierce & Ward 2007, Sturkie 1987). These raptors are cone dominated and rod dominated respectively, as cones have photoreceptors (Fig 5.) allowing them to filter out different wavelengths of light depending on the oil droplet they contain, whereas rod photoreceptors in low light environments discern shapes and movements (Jones, Pierce & Ward 2007, Sturkie 1987). 

The fovea (Fig 4.) is characteristically deep in diurnal raptors and is, in fact, something that most taxa lack, but raptors have two foveae, being the area that aids in the acuity of vision, this allows diurnal raptors see on more than a single plain stationary and whilst hunting (Jones, Pierce & Ward 2007, Sturkie 1987).  

Fig 3. Obtained from Glasser & Howland (1996 p.479)

Fig 4. deep (A) and shallow fovea (B) from Jones, Pierce & Ward (2007 p.77)

Fig 5. Labeled diagram of cone photoreceptor from Jones, Pierce & Ward (2007 p.77)

Reference:


Glasser, A. & Howland, H.C. 1996, "A History of Studies of Visual Accommodation in Birds", The Quarterly Review of Biology, vol. 71, no. 4, pp. 479.

Garamszegi, L.Z., Møller, A.P. & Erritzøe, J. 2002, "Coevolving avian eye size and brain size in relation to prey capture and nocturnality", Proceedings of the Royal Society of London. Series B: Biological Sciences, vol. 269, no. 1494, pp. 961.

Iwaniuk, A.N., Heesy, C.P., Hall, M.I. & Wylie, D.R.W. 2008, "Relative Wulst volume is correlated with orbit orientation and binocular visual field in birds", Journal of Comparative Physiology A, vol. 194, no. 3, pp. 267-268.

Jones, M.P., Pierce, K.E. & Ward, D. 2007, "Avian Vision: A Review of Form and Function with Special Consideration to Birds of Prey", Journal of Exotic Pet Medicine, vol. 16, no. 2, pp. 69-79.

Martin, G.R. 2009, "What is binocular vision for? A birds' eye view", Journal of Vision, vol. 9, no. 11, pp. 14-14.

Sturkie, P. D., 1987, Avian Physiology: Fourth Edition, Springer-Verlag, New York, pp. 39-45.

Distinguishing genetic relations and associations with Diurnal birds of prey and owls opposed to morphological evidence, is it important?

Birds of prey (Raptors) are characteristically carnivorous birds that hunt their prey and stated by an adaption of Baird are in the Order: Raptores, consisting of Vultures, Diurnal raptors (osprey, kites, falcons, eagles) and nocturnal raptors being owls (Sibley, C.G. & Ahlquist, J.E 1990).

The raptors classifications are generally distinguished by physical (morphological) characteristics for instance, some eagles and their feathered "booted" legs, also behavioural features that can be used in the field (Morris, F.T 1976, Weick, F. & L.H, Brown 1980). Because of this early scientists may have associated the two orders closely, as mentioned by Sibley& Ahlquist (1990 p.178) in their adaption of Bairds classification. But with the development of DNA analysis, scientists were able to classify organisms based on not just characteristics but genetics as well. This classification is determined specifically by the closeness and relationship between sequences of genes found along the DNA of organisms (Sibley, G. C. & Ahlquist, J. E. 1990, King, R.C. & Stansfield, W.D. 2006).

Thus leading to the classification of diurnal raptors and owls, looking at Order classification. Diurnal raptors are classified under the Order: Falconiformes, with five families within that order, sharing a common ancestor, whereas owls are classified by the Order: Strigiformes, with two families (Wink, M. and Sauer-Gürth, H., 2004, Brown, L. & Amadon, D. 1968, Christidis, L. & Boles W.E. 2008). Although with DNA analysis, the issue of properly distinguishing the relationships of owls and diurnal raptors was apparent. As mentioned by Christidis & Boles (2008 p.54), scientists formed associations with nightjars and owls,  others suggested that Strigiformes might have a weak relation with Falconiformes based on genetics. Further more with similarities in bone structures of the owl and diurnal raptors, Mayr (2005 p.639) suggested Strigiformes are a sister group to Falconiformes, although without sufficient data as proof for this, it was not conclusive.

Therefore determining an organisms’ relationship with DNA analysis, such as the owls and Diurnal raptors, is definitely something that should be of importance as it could indicate the evolutionary history of that particular organism. Overall determining how they might have evolved specific features and the relationships that they have with other organisms.

Reference:

Books:

• Brown, L. & Amadon, D. 1968, 'cLASSIFICATION AND dISTRIBUTION', in Balding and Mansell (eds), Eagles, Hawks and Falcons of the World, Hamlyn Publishing, Great Britian. pp.17-21
• Christidis, L. & Boles W.E. 2008, 'Higher-level avian systematics', in J  Kelly (eds), Systematics and Taxonomy of Australian Birds, CSIRO, Victoria. pp. 52-54.

• King, R.C. & Stansfield, W.D. 2006, A dictionary of genetics, 7th edn, Oxford University Press, New York. 

• Morris, F.T 1976, Birds of Prey of Australia, Lansdowne Editions, Melbourne. pp.14-23.

• Sibley, G. C. & Ahlquist, J. E. 1990, 'Structure and Properties of DNA', in J J Johnson (eds), Phylogeny and Classification of Birds: A study in Molecular Evolution, Yale University Press, United States of America. pp. 27-29.
• Sibley, G. C. & Ahlquist, J. E. 1990, 'A Chronological Survey of the Classification of Birds',  in J J Johnson (eds), Phylogeny and Classification of Birds: A study in Molecular Evolution, Yale University Press, United States of America. pp.187
• Sibley, G. C. & Ahlquist, J. E. 1990, 'Structure and Properties of DNA', in J J Johnson (eds), Phylogeny and Classification of Birds: A study in Molecular Evolution, Yale University Press, United States of America. pp. 245
• Weick, F. & L.H, Brown 1980, 'Topography of diurnal bird of prey (Heterospizias)', in P Parley (eds), Birds of Prey of the World, Collins, London. pp.12
• Weick, F. & L.H, Brown 1980, 'Key for identifying', in P Parley (eds), Birds of Prey of the World, Collins, London. pp.124

Scientific papers:

• Mayr, G. 2005, "The postcranial osteology and phylogenetic position of the Middle Eocene Messelastur gratulator Peters, 1994—a morphological link between owls (Strigiformes) and falconiform birds?", Journal of Vertebrate Paleontology, vol. 25, no. 3, pp. 639-640.

• Wink, M. and Sauer-Gürth, H., 2004. “Phylogenetic relationships in diurnal raptors based on nucleotide sequences of mitochondrial and nuclear marker genes”, Raptors worldwide. WWGBP, Berlin, pp.483-487.