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.
It’s interesting to try and understand why females are larger in males in these species. Has anyone considered the energetic costs of producing eggs as a possible reason for larger females? Particularly with raptors, where siblicide is common, greater investment and hence bigger body size, could be beneficial.
ReplyDelete