Understanding colour polymorphism in birds
Many species show a variety of different phenotypes. How such diversity is maintained is one of the main questions in evolutionary biology. Colour poly-morphism represents an ideal system to explore these issues because the different phenotypes represent their genotypes. Researchers have long been fascinated by colour polymorphism because the occurrence of two or more phenotypes in the same population runs counter to the notion that selective pressure should favour the optimal form for an environment, and any lesser quality individuals should be eliminated through natural selection. Colour polymorphism occurs in around 3.5% of bird species, but is more common in raptors and particularly within Accipiter hawks where about 25% of species are polymorphic.
Colour morphs influence the performance and fitness of individuals through direct effects of pigment production (e.g. camouflage or thermoregulation) and indirect effects (e.g. physiological and behavioural traits). Hence, it is unlikely that different morphs are distributed randomly in the environment. Our focus species is the Black Sparrowhawk Accipiter melanoleucus, which occurs as either a dark or a light adult morph. The morph distribution of this species across South Africa follows a cline associated with rainfall seasonality and the intensity of solar radiation, with a far higher proportion of dark morphs in the Western Cape than the rest of the country. Our research has shown that provisioning rates differ depending on ambient light levels, with dark morphs providing more food to their chicks in duller light conditions than the light morph, possibly due to improved background crypsis. Thus, when the two morphs breed together in a ‘mixed’ pair, they might complement each other by expanding the conditions (daylight hour, prey or habitat types) in which parents can forage successfully. Indeed, pairs consisting of mixed-morph parents produce more offspring than parents of the same morph, and their offspring show higher survival rates. The idea that pairs consisting of the two morphs may complement each other has been termed the complementarity hypothesis.
We have extended the project from correlative studies to conducting field experiments and behavioural tests under laboratory conditions to understand the mechanisms underpinning the fitness advantages for breeding with a partner of the opposite morph. We are also developing an Integral Projection Model (IPM) to understand the different components of the system.
Additional research has focused on the genetics behind plumage polymorphism. In many avian and non-avian species discrete colour polymorphisms have been linked to the coding region of MC1R, a gene involved in the regulation of melanin production. However, we have established that this is not the case for Black Sparrowhawks. Several other potential candidate genes also have been excluded. Our research suggests that the differences in melanin production characteristic of the two morphs in this species is associated with different regulation of key genes involved in melanin production. Interestingly, variation in pheomelanic plumage in juveniles appears to be regulated by differences in expression of the same genes. We are now testing for differences in telomere dynamics between the two adult morphs to determine if there are any pleiotropic effects of morph. We also have developed an assay to quantify haemosporidian blood parasite prevalence and intensity from blood samples using qPCR, which we are using to supplement microscopy data to assess morph specific differences in blood parasite prevalence and intensity in the species.
Activities in 2019
- We performed an immune challenge on 23 Black Sparrowhawk chicks from 15 nests to understand the strength of the innate immune function in relation to parental morphs.
- We initiated a collaboration with Dr Dylan Childs (U. Sheffield) using Integral Projection Models. Dr Childs visited UCT in September, funded by UCT’s Visiting Scholars Fund and a Royal Society International Exchange grant.
- PhD student Carina Nebel visited Lund University to analyse blood samples. Data on the innate immune system and oxidative stress levels will be related to parental morph combinations and urban stressors.
- PhD student Ed Rodseth quantified the expression of six candidate genes in developing feather tissue from adult and juvenile Black Sparrowhawks to characterise different regulation genes involved in melanin production. Ed measured melanin levels using HPLC and linked differences in melanin production to expression of the candidate genes.
- We measured relative telomere length in adults using qPCR to determine if we can detect pleitropic effects of morph and developed a qPCR based assay to quantify haemosporidian prevalence and intensity from adult blood samples.
- Dr Chima Nwaogu joined the research group in March. Part of his research will explore timing of Black Sparrowhawk breeding in relation to weather parameters and their colour morph.
- In collaboration with Dr Chevonne Reynolds (Wits) and Dr Chris Briggs (Hamilton College, USA), a paper was published in the Biological Journal of the Linnean Society on colour polymorphism in Swainson’s Hawk Buteo swainsoni.
- In collaboration with Dr Alexandre Roulin and colleagues from Lausanne University, we published a paper in Nature Ecology & Evolution on Barn Owl Tyto alba hunting success under different phases of the Lunar cycle.
- Carina Nebel published her first PhD chapter as a paper in Royal Society Open Science in which she tested whether prey reaction times differed under varying environmental conditions and in relation to predator morph.
- In collaboration with Dr Josef Harl (Austria), a paper was published in Parasitology Research on blood parasites of Feral Pigeons Columba livia, which explored how the degree of melanism influences blood parasite intensity and prevalence.
- Petra Sumasgutner and Carina Nebel presented research from the project at the European Ornithologists’ Union (EOU) in August 2019 in Cluj, Romania.
Impact of the project
This project will add to our understanding and theory about the maintenance of genetic diversity in populations. It provided the first empirical evidence for the light level hypothesis for the maintenance of colour polymorphism in birds that is now experimentally tested.
DST-NRF CoE grant, UCT Visiting Scholars Fund, Royal Society International Exchange Grant.
Research team 2019
Dr Arjun Amar (FIAO, UCT)
Dr Petra Sumasgutner (FIAO, UCT)
Dr Chima Nwaogu (FIAO, UCT)
Dr Rob Ingle (MCB, UCT)
Dr Dylan Childs (University of Sheffield)
Dr Arne Hegemann (MEEL, Lund University)
Dr Gareth Tate (EWT)
Dr Jacqui Bishop (Biological Sciences, UCT)
Dr Chevonne Reynolds (Wits)
Dr Chris Briggs (Hamilton College, USA)
Students: Carina Nebel (PhD, UCT); Edmund Rodseth (PhD, MCB, UCT); Adrien Pajot (M.Eng, U. Bordeaux).
Research Assistants: Dr Shane McPherson, Burghen Siebert
Volunteers: Ann Koeslag, Margaret MacIver, Antje and Bernard Madden