Issue: January/February 2004
Research in Action
Birds, Bugs & Sex Appeal
Studies into viruses and their avian hosts
What do the sudden SARS scare, the rise in numbers contracting new strains of avian influenza viruses and more West Nile encephalitis cases mean?
According to La Trobe University Mildura ecologist and ornithologist, Dr Aldo Poiani, they mean that micro-organisms are making a comeback, with a vengeance.
'And one thing that viruses causing these diseases have in common is that they find a natural reservoir in wild birds,' says Dr Poiani.
Dr Poiani combines his areas of expertise to study host-parasite relationships in birds from both a theoretical-evolutionary perspective and applied perspectives such as medical, veterinary and conservation.
He has studied relationships between parasites and their avian hosts since he was a PhD student in La Trobe's Department of Zoology in the late eighties because parasites can be a major factor shaping many aspects of the physiology, behaviour and ecology of birds.
Historically, parasitism was seen as a major cost of sociality in birds, as group living - such as that of colonial and cooperative breeding species - was thought to increase the chance of harbouring contagious ectoparasites such as bird lice and mites.
However, Dr Poiani's work on Australian co-operatively breeding birds (those that breed in groups rather than in pairs) suggest that contagious ectoparasites may not be more common in social hosts when other ecological variables are considered. This is because hosts are not passive victims of their parasites, as they develop mechanisms to keep them at bay. Interest in these anti-parasitic defences, has led Dr Poiani to study immunology and endocrinology of birds.
He says recent theoretical developments in the field of sexual selection suggest a central role of parasites and parasite avoidance in the process of mate choice and evolution of mating systems. Bright plumage and secondary sexual structures such as combs and wattles in males can be used by females to assess the health status of their potential partners.
If the male does not help to rear chicks - as it is the case in many bird species - the female tends to be choosy about the father of her offspring. Female choosiness selects for external evidence of health in males like bright plumage and elaborated displays.
But how is the female going to be sure that her partner is not a cheat? Is he really as good as he seems to be, or is he a fake? Some recent studies have suggested that conspicuous male secondary sexual traits may be developed only by males in good body condition.
Producing and carrying those structures may be energetically costly and the hormones that control the expression of those traits may suppress the immune system, thus making the male more susceptible to parasitic infections.
In collaboration with Dr Matthew Evans of the University of Stirling, Scotland, Dr Poiani contributed to the understanding of the complex relationship among secondary sexual traits, immunity and hormones by carrying out experimental tests of the well known 'Immunocompetence Handicap Model'.
They suggested that several hormones that may contribute to the darkening of male house sparrow plumage - which may be attractive to females - can be either immuno-suppressive (corticosterone) or immuno-enhancing (testosterone, thyroxine), thus throwing doubt on the theory that conspicuous secondary sexual traits are always associated with immune suppression.
More recently, Dr Poiani has become one of the most active ornithologists studying the effect of sexually transmitted diseases on mating systems and mate choice in birds. He has found that polygamous Australian birds are more likely to harbour sexually transmitted bacteria and yeast such as Chlamydia and Candida albicans than monogamous species. He also found that more attractive male birds may be more infected with sexually transmissible parasites - a cost of being sexy!
Dr Poiani says conservation programs should consider the role of parasites in reproduction and survival of endangered host species. His recent study of chlamydia infection of birds in the South-eastern suburbs of Melbourne, has suggested that the bacterium may be in decline in wild bird populations.
This may be good news for the Superb Lyrebirds of the Dandenong Ranges National Park, which have been found to be prevalently chlamydia-negative in spite of some early concerns.
Dr Poiani is planning to develop a strong research program on theoretical and applied issues in bird ecology at the Mildura Campus which he joined recently.
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