How Greenfinches Help Our Understanding of the Immune System

Tuul Sepp

Tuul Sepp. Image credit: Jaak Nilson.

Tuul Sepp is a researcher at the UT Institute of Ecology and Earth Sciences. Her PhD thesis on greenfinches and their immune system won the grand prize at the national competition of students’ academic works in 2012. Last year Tuul also received first prize in the Estonian Science Communication Award.

The majority of all living organisms are parasites – they grow, feed, and are sheltered on or in a different organism (host) while contributing nothing to the survival of their host. Does this sound like an exaggeration? Surely there must be more organisms trying to make their way in this world by ‘honest’ methods? Unfortunately not. Simple logic proves it: All organisms in this world are infected with several parasite species and most parasites are species-specific (feeding only on one host species).

So there are hosts, parasites, parasites of parasites, and so on. Parasites rule this world.

Of course, hosts do not go down without a fight. We have several layers of protection, starting with mechanical barriers (skin, mucus layers) and ending with a complex and highly integrated system of organs, cells, and molecules – the immune system. It has to be big and sophisticated – the variety of parasites is huge and there must be defence mechanisms against all possible intruders. Having this kind of protection is not cheap. It is expensive to maintain, it is expensive to train, and it is expensive to use. Is there a choice not to use it?

Well, we all see what happens when organisms die and their immune system is switched off – there is a parasite feast that leads to decomposition of the body in only a few days. Even if our defence systems are working, it is not easy to control parasites – according to the World Health Organization, almost one in every four people dies of parasitic or infectious diseases.

If parasites are so dangerous, then why don’t we have better protection? After all, natural selection should lead to development of better and better defence mechanisms. Only the fittest and most resistant to parasites should survive and give its good genes to offspring. But still, after some 3.5 billion years of evolution, we routinely get sick and die because of parasites. What is happening?

There are two answers to this question. Firstly, parasites also enjoy the power of natural selection and develop better and better mechanisms for abusing the hosts. And secondly, immune defences truly are costly. If we keep our guardian cells super alert and ready to attack every single intruder, we risk mistaking harmless substances (for example pollen or cat saliva) for infectious agents and develop allergies or autoimmune diseases. If we use toxic and reactive molecules to destroy parasites, we also damage our own cells. The immune reactions might damage us more than the parasite ever would – think of high fever, for example. Immune defences going berserk. Really, sometimes it is better to tolerate the parasite than to eliminate it. After all, it is not in the best interest of the parasite to kill the host: If the host is dead, the parasite cannot spread.

In nature, resources are always limited. The energy and matter we “waste” to fight off diseases is energy and matter we are not able to use for growth or reproduction. So, what does it really cost to use the immune system? What is the optimal level of immune defences? Why does susceptiblity to diseases vary among organisms to a great extent?

The role of parasites and parasite defences in shaping the life-course of organisms is not something the scientific fields of medicine or immunology are focused on. No – these disciplines are only interested in the outcome of a parasite attack or immune response – the questions “how?” and “when?”, but not the question “why?”. To answer this question, we must study the relationships between living organisms and their natural environment, and this falls under the jurisdiction of ecology, giving rise to a new exciting field of science – ecoimmunology. Although this multidisciplinary area is still very young, the amount of research conducted in this field has virtually exploded over the past ten years.

Of course, ecoimmunological studies are rarely conducted on human beings. Firstly, humans have an annoyingly long lifespan for scientists to study, and secondly, there are, of course, all kinds of ethical limitations. For example, we cannot induce immune responses in humans and then see if reproductive success or lifespan is reduced. Fortunately, all animals are somewhat similar. Humans and mice share about 90 per cent of their genome and about 60 per cent of chicken genes correspond to similar human genes. Therefore, when studying ecoimmunological questions on mice, birds, or even insects, we can (cautiously) interpret the results obtained also for humans.


Greenfinch – not a friendly bird, but a good model organism for ecoimmunological studies. Image credit: Jaanis Lodjak.

My work group uses greenfinches as model organisms in studying ecoimmunological questions. What makes greenfinches a good ecoimmunological model species? Firstly, there are plenty of greenfinches wintering in Estonia. They gather in groups of 50, or even up to 100 birds, and sit stoically in treetops. However, there is an easy trick to bring them down – they are crazy about sunflower seeds. Put some down in your garden and you’ll have hundreds of greenfinches feeding there every morning. Then we can catch them with mist-nets and bring them into our aviary.

We keep the birds in the aviary for a few winter months and then release them. They tolerate captivity well – they remain in good condition and show no increase in stress markers (studying this was part of my PhD studies). Their survival in the laboratory is much better than in the wild, where cats, sparrowhawks, and harsh winter conditions decimate their populations. Life in the lab is easy – they have an endless supply of sunflower seeds and protection from predators. Our experiments are not too big of a burden for the birds. Modern methods allow us to study many different indices of condition and immune status from a small blood sample; from just one feather we can analyze stress hormone levels, and determine parasite infection intensity from faeces.

Catching greenfinches with mist-nets

Catching greenfinches with mist-nets. Image credit: Peeter Hõrak.

Secondly, all greenfinches in the wild are chronically infected with intestinal protist parasites – the coccidians. It is quite easy to assess the infection level by counting shredded oocysts from faecal samples – this is the way the parasites plan to spread. It is also possible to manipulate parasite levels by administring anticoccidian medicine or by reinfecting the birds. This way we can investigate what makes an organism more or less resistant to parasites or what it costs to tolerate or control the infection.

For example, we have shown that fighting off coccidians results in greater damage to the birds’ own cell membranes caused by reactive molecules of the immune system. Also, we can study the costs of immune response and its associations with other traits, such as different biochemical indices or plumage ornamentation.

Why are plumage ornaments important? Usually brighter feathers indicate higher quality, whether it be better genes, good health or supreme foraging ability. This is also something we have established in our studies: Birds with brighter plumage are better at fighting off parasites. Only the best individuals have enough resources to maintain good health and grow bright feathers. Therefore, plumage ornaments are used in choice of mate. Obviously, birds who are preferred as mates enjoy a higher number of offspring. So if a bird cannot tolerate or fight off parasites and therefore has no resources to invest in its appearance, it must pay a price in terms of reproductive success. In greenfinches, bright yellow feathers in the tail and wings are used as this kind of quality signal. By studying the color of feathers, we can also make assumptions about the reproductive success of individuals.

Greenfinches eating sunflower seeds.

Greenfinches eating sunflower seeds. Image credit: Peeter Hõrak.

So, will we find the cure for cancer, a vaccine for AIDS, or elixir of immortality by studying greenfinches? Probably not. However, our studies will help to understand the evolution of immune defences. This understanding is crucial for answering questions about variation in parasite tolerance and disease resistance among individuals. And, along the road, we will learn a lot about the immunology and ecology of a wild bird species.

This is why I love multidisciplinary fields of science: If you don’t find answers to questions in one field, you may accidentally stumble upon answers to the questions of another field. We are still in the very early days in the development of ecoimmunology and every day may bring along breakthrough discoveries, possibly with the help of Estonian greenfinches.

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