Immunology is traditionally thought of as a molecular pursuit. It is, after all, dependent upon binding of antigen molecules to a variety of different receptors (immunoglobulins, T cell receptors, Tolls, MHC, etc). And, of course, it deals with a host’s ability to defend itself from microbial challenges. However, increasingly, immune function is being viewed in an ecological sense.
Why is this unique? It’s rare for ecology to have this kind of fine resolution. Generally speaking, interactions between macro-organisms, especially metazoans, are only examined in as much detail as genomic/phylogenic studies allow. This certainly gives relevant information, but it is more evolutionary than functional. However, as our understanding of science increases, so does the ability to interconnect the disparate worlds of molecular and environmental biology. Organisms, of course, exist not just as a collection of functional proteins and nucleic acid code, but as whole creatures relating with their surroundings. In a sense, this new research allows science to ask bigger “whys” and “hows”.
The immune system, in particular, is a very personalized concept in that there are far fewer and more abstract relationships between it and the ecological environment. In contrast, other molecular biosciences, for example developmental bio or endocrinology, are very readily linked to external cues.
So instead of just considering immunology in a few specialized laboratory cases, one can look at the bigger picture. But how does one go about doing that? While there are numerous avenues of this new field being explored, let us take a look at one of the least concrete ecological concepts (mate choice) and tie that in to an immunology framework.
Intersexual selection is where organisms of one sex actively choose mates based on the strength on sexual displays. This prevents taxing and potentially damaging confrontation between members of the same sex. Nowhere is this concept more familiar than it is in the bird world. Indeed, some of the features most attributed to birds are their sexual displays/activities—complex songs, elaborate plumage, nest building, and brooding behaviors.
This being a courtship issue, it is appropriate to look at this through hormones. Specifically, testosterone. There is a long precedence of experimental immunosuppression in birds when testosterone is overexpressed. In general, the antibody response is curtailed, with some data showing an increase in certain lymphocyte subpopulations such as killer (CD8+) T cells.
Now we ask, as one often does in ecology, “why is this so?”. Testosterone increases risk taking ability and aggression, and other stereotyped masculine behavior. During the competition for mates, a male must be in top functional form. While the immune system is used to clear infective agents, inflammation and other immunogenic effects greatly curtail these competitive abilities. Thus, it is only after brooding occurs, when mates are secured, and chicks are not yet hatched, do testosterone levels go down and immune function returns, full force.
What’s more, carotenoids, pigments used in sexual displays and not synthesized de novo by birds have been shown to boost the immune system. Increased testosterone works to increase foraging aggression, giving these males the most access to the pigments. This both strengthens their sexual displays and helps immunity.
Drastic effects of carotenoid limitation in Carpodacus mexicanus (J Exp Biol).
To further explore this concept, we might further look at parasite effects on immunosuppressed males, androgen effects on females and young, oestrogen mediated immunity, immune restructuring under androgen presence, and cost benefit analyses of immunosuppression and mate desirability.
Is this a correct analysis of testosterone immunosuppression? Maybe, maybe not. But it does give an overview of what ecological immunology entails. The problem here is that many of these concepts can be difficult to test, but the scope of such an analysis is great, and it gives a whole new perspective to biology.