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Telling someone, "Animals cheat just like people" is unlikely to cause uproar. In fact we can all point to little cons in the animal kingdom, to frauds that cleverly manipulate other animals, not just those from their own species, in order to achieve their own goals. However, if we add that we mean marital infidelity, most people will certainly look intrigued, expecting more details.

It's true – cuckoldry occurs among animals and one might even say that cheating is quite common! While watching a pair of birds known as "great tits", where the male continuously supports his female partner who is incubating eggs, one might think: "This is most certainly an example of faithfulness." Nothing could be more wrong. Scientists have been gathering evidence of the existence of marital infidelity in numerous groups of animals for years. Evolutionary biologists from the Institute of Environmental Sciences of the Jagiellonian University have also contributed to those studies. The Group of Population Ecology not only looks for examples of infidelity in birds; first of all the group tries to find the answer to the question of why it happens.

Disloyalty – risky or beneficial?

Without any doubts, infidelity can be costly. In birds, it is the female who looks for additional partners, but by doing so she risks being abandoned and having to raise the offspring on her own as soon as the actual partner finds out that he has been cheated on. Moreover, the extra partners will not support the abandoned female in raising offspring "out of wedlock," either. What is interesting is the fact that the extent of such infidelity can often be shocking! Females of Polish bird species – such as tits, flycatchrs or warblers – can have 10–40% of their offspring sired by additional male partners. However, there are species such as the American tree swallow, where whole nests (i.e., 100% of the offspring!) may originate from extra-pair mating. Such female obviously takes a significant risk when she decides to be unfaithful to such an extent. If it's so risky, what drives infidelity in birds?

It turns out that potential partners are divided into "better" ones and "worse" ones, particularly in terms of their genetic quality. Genetically better partners can transfer more beneficial genes to their offspring (the "good genes" hypothesis), thus providing the offspring with the ability to survive in adverse environmental conditions, if only through the improvement of physical condition or resistance to pathogens. The female may want to improve the genetic quality of at least some of her offspring, enabling another male to become the father of some of her children instead of her actual partner.

Looking for traces of advantageous genes is not a trivial issue, as it requires the scientists to recreate the "track record of infidelity" in a population of bird families in order to be able to identify all of the "illegitimate" hatchlings and then compare the legitimate offspring to those conceived "out of wedlock" (so-called extra-pair offspring). Such comparisons may point directly to differences in the "quality" of extra-pair offspring as compared to those that were conceived by "faithful couples." In spite of the considerable effort required, research on infidelity in birds and the "good genes" that control it is carried out by a large group of scientists throughout the world.

The question concerning the contribution of "good genes" to infidelity in animals has become one of the biggest puzzles of evolutionary biology, mainly due to the evasiveness of these genes and the difficulty in precisely measuring their effects. The Institute of Environmental Sciences of the Jagiellonian University is also taking steps towards solving this issue by conducting intensive studies.

Eeny, meeny, miny, moe – and your Dad is…

"The methods used in our studies are quite similar to the operations of a forensic laboratory. Analyzing a large wild population of the blue tit on the Swedish island of Gotland allows us to obtain information about infidelity in hundreds of birds' nests. It all starts with collecting evidence of cuckoldry, which is done using a small amount of blood collected from all analyzed birds: parents and their offspring," explained Szymon Drobniak, PhD, from the Population Ecology Group of the Jagiellonian University. Blood samples are then used as a source of DNA, which, in turn, through specifically selected genes, enables us to precisely identify the real fathers of each hatchling in the given population. The family tree obtained in this manner covers more than eight hundred juvenile birds and four hundred adults each year. It proves that over 1/3 of analyzed bird families have at least one hatchling that is the fruit of infidelity. Each young bird in the population is also thoroughly measured and the traits connected with size or immunological resistance and condition become the most important. And, although searching for "good genes" is so tedious that it resembles looking for a needle in a haystack, it seems likely that we have succeeded in detecting traces of their activity.

Male blue tit caught at the nest for the purposes of collecting genetic material.
Photo: S. Drobniak

Combination of genes and environment

The potential effects of "good genes" have been manifested most visibly in the so-called immunological response of hatchlings, which indicates how well the given individual copes with infections. It was proven that, as the scientists expected, extra-pair offspring had a stronger immunological response than those whose father was the actual partner of the female. This means that the blue tit females, when choosing additional partners, may "enhance" the immunological response in some of her offspring, which will provide them with better ability to fight natural infections. This is quite an important piece of information, considering the fact that such individuals have a higher chance of survival. So, have we succeeded in detecting the "good gene" effect that would provide extra-pair offspring with an advantage?

It turns out that the answer was not that obvious. Indeed, hatchlings from infidelity had higher immunity but only in those nests where development conditions were adverse or stressing (e.g., as a result of limited amount of food or strong competition between hatchlings in the nest). Thus, it seems that "good genes" are not a simple mechanism that always influences the individuals in the same way. The outcome of their activity also depends on the external environment. Such co-operation of "good genes" and the environment is not a new idea. Not all scientists who took up research on infidelity in birds succeeded in proving the advantage of "illegitimate" hatchlings over the rest of the offspring. This has been explained by the fact that the environment itself "jams" the effects of "good genes." "Our research proved directly for the first time that the environment may decide whether "good genes" will actually be ‘good,'" Drobniak concluded.

More questions to come…

"Our research is only at the beginning of its path. We continue to ask ourselves the questions: What exactly are ‘good genes?' Why are some versions of ‘good genes' good and others bad? How do genes ‘sense' the environment in which their carriers live? The research carried out in the Institute of Environmental Sciences of the Jagiellonian University is slowly leading us towards addressing these questions," Drobniak explained. One thing is without a doubt – these studies have a chance to contribute not only to the development of science but also to livestock farming, for example in the breeding of animals, where "good genes" may constitute ideal goals of effective selection of desirable traits.

This research is also a step toward a change in the perspective of evolutionary biology. It should be remembered that evolution is not only about fossils or our prehistoric ancestors. Evolutionary biology is a living field of science, which answers the most current questions in biology, while at the same time being applicable to medicine, agriculture or genetics. The research conducted by the Population Ecology Group has already reached a wide group of recipients. "If our work gains new advocates for evolutionary biology, we will be even more proud, as such achievement is equally important as breakthrough publications in scientific journals," Drobniak concluded.

Research team: Professor Mariusz Cichoń – Team Manager; Szymon Drobniak, PhD; Aneta Arct, PhD