Are birds faithful to their sexual partners? This question has long fascinated researchers and nature lovers. New light on that complex issue may be shed by research conducted by Dr hab. Aneta Arct from the JU Institute of Environmental Sciences, who studied the relationship between environmental conditions and frequency of extra-pair copulation among birds within the framework of a project funded by the National Science Centre.
Why are bird females unfaithful? Extra-pair copulation, that is, female’s copulation with a partner other than her social mate is a breeding strategy very common among birds. Although birds, like humans, form monogamous couples, molecular biology methods indicate that extra-pair offspring is found in over 90 percent of bird species.
In nature, every type of behaviour brings about certain costs and benefits. Hence, it is legitimate to ask a question what benefits are gained by females by cheating on their partners? The costs are obvious – by cheating, the female risks losing the partner and his indispensable help in raising offspring. Besides, extra-pair copulations can also increase the risk of contracting diseases and require additional time and effort which are worth their weight in gold during the short breeding season.
One of the hypotheses, known as the ‘bet-hedging hypothesis’, assumes that females can engage in extra-pair copulations in order to diversify the genes of their offspring and improve the chances of their survival in uncertain environmental conditions. In that way, at least some of the offspring can have genes that may prove useful under the changing conditions and are not in possession of their mother’s social mate. In addition, the offspring resulting from extra-pair copulations can be of better ‘quality’ and hence better suited to unfavourable environmental conditions. A number of studies, including those conducted by Dr hab. Aneta Arct and published in the journal Behavioural Ecology and Sociobiology, confirm that the offspring resulting from extra-partner copulation is better adapted to the environment and, consequently, has better chances of survival and reproduction. The females can also cheat on their partners to avoid inbreeding, that is, mating of individuals closely related genetically, which may lead to a number of negative consequences for the offspring. The results of meta-analytic studies presented by Dr hab. Aneta Arct’s team in Behavioral Ecology journal in 2015 indicated that those bird females who mated with males more genetically similar to themselves more often had extra-pair offspring.
The hypothesis that in unfavourable circumstances, such as draught, food shortage or extreme temperatures, the females may look for genes from an extra-pair partner, which could increase their offspring’s chances of survival, was tested by synthesising the results of research papers published to date.
Based on the analysis of metadata from 240 studies on 165 bird species, it was indicated that environmental conditions (such as climate conditions, geographic location, or primary production) have an impact on the probability of birds’ involvement in extra-pair copulations. This significant part of the project provided some insight into ecological processes regulating different patterns of extra-pair partner choice.
As part of the project, Dr hab. Aneta Arct’s team has also analysed the available weather data and compared them to long-term data documenting the natural population of blue tit (Cyanistes caeruleus) nesting on the Swedish Island of Gotland. This ongoing research is carried out by Dr hab. Aneta Arct in collaboration with Prof. Mariusz Cichoń and Dr. hab. Szymon Drobniak from the JU Institute of Environmental Sciences and Prof. Lars Gustafson from Uppsala University.
The results of the joint research, recently published in Behavioral Ecology, show that blue tit females more often have extra-pair offspring when they experience large fluctuations of air temperature.
The blue tit is a common bird popular with researchers, which frequently visits gardens and bird feeders. Tits, like humans, form monogamous relationships, however, as indicated by Dr hab. Aneta Arct’s studies, they also lead a secret life, which may be a strategy of coping with changing environmental conditions.
The research conducted as part of the project shed new life on birds’ complex reproductive strategies, which can result from their adaptation to environmental conditions. The results of the project can contribute to solving one of the greatest evolutionary mysteries, consisting in answering the question why females so often engage in extra-pair mating. The studies can also shed new light on the evolution of sexual behaviour of humans and other animal species, also contributing to the better understanding of limitations to phenotypic plasticity, which is key to maintaining biodiversity.
