It is well known that males and females differ in size, appearance, and behaviour. However, several aspects of this phenomenon, known as sexual dimorphism, remain unresolved. For example, it is not quite clear how environmental conditions influence the degree of the difference between female and male behaviour.
UT Professor Raivo Mänd, Head of the Bird Ecology Research Group, his students, and colleagues Elo Rasmann and Marko Mägi ran a behaviour experiment with the pied flycatcher. They observed how both parents took care of their nestlings.
Young pied flycatcher in a nesting box. Image credit: Miika Silfverberg / Wikimedia Commons.
Professor Raivo Mänd and Marko Mägi in the field. Photo from personal archive.
In normal feeding conditions, female parents provided nestlings with relatively more food collected from the tree canopy (caterpillars), whereas males brought more food caught in flight (adult lepidopterans). The food provisioned by females was more filling and nutritious compared to that brought by males. Male parents tended to keep better pieces for themselves and spent more time in flight, overseeing the surroundings and demonstrating their presence.
The bird ecologists then imitated a temporary worsening of environmental conditions by experimentally increasing the hunger level of nestlings. While female behaviour remained unchanged in this situation, male parents changed their ways. In response to environmental stress, males started to provision more valuable food, so that the sex difference in the parents’ provisioning behaviour diminished.
Doctoral student Elo Rasmann checking the nesting box. Photo from a personal archive.
The results of this experiment are in accordance with the general pattern previously found for sex differences in animal size and certain other traits; namely, that sex differences tend to diminish in harsh environmental conditions. For example, in some species, environmental stress tends to affect the larger sex disproportionately and mute costly secondary sexual characteristics (like male birds displaying their plumage).
It is possible that the decrease in sex differences in harsh conditions represents a more general pattern than previously assumed. Some years ago, UT psychologists Anu Realo and Jüri Allik participated in a cross-cultural study with a rather sensational outcome; namely, it turned out that personality differences between men and women are smaller in poorer societies like those in India or Zimbabwe than in the more prosperous societies of Western industrial countries. It is not so much the women, but men’s personality that changes. Men in traditional agricultural societies and poorer countries seem more cautious and anxious, less assertive, and less competitive than men in the most progressive and affluent countries of Europe and North America.
Researchers explain this somewhat counter-intuitive result by turning to animal ecology and studies similar to the experiment with the pied flycatcher. It seems that similarly to birds, environmental and social stress mute biological sex differences in humans. This, for example, explains the relatively small difference in height between women and men in poorer countries where malnutrition and disease are serious problems. This also explains why both parents put great effort into raising their offspring when resources are scarce.
Professor Mänd reasons: “Why is gender inequality such an issue nowadays, and why are women sad about men not willing to contribute or avoiding their parental responsibilities? The obvious reason lies in the high level of well-being in our Western society. Men feel that women can manage to raise children perfectly well, particularly with all the social aid available”.
Raivo Mänd, Elo Rasmann and Marko Mägi (2013). When a male changes his ways: sex differences in feeding behavior in the pied flycatcher Behavioral Ecology, 24 (4), 853-858 DOI: 10.1093/beheco/art025
