Geoffrey Beattie: The leaders locked horns, as big beasts do

The party leaders may work hard on their presentation, but the unconscious, animal signals they send are the most revealing

Human beings are social animals that evolved in small social groups. Our brains are adapted, through the processes of evolution, to pick up many of the signals that indicate the position of an individual within the group. Often we pick up these signals unconsciously – and very quickly – but this can have a profound effect upon our social judgements. In the animal kingdom, the leader is open to (sometimes violent ) challenge. And the rest of us gaze on, watching the battle, drawn to the emerging victor in ways that we barely understand. This is what made the leadership debates so fascinating; it was the sheer primitiveness of the whole thing. Evolution meets technology here in our living rooms.

We have all seen Cameron taking on Brown at the dispatch box during Prime Minister's Questions, but this often has the feeling of political theatre, or panto, with booing, jeering and so on. But the televised leaders' debates were always going to be different. It was stags locking horns in a battle to the political death, with the rest of the herd deathly quiet.

Brown had the square jaw, and the low furrowed brow, in other words, with some of the right physiognomic attributes for the role of dominant male, regardless of how attractive we find these features. Then, there was Cameron and Clegg, much more boyish looking, not such square jaws, but not weak either, higher eyebrows, much weaker primitive signals when it comes to being dominant.

Their wives were paraded before us so that we could see how desirable the three candidates were in crude sociobiological terms. The consensus was that Clegg's wife was the most attractive with thick luxuriant hair. All three candidates stood at the podium and all were careful to straddle the ground, taking up considerable room, like alpha males saying: "I will not be shifted, I will not be knocked over, I will emerge triumphant from this battle."

And then they started talking. We all knew that Brown had a deep, powerful voice that makes him sound like a dominant leader. Cameron's movements were more vigorous, there was a lot of symbolic punching and prodding and gripping and chopping. When Cameron spoke, Clegg did not just glance his way, he gazed at him and we could decode this primitive signal for what it is. Subordinates gaze more at high dominance individuals than the converse. So it looked, in the first debate, as if there might be a clear pattern in the positioning of the three individuals in the dominance hierarchy.

But then Clegg did something remarkable, something unthinkable in the animal kingdom, he stopped trying to lock antlers. He withdrew from this particular contest and instead worked on building a relationship with the audience. He literally and symbolically stepped to one side and gestured towards the other two so that he could be a spectator, like the audience itself, on Brown and Cameron going through with their political battle. He attacked Brown, but before Brown could reply Clegg put his hands in his pocket as if to signal: "You can't attack me now, you're too late, old man." Clegg positioned himself beautifully: he wasn't like them, he was more like us, this was the future, the end of two party politics, the end of chest-thumping and dominance displays, the end of Tory and Labour.

But that was then. The second debate was always going to be different, and not just because Clegg had been at the centre of sustained media attacks in the intervening week. He was no longer the new kid on the block; he was now a serious challenger and had to be confronted head on. Critically, he was also positioned in the middle of the three and could no longer symbolically step to one side, as he had done so effectively in the first debate. Brown was now more serious, explicitly so. "If it's all about style, count me out," he said in his opening statement.

In the first debate, Brown had made a number of overtures towards the Liberal Democrats and had been rebuffed by Clegg, and every time he was rebuffed, that false, masking smile that Brown sometimes likes to wear, faded abruptly, exposing the negative emotion underneath (a blend of apprehension and uncertainty). But this time, the masking smiles were reduced. Now, Clegg was engaging in the dominance battle, he interrupted Cameron by overlapping the end of his turn. This is the form of interruption that most clearly signals dominance in humans. And in case we fail to understand that this was a dominance display rooted in evolution and sociobiology and unconsciously signalled through nonverbal communication, Clegg brought the unconscious into the conscious. When discussing Europe he said that this is one area where "size does matter". This was now a swinging-dicks contest, explicitly as well as implicitly.

Clegg was now locking horns with Cameron. He tried to interrupt Cameron but failed, and then Brown came out with a master stroke. It was his turn to step aside and redefine what was happening. Commenting on Cameron and Clegg arguing in this way, he said: "They remind me of my two young children squabbling at bath time." It reminded us all, if we needed it, that human beings may be social primates with many core processes rooted in basic biology, but we also uniquely have language to allow us to position ourselves in the social hierarchy. So when Cameron told us that he had been to Afghanistan four times and that he had been for a run that very morning with someone who had served in Afghanistan, I couldn't stop thinking about the self presentational tactics of young adults desperate to impress.

At one point, Cameron lost his temper at "Brown's lies". We like to see some emotion from our politicians but we also like to see them in control. Clegg's forehead was becoming a little shinier. Clegg fired a whole series of questions at Cameron, so called machine-gun questions, a clear attempt at dominance. Cameron side-stepped this attack with a joke and said it was like a replay of last week.

This time there was no clear winner. If you watched the nuance of dominance displays as I have to do, then I thought that Brown had it because he has a natural authority. He wasn't trying in the desperate way that he did in the first debate, and he emerged like someone who knew and understood the challenges ahead. Many viewers, however, would have been impressed with Cameron's performance, his relative youth and vigour, his energy for change (signalled both through his body language and his references to his trips to Afghanistan, his running etc). Nick Clegg locked horns in the second debate and didn't do nearly as well.

Human beings are rational creatures and we want to elect the party with the best policies and the best ideas. But we also want a prime minister who can lead us and who can reassure us in the battles that we will have to fight for our economic, social, and political survival. There was, however, no obvious victor.

The stags are resting now. They need one final push.

Professor Geoffrey Beattie is Head of School and Dean of Psychological Sciences at the University of Manchester