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How animals flirt with each other

How do animals choose their partners? The answer is simple: it’s all about quality. While humans tend to wear clothes that happen to be in fashion, animals do nothing without a reason. Behind beautiful plumage or a deafening roar is only one message: I am in great shape. The evolutionary courtship displays of animals are explained by Leiden behavioural biologist Carel ten Cate.


First, the appearance. Why do birds have such bright colours? ‘Only animals that are in excellent health can produce such intense colours. At the same time, they are saying something else with those bright colours. Obviously, such colourful plumage makes them stand out in nature, and that makes them vulnerable. By sporting such bright colours, the animal is saying, “Look at me! I’m so strong that I can afford an extravagant appearance.”’


Let’s move on to muscles. To what extent does strength play a role in the choice of a partner? ‘A very great extent. Think about the belling of red deer in the September mating season. This sound gives information about how large they are. The deer that produce the lowest cries are the largest. The other males know that, too. From the sound of the belling, they can judge whether they can acquire their competitor’s harem.’

Something to say

Now we come to the vocal aspect. Animals croak, roar and sing themselves into a frenzy. Anything to attract attention. ‘Producing sound takes energy. You can only do that if you are in good shape. This is especially true if you can keep up the sound for a very long time, like some frogs, or are able to make a very loud sound, like howler monkeys. The animals are saying: “I am in top condition and am therefore very fit for breeding.”’

‘And then there are the songbirds. The vocal organ of birds can produce different tones, called syllables. Canaries, for example, make a special syllable, known as the sexy syllable. This sound is difficult to produce and some males do this better than others. Basically, they are saying: “Look what I can do. Choose me!” And it works. The females react very strongly to it.’

Female budgies prefer a smart male to a handsome one, discovered Professor of Animal Behaviour Carel ten Cate.
Female budgies prefer a smart male to a handsome one, discovered Professor of Animal Behaviour Carel ten Cate.


We all want a meaningful relationship, but don’t forget about smell, biologists say. ‘Some odours tell us something about the genome involved in recognising virus proteins, the MHC complex. Animals – as well as humans – choose a partner with a slightly different MHC complex than ourselves. This guarantees the greatest possible immunity in the offspring. The odours involved are quite subtle. There are also animal species that emit all kinds of olfactory flags. Cats do this, for example. A territorial male urinates in various places. That’s not just a signal to other males; his urine also contains substances that can tell a female about the quality of his genes.’


Finally, a little brainpower: does that still count in the animal kingdom? That is what Carel ten Cate investigated with budgies. ‘Pure intelligence had never really been looked at as a possible factor in mate choice. It that respect, our experiment was unique. We let female budgies choose between two males. We then taught the less popular male to open a puzzle box that contained food. That took a few days. First, he had to learn to lift a lid a little, then to open a door, and finally to pull open a drawer. We left the popular male artificially “stupid”. As it turned out, when the female had the choice between the same males again, she chose the one she had seen was capable of opening the box.’

Smart scores. But does this also apply to other animals? ‘That’s very likely. Especially in animals that are known to have highly developed cognitive abilities. Among birds, these are the parrots and crows. The same applies to great apes, although that will be more difficult to investigate. And there also are indications that among us humans, smarter partners are preferred over those who are not as smart.’

Text: Nicolline van der Spek

This article was previously published in Leidraad, the Leiden University alumni magazine (in Dutch).

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