Behavioural Biology: Scare off

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Behavioural Biology: Scare off
Behavioural Biology: Scare off

Fright ease off

Freshly mated male locusts emit the pheromonal command: "You shall not covet your neighbor's wife!" - Does it work?


Excess males? Oh dear - this can lead to conflicts as we know. Desert locusts should chirp a song about it: In a swarm of the species Schistocerca gregaria, no matter how large it may be, there are usually more males than females willing to mate, because only the currently fertile females let males approach them.

After a successful tête-à-tête, a locust man guards his conquest for quite a while after copulation – nine hours on average. And not out of a protective instinct, but because it wants to ensure that all the eggs of its chosen ones are fertilized by itself and its own genes in the form of offspring mix as successfully as possible among the people.

In order to make their claim to ownership clear, the males initially rely on a very subtle, non-violent strategy: If a possible rival approaches, they greet him with a pheromone cloud made of phenylacetonitrile (PAN). The message is clear: "Fingers or feelers away! Already taken! Pull away!" PAN signals a willingness to defend if the rival doesn't flee the desert dust and tries his luck anyway.

Karsten Seidelmann from the University of Halle was interested in how reliable this signal actually works. To aggravate the situation, he separated groups of ten males from female locusts and let a few of them pine for females for different lengths of time. Then he exposed them to the atmosphere of a romantic evening for couples, in which they only met alert, PAN-secreting sex comrades who vehemently guarded their six-legged friends, or rather, secured their offspring.

In the case of a very short separation of less than an hour, the PAN had the desired effect and the single males respected the idyll of the other couple that was signaled in this way. After being deprived of love for a day - in a locust's life already a small eternity - the pheromonal warning was of little interest: almost half of the love-hungry males forgot their good manners and tried to stir up the cozy togetherness. Despite the threat of a fight, the couples attacked, hoping to drive away a lover and replace him.

If the period of their enforced abstinence lasted even two days, over 60 percent of the males found it worth the use of force to conquer a female. A full three days of abstinence made the locusts seem mad. The meanwhile love-mad insects now completely ignored the warning signal. They invariably attempted to rush headlong into an affair with someone else's wife and all went on the offensive. They were also much more impatient and persistent than before in their stormy attempts to get close to the females that had already been taken. They now tried much more often – around nine times – to literally drag their opponent out of the bed of their new beloved.

Despite everything, these desperate attempts at conquest were only seldom crowned with success: On average over all periods of abstinence, the success rate was just a little over five percent, with victory occurring after around 2.4 attacks. Incidentally, the females were unimpressed by the behavior of the machos: they behaved completely passively.

What sounds like passionate love drama here, Seidelmann explains with a simple cost-benefit calculation. The animals compare which behavior will best get them the most numerous offspring: patience and perseverance, or accepting a fight with little chance of success and a female who probably only has a few unfertilized eggs to offer. After long waiting times, which would correspond to long search times in the wilderness, choosing the second strategy can make more sense.

So the pheromone commandment only works to a limited extent: if you press it, on the other hand, it's not just insects that like to cross borders, even when threatened with a beating.

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