You can't have everything
What don't human and animal men do to convince the dream woman? They rely on horn and feather jewelry or powerful vocal chords, on muscle packs or self-written poems - they all want to show off in their own way. But beetles show that investments in invisible places are sometimes worthwhile.
Whether in Austria's Donau-Auen, the Berchtesgadener Land National Park or in the forests of Mecklenburg-Western Pomerania - where there are still red deer, you can now hear their characteristic roaring during the rut. Ten or twelve bucks draw attention to themselves loudly, vie for the favor of the females or try to beat rivals out of the field acoustically in advance. If they are not willing, a fight ensues – antlers crash into each other and get caught, competitors try to push each other out of the ring. Finally, the victor beckons with a harem and unlimited procreative joy.
As on a large scale, this spectacle also works on a small scale - for example in the kingdom of the stag, fork or rhinoceros beetle. They also wear jagged headdresses to ward off challengers and to beguile the lovely femininity. However, their veritable protuberances as secondary sexual characteristics come at a high price, as research by Leigh Simmons of the University of Western Australia at Crawley and Douglas Emlen of the University of Montana in Missoula now shows.
Because, as in many animal species, competition among males continues after copulation: then as a proxy fight for the released sperm if the females mate more than once. The horned reproductive partners are therefore, at least theoretically, faced with the dilemma of investing more energy in the quality of their seed material in order to ultimately come out on top in this race – or opting for their optical trump card in order to copulate first with the wife and thereby to give her sperm a head start.
The two researchers used dung beetles of the genus Onthophagus to test whether and how the ability to present and the actual fertility of the Kerfe influence each other. Often only the largest and strongest males of each species carry various horns, with which they first want to capture the females and then block the entrances to their breeding burrows in the ground. As is to be expected, these alpha animals produce significantly more offspring than their submissive relatives. All good then?
Not necessarily, because the less equipped conspecifics don't go away completely empty-handed either. Again and again, despite everything, they succeed in mating with a guarded female and producing offspring. The reason: In larger than average testicles, they produce increased amounts of sperm, which literally flood the competing material in the partners - a strategy that is clearly related to the missing headdress.
While Simmons and Emlen prevented the horn growth of young Onthophagus nigriventris, these manipulated beetles ultimately compensated for their lack of ability to present themselves with increased body growth and, above all, enlarged testicles compared to the normally developed companions. In the species Onthophagus taurus, on the other hand, a manipulative reduction of the testicles led to an increased development of the top-heavy ornamentation.
The beetles can thus compensate in some way for physical disadvantages of the primary or secondary sex characteristics by special promotion of the other area - a process that is very likely hormonally controlled. But how can all this be brought into an evolutionary context? Why doesn't a line prevail sooner or later?
In principle, the development of the species encourages the most magnificent males, who appear to be the he althiest and most assertive to the outside world. If, however, the development of body decorations is paralyzed for a specific reason - for example due to a food shortage or external hormonal influences - this could endanger the survival of the species: Because if only a few existing alpha nuclei then proceed to mate, a large part of the genetic potential would be wasted and devastating late effects such as an increased risk of extinction would be more likely.
By the way, the research of the two biologists brought further interesting insights. The beetles with the largest antlers on their heads also had the smallest eyes, and those with the largest spines on their carapace had the shortest wings. Possible analogies in humans - such as the connection between car spoiler volume or private library size, courtship behavior and partner choice - are of course out of the question.
