Evolution stinks

An interesting evolutionary competition takes place between animals that feed on detritus and bacteria and fungi that feed on the same source * Recently a researcher from the University of Manchester tested a hypothesis on the subject from the XNUMXs and confirmed this conclusion

When we talk about natural selection and evolution, we often talk about competition between individuals of the same species or related species - the deer that can run faster than its members, the tiger with the sharpest senses, the snow fox with the longest fur - will survive and thrive at the expense of their relatives with less successful genes .

But animals compete not only with members of their own species, but also with creatures very different from them - sometimes so different that they are not animals at all. The American ecologist Daniel Jansen already realized in the 70s that there is competition between animals that feed on detritus and bacteria and fungi, which also feed on the same source. The competition between the carrion-eating animals and themselves is quite visible - whether they are birds, such as the goat, which developed sharp vision in order to identify and reach the carcass first, or the more direct competition between jackals, hyenas and other mammals for control of the food source.

Jensen suggested that the competition between the bacteria and the animals is done using other weapons: the bacteria produce compounds that cause the meat to rot, so that it becomes inedible and even toxic to the animals. The smell of decay, which also comes from compounds produced by the bacteria, is meant to make it clear to the scavengers that this carcass is already "occupied": the bacteria have taken hold of it. But if this be the case, what are the weapons of the animals in this contest? What do they do to ensure that the bacteria do not beat them in the race? The answer is that they usually do nothing, except to avoid a carcass whose smell indicates that it is occupied by the bacteria.

The reason for this is that animals can go and look for another food source, but if the bacteria lose the carcass they are in they will die - so during evolution, the "motive" of the bacteria to develop tools to win this race was much stronger than that of the animals. That is, of most animals - but there is at least one animal whose ability to reproduce depends on finding a suitable carcass, and therefore its motivation to fight bacteria is stronger. This is the beetle called the "Burying beetle, Nicrophorus vespilloide. The beetle lays its eggs on the carcasses of small mammals, and raises the larvae that hatch from them - feeding them digested food and protecting them from competitors or predators. Dr. Daniel Rozen from the University Manchester in England and other researchers, in an article they recently published in the PNAS magazine, examined Jensen's competition theory - does (and how) the beetle fight bacteria?

As a first step, the researchers checked whether the beetle larvae and the bacteria are indeed in competition - whether the larvae are harmed by the fact that the carcass on which they grow also feeds smaller creatures. For this purpose, they raised some of the larvae on fresh carcasses of mice, and some - on week-old carcasses. The researchers testify in their article that the old carcasses were indeed colonized by bacteria, as they were swollen and "produced very unpleasant smells of decay". It turned out that the larvae raised on these carcasses were smaller - the size of the larva is known to affect its chances of reproduction in the future. In addition, they spent more time begging for food from their mother, indicating poor nutrition, and also left the carcass later than larvae reared on fresh carcasses.

If so, the larvae are indeed damaged by the presence of the bacteria. What do the mothers do to help them? First of all, when given a choice, they prefer to lay their eggs on the fresh carcasses, which had less bacteria. Surprisingly, beetles that were looking for food and not a place to breed were initially attracted to the rotting carcasses: this may be due to different types of bacteria present in the carcass at different stages, and their toxicity to the beetle or larvae. However, when they were about to lay their eggs, they chose the better option for rearing the larvae.

Even when the researchers did not allow the beetles to choose and forced them to lay their eggs on rotting carcasses, they had ways to improve the condition of the larvae. The beetles usually cover the carcass before laying with special secretions, which apparently contain a substance that fights bacteria. It is still not clear what exactly is in this substance, but previous studies have shown that carcasses treated with the substance were more resistant to fungi and bacteria. The researchers found that the maternal treatment is able to compensate, at least to a certain extent, for the rotten condition of the carcass - larvae that grew on a rotten carcass that underwent treatment were larger than those that grew on a rotten carcass that was not treated. If the carcass was fresh, there was no difference between the groups - a fact that strengthens the assumption that the treatment is intended to fight bacteria. This is how the beetles manage to take care of the larvae, even if they find themselves in less than ideal conditions.

The study presents a relatively rare example of antibiosis relationships - relationships between living beings in which each party invests resources and effort not to benefit itself, but to harm the other party. Such relationships can only exist when the resource at the center of the competition - in this case, the carcass - is equally essential to both parties. In addition, the research shows that competition can exist anywhere in the animal world - and that there is no limit to what certain people are willing to do (and smell) for the sake of science.