Parasites usually harm their hosts because they live at its expense. Obviously, this does not always apply, as our native ant species is now being proven. In this species, workers infected with the parasitic tapeworm lived longer than their nesting mates. This may be due to increased care by their peers, as well as an altered metabolic rate and a different fat content.
There is a clear division of roles in ant colonies: only queens breed, are naturally well protected from diseases, for example, and spend almost their entire lives in a safe nest. There they are looked after by the workers, their daughters: they take care of the brood and, in old age, also risky fodder outside the colony. Result: The queen ant usually lives for several decades, while her daughters live only from a few weeks to months, or rarely a few years.
Parasitic ants under surveillance
In the case of a species of ant (Temnothorax nilandri) occurring in Central Europe, however, it has been observed that some workers live abnormally long and normally reach a maximum age of more than two years. Scientists working with Sarah Beros from the Max Planck Institute for Biology of Aging in Cologne have now investigated in more detail what is behind it. The ants of the species live only a few millimeters in size, in small colonies on the forest floor, in acorns or in the dead wood of trees. They are known as intermediate hosts for a parasitic tapeworm (Anomotenia brevis), allowing one ant to attack up to 70 parasitic larvae. The parasites survive in the worm’s body liquid hemolymph, and complete their complex life cycle as soon as the ant is eaten by a suitable end host, such as a woodpecker.
The research team now investigated whether parasitism may have anything to do with the increase in survival rate of some ant workers of this species, based on about 60 ant colonies that they had found in the vicinity of Mainz Collected in the jungles and looked further into the laboratory. “We followed the survival rates of workers and queens in both infected and non-infected ant colonies over three years,” explains co-author Suzanne Fitzik of the University of Mainz. The team was able to distinguish ants on the basis of their color: infected animals are light-colored because their rigid body cover – the so-called cuticle – is less pigmented than brown conspiracies that are not infected with tapeworms.
Until a queen
It turns out: “The life expectancy of infected ants is quite long,” reports Phoitjik. After the three-year observation period, more than 95 percent of workers not infected with tapeworm had died, but more than half of the parasitic workers were still alive. “According to our observations, the survival rate of the workers is similar to that of the queens,” Foitic explains. “It is very exciting that a parasite can cause such positive changes in its host. The extension of life span is very unusual,” the researcher says.
But how can a parasite considered harmful increase its host’s life expectancy? Scientists looked closely at the behavior of infected ants and their classmates to find out the cause. It was worth noting that parasitic worms were less active and could be cared for by other workers in the nest. “Infected animals receive more attention, are fed better, cleaned, and taken care of,” says Phoetic. “They also receive a little more care than the nesting queen.” However, workers are not actively begging for this increased care for their nest mates. Instead, chemical signaling substances on the cuticle of parasitic ants could have stimulated the attention of their nesting peers, the researchers suspect. In another experiment, they were able to determine that uninfected nest mates, in contrast to infected ones, showed a particularly high level of interest in tapeworm-infected workers.
Tapeworm protein as a “fountain of youth”
In addition to enhanced care, scientists suspect that other factors should be responsible for the high life expectancy of parasitic ants. “Infected animals live in a land of milk and honey, but only good care may not explain the high life expectancy,” says Foitzic. In fact, studies have shown that the metabolic rate of female ant workers affected by tapeworms was uniformly increased and a comparable fat content as much smaller animals. Apparently, ants remain permanently in their young as a result of infection, Beros and his colleagues conclude. This may contribute to the fact that tapeworm larvae release proteins with antioxidants in the hemolymph of ants and alter the activity of ant genes that affect aging. Future research should clarify what other factors play a role in the aging of parasitic workers.
But it can already be explained what the parasite benefits from the increased life expectancy of its intermediate host: predators such as woodpeckers, which are the ultimate hosts of tapeworms, prefer to open ant nests to catch their prey. Huh. If infected ants live too long and are primarily safe in the nest, the likelihood increases that the woodpecker will eat the infected ants and pass the parasite to the final host.
Source: Johannes Gutenberg University Mainz, article: Royal Society Open Science, DOI: 10.1098 / rsos.202118