Parasites usually harm their host because they live at its expense. Obviously, this does not always apply, as an ant species native to us is now proving. In this species, workers infected by a parasitic tapeworm lived much longer than their nest mates. Reasons for this could be increased care by their comrades as well as a changed metabolic rate and a different fat content.
There is a clear division of roles in ant colonies: the queens are the only ones to reproduce, are naturally well protected from diseases, for example, and spend almost their entire life in a safe nest. There they are looked after by the workers, their daughters: They take care of the brood and, in old age, the risky foraging outside the colony. The consequence: the queen ant usually reaches a lifespan of several decades, while her daughters only live a few weeks to months or rarely a few years.
Parasitized ants under observation
In the case of a species of ant (Temnothorax nylanderi) occurring in Central Europe, however, it has been observed that some workers live unusually long and become older than the usual maximum of two years. Scientists working with Sara Beros from the Max Planck Institute for Biology of Aging in Cologne have now examined what is behind this in more detail. The ants of the species, only a few millimeters in size, live 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 (Anomotaenia brevis), whereby a single ant can be attacked by up to 70 parasitic larvae. The parasites survive in the hemolymph, the body fluid of the insects, and complete their complex life cycle as soon as the ant is eaten by a suitable ultimate host such as a woodpecker.
The research team now examined whether the parasitization could have something to do with the increased survival rates of some ant workers of this species on the basis of a total of around 60 ant colonies that they collected in the forests around Mainz and further observed in the laboratory. “We followed the survival rate of workers and queens in both infected and non-infected ant colonies over three years,” explains co-author Susanne Foitzik from the University of Mainz. The team was able to distinguish the ants based on their color: the infected animals are lighter in color because their hard body cover – the so-called cuticle – is less pigmented than the brown-colored conspecifics that are not infected by tapeworms.
As long as a queen
It turned out: “The life expectancy of infected ants is significantly longer,” reports Foitzik. After the three-year observation period, more than 95 percent of the workers not infected by tapeworms had died, but significantly more than half of the parasitized workers were still alive. “According to our observations, the workers have a survival rate that is similar to that of queens,” explains Foitzik. “It is extremely exciting that a parasite can cause such a positive change in its host. The extension of the lifespan is very unusual, ”says the researcher.
But how can a parasite considered harmful extend the life expectancy of its host? To find out the reasons for this, the scientists observed the infected ants and the behavior of their nestmates more closely. It was noticeable that the parasitized insects were less active and could be looked after by other workers in the nest. “The infected animals receive more attention, are better fed, cleaned and cared for,” says Foitzik. “They even receive a little more care than the queen of the nest.” However, the workers do not seem to be actively begging for this increased care for their nestmates. Instead, chemical signaling substances on the cuticle of parasitized ants could have encouraged the attention of their nestmates, 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 the workers infected by the tapeworm.
Tapeworm proteins as a “fountain of youth”
In addition to the increased care, the scientists suspect that other factors must be responsible for the high life expectancy of the parasitized ants. “The infected animals live in the land of milk and honey, but good care alone cannot explain the high life expectancy,” says Foitzik. In fact, the studies showed that the female ant workers affected by the tapeworm had a similarly increased metabolic rate and a comparable fat content as very young animals. Obviously, the ants remain permanently in a youth stage as a result of the infection, conclude Beros and her colleagues. This could contribute to the fact that the tapeworm larvae release proteins with antioxidants into the ants’ hemolymph and alter the activity of ant genes that influence aging. Future research should clarify which other factors play a role in the old age of the parasitized workers.
But it can already be explained what advantage the parasite has from the increased life expectancy of its intermediate host: Predators such as woodpeckers, which are the final hosts of the tapeworm, prefer to open the ant nests to catch their prey. If the infected ants live for a very long time and mainly stay protected in the nest, the probability increases that the woodpeckers will eat the infected ants and the parasite will be transmitted to the ultimate host.
Source: Johannes Gutenberg University Mainz, Article: Royal Society Open Science, doi: 10.1098 / rsos.202118