The most recent of a series of studies has confirmed that woodpecker tapeworm larvae induce changes in their ant host’s life histories, including remarkably increased lifespans and the ability to receive more social welfare than the colony queen from non-parasitized nestmates.
Temnothorax nylanderi, worker ant. Source: Wikipedia, attribution April Nobile / © AntWeb.org https://en.wikipedia.org/wiki/Temnothorax_nylanderi
Temnothorax nylanderi, is a tiny species of ants found in forests in Western Europe. Colonies live on bark or are found in the leaf litter in small cavities such as in the interior of twigs or acorns. Intercaste ants with traits of both worker and queen are common and increase in weight once the colony has become infected with the larvae of the tapeworm. Anomotaenia brevis.
As an adult, this tapeworm lives in the intestines of a woodpecker. When its eggs, which are excreted with the bird’s droppings, from T. nylanderi Collect ants and feed them to the larvae of the colonies, then the life cycle continues. The parasites hatch, burrow through the intestines of the larva and develop in the body cavity. The life cycle of the tapeworm is completed when an infected ant is eaten by another woodpecker.
At first glance, transmission from the woodpecker to the ant and back again seems unlikely. However, as with other parasites with complex life cycles, these intermediate hosts experience changes in life history features that could increase the likelihood of transmission.
Sarah Beros and colleagues from Johannes Gutenberg University Mainz have investigated the effects of this tapeworm infection on the life history of these wood ants and published a series of articles reporting their results. Infected workers show morphological changes and can develop ovaries in the absence of a queen. These parasitized ants are tolerated by their nestmates, although chemically different from non-parasitized workers, and are also more commonly fed.
In 2015, the group reported that the presence of infected individuals in a colony had a profound effect on all workers in the colony. In a parasitized nest, uninfected workers became less aggressive and showed a decrease in lifespan, while infected ants showed a reduced flight response and did not take part in foraging as they age and survive longer. I discussed these results on a BugBitten blog in 2015.
This research group recently published further results showing the great extent of this tapeworm in the life of the entire colony.
The new investigation: survivorship
Over a period of three years, newly hatched (Kalows), ants (young workers in brood care), foragers and queens from parasitized and non-parasitized colonies (57 in total) were observed in the laboratory. Infected ants could be recognized by their unpigmented cuticles.
Several individuals from each colony were wire tagged and their survival monitored. At the end of three years, 21% of the colonies were dead. In the surviving parasitized colonies, all of the marked non-parasitized workers were dead, while just over half of the parasitized survived. None of the nurses and only 2% of the labeled collectors survived in the non-parasitized colonies. This confirmed the remarkable lifespan extension that resulted from the infection.
Metabolic Rate and Other Comparisons
When examining 14 parasitized and 14 non-parasitized new colonies, the researchers found that queens had the lowest metabolic rate and non-parasitized foragers had the highest. Infected workers had similar metabolic rates as nurses, that is, between queens and foragers. Unsurprisingly, queens had much greater body mass than any other ants, and in this case there was no difference in body mass between parasitized workers and non-parasitized keepers and gatherers.
Non-parasitized wet nurses had by far the highest lipid content, while the lipid content of parasitized workers ranged between that of foragers and queens.
In non-parasitized colonies, the queen received more social care than nurses more care than foragers. But in parasitized colonies, the infected workers were even more cared for than the queens.
This additional attention is likely related to chemical cues emanating from the infected workers, as demonstrated by measurements of nestmate responses to cuticular hydrocarbon-coated glass beads given by parasitized or non-parasitized brood carers. Cuticular hydrocarbons from parasitized nestmates received more attention than non-parasitized ones. It is unclear whether this is caused by a change in a particular hydrocarbon or the general profile, or how that change is achieved.
Longevity in sight
The remarkable increase in life expectancy in infected workers is by no means an uncommon consequence of parasitization. Work from my laboratory, for example, showed a highly significant increase in the lifespan of beetles infected with rat tapeworm larvae. This effect was accompanied by a decrease in fertility, suggesting that a resource trade-off was taking place. However, this is not the case with this woodpecker tapeworm / ant association. Previous work by the group has shown that when the queen is removed from a parasitized colony, the parasitized workers respond most quickly by developing ovaries.
Could their longer lifespan instead be due to the extra care that parasitized workers receive, possibly at the expense of their non-parasitized nestmates?
Is a longer host life a benefit to the parasite?
The tapeworm only ends its life cycle when an infected ant is eaten by a woodpecker. The authors of this fascinating series of articles suggest that these tiny ants foraging in leaf litter would be hard to find. Woodpeckers are more likely to be able to successfully precede the ants by searching for their nests. Both the altered behavior that parasitized ants keep in the nest and foraging for food, as well as their extended lifespan, are likely to increase the likelihood that they will be predated and Anomotaenia brevis completes its life cycle.