How parasites affect the survival of specimens. The example of the wolf

We have already dealt with, in these pages, some evolutionary implications linked to the interactions between parasites and hosts, both in terms of natural selection and in terms of the generation of new varieties through the integration of viral genomes and their subsequent reworking in the genome of the species guests.
There is now a further point that I would like to discuss, inspired by a research that has just been published on an iconic predator, an animal that has always populated our culture and our imagination: the wolf.

This animal can be infected by a parasite, a protozoan that has an interesting propagation strategy: Toxoplasma gondii, the causative agent of toxoplasmosis. To reproduce sexually, Toxoplasma must reach a cat’s body, usually when its host is eaten by the feline.
To increase the probability of this happening, the parasite is able to alter the behavior of the rodents it infests: in these animals, the infection is generally related to a decrease in fear of cats and an increase in exploratory behaviour.

The alteration of behavior depends on the manipulation of molecular mechanisms fairly conserved in mammals: in fact, behavioral changes have also been found in people accidentally infected with Toxoplasma, due to the increase in testosterone and dopamine and the consequent greater propensity to take risks.
Infection of species other than rodents and cats is accidental: all mammals can be parasitized by eating an infected animal or by ingesting Toxoplasma cysts present in the faeces of parasitized cats. After a period of acute illness, semi-dormant cysts form in muscle and brain tissue and persist for the rest of the host’s life.

Now, some American ecologists have thought about studying the occurrence of the infection and its effects in wolves, collecting data on wolves for 27 years in Yellowstone National Park, in Wyoming. Indeed, some packs of Yellowstone wolves live near, and sometimes steal prey from, cougars (Puma concolor), which are known to carry the parasite, like our cats. In addition to stealing their prey, wolves sometimes prey on pumas, so they are exposed to contamination both from their feces and directly from the consumption of their meat.
The researchers looked at 256 blood samples from 229 wolves who had been closely observed throughout their lives, recording their life histories and social status. Thus, it was found that infected wolves were 11 times more likely than uninfected ones to leave their birth family to start a new pack and 46 times more likely to become pack leaders, which means – considering that often only pack leaders reproduce – which were more likely to leave uninfected offspring of wolves.

I want to underline the concept: the Toxoplasma parasitosis produced an effect which, net of the overcoming of the acute disease phase (very probable in wolves), was an increase in reproductive success, or fitness, of infected animals, because, contrary to what happens for the preys of cats or pumas – whose life can end prematurely and before reproduction due to excessive recklessness – in wolves there are no predators that are able to carry out a selection of infected individuals; the bravest and least fearful animals, therefore, move up the social ladder and tend to be at an advantage.
We therefore observe how a parasite can modify the diffusion in the host population of genomes that do not have specific advantageous characteristics, through the modulation of the behavioral phenotype; in the long run, this can translate into important evolutionary and ecological effects, if we think that the offspring of an infected individual are more likely to become infected themselves (since the parent’s infection lasts a lifetime).

Once again, the natural history and evolution of disparate organisms can be subjected to unexpected effects of selection and evolutionary drive, which are not simply connected to the immunological contest between a host and its parasites, but are instead the result of ecological adaptations. – functional details of the parasite.
Yet another example of how life on this planet would be very different without the profound shaping action of parasites.