Whether it's a new taste for salt or a sudden hunger for sugar, animals infected with parasites often develop new cravings. But it's not always clear whose cravings the animal is responding to. On the one hand, the parasite could be an overly pushy guest manipulating its host to achieve its own needs. On the other, the animal's own body could prioritize an immune system-boosting diet to fight the infection. That's why Enikő Csata from the University of Toulouse, France and an international team of scientists explored the dietary preferences of Argentine ants (Linepithema humile) and a fungus (Metarhizium brunneum) that infects them. They wanted to know if the dietary choices of infected ants reflected the preferences of the fungus and if the fungus itself was responsible for its host's choices.

First, the researchers had to figure out what kind of diet the fungus liked best; does the fungus like something high in protein, or does it prefer a high-carbohydrate diet? After whipping up different mixtures of nutrients with varying ratios of sugars and amino acids, the scientists grew the fungus in Petri dishes containing one of each of the mixtures. For a month, they let the fungus spread over the nutrients on each dish. As it grew, the fungus produced spores, the reproductive cells that make more fungi and also infect the ants. At the end of the experiment, Csata and her team compared the growth of the fungus and the number of spores in each Petri dish. They found that the fungus grew best and produced the most spores when there was a 1:4 ratio of amino acids to sugar molecules, making this protein-heavy meal the optimal fungal diet.

This diet was far from optimal for the ants, though. The researchers fed individual ants either the optimal fungal diet or a high-carbohydrate diet for 2 months and found that the ants tended to die sooner when they ate only the optimal fungal diet. However, this protein-packed diet did not make fungal infections any more deadly. During the same period, the researchers applied fungal spores to the exoskeletons of a different group of ants and compared the effects of the two diets on the ants. The infected ants generally died sooner than their uninfected peers, regardless of what they ate. Infected ants even developed a taste for the ordinarily unhealthy, protein-heavy diet. When the scientists allowed worker ants to forage for food in a spore-ridden environment, the insects preferred bringing the optimal fungal diet back to their colonies, rather than carbohydrate-heavy meals. At first glance, this dietary choice would appear to benefit the infectious fungus. While the parasite could be manipulating its host's food choices, an ant's immune system also relies heavily on amino acids to make all the proteins it uses to recognize and kill parasitic invaders.

To figure out which party was in charge, Csata and her team injected individual ants with a piece of the fungus that was incapable of growing or reproducing to fool the ants’ immune systems into making defensive proteins. Then, they let the ants choose what to eat. Even though the ants weren't truly infected, the needs of their amino acid-hungry immune systems still drove their cravings towards the protein-rich optimal fungal diet. The researchers concluded that the infected ants ate more protein despite their parasite's preferences. Whole colonies ate more protein when their workers were infected, suggesting that ants use food to self-medicate and limit the spread of infection. Bringing more protein back to the colony may not cure what ails any one worker, but it could give the rest a fighting chance.

Csata
,
E.
,
Pérez-Escudero
,
A.
,
Laury
,
E.
,
Leitner
,
H.
,
Latil
,
G.
,
Heinze
,
J.
,
Simpson
,
S.
,
Cremer
,
S.
and
Dussutour
.
A.
(
2024
).
Fungal infection alters collective nutritional intake of ant colonies.
Curr. Biol.
34
,
902
-
909
.