Living organisms are constantly confronted by a broad diversity of parasites of every sort. The last several months have shown us that we humans are hardly immune either. Nevertheless, targeted organisms are not without means when facing these attacks, since a number of mechanisms exist to prevent and limit these infections. And monarch butterflies have more than one trick up their sleeves!
Different mechanisms for preventing infection
There are two major mechanism groups for fighting these attacks: physiological mechanisms (a biological phenomenon inside an organism) and behavioral mechanisms.
The best-known physiological defense mechanism against illnesses is the immune system. That complex system allows the organism to recognize and to eliminate the foreign body attempting to infect it. Still, it happens that the immune system may not be sufficient to defend itself against illnesses and parasites.
Certain animals in that case modify their behavior in order to prevent or restrict infections.1 Among behaviors studied, what has been observed are avoidance (healthy individuals avoid being in contact with sick individuals) and self-medication (through consumption of products that are not found in their usual diet). In other words, certain animals “distance” themselves or take “medications” to fight off infections. Find these behaviors familiar? Fascinating, isn’t it?
So where does the monarch fit in?
Monarchs are not exempt from infections: the protozoan parasite Ophryocystis elektroscirrha (OE) is currently being found among bothNorth American populations. Infected butterflies are covered with spores that they’ll disperse for the duration of their lives. When an infected female lays an egg, she involuntarily disseminates a few spores of OE on the capsule of the egg and on the surface of the leaf. Infection of the new monarch takes place after hatching, when the caterpillar ingests the protozoan sporers. When the infection rate is high, monarchs suffer a number of negative effects, including a decrease in their llifespan, and their ability both to reproduce and to fly.
An unusual mechanism
In order to avoid overly high infection rates and being adversely affected by this parasite, the monarch uses a mechanism rarely documented in the animal kingdom.2 Infected females lay their eggs on milkweed species known as “anti-parasitic,” rather than lay them on more nourishing species.2 We call them “anti-parasitic” milkweeds because they’re rich in cardenolides. These molecules, stored in caterpillar tissues, limit the multiplication of the parasite (to learn more about cardenolides, see the blog Monarchs and milkweed, a complex relationship). Thus, ailing mothers help their young by depositing them on therapeutic milkweed, as though they were giving them medicine!
Monarchs, just like humans, adjust their behavior to better fight off illnesses and to heal. A lot of questions about the phenomenon of self-medication in animals remain to be answered at this point. But surprising discoveries are no doubt close by. It’s therefore important to protect this model insect so that we can continue to study it – just one more good reason to help the monarch!
1Lefèvre, Thierry, Allen Chiang, Mangala Kelavkar, Hui Li, James Li, Carlos Lopez Fernandez de Castillejo, Lindsay Oliver, Yamini Potini, Mark D. Hunter, et Jacobus C. de Roode. ‘Behavioural Resistance against a Protozoan Parasite in the Monarch Butterfly’. Journal of Animal Ecology 81, no. 1 (2012): 70–79.