A stickleback is heading for a warm bath. While its peers prefer to swim in lukewarm water at around 16 degrees Celsius, this individual likes it hotter. That's not because of a personal preference – instead, it is being steered by a parasite. A tapeworm has lodged in its guts, and it needs warmer temperatures to grow as large as possible. The stickleback becomes little more than a living car that drives the worm to the heated pools that it prefers.
The bird tapeworm Schistocephalus solidus, like many parasites, has a complicated life cycle. As its name suggests, it infects birds, and particularly those that live by water. It reproduces in its host's intestines and its eggs plop into the water along with the bird's droppings. There, they hatch into larvae that infect small crustaceans call copepods. These are then eaten by sticklebacks, which are, in turn, eaten by birds. Once inside its final host again, the tapeworm can mature into an adult and lay new eggs.
To ensure that it completes its life cycle, the tapeworm manipulates the behaviour of its stickleback host. The fish becomes less averse to risks, more likely to swim alone, and less likely to flee from a predator. With such a laissez-faire attitude, it inevitably gets eaten by a bird.
But the tapeworm needs something else from the stickleback – time to grow. They have to get to a weight of 50 milligrams to be sure of successfully infecting a bird. Beyond that, the heavier they are, the more eggs they can lay. The fish, obviously, would prefer it if a smaller worm was writhing inside their bodies, rather than a large one. They don't often get their way; that much is obvious if you look at infected fish. The tapeworms can get so big that they outweigh their hosts, whose bellies are swollen and distended by their unwanted passengers.
Vicki Macnab and Iain Barber from the University of Leicester found that temperature is the deciding factor in this conflict of interests. At 15 degrees Celsius, the worm grows to an average weight of 26 milligrams within 8 weeks but at 20 degrees, it can reach a whopping 105 milligrams. This difference in weight has a big impact on the worm's future success. The smaller one can lay around 12,000 eggs in its final bird host, but the larger version can lay almost 200,000.
So, it's clear that worms prefer the heat, but so do the infected fish. Even though it would suit their interests to stay in cooler water where their passengers will stay as small as possible, they prefer to swim towards warmth. The temperature preference is most stark in fish that harbour the smallest worms, which have the furthest to grow. You could portray this as the fish's attempt to "burn out" its parasite with the behavioural equivalent of a fever, but that is unlikely given that the tapeworm positively thrives at such temperatures. Instead, Macnab and Barber think that the worms are somehow manipulating the fish.
Tapeworms are just one of several parasites that do better in warmer conditions, and Macnab and Barber suggest that our warming world could have unforeseen consequences on the delicate balance between these freeloaders and hosts. After all, a rise of just five degrees can have a disproportionately dramatic impact on the growth of a parasite and its ability to spawn future generations. I'm not convinced: surely a blossoming population of parasites may lead to fewer viable hosts, which would in turn limit the parasite population. These bust-and-boom cycles are nothing new. Regardless of the wider relevance, this is still a fascinating piece of animal behaviour.
Reference: Macnab & Barber. 2011. Some (worms) like it hot: fish parasites grow faster in warmer water, and alter host thermal preferences. Global Change Biology http://dx.doi.org/10.1111/j.1365…