Biology
Influence of Parasitic Infections on Host Reproductive Behavior in Sticklebacks
Quick fact
In three-spined sticklebacks infected with the tapeworm Schistocephalus solidus, males lose their bright red breeding coloration and their eagerness to build nests, while females become less picky about mates—a behavioral takeover that helps the parasite get eaten by a bird.
Why this is interesting
Imagine a fish that, instead of fleeing from predators, starts swimming lazily near the surface—what if a parasite is steering it?
Read the full explanation
Understanding Influence of Parasitic Infections on Host Reproductive Behavior in Sticklebacks
Three-spined sticklebacks are small fish living in rivers and lakes. When infected by a tapeworm larva, their normal reproductive behaviors are drastically altered. Healthy males perform an elaborate courtship dance and build a nest, using bright red coloration to attract females. Infected males lose this coloration and become less active, often neglecting nest-building. Similarly, infected females show reduced choosiness, accepting less attractive males. These behavioral changes are not accidental; they are induced by the parasite to increase its own chance of completing its life cycle. The tapeworm lives inside the fish's body cavity, growing large enough to burst the belly, and its final host is a fish-eating bird. By altering the fish's behavior, the parasite makes the fish more vulnerable to predation by birds, allowing the tapeworm to reach its reproductive stage inside the bird's gut.
A deeper explanation
The mechanism behind this manipulation is complex and not fully understood, but it involves the parasite interfering with the host's endocrine and neurochemical systems. In sticklebacks, infection with Schistocephalus solidus reduces levels of androgens like 11-ketotestosterone, which are crucial for male breeding coloration and behavior. The parasite likely secretes substances that suppress the host's reproductive hormones, redirecting the host's energy and behavior away from reproduction. Additionally, infected fish show reduced responses to stressors and a decreased ability to shoal with healthy fish, making them more susceptible to predators. The ultimate evolutionary driver is parasite transmission: the tapeworm cannot reproduce until it enters a bird's digestive system, so any behavioral change that increases the likelihood of being eaten by a bird is favored by natural selection. This phenomenon is a classic example of extended phenotype, where a parasite's genes influence the host's phenotype to the parasite's advantage. Understanding this manipulation helps scientists appreciate how parasites can shape host population dynamics and even influence the evolution of host behaviors through strong selective pressures.