How Parasites Can Make Their Hosts Grow Bigger
Some parasites can make infected animals grow unusually large by shutting down reproduction and redirecting resources toward body growth.
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Parasite gigantism is a striking effect seen in some host and parasite relationships, especially among certain mollusks and other animals. The basic pattern is that an infection interferes with the host’s normal reproduction, sometimes by damaging or taking over reproductive tissue. With reproduction reduced or stopped, resources that would normally be invested in producing offspring can instead contribute to continued body growth, while the parasite also uses the host as a source of energy and shelter. In some systems, the larger infected host may give the parasite more room or help it survive long enough to complete its life cycle. Research on parasitic trematodes in snails has documented changes in shell growth and shape associated with infection, with the altered body providing additional space for parasite development. Scientists have proposed several explanations for why gigantism evolves, and the exact mechanism can vary between host and parasite systems. Parasite-induced gigantism therefore is not simply a case of an animal becoming healthier or stronger after infection. It is a consequence of a complicated struggle over the host’s energy, reproduction, growth, and survival.
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