The beginning is invisible. A freshwater flea infected with the parasitic bacterium Pasteuria ramosa could happily eat green algae in a lake for a week before it starts swelling and turning pink, like the one on the right in the picture above. That’s also when eggs stop popping up in the flea’s brood pouch. The castrated flea then continues ballooning for up to 2 months, until it dies weighing twice as much as a healthy flea. Scientists knew that P. ramosa must benefit from castrating its host and causing it to grow to gigantic proportions—why do it otherwise? What they didn’t know is how influencing the amount of energy the host flea uses for reproduction versus growth helped the parasite maximize its own energy gains. So researchers put infected fleas on different feeding schedules and observed the impact on the populations of parasites inside them. All fleas received the same total amount of green algae, but some were fed once a day whereas others had to go hungry for up to 6 days in between meals. The starved fleas grew as fast as well-fed ones, but spent much less energy reproducing. The research, published online today in the Proceedings of the Royal Society B, found that the less energy the flea allocates to reproduction, the faster the parasite grows. It seems that by castrating its host as early as possible, P. ramosa forces the flea to concentrate on growing, gaining more energy for itself in the process. The researchers hope to apply the framework to studying similar host-parasite systems, such as parasites on snails.