Even for fish, it can be a drag to swim while pregnant

For fish like the porthole livebearer (Poeciliopsis gracilis, above), pregnancy might be less of a drag if they take it slow. To find out how pregnancy might affect a fish’s swimming speed—and maybe its ability to escape predators—researchers made 3D-printed models of expectant livebearers in their lab, whose ballooning bellies increased their size by as much as 43% during pregnancy. They then measured the drag forces the models experience in water flowing at various speeds.

Unsurprisingly, “swimming” was much harder for the bigger models. Drag forces grew exponentially with the 3D models’ volume increases. This implies the fish pay a price for bearing more offspring, the researchers report today in the Journal of the Royal Society Interface . It might seem straightforward that a pregnant, less-streamlined fish suffers stronger drag forces, but this study is the first to quantify how much a pregnant animal’s body changes add to drag forces when swimming.

But the speed of water flow also matters. At high speeds, the researchers say, drag forces increase much more rapidly than at lower speeds. That means pregnant fish may not have it too bad if they slow down and swim at a more leisurely pace.