With seahorses and pipefish, males are responsible for the pregnancy. They’re the only animals on the planet where that’s the case, but as it turns out, their breeding behaviors aren’t that peculiar. They have a lot in common with vertebrate pregnancies, like that of lizards and even humans. The work, published in Molecular Biology and Evolution this week, is the first to track how genes switch on and off across the full course of pregnancy in any animal.

Male seahorses and pipefish have special brooding pouches that provide protection, gas exchange, osmoregulation, and some amount of provisioning for embryos developing inside during their 24-day gestation period. Fathers aren’t just pouch providers, they nurture too. Energy-rich lipids and calcium for tiny seahorse skeletons are secreted into the brood pouch, and then absorbed by the embryos.

"Surprisingly, seahorse dads do a lot of the same things human mums do," University of Sydney’s Camilla Whittington said in a statement. "Seahorse babies get a lot of nutrients via the egg yolk provided by their mothers but the pouch of the fathers has also evolved to meet the complex challenges of providing additional nutrients and immunological protection, and ensuring gas exchange and waste removal." 

However, the physiological and genetic changes facilitating these male pregnancies are largely a mystery. To investigate, Whittington and colleagues examined pouch gene expression at successive gestational stages in the pot-bellied seahorse Hippocampus abdominalis. Key elements of male gene expression during pregnancy, they found, were similar to that of pregnant mammals and reptiles.

"Regardless of your species, pregnancy presents a number of complex challenges, like ensuring you can provide oxygen and nutrients to your embryos," Whittington added. "We have evolved independently to meet these challenges, but our research suggests that even distantly related animals use similar genes to manage pregnancy and produce healthy offspring." The findings suggest that a similar toolkit of genes for regulating pregnancy is shared among divergent evolutionary lineages.