Parenting is hard, and everyone takes a different approach. Infant mice are usually reared by mom, whereas songbirds tend to be exemplar co-parents. While scientists know how parenting gets kick-started in feathered and furry vertebrates, other animals, such as poison frogs (Dendrobatidae), have been largely under-studied. Despite their alarming name, poison frogs are superb caretakers who travel long distances to safely hide and feed their young. Recently-hatched tadpoles need to be in water to develop, so mom or dad give them a ‘piggyback ride’ to nearby pools on the forest floor. While some poison frog species do this as a single parent (either just mom or dad), others take after the birds and share the carpooling duty. However, it was not known how these different parenting arrangements are evoked.
Given poison frogs’ diverse parenting styles, Eva Fischer in the lab of Lauren O'Connell at Stanford University, USA, along with a team of collaborators in the USA and Ecuador, explored whether the brains of solo parenting versus co-parenting frog species differ. In their paper published in the Proceedings of the Royal Society B in July 2019, Fischer and her team focused on three poison frog species with different parenting tactics: single dads (Dendrobates tinctorius), single moms (Oophaga sylvatica) and co-parents (Ranitomeya imitator). To accomplish this, Fischer intercepted parent frogs giving tadpoles a piggyback ride and compared how many brain cells were active in the brain of the caretaker and the uninvolved parent. Two brain regions stood out: the hypothalamus, which controls a range of social behaviours – including parenting – in other animals, and the hippocampus, which is important for navigating and storing memories. Overall, giving a piggyback ride to their kid triggered more activity in both the hippocampus and hypothalamus of mama and papa frogs compared with levels in childless adults or uninvolved partners across species, suggesting these areas are conserved in parenting frogs. But while the boost in activity pinpointed the brain regions excited by parenting, it was unclear which specific cells were active when the frogs dropped off their kids.
Focusing in further, Fischer scrutinized the expression pattern of genes in brain cells that had been activated while the frogs were parenting and how those expression patterns varied between the different parenting styles. Of the many genes in the brain that increased during piggyback rides, one stood out from the crowd: galanin. Galanin is a hormone found in the hypothalamus that is critical for mouse parenting, but its presence outside rodents was unknown. In co-parenting frogs, transport of tadpoles sparked more activity in galanin-expressing brain cells than in frogs without kids. Interestingly, having kids generated more brain activity in galanin brain cells in the uninvolved female partner of solo dads, suggesting that moms are ready to jump in and help if dad fails at childcare. Oddly, dropping the kids at the pool didn't change solo mom frogs much – their galanin-expressing cells were just as active as those in their non-parenting counterpart.
It seems that the hypothalamus – with its galanin-expressing nerve cells – is part of an important and conserved parenting mechanism in many animals, as it exists in amphibians that are evolutionarily more ancient than their avian and mouse relatives. Fischer and her team suggest that this work lays an exciting foundation for exploring future questions about the evolution of parenting in poison frogs and expands our understanding of the complex concoction of genes, brain circuits and hormones that combine to make ‘Parents of the year’.
