Most mammals relegate all parental duties to mom. However, some mammal dads, such as common house mice (Mus musculus), do share caregiving duties. Male mice normally bully unrelated babies, but soon after they sire their own offspring, they pitch in with caretaking. In contrast to mice, rats (Rattus norvegicus) never develop a parenting instinct, even after having a litter of their own. Intrigued by this disconnect in fatherly involvement between murines, Stefanos Stagkourakis from Christian Broberger's lab at the Karolinska Institutet, Sweden, in collaboration with an international team of colleagues from Austria and New Zealand, explored why these rats and mice take on such different dad duties.
The hormone prolactin is well known for spurring milk production and parental care in mammalian moms, but its role in paternal behavior was largely unknown. Stagkourakis wondered whether prolactin might explain why male rats, but not male mice, neglect their young. Knowing that dopamine prevents prolactin release, Stagkourakis measured how often male rats and mice release this neurotransmitter. They found that the rat neurons released more dopamine, resulting in dramatically lower prolactin levels in the blood compared with male mice. These results suggest that rats’ higher dopamine and consequently lower prolactin levels make them unmotivated fathers. As such, perhaps all that rat dads need to make them as fatherly as mice is more prolactin.
Based on these findings, Stagkourakis and his team next asked a fairly straightforward question: does boosting rats’ prolactin levels to rival that of male mice make rats better dads? To test this, Stagkourakis placed virgin or recent rat dads in an arena with several pups and gave some of the dads a shot of prolactin to see how attentive they were towards the needy pups while parenting solo. Compared with rat dads that didn't get a prolactin boost, rats that did spent more time grooming pups and huddling over them. Importantly, the virgin males were unaffected by prolactin injections, which suggests that sexual experience is required to make male rats receptive to prolactin's parental influence. In other words, prolactin can quickly induce parental behavior; however, it was unclear whether preventing prolactin's action would cause fatherly mice to relapse into deadbeat dads.
For their final set of experiments, Stagkourakis and colleagues determined whether prolactin inspires parental involvement by mouse fathers. To do this, the team removed the prolactin receptors from the hypothalamus of some mouse dads and checked the animals’ parenting skills. Instead of their usually high involvement, these dads became less fatherly, spending less time attending to the pups. In addition, Stagkourakis used a technique known as optogenetics to activate dopamine neurons precisely in the brains of a few male mice to mimic the rat's pattern of dopamine release, triggering rat-like prolactin release, and again the mice reduced the care they gave their pups. So, rodent dads rely on prolactin to activate parental care-inducing brain cells in the hypothalamus.
Overall, Stagkourakis and colleagues have found that rats and mice are both equipped with the tools to be either deadbeat dads or fathers of the year. However, it turns out that it's prolactin that makes the parent.