Becoming a parent can be hard and daunting, especially the first time around. Perhaps you’re a parent who knows this intimately, or perhaps you’ve seen your friends struggle to adjust to pregnancy pains or the stressful sleep-schedules of a newborn.  In the parenting community, memes joke about the difference between raising a first-born versus the next kid. Like many things, parenting seems to get easier, or at least smoother, with experience.

Does “practice make perfect” for parents? Possibly. Parental behavior requires a complex suite of changes to an animals’ body and brain. Many of these changes involve hormones, small chemicals released into the blood to affect the brain and system as a whole. Hormones are the infamous culprits behind bloating and nausea during pregnancy, including “sympathy symptoms” in non-pregnant partners. Hormones get blamed for negative side-effects, but they also help form the deep bonds between parent and offspring and turn on key behaviors that make parenting functional. You might recognize some of their household names: oxytocin (the “love” hormone) estrogen and progesterone (the components of birth control), or testosterone (the “male” hormone, but that’s a common misconception, as both males and females produce it in different quantities).

One of my favorite hormones, and the center of my research, is prolactin. Prolactin gets most of its fame as the “got milk” hormone, as it drives milk production in mammals.  But prolactin goes beyond promoting lactation. Prolactin also helps keep fertility in check while new moms are lactating (since the body doesn’t want a new pregnancy to occur while it’s spending tons of energy feeding a newborn), maintains energy levels, regulates water balance, can protect brain cells involved in memory, along with nearly 200 other functions (including a possible role in human male orgasms) [1]. Clearly, prolactin is important for parenting and reproduction.

So, I wondered: do animals that have had kids differ in their response to prolactin, compared with animals that have never reproduced before?  Studies in mammals and birds show that levels of prolactin tend to be higher in experienced parents compared to first-timers or animals that had never been parents at all [2-4].  While levels of a hormone are important, they’re not the whole story. I wanted to understand how an animal’s ability to respond to prolactin changes, or how quickly they could turn on parental behavior when the required hormones are present. This requires looking at hormone receptors – small molecules that act as “gates” when unlocked by hormones’ “keys” – on areas where behavior is controlled in the brain.

To study this, I’m looking at one of the most ubiquitous animals in the world – the pigeon! Also known as rock doves (Columba livia), pigeons are likely more famous for their ability to poop on property than their ability to parent. But pigeons are excellent parents! Unlike most mammals, pigeons show biparental care. This means both males and females take care of the young, and in pigeons, this load is shared almost equally between the sexes. Both parents build nests, sit on eggs, and lactate to feed chicks. That’s correct, both sexes lactate. No, pigeons are not mammals, and so this is technically “pseudo-lactation”, in that it doesn’t come from mammary glands, but instead a modified part of their esophagus called the crop sac. While “crop milk” is a lot grosser than mammalian milk (imagine two-week old rotten cottage cheese), it plays a similar role and is also driven by prolactin. So, pigeons make perfect model systems to answer questions about parental care.  

One of the hardest parts of understanding the effect of parental experience is the problem of time. Is there any way to successfully raise kids without getting older? I’m sure many parents wish the answer was yes, but unfortunately, there isn’t. Age and experience thus tend to be intertwined, so as our birds gain experience, they also get older than birds that have never reared chicks. What if it’s just aging, and nothing to do with parenting, that changes hormone receptors? In the wild, this would be tricky to get around, since it’s hard to a) get ages of wild animals, and b) reliably determine if they have been parents in the past. However, the Calisi lab has a thriving population of pigeons breeding in captivity, allowing me to control for age and known levels of parenting experience (i.e. never had chicks, or have had chicks). Plus, we have a secret weapon: amazing undergraduate research assistants.

Every day, our team of undergraduate research assistants brave rain, heat and dust to check on the breeding status of our birds. Pigeons breed all year round, so eggs can appear any morning in the wooden nest boxes at our aviary. Students carefully check each of these boxes for fresh eggs or hatched chicks, and note the parents – who are identified with color-banded “bracelets” – so we can have careful records of when birds were born and how many chicks they’ve had.

To understand how experience changes the brain’s ability to respond to hormones, I’m combining the breeding data our students collect with genetic information from the brain of parental birds. Specifically, I’m looking at a corner of the brain that drives many parental care behaviors, called the preoptic area. From this small, but important area, I’ll measure whether the gene for the prolactin receptor is being “turned on” at higher or lower levels.

To find this corner of the brain, I process my samples at a multi-thousand-dollar machine called a cryostat. This might sound super science-y, but in reality, it’s more like a very expensive deli-slicer. From there, I use a map of the brain, called an atlas, to use the landmarks made by other brain regions and neurons to locate the preoptic area. I use these markers to find my way around the tiny pink-and-white pigeons brains, each about 1 inch long.

Once we have the preoptic area in tubes, we measure the prolactin receptors with molecular genetic methods very similar to the lab diagnostics used for COVID-19 saliva testing. I won’t go into the details, but instead of looking for viral genetic material in a person’s spit sample, I’m looking for the gene underlying the prolactin receptor. One of our hypotheses is that animals with parental experience will have more prolactin receptors than animals without experience of the same age. This work takes hours of time, carefully pipetting samples and then running them on machines that count of how many copies of the genes are present. Luckily, I collaborate with talented student mentees, such as Alison Ramirez. Alison excels at carefully extracting the teeny-tiny genetic molecules in to measure gene counts. With the skills she’s learning in lab, she hopes to go to graduate school to study the neurobiological basis of mental health disorders. Her work exemplifies how the tools we use, while used here to understand parental care in birds, can translate to many fields. These genetic tools can answer a variety of questions, from COVID testing to identifying whether neurons are turning on genes of depression or Alzheimer’s in humans.  

We are hoping to present our findings in the next year, so stay tuned for what pigeons can teach us about the parental brain!

Victoria Farrar is a 5th year Ph.D. candidate in the Animal Behavior Graduate Group, working with Dr. Rebecca Calisi Rodríguez. When not becoming a “pro” on prolactin, she really enjoys working with and learning from undergraduate students.

References:

[1] M. E. Freeman, B. Kanyicska, A. Lerant, G. Nagy, Prolactin: Structure, Function, and Regulation of Secretion. Physiol. Rev. 80, 1523–1631 (2000).

[2] K. O. Smiley, E. Adkins-Regan, Relationship between prolactin, reproductive experience, and parental care in a biparental songbird, the zebra finch ( Taeniopygia guttata ). Gen. Comp. Endocrinol. 232, 17–24 (2016).

[3] D. Christensen, C. M. Vleck, Effects of age and reproductive experience on the distribution of prolactin and growth hormone secreting cells in the anterior pituitary of a passerine. Gen. Comp. Endocrinol. 222, 54–61 (2015).

[4] R. E. A. Almond, T. E. Ziegler, C. T. Snowdon, Changes in prolactin and glucocorticoid levels in cotton-top tamarin fathers during their mate’s pregnancy: the effect of infants and paternal experience. Am. J. Primatol. 70, 560–565 (2008).

[Edited by Josie Hubbard and Maggie Creamer]