Hormonal and neural correlates of care in active versus observing poison frog parents
Graphical abstract
Introduction
Sex differences in behavior and neuroendocrine function are a fundamental feature of most animals. Although sex-reversed behavior is relatively uncommon in adult animals, its existence suggests that neural circuits underlying behavior are conserved across sexes and can be activated under some circumstances, despite physiological and morphological differences established during development (Pereira and Ferreira, 2016). Exploring sex-reversed behavioral flexibility provides potential inroads to understanding how sex-specific behavioral patterns are coded in the brain, the conditions under which these patterns can be altered or reversed, and how underlying mechanisms are environmentally, developmentally, and evolutionarily tuned to give rise to sex-specific behavior.
Parental care is a behavior for which many species exhibit sexually dimorphic patterns. Parenting is ubiquitous across the animal kingdom, having arisen independently multiple times and taking different forms across species (Clutton-Brock, 1991; Royle et al., 2012). Species vary in the extent to which males and/or females provide care to offspring, and care behavior may be similar between sexes or sex-specific. Aside from sex-typical parental behaviors, some biparental species show flexibility in behavior, where a parent will compensate care effort when their partner disappears. This behavioral flexibility has been described in beetles (Smiseth et al., 2005), fish (Lavery and Reebs, 2010; Smiseth et al., 2005), birds (Harrison et al., 2009), and primates (Harrison et al., 2009; Zahed et al., 2009). However, less is known about flexibility in uniparental species where the normally non-parenting sex will occasionally care for offspring. Moreover, despite the phylogenetic diversity in parental behavior, mechanistic neuroendocrine research is heavily biased toward rodents that are female uniparental in the wild, although some can exhibit biparental care under laboratory conditions (Kohl et al., 2017; Numan and Insel, 2006). Maternal care evolved at the base of the mammalian lineage and shows little flexibility given the dependence of offspring survival on lactation. As a result, studies of male parental care come almost exclusively from biparental systems (Rilling and Mascaro, 2017), where parental behavior cannot easily be dissociated from pair bonding.
A fundamental question left open by heavily female-biased parental care research is how sex differences in offspring care arise and evolve. Despite marked sex differences in behavior, the neural circuits governing parental care appear largely conserved across sexes (Kohl and Dulac, 2018; Pereira and Ferreira, 2016), as are the expression patterns of key neuromodulators, such as galanin (Kohl et al., 2018; Wu et al., 2014) and prolactin (Angelier et al., 2016; Angelier and Chastel, 2009; Hashemian et al., 2016). Some sex-typical care behaviors are linked to differences in circulating hormones (Adkins-Regan, 2005), expression of key signaling molecules (Albers, 2015), and variations in receptor distributions in the brain (Dumais and Veenema, 2016). Nonetheless, the existence of flexible parenting suggests that underlying neural circuits are largely conserved and that there are alternative routes of access to their activation (Kohl and Dulac, 2018; Pereira and Ferreira, 2016). It remains an open question how and in response to which cues the modulation of these circuits leads to sex-reversed behavior.
Dendrobatid poison frogs show diversity in parental care across closely related species, including male uniparental, female uniparental, and biparental care (Summers and Tumulty, 2014). Parental care in poison frogs involves defense, hydration, and cleaning of embryos during development, and transportation of tadpoles piggyback to pools of water upon hatching (Brown et al., 2010; Pröhl and Hödl, 1999; Weygoldt, 1987). Importantly, males and females exhibit similar care behaviors within and across species, and both male and female care occur with and without pair bonding in this clade (Summers and Tumulty, 2014). Moreover, a number of species exhibit plasticity in parental care behavior, where the typically non-parenting sex will occasionally perform parental duties. For example, in brilliant-thighed poison frogs (Allobates femoralis) males typically perform tadpole transport, but females will do so if their mate disappears in both the wild and the laboratory (Ringler et al., 2015). Conversely, in the strawberry poison frog (Oophaga pumilio), females typically perform tadpole transport, but males have been observed doing so as well (Killius and Dugas, 2014). In poison frogs, the diversity of behavioral care strategies between species combined with behavioral flexibility within species affords a unique opportunity to identify physiological, neural, and molecular contributions to parental care and its evolution (Roland and O'Connell, 2015). Moreover, male parental care is ancestral in the clade (Summer et al., 1999), providing an important evolutionary comparison for mammalian systems in which female care is ancestral and male uniparental care is absent.
In the current study, we took advantage of behavioral flexibility in the typically male uniparental poison frog Dendrobates tinctorius to characterize hormonal and neural correlates of sex-typical and sex-reversed parental care. We first characterized hormone levels and whole brain gene expression during sex-typical parental behavior across egg attendance and tadpole transport care stages. We then took advantage of behavioral plasticity in female D. tinctorius to compare hormonal changes and neural activity associated with parental care in males and females during sex-typical and sex-reversed care. Flexible parenting in poison frogs provides an exciting opportunity to explore how sex-specific behavioral patterns are coded in the brain and how neuroendocrine mechanisms are evolutionarily tuned to give rise to sex- and species-specific variations in parental care.
Section snippets
Animals husbandry
All D. tinctorius used in the study were sexually mature individuals housed in our breeding colony. Animals were housed in 18x18x18 inch glass terraria (Exoterra, Rolf C. Hagen USA, Mansfield, MA) with sphagnum moss, live philodendrons, coconut shelters with petri dishes for egg laying, and a single water pool for tadpole deposition. Frogs were kept on a 12:12 h light cycle and fed wingless Drosophila fruit flies dusted with vitamin supplements three times weekly. All procedures were approved
Hormone differences across parental care stages and sexes
We quantified cortisol, testosterone, estradiol, and prostaglandin levels in parental male frogs and their non-caregiving female partners between parental bouts (no care), during egg care, and during tadpole transport (Fig. 1). Testosterone was overall higher in males (sex: F1,41 = 40.64, p < 0.0001, approximate d = 1.88), but decreased in both sexes during tadpole transport (group: F2,41 = 10.39, p = 0.0002, approximate d = 0.95). Cortisol differed marginally across care stages (group: F2,40
Discussion
We compared hormone levels, whole brain gene expression, and neural activity across care stages in parental males, their non-caregiving female partners, and behaviorally sex-reversed females of the typically male uniparental poison frog Dendrobates tinctorius. During sex-typical care, hormone and gene expression signatures were most distinct during the tadpole transport stage of parental care in both males and females, even though females are not directly involved in sex-typical parental care.
Conclusions
We found that similarities in neural activity were more closely related to active performance of parental behavior than hormone levels or whole brain gene expression, which were similar in parental males and their non-caregiving female partners. We suggest that hormonal and brain gene expression changes prime neural circuits for parental care, but additional factors trigger the changes in neural activity required for the ultimate performance of care behavior. Thus, the hormone and brain gene
Acknowledgements
We thank the O'Connell Lab frog caretakers for help with animal care and Alexandre Roland for technical assistance. We also thank the members of the O'Connell Lab for discussions and Beau Alward and Kristina Smiley for comments on early versions of this manuscript.
Funding
We gratefully acknowledge support from a Harvard University Bauer Fellowship (LAO), the International Society for Neuroethology Konishi Research Award (LAO), the Graduate Women in Science Adele Lewis Grant Fellowship (LAO), and an NSF postdoctoral fellowship (NSF-1608997 to EKF).
Declaration of competing interest
The authors have no competing interests to declare.
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