Integrating adaptive trade-offs between parental care and feeding regulation

Transcriptomic changes associated with maternal care in the brain of mouthbrooding cichlid Astatotilapia burtoni reflect adaptation to self-induced metabolic stress

Parental care in Astatotilapia burtoni entails females protecting eggs and developing fry in a specialized buccal cavity in the mouth. During this mouthbrooding behavior, which can last 2-3 weeks, mothers undergo voluntary fasting accompanied by loss of body mass and major metabolic changes. Following release of fry, females resume normal feeding behavior and quickly recover body mass as they become reproductively active once again. In order to investigate the molecular underpinnings of such dramatic behavioral and metabolic changes, we sequenced whole-brain transcriptomes from females at four time points throughout their reproductive cycle: 2 days after the start of mouthbrooding, 14 days after the start of mouthbrooding, 2 days after the release of fry and 14 days after the release of fry. Differential expression analysis and clustering of expression profiles revealed a number of neuropeptides and hormones, including the strong candidate gene neurotensin, suggesting that molecular mechanisms underlying parental behaviors may be common across vertebrates despite de novo evolution of parental care in these lineages. In addition, oxygen transport pathways were found to be dramatically downregulated, particularly later in the mouthbrooding stage, while certain neuroprotective pathways were upregulated, possibly to mitigate negative consequences of metabolic depression brought about by fasting. Our results offer new insights into the evolution of parental behavior as well as revealing candidate genes that would be of interest for the study of hypoxic ischemia and eating disorders.

Neurotranscriptomic changes associated with chick-directed parental care in adult non-reproductive Japanese quail

For many species, parental care critically affects offspring survival. But what drives animals to display parental behaviours towards young? In mammals, pregnancy-induced physiological transformations seem key in preparing the neural circuits that lead towards attraction (and reduced-aggression) to young. Beyond mammalian maternal behaviour, knowledge of the neural mechanisms that underlie young-directed parental care is severely lacking. We took advantage of a domesticated bird species, the Japanese quail, for which parental behaviour towards chicks can be induced in virgin non-reproductive adults through a sensitization procedure, a process that is not effective in all animals. We used the variation in parental responses to study neural transcriptomic changes associated with the sensitization procedure itself and with the outcome of the procedure (i.e., presence of parental behaviours). We found differences in gene expression in the hypothalamus and bed nucleus of the stria terminalis, but not the nucleus taeniae. Two genes identified are of particular interest. One is neurotensin, previously only demonstrated to be causally associated with maternal care in mammals. The other one is urocortin 3, causally demonstrated to affect young-directed neglect and aggression in mammals. Because our studies were conducted in animals that were reproductively quiescent, our results reflect core neural changes that may be associated with avian young-directed care independently of extensive hormonal stimulation. Our work opens new avenues of research into understanding the neural basis of parental care in non-placental species.

Neural Activation Patterns Associated with Maternal Mouthbrooding and Energetic State in an African Cichlid Fish

Parental care is widespread in the animal kingdom, but for many species, provisioning energetic resources must be balanced with trade-offs between self-promoting and offspring-promoting behaviors. However, little is known about the neural mechanisms underlying these motivational decisions. Mouthbrooding is an extreme form of parental care most common in fishes that provides an ideal opportunity to examine which brain regions are involved in parenting and energetics. The African cichlid fish Astatotilapia burtoni is a maternal mouthbrooder in which females hold developing young inside their mouths for 2 weeks. This brood care makes feeding impossible, so females undergo obligatory starvation. We used immunohistochemistry for the neural activation marker pS6 to examine which brain regions were involved in processing salient information in mouthbrooding, starved, and fed females. We identified brain regions more associated with maternal brood care (TPp, Dc-4/-5), and others reflective of energetic state (Dl-v, NLTi). Most nuclei examined, however, were involved in both maternal care and energetic status. Placement of each of the 16 examined nuclei into these functional categories was supported by node by node comparisons, co-activity networks, hierarchical clustering, and discriminant function analysis. These results reveal which brain regions are involved in parental care and food intake in a species where provisioning is skewed towards the offspring when parental feeding is not possible. This study provides support for both distinct and shared circuitry involved in regulation of maternal care, food intake, and energy balance, and helps put the extreme parental case of mouthbrooding into a comparative and evolutionary context.

Nonapeptides mediate trade-offs in parental care strategy

Parental care represents a suite of distinct behaviors performed by parents to maximize fitness. Dynamic shifts in parental care behaviors, such as between nest defense and direct provisioning of the offspring, are required in response to environmental variation. However, the neural mechanisms which mediate such behavioral shifts remain a mystery. The anemonefish, Amphiprion ocellaris, represents an experimentally valuable model in social neuroscience which is conducive to manipulating the environment while simultaneously measuring parental care. The goal of this study was to determine the extent to which arginine vasotocin (AVT) and isotocin (IT) signaling are necessary for males to shift between direct egg care and aggressive nest defense in the presence of intruders, Domino damselfish (Dascyllus trimaculatus). The IT receptor antagonist desGly-NH₂-d(CH₂)₅[D-Tyr²,Thr⁴]OVT, significantly reduced direct egg care, while at the same time increased levels of aggressive nest defense relative to vehicle. Conversely, blockade of AVT using the antagonist d(CH₂)₅[Tyr(Me)²]AVP, reduced aggression and tended to increase egg care. Results demonstrate that male anemonefish alter their parental strategy in response to allospecific intruders, and that IT and AVT signaling oppositely regulate parental care displays of aggression versus egg care.

Hormonal and neural correlates of care in active versus observing poison frog parents

The occasional reversal of sex-typical behavior suggests that many of the neural circuits underlying behavior are conserved between males and females and can be activated in response to the appropriate social condition or stimulus. Most poison frog species (Family Dendrobatidae) exhibit male uniparental care, but flexible compensation has been observed in some species, where females will take over parental care duties when males disappear. We investigated hormonal and neural correlates of sex-typical and sex-reversed parental care in a typically male uniparental species, the Dyeing Poison Frog (Dendrobates tinctorius). We first characterized hormone levels and whole brain gene expression across parental care stages during sex-typical care. Surprisingly, hormonal changes and brain gene expression differences associated with active parental behavior in males were mirrored in their non-caregiving female partners. To further explore the disconnect between neuroendocrine patterns and behavior, we characterized hormone levels and neural activity patterns in females performing sex-reversed parental care. In contrast to hormone and gene expression patterns, we found that patterns of neural activity were linked to the active performance of parental behavior, with sex-reversed tadpole transporting females exhibiting neural activity patterns more similar to those of transporting males than non-caregiving females. We suggest that parallels in hormones and brain gene expression in active and observing parents are related to females' ability to flexibly take over parental care in the absence of their male partners.

The intestinal environment as an evolutionary adaptation to mouthbrooding in the Astatotilapia burtoni cichlid

Many of the various parental care strategies displayed by animals are accompanied by a significant reduction in food intake that imposes a substantial energy trade-off. Mouthbrooding, as seen in several species of fish in which the parent holds the developing eggs and fry in the buccal cavity, represents an extreme example of reduced food intake during parental investment and is accompanied by a range of physiological adaptations. In this study we use 16S sequencing to characterize the gut microbiota of female Astatotilapia burtoni cichlid fish throughout the obligatory phase of self-induced starvation during the brooding cycle in comparison to stage-matched females that have been denied food for the same duration. In addition to a reduction of gut epithelial turnover, we find a dramatic reduction in species diversity in brooding stages that recovers upon release of fry and refeeding that is not seen in females that are simply starved. Based on overall species diversity as well as differential abundance of specific bacterial taxa, we suggest that rather than reflecting a simple deprivation of caloric intake, the gut microbiota is more strongly influenced by physiological changes specific to mouthbrooding including the reduced epithelial turnover and possible production of antimicrobial agents.

Gene expression signatures of mating system evolution

The diversity of mating systems among animals is astounding. Importantly, similar mating systems have evolved even across distantly related taxa. However, our understanding of the mechanisms underlying these convergently evolved phenotypes is limited. Here, we examine on a genomic scale the neuromolecular basis of social organization in cichlids of the tribe Ectodini from Lake Tanganyika. Using field-collected males and females of four closely related species representing two independent evolutionary transitions from polygyny to monogamy, we take a comparative transcriptomic approach to test the hypothesis that these independent transitions have recruited similar gene sets. Our results demonstrate that while lineage and species exert a strong influence on neural gene expression profiles, social phenotype can also drive gene expression evolution. Specifically, 331 genes (∼6% of those assayed) were associated with monogamous mating systems independent of species or sex. Among these genes, we find a strong bias (4:1 ratio) toward genes with increased expression in monogamous individuals. A highly conserved nonapeptide system known to be involved in the regulation of social behavior across animals was not associated with mating system in our analysis. Overall, our findings suggest deep molecular homologies underlying the convergent or parallel evolution of monogamy in different cichlid lineages of Ectodini.

Modification of feeding circuits in the evolution of social behavior

Adaptive trade-offs between foraging and social behavior intuitively explain many aspects of individual decision-making. Given the intimate connection between social behavior and feeding/foraging at the behavioral level, we propose that social behaviors are linked to foraging on a mechanistic level, and that modifications of feeding circuits are crucial in the evolution of complex social behaviors. In this Review, we first highlight the overlap between mechanisms underlying foraging and parental care and then expand this argument to consider the manipulation of feeding-related pathways in the evolution of other complex social behaviors. We include examples from diverse taxa to highlight that the independent evolution of complex social behaviors is a variation on the theme of feeding circuit modification.

The role of neuropeptide F in a transition to parental care

The genetics of complex social behaviour can be dissected by examining the genetic influences of component pathways, which can be predicted based on expected evolutionary precursors. Here, we examine how gene expression in a pathway that influences the motivation to eat is altered during parental care that involves direct feeding of larvae. We examine the expression of neuropeptide F, and its receptor, in the burying beetle Nicrophorus vespilloides, which feeds pre-digested carrion to its begging larvae. We found that the npf receptor was greatly reduced during active care. Our research provides evidence that feeding behaviour was a likely target during the evolution of parental care in N. vespilloides Moreover, dissecting complex behaviours into ethologically distinct sub-behaviours is a productive way to begin to target the genetic mechanisms involved in the evolution of complex behaviours.

Adopt, ignore, or kill? Male poison frogs adjust parental decisions according to their territorial status

Systematic infanticide of unrelated young has been reported in several animal taxa. Particular attention has been given to carnivores and primates, where infanticide is a sexually selected strategy of males to gain increased access to female mating partners. Cannibals must ensure avoiding their own offspring and targeting only unrelated young. Therefore, decision rules are needed to mediate parental and cannibalistic behaviour. Here we show experimentally that male poison frogs adjust their parental responses – care or infanticide – towards unrelated clutches according to their territorial status. Male frogs followed the simple rule ‘care for any clutch’ inside their territory, but immediately switched to cannibalism when establishing a new territory. This demonstrates that simple cognitive rules can mediate complex behaviours such as parental care, and that care and cannibalism are antagonistically linked. Non-parental infanticide is mediated by territorial cues and presumably serves to prevent misdirected care in this poison frog. Our results thus prompt a re-consideration of evolutionary and causal aspects of parental decision making, by suggesting that selective infanticide of unrelated young may generally become adaptive when the risks and costs of misdirected care are high.