Support for the parental practice hypothesis: Subadult prairie voles exhibit similar behavioral and neural profiles when alloparenting kin and non-kin

Parental care is critical for offspring survival in altricial species. Although parents are the most common caregivers, other individuals (e.g., older siblings) can also provide alloparental care. Some have argued that animals engage in alloparental behavior to practice providing care for their eventual offspring, whereas others have argued that alloparental behavior enhances indirect fitness. Proximate measures have the potential to test ultimate functions of behavior. A focus on neural expression of oxytocin and vasopressin (two neuropeptides modulating alloparental care) or neural activation following exposure to related and unrelated individuals could reveal whether practice or investment in indirect fitness explains alloparental behavior. This study examined alloparental behaviors and neural responses in prairie voles (Microtus ochrogaster), a species that engages in alloparental behavior. Subadult (independent, yet sexually immature) male prairie voles were exposed to one of four stimuli: same-age sibling, neonatal sibling, unrelated neonate, or inanimate neonate-sized object. We assessed alloparental behaviors and quantified cFos protein expression in oxytocin and vasopressin neuronal populations of the paraventricular nucleus of the hypothalamus and the supraoptic nucleus of the hypothalamus in response to stimulus exposure. We detected no differences in cFos and nonapeptide co-localization among stimulus groups. Subjects performed similar amounts of alloparental care toward related and unrelated neonates, but not other subadults or inanimate objects. Notably, caregiving did not differ based on kin-status. The lack of difference in alloparenting toward related and non-related neonates suggests that alloparental care in prairie voles primarily serves to provide subadults with parental practice.

The Role of Olfactory Genes in the Expression of Rodent Paternal Care Behavior

Olfaction is the dominant sensory modality in rodents, and is crucial for regulating social behaviors, including parental care. Paternal care is rare in rodents, but can have significant consequences for offspring fitness, suggesting a need to understand the factors that regulate its expression. Pup-related odor cues are critical for the onset and maintenance of paternal care. Here, I consider the role of olfaction in the expression of paternal care in rodents. The medial preoptic area shares neural projections with the olfactory and accessory olfactory bulbs, which are responsible for the interpretation of olfactory cues detected by the main olfactory and vomeronasal systems. The olfactory, trace amine, membrane-spanning 4-pass A, vomeronasal 1, vomeronasal 2 and formyl peptide receptors are all involved in olfactory detection. I highlight the roles that 10 olfactory genes play in the expression of direct paternal care behaviors, acknowledging that this list is not exhaustive. Many of these genes modulate parental aggression towards intruders, and facilitate the recognition and discrimination of pups in general. Much of our understanding comes from studies on non-naturally paternal laboratory rodents. Future studies should explore what role these genes play in the regulation and expression of paternal care in naturally biparental species.

Social novelty increases tyrosine hydroxylase immunoreactivity in the extended olfactory amygdala of female prairie voles

The monogamous social behaviors of prairie voles (Microtus ochrogaster) require olfactory inputs, which are processed by the posterodorsal medial amygdala (MeApd) and principal bed nucleus of the stria terminalis (pBST). The male prairie vole MeApd and pBST contain hundreds of cells densely immunoreactive for tyrosine hydroxylase (TH-ir). Female prairie voles have relatively few of these cells, but we previously found that the number of these TH-ir cells is greatly increased in females by exogenous estradiol. We here hypothesized that the number of TH-ir cells in their MeApd and pBST would also increase during the natural hormone surges associated with females' induced estrus. We found that the number of TH-ir cells in both sites did significantly increase after females cohabitated for two days with an unfamiliar male. However, this increase did not require the presence of ovaries and even tended to occur in the pBST of females cohabitating for two days with unfamiliar females. We then determined if the greater number of TH-ir cells after heterosexual pairing was transient by examining two groups of long-term pairbonded females (primiparous and multiparous), and found these females also had significantly more TH-ir cells in the pBST and/or MeApd compared to unmated controls. Thus, social novelty arising from cohabitation with unfamiliar conspecifics produces a reoccurring increase in the number of TH-ir cells in the female prairie vole extended olfactory amygdala. Ovarian hormones are not necessarily required. This increase in catecholaminergic cells may facilitate acquisition and retention of olfactory memories necessary for social recognition in this species.

Social interactions, cross-fostering, and sibling recognition in prairie voles, Microtus ochrogaster

Sibling recognition in prairie voles (Microtus ochrogaster) has been traditionally inferred from incest avoidance or lack of reproduction among littermates. Researchers have concluded that when sibling prairie voles are isolated from one another there is a breakdown of incest avoidance (and therefore of sibling recognition). In a reevaluation of these studies, using social interactions rather than incest avoidance, we found that the breakdown of incest avoidance was not equivalent to a breakdown of sibling recognition. We explored the effects of cross-fostering on sibling recognition by looking at changes in amicable or agonistic behaviors that were then used to infer sibling recognition. Prairie voles that were cross-fostered with nonsiblings were not able to recognize their own siblings when reunited and tested for social interactions in adulthood. Siblings reared apart treated their own siblings as if they were not closely related and nonsiblings reared together treated nonsiblings as if they were their own siblings. The mechanism of sibling recognition in prairie voles is association or familiarity. The ability to recognize siblings may be limited to individuals that are raised together. Sibling recognition in prairie voles may be a by-product of familiarity in early life and not a mechanism restricted to genetic relatedness as predicted by kin-selection theory.

Effects of exposures to siblings or sibling odors on sibling recognition in prairie voles (Microtus ochrogaster)

Determining the mechanisms of sibling recognition is important for understanding social behavior and the basic parameters of population dynamics (cycles) in microtine rodents. Previous studies have shown that, after relatively short periods of isolation, prairie voles (Microtus ochrogaster) no longer recognize their siblings. In this study we tested the hypothesis that brief encounters of prairie voles with siblings or sibling odors during a period of isolation can maintain social memory and the ability of animals to recognize their siblings over time. Six-week-old voles of both sexes that were isolated for 21 days and exposed (on days 3, 6, 9, 12, 15, and 18 of this 21-day isolation period) to brief encounters (30 min for each encounter) with their siblings continued to recognize their siblings when tested for social interactions at 42 days of age. Only females exposed to odors of same-sex siblings during the period of isolation continued to recognize their sisters. Males exposed to odors of same-sex siblings did not recognize their brothers when tested for social interactions at 42 days of age. These results demonstrate that after dispersal prairie voles need occasional encounters with their siblings, or their siblings' odors (at least in females), to reinforce social memory and the ability to recognize kin over prolonged periods of time. This study is the first to provide empirical evidence that helps us understand the mechanism by which sibling prairie voles, which are philopatric or often settle in home ranges close to one another, almost never form breeding pairs.