Effects of sex and age on parental motivation in adult virgin California mice

Highlights

• We compared parental motivation in male and female adult virgin California mice.

• Males were more parentally motivated than females in two of three behavioral tests.

• Parental motivation did not differ markedly between middle-aged and older adults.

• T-maze tests provide complementary information to standard parental-behavior tests.

Abstract

Female mammals often demonstrate a rapid initiation of maternal responsiveness immediately after giving birth, as a result of neuroendocrine changes that occur during pregnancy and parturition. However, fathers and virgins of some species may display infant care similar to that performed by mothers but without experiencing these physiological events. In biparental species, in which both mothers and fathers care for their offspring, both sex and age may affect parental motivation, even in adult virgins. We examined the effects of sex and age on parental motivation in the California mouse, a monogamous, biparental rodent. We compared parental motivation of male and female virgins in both mid- and old adulthood using two new tests – a T-maze test and a rain test – as well as in standard parental-behavior tests. Adult virgin males were more parentally motivated than adult virgin females in both the T-maze test and the parental-behavior test, but parental motivation did not differ markedly between middle-aged and older adults of either sex. These findings suggest that sex differences in parental motivation in adult virgins are similar to those observed in other biparental rodents, and indicate that the T-maze test may be useful for evaluating parental motivation in this species.

Introduction

To reproduce successfully, female mammals must provide care to offspring through species-specific patterns of caregiving behavior, often beginning immediately after the birth of their infants. In some species, the rapid onset of maternal behavior in new mothers is brought about through hormonal changes occurring during pregnancy, parturition, and the initiation of lactation. These changes and the neuroendocrine mechanisms governing maternal behavior have been studied extensively, especially in females of uniparental species – i.e., species in which only one parent provides offspring care (Neumann, 2008; Numan and Insel, 2003).

In addition to mothers, fathers show caregiving behavior (i.e., “parental behavior”) toward young in some species. Approximately 5-10% of mammalian species are biparental, meaning that both mothers and fathers consistently provide care to offspring under natural conditions (Kleiman and Malcolm, 1981). Males of these species can engage in parental behavior to a similar extent as females, potentially providing all the same types of care as mothers, with the exception of lactation. Although an increasing number of studies have focused on the initiation of paternal behaviors in fathers in the last 30 years, the underlying mechanisms are not well understood. Systematic changes in several endocrine signaling systems (e.g. prolactin, testosterone, and vasopressin) have been observed in fathers, yet the functions of these changes are not clear (Horrell et al., 2019a; Saltzman and Ziegler, 2014). Furthermore, in contrast to females, such hormonal changes may not be necessary for the rapid onset of paternal care in new fathers (Numan and Insel, 2003).

Similar to fathers, virgin animals of some uniparental and biparental species may display care towards young without undergoing the extensive hormonal changes observed in mothers. (Note that caregiving by virgin animals is a form of alloparenting [i.e., care provided to non-descendant young]; however, we will refer to care provided by virgins as “parental care” throughout this paper for consistency with many relevant papers.) For example, adult virgin females in several strains of house mice (Mus musculus) are often spontaneously “maternal,” exhibiting parental behavior upon their first exposure to pups (Gandelman, 1973; Martín-Sánchez et al., 2015). In contrast, adult virgin female prairie voles (Microtus ochrogaster) and rats (Rattus norvegicus) may attack, ignore, or care for foster pups at first exposure (Bales et al., 2007; Jakubowski and Terkel, 1985; Lonstein and De Vries, 2001). Sex differences in adult virgins’ responses to pups have been observed in several rodent species. For example, in prairie voles, golden hamsters (Mesocricetus auratus), and Mongolian gerbils (Meriones unguiculatus), virgin adult males are more spontaneously parental than virgin adult females (reviewed in Lonstein and De Vries, 2000; Saltzman et al., 2009).

Age, in addition to sex, can affect motivation to perform parental care. Although parental responsiveness often wanes around the time of puberty, it may increase again in aged adults. In post-pubertal rodents (e.g. Mongolian gerbils and dwarf hamsters [Phodopus campbelli]), for example, parental responsiveness may be lower during young adulthood than during both the prepubertal period and advanced age (Clark and Galef, 2001; Clark et al., 2002; Elwood, 1980; Vella et al., 2005). The mechanisms underlying these age-related changes in parental motivation are not fully understood but could involve changes in levels of gonadal steroid hormones. Puberty is characterized by increased production of gonadal steroids that can potentially modulate parental care (Horrell et al., 2019a), whereas senescence of endocrine function in old age may decrease the production and/or activity of these hormones (Clark and Galef, 2001; Olazábal and Alsina-Llanes, 2016). These changes in gonadal hormone levels may thus lead to differences in parental motivation between old and young individuals. In laboratory settings, animals are usually bred soon after puberty and breed during their reproductive prime; however, wild animals may not necessarily gain the opportunity to breed until later in life, which may be attributed to seasonality, intrasexual competition, and population density (Antor et al., 2007; Rood and Weir, 1970). Therefore, comparing parental motivation in old and young adults may provide insight into the ontogeny of parental motivation across an individual’s lifespan.

The California mouse (Peromyscus californicus) is a valuable model of biparental care because it is monogamous and biparental in both field and laboratory conditions (Bredy et al., 2007; Cantoni and Brown, 1997; Dudley, 1974; Gubernick and Nordby, 1992; Gubernick and Teferi, 2000; Wright and Brown, 2002). Male California mice exhibit paternal care immediately after the birth of their offspring and spend as much time caring for offspring (e.g., huddling, grooming, and retrieving pups) as mothers (Gubernick and Alberts, 1987; Lee and Brown, 2002; Jašarević et al., 2013; Rosenfeld et al., 2013). When experimentally presented with unrelated pups, fathers usually engage in parental behavior, while adult virgin males may either attack, ignore, or care for the pups (Chauke et al., 2012; de Jong et al., 2009; Gubernick and Alberts, 1987). In adult virgin males, previous experience with pups – as little as two 20-minute exposures – can facilitate the onset of parental behavior (Horrell et al., 2017). The initiation of parental behavior in virgin females of this species has received little attention, as compared to virgin males. Gubernick and Laskin (1994) found that across the transition to adulthood, both male and female California mice show pronounced decreases in parental behavior and increases in infanticide toward an unfamiliar pup. These studies found no sex differences in parental responsiveness.

The most common method of testing parental behavior in rodents is the parental-behavior test, in which one or more related or unrelated pups are introduced to an animal in its home cage or a neutral cage and the subject’s behavioral responses to the pup(s) (e.g. latency to approach, duration of huddling, retrieval, pup-directed aggression) are quantified (Lambert et al., 2011). However, the parental-behavior test may not always provide a sensitive measure of motivation (Fraser and Matthews, 1997) because the subject does not have to work or overcome any obstacle in order to interact with a pup. This can lead to a ceiling effect, potentially preventing detection of subtle differences among subjects with generally high levels of motivation.

Previous studies have demonstrated that different tests for parental motivation may yield different results. For example, in both rats and house mice, mothers and sensitized virgin females display similar levels of maternal behavior in the home cage (rats: Fleming and Rosenblatt, 1974; house mice: Noirot, 1964), but mothers retrieve pups more frequently than sensitized virgins in a T-maze and a straight alley (Bridges et al., 1972; Gandelman et al., 1970). Together, the results of these and other studies suggest that differences in parental motivation may emerge when animals are required to overcome an obstacle (e.g. novel environment, completing a task) to gain access to pups or when animals must choose between pups (or pup-associated stimuli) and another stimulus. Thus, these tests may provide information complementary to measures of parental motivation obtained under baseline conditions.

We propose two new tests to complement the standard parental-behavior test for characterizing parental motivation. Recently, our lab used a modified open-field paradigm with a pup in the center of the open-field arena to measure paternal motivation under presumably anxiogenic conditions, and found that fathers had shorter latencies to approach pups and spent more time engaging in parental behaviors than virgin males (Perea-Rodriguez et al., 2018). In the present study we developed a new behavioral test, the rain test, in which dripping water serves as an obstacle to interacting with a pup. Wet bedding is commonly used as a stressor for rodents, including this species (Harris and Saltzman, 2013; Huang et al., 2017; Kompagne et al., 2008), and pilot tests indicated that California mice tend to avoid dripping water (unpub. data). A more nuanced examination of parental motivation may also be accomplished by comparing an animal’s attraction to a pup vs. another rewarding stimulus. Thus, we developed a T-maze test to compare parental motivation with motivation to interact with palatable food.

The aims of this study were two-fold. First, we evaluated possible effects of sex and age on motivation to care for pups in adult virgin California mice. We hypothesized that 1) old adult virgins will display more parental motivation than younger adult virgins, and 2) adult virgin males will be more parentally motivated than adult virgin females. Second, we assessed the validity of two novel tests of parental motivation in adult virgin California mice by determining animals’ 1) preference for interacting with an unfamiliar pup compared to ingesting palatable food, and 2) willingness to overcome an aversive stimulus (simulated rain) to reach a pup.