Birth spacing in the mouse communal nest shapes adult emotional and social behavior

Abstract

The interactions with the mother and with peers are among the most relevant early environmental factors shaping adult brain function and behavior. In order to investigate the role of these factors, we exploited a novel early manipulation, the Communal Nest (CN), consisting in a single nest where three mothers give birth, keep their pups and share care-giving behavior from birth to weaning. In particular, we reared CD-1 swiss mice in three different CN conditions, each one characterized by a different interval between the three deliveries (Birth Spacing) of 3, 5 or 7 days (respectively, CN ± 3, CN ± 5, CN ± 7). Length of birth spacing affected maternal behavior, CN ± 7 pups receiving the highest levels. At adulthood, mice reared in the different conditions showed differences in emotional response and social skills. In the plus maze test, short birth spacing was found to be associated with enhanced emotionality, CN ± 3 mice showing highest levels of anxiety-like responses in the plus maze compared to the other two CN groups. In the social interaction test, the strategies to achieve dominance differed among the three groups. While CN ± 3 mice appeared to have a more aggressive strategy, displaying high levels of attack behavior in the first encounter, CN ± 5 and CN ± 7 mice displayed a more affiliative strategy based on social investigation. Overall, these findings show that birth spacing shapes the early mouse social environment and, in turn, affects the development of social skills and emotional responses.

Introduction

The early environment has been shown to exert a major impact in determining the developmental trajectories of the organism. A key role has been attributed to the mother, representing for the mammalian infant the most important source of early experiences. Indeed, altricial mammals depend on her for essentially all of their needs, especially feeding [1]. Changes in maternal behavior have been related to modifications in the physiological and behavioral response of the offspring later in life [2], [3], [4], [5]. Studies carried out in different species, ranging from non-human primates to laboratory rodents, have shown that high levels of maternal behavior are associated with reduced emotionality and improved social skills in the adult offspring, in addition to lower hypothalamic-pituitary-adrenal (HPA) axis activation following stress [2]. These results have been interpreted according to the maternal mediation hypothesis [6], stating that the quantity and quality of maternal behavior is a key factor in shaping the adult individual. More recently, an increasing amount of data suggests that factors other than the mother mediate the long-term consequences of the early environment [7], [8], [9], [10]. For instance, interaction with peers appears extremely relevant. Indeed, according to their number, gender and age, they determine the quality and quantity not only of peer interactions but also of interaction with the mother [10], [11], [12]. In non-human primates, peer interactions during plastic post-natal phases are of fundamental importance for the development of sophisticated social behavior and emotional responses [13], [14], [15]. In rodents, though it has been reported that high social complexity is a feature of the ecological niche of the developing pup [16], [17], peer interaction and its consequences on the adult individual have been rarely investigated [12], [18], [19], [20]. For instance, it has been shown that litter gender composition affects behavior at long-term, mice reared in a balanced sex ratio condition showing higher levels of exploration compared to mice reared in isosexual litters [12].

In the mouse communal nest (CN), mothers combine their litters in a single nest and nurse them. In this condition, mothers take care of the pups in an apparent indiscriminate fashion, though this is still a debated issue [21], [22], [23]. Furthermore, more than one mother can be simultaneously on the nest (in a CN procedure involving three mouse mothers, when the nest is attended, an average of 1.5 mothers has been observed [24]). The CN occurs when deliveries are either synchronous or not, since it has also been observed when the discrepancy in age of pups was of 12 days [25]. When deliveries are asynchronous, birth spacing—i.e., the interval between deliveries—has an impact in characterizing the early social environment of the pup. Birth spacing affects pup development mainly by modulating two different features of its early environment: maternal behavior and peer competition. Indeed, the number and age of pups present in the nest may determine the amount of care-giving behavior provided by dams [12], [26], [27] as well as the competition among pups to access limited resources [28], [29], [30], [31].

The aim of the present study is to investigate the effects of variations of the social complexity of the mouse nest environment on pup development. In particular, we evaluated the effect of birth spacing, analyzing the long-term consequences on emotional and social behavior of being reared in a CN in which three CD-1 mouse mothers give birth 3, 5 or 7 days apart (respectively, CN ± 3, CN ± 5, CN ± 7). The birth spacing determines the age difference of the three litters belonging to the same CN. In the CN ± 3 condition, on the day the last litter was born, the other two litters are respectively, 3- and 6-days old. For the CN ± 5 and CN ± 7 conditions, the other two litters are, respectively 5- and 10-days old and 7- and 14-days old (Fig. 1). Previous studies exploiting the synchronous CN manipulation have shown that, when tested in the social interaction test, in which the experimental mouse faces a conspecific in a neutral cage, adult mice reared in CN have more elaborate social competences, promptly playing a given social role. Furthermore, CN mice display an increased anxiety-like response in the elevated plus maze [24], [32], [33]. Our hypothesis was that, depending upon birth spacing, these behaviors would be differently affected. In particular, we expected that the CN ± 3 group that, among the CN conditions here investigated, is the closest to synchronous CN, would show more elaborate social competences and higher levels of anxiety-like behavior compared to the CN ± 5 and CN ± 7 groups. The results are discussed in terms of the role of the many features of the early environment, including maternal behavior, in shaping the developing organism [7], [10].