Comparison of the Parental Behavior of Pair-Bonded Female and Male Prairie Voles (Microtus ochrogaster)

Physiology & Behavior, Vol. 66, No. 1, pp. 33–40, 1999 © 1999 Elsevier Science Inc. Printed in the USA. All rights reserved PIIS0031-9384/99/$–see front matter

PII S0031-9384(98)00270-4

Comparison of the Parental Behavior of Pair-Bonded Female and Male Prairie Voles (Microtus ochrogaster) JOSEPH S. LONSTEIN1 AND GEERT J. DE VRIES Center for Neuroendocrine Studies, Tobin Hall, Box 37720, University of Massachusetts Amherst, MA 01003 Received 20 April 1998; Accepted 1 September 1998 LONSTEIN, J. S. AND G. J. DE VRIES. Comparison of the parental behavior of pair-bonded female and male prairie voles (Mictrotus ochrogaster). PHYSIOL BEHAV 66(1) 33–40, 1999.—The behavior of primiparous lactating prairie voles (Microtus ochrogaster) and their mates individually interacting with pups was continuously assessed for 45 min after a 2-h parent–litter separation on days 3–4 and 10–11 postpartum. Both sexes were highly parental after reunion with the young, and their general pattern of behavior consisted of bouts of quiescence interspersed with bursts of heightened activity. Lactating females spent more time than males in contact with pups, and more time being quiescent, most often in the kyphotic (upright crouched) nursing posture. Even in the absence of nipples upon which the pups could suckle, males also displayed kyphosis, although for shorter durations than females. Males spent more time, however, huddled over the litter in a hunched position than their mates. In accordance with their decreased quiescence, male voles licked and carried pups more and were more exploratory than females. Compared with the first week postpartum, bouts of kyphosis were shorter during the second week postpartum for both sexes, while laying prone on the pups increased. Males spent less time licking and more time carrying older pups than younger ones, and were more exploratory during the second week postpartum. Sex differences in the parental behavior of prairie voles may reflect differences in the somatosensory stimulation that females and males receive from pups. Furthermore, the display of kyphosis by male voles indicates that the sensorimotor organization of this posture in voles differs from that of lactating rats, which require suckling stimulation for its regulation. © 1999 Elsevier Science Inc. Lactation

Maternal behavior

Monogamy

Paternal behavior

Suckling

ties because there have been few studies in which parental males and females have been tested concurrently under identical test conditions [36a]. It is likely, though, that such differences exist. First, the neurochemical mechanisms underlying the onset of parental behavior in female and male prairie voles differ, with fluctuations in ovarian hormones during gestation and parturition most likely responsible for the transition from infanticide to nurturance in adult females (22) and sexually dimorphic forebrain projections containing the neuropeptide arginine-vasopressin (AVP) possibly involved in promoting high levels of parental responsiveness in both virgin and sexually experienced males (1,2,23,45). Second, suckling stimulation provided by pups is crucial for the display and patterning of most maternal behaviors in lactating rats [see (37)]. In the absence of prehensile nipples onto which pups may suckle, stimuli that

PRAIRIE VOLES (Microtus ochrogaster) display several morphological (6,34), neuroanatomical (2,20,36), and behavioral [see (4)] attributes that have been suggested to be indices of social monogamy (4,21). One such behavioral distinction of monogamous mammals is the sharing of parental activities, the existence of which has been inferred for prairie voles from field studies where both parents can be found with offspring in the natal nest (10–12), as well as from direct observations of pair-bonded males or females in seminatural (15,30,32,43,50) and laboratory (1,2) environments. Previous comparisons of parental behavior in voles have focused primarily on differences between vole species (15,30, 32,50), or the effects of different social environments on their parental behaviors (46,47). Less is known of the similarities or differences that may exist between female and male prairie voles in the components and patterns of their parental activi-

1To

Somatosensation

whom requests for reprints should be addressed. E-mail: [email protected]

33

34

LONSTEIN AND DE VRIES

male prairie voles receive from pups are necessarily different than those received by their lactating counterparts. To test the hypothesis that sex differences in neuroendocrinology and somatosensory inputs produce sex differences in the quality of parental behavior, we observed the behavior of female and male prairie voles interacting with their litters for periods during the first and second weeks postpartum. METHODS

Subjects Subjects were nine female and nine male F1 generation prairie voles (Microtus ochrogaster) born and raised in our colony, which was established in 1996 at the University of Massachusetts, Amherst, from breeding stock provided by Dr. Betty McGuire (Smith College, Northampton, MA) and Dr. Zuoxin Wang (Emory University, Atlanta, GA) that originated from offspring of wild-caught voles from Urbana, IL. Animals were maintained on a 14:10-h light:dark cycle at an ambient temperature of 218C. Animals were housed in plastic cages (48 3 28 3 16 cm) containing wood chips, wood shavings, and substantial hay covering. Water and a food mixture containing cracked corn, whole oats, sunflower seeds, and Purina rabbit chow (ratio of 1:1:2:2) were available ad lib. Subjects were weaned at 20 days of age and housed with their littermates in mixed-sex groups until sexing approximately 10 days later. At 70–90 days old, subjects were removed from their littermates and rehoused alone in plastic cages for 3 days. Females were then placed in the home cage of isolated males. Twenty-one days after pairing, cages were checked daily for the presence of litters with the day after parturition 5 day 1 postpartum (PP). Litters were culled to five pups on Day 1 PP. Procedure On the morning of Day 3 PP, litters were removed from their parents and placed in a humid incubator maintained at 328C. Either the dam or sire was removed from the home cage and placed in a similarly sized clean plastic cage with wood chips and shavings for bedding, which also contained a small amount of hay, food mix, and water. After a 2-h parent–litter separation, pups were expressed of feces and urine, weighed to the nearest 0.01 g, and scattered in the test cage opposite to where the subject was sitting. Behavior of subjects and litters was continuously observed for 45 min. Pups that remained excluded from parental attentions 10 min after reunion were moved to the subject by the experimenter. Pups were removed after testing, weighed, and both parent and pups were returned to their home cage. Relative litter weight changes were expressed in mg as {[(postobservation wt) 2 (preobservation wt)]/preobservation wt} 3 1000. The other parent was tested similarly on Day 4 PP (in counterbalanced manner). On Days 10 and 11 PP, litters were again removed from their parents, incubated for 2 h at 298C, and the behavior of dams and sires were again measured for 45 continuous minutes. Within a particular pair, subjects were tested in the same order as the previous week. A custom-made computerized data acquisition system was used to record latency, frequency, and duration of numerous behaviors. Active behaviors directed toward pups included sniffing, licking, and carrying them from one place to another. Nonpup-oriented activities recorded were self-grooming, exploration away from the pups, digging or nest construction, and eating or drinking. Measures of nursing behavior of

dams, and “huddling” behavior of sires, included actively hovering—defined as hovering over the pups while performing other activities—and being quiescently positioned over the litter in one of three mutually-exclusive postures: 1) high or low kyphosis (upright crouch) characterized by all limbs placed on the ground in a splayed and rigid manner, depression of the head, and pronounced dorsal arch that was strikingly similar to that observed in lactating rats (39); 2) hunched over the litter with body weight resting only on the hindlimbs and hindflanks, forelimbs elevated and drawn toward the body, and head passively resting on top of the litter; and 3) laying prone on the litter mass with little or no limb support (24). Two quiescent positions that were found to be present only for very short durations were also recorded: females laying on their side while supine with the pups attached to her nipples (40), and males laying adjacent to the litter mass in a side-by-side manner. Total activity was the sum of the duration of all active behaviors recorded, and total time with pups was the sum of time spent hovering over pups while active plus the time spent during quiescent nursing/huddling. Total quiescence was the duration of time spent in all quiescent postures. A quiescence bout was defined as a period >30 s spent in one or more of the quiescent postures defined above without interruptions longer than 5 s. A kyphosis bout was defined as any period of time in which the high or low kyphotic posture was sustained for >5 s and not interrupted for more than 2 s. Data Analyses Behavioral and litter weight change data were analyzed with repeated measures ANOVA followed by Fisher’s t post hoc analyses. Pearson’s r correlation coefficient was used to compare data within prairie vole pairs and to determine its consistency for individual subjects across test days. Chi-square analysis was used to analyze categorical data. Statistical significance was indicated in all cases by p < 0.05. RESULTS

As expected, females and males received different ventral somatosensory inputs during interactions with pups. All five suckling pups were each firmly attached to one of their dam’s six nipples when females were removed from the test cage at the completion of behavioral observations. In contrast, pups were never seen to be attached to the penile sheath, fur, or skin of their sire during testing or when he was removed from the observation chamber after testing. Active Behaviors Both female and male prairie voles were highly parental at reunion (Table 1), with a similar latency to initiate contact with pups, which was almost always in the form of sniffing. None of the males and only one female retrieved the pups to a particular location; instead most subjects attended to the scattered pups sequentially. In fact, grouping of pups that eventually occurred after reunion was typically the result of the pups moving to the parent. Carrying of pups for any distance was very infrequent for females during either test week. The same was true for males during the first week PP. During the second week PP, however, males often retrieved the highly mobile pups [sex: F(1, 16) 5 4.7, p < 0.05; sex 3 day: F(1, 1) 5 8.7, p < 0.01; Fig. 1a]. After initial contact with pups, subjects soon began hovering over them while performing activities such as facial and anogenital licking of the pups and self-

PARENTAL BEHAVIOR IN PAIR-BONDED PRAIRIE VOLES

35

TABLE 1 MEASURES (MEAN 6 SEM) OF ACTIVE PARENTAL BEHAVIORS OF FEMALE AND MALE PRAIRIE VOLES DURING 45-MIN INTERACTIONS WITH PUPS ON DAYS 3 OR 4 POSTPARTUM AND DAYS 10 OR 11 POSTPARTUM

Latency (s) Contact pups Female Male Duration (s) Prequiescence pup licking Female Male Nest/burrow Female Male Self-groom Female Male Explore Female Male Total activity* Female Male

Days 3–4

Days 10–11

965 764

562 663

59 6 10a 155 6 33b

53 6 15a 91 6 21b

33 6 13 105 6 60

80 6 40 146 6 54

51 6 13 52 6 13

38 6 12 59 6 12

4 6 2a 61 6 31b

5 6 3a 218 6 73c

503 6 41a 796 6 85b

483 6 73a 847 6 114b

Fsex

Fday

Fsex 3 day

0.20

0.70

0.15

8.79‡

2.94

2.06

1.62

1.73

0.01

0.50

0.08

1.28

7.46†

8.98‡

8.71‡

12.16‡

0.05

0.27

*Includes eating and drinking which were of very short duration and not displayed by all subjects, and sniffing the pups, which was also of short duration. a,b Significant post hoc differences between groups indicated by different superscript letters (p < 0.05). † 5 ANOVA ps < 0.01. ‡ 5 ANOVA ps < 0.01.

grooming. Perioral stimulation received by the subjects from licking the litter prior to the onset of the first bout of quiescence was greater in males than females, particularly on Days 3–4 PP, and did not change for either sex when interacting with older pups (Table 1). Total licking (before and after the onset of quiescence) was 35% greater in males than in females on Days 3–4 PP [sex: F(1, 16) 5 8.3, p < 0.01; day: F(1, 1) 5 18.8, p < 0.001; sex 3 day: F(1, 1) 5 9.3, p < 0.008; Fig. 1b], but did not differ between sexes later PP. Males also spent more time during both tests displaying nonpup-oriented activities such as digging/nesting, self-grooming, and exploring, though only the latter was significantly different from females. Accordingly, total activity was 60% greater in males than females (Table 1). Inactive Nursing/Huddling Behaviors

FIG. 1. Duration of time (mean 6 SEM) spent by female and male prairie voles: (A) carrying and (B) licking pups during 45-min parent–litter interactions on Days 3–4 and 10–11 postpartum. Significant differences between groups indicated by different letters above bars.

The time between the first contact with pups to the onset of actively hovering over them was shorter for females than males (Table 2), and females spent approximately 10% more time in physical contact with pups than males [sex: F(1, 16) 5 14.2, p < 0.002; Fig. 2]. Pups typically became attached to the nipples of females within 45–60 s after reunion, and dams tended to spend less time actively hovering over the litter [sex: F(1, 1) 5 2.8, p < 0.12] and more time quiescently positioned over them than males (Table 2). After actively hovering over pups for approximately 5 min, females as well as males displayed their first bout of quiescence with a similar latency. All subjects displayed kyphosis (upright crouched posture). Although kyphosis was the most typical quiescent posture in both sexes, females spent more time than males in this posture during both tests [sex: F(1, 1) 5 51.4, p < 0.0001; Fig. 2]. Females also displayed longer bouts of kyphosis (Table 2). Furthermore,

36

LONSTEIN AND DE VRIES TABLE 2 MEASURES (MEAN 6 SEM) OF NURSING/HUDDLING BEHAVIOR IN FEMALE AND MALE PRAIRIE VOLES, AND RELATIVE LITTER WEIGHT CHANGES, DURING 45-MIN INTERACTION WITH PUPS ON DAYS 3 OR 4 POSTPARTUM AND 10 OR 11 POSTPARTUM Days 3–4

Latency (s) Hover over from first contact Female Male Quiescence Female Male Number of quiescence bouts Female Male Quiescence bout duration (s) Female Male Total quiescence (s) Female Male Interbout interval (s) Female Male Number of kyphosis bouts Female Male Kyphosis bout duration (s) Female Male Longest sustained kyphosis (s) Female Male RLWC (mg) Female Male

2 6 1a 5 6 1b

Days 10–11

Fsex

Fday

Fsex 3 day

4.73†

1.39

0.37

2.86

0.66

0.98

0.36

4.47*

0.08

3.49

4.56†

0.83

8.05‡

0.01

0.15

0.89

0.86

0.08

1.37

0.25

4.14*

9.30‡

5.93†

0.68

16.93§

5.29†

1.10

23.05§

3.05

1.35

1 6 1a 3 6 1b

308 6 65 394 6 56

216 6 42 403 6 102

7.3 6 0.5 8.1 6 1.1

6.2 6 0.8 6.7 6 0.9

260 6 22a 200 6 32b

343 6 50c 233 6 42ab

1906 6 53a 1518 6 113b

1927 6 111a 1478 6 169b

105 6 14 143 6 28

128 6 29 155 6 36

11.3 6 1.1 12.3 6 1.9

13.6 6 1.3 8.7 6 1.7

161 6 17a 100 6 21b

123 6 10bc 73 6 13b

554 6 70a 350 6 46b

495 6 49a 191 6 51c

11.1 6 2.8a 24.3 6 1.3b

5.5 6 2.9c 25.4 6 2.1b

RLWC 5 relative litter weight change. a,b Post hoc differences between groups indicated by different superscript letters.* 5 ANOVA ps < 0.06. † 5 ANOVA ps < 0.05. ‡ 5 ANOVA ps < 0.01. § 5 ANOVA ps < 0.001.

males tended to spend more time in kyphosis on Days 3–4 PP than on Days 10–11 PP [sex 3 day: F(1, 1) 5 4.1, p < 0.06]. Kyphosis was often the first quiescent nursing/huddling posture assumed, with six of nine and eight of nine females and half as many males initiating their first bout of quiescence with this posture on Days 3–4 and 10–11 PP, respectively (females vs. males weeks 1 and 2: x2 5 1.0, 1.3, respectively, ps > 0.1). Time spent nursing/huddling over the litter in the hunched posture was greater for males than females on both test days [sex: F(1, 16) 5 21.5, p < 0.0003; Fig. 2], whereas time spent laying on the litter while prone was similar for both sexes [sex: F(1, 16) 5 0.06, p > 0.8] and greater on Days 10–11 PP [day: F(1, 1) 5 8.7, p < 0.01; Fig. 2]. A total of 66 and 73 bouts of quiescence >30 s in length were observed for females and males, respectively. Females tended to remain quiescent longer during each bout (Table 2; Fig. 3). For both sexes, most of these bouts lasted either less than 60 s or were between 2–5 min in length. The average length of quiescence bouts in any posture was longer for both

sexes on Days 10–11 PP than Days 3–4 PP, but only significantly so for females (Table 2; Fig. 3). Duration of time between bouts of quiescence (interbout interval, IBI) was similar for males and females (Table 2). Similar to bouts of general quiescence, frequency of bouts of kyphosis >5 s in length did not differ between females and males, but bouts were longer in duration for females on both test days (Fig. 4). Furthermore, individual bouts of kyphosis were longer for females on Days 3–4 PP compared with Days 10–11 PP. Changes in Litter Weight Weights of litters interacting with males and females were similar [sex: F(1, 16) 5 0.2, p > 0.8, data not shown], and all litters gained weight between Days 3–4 and 10–11 PP [day: F(1, 1) 5 77.0, p < 0.0001, data not shown]. Simultaneous stretching and wriggling of the entire litter in response to milk letdown, such as that seen in suckling rats (7), was not observed in prairie vole pups. Nonetheless, eight of nine litters

PARENTAL BEHAVIOR IN PAIR-BONDED PRAIRIE VOLES

FIG. 2. Duration of time (mean 6 SEM) spent by female and male prairie voles hovering over their litter while active and nursing/huddling over them in the kyphotic, hunched, and prone postures during 45-min observations on Days 3–4 and 10–11 postpartum. TTWP 5 total time with pups, which includes the adjacent and supine positions which were of very short duration and not displayed by all subjects (data not shown).

interacting with their dams on Days 3–4 PP showed small weight gains (range 1.2–23.4 mg) by the end of the 45-min observation with the exceptional litter showing no weight change (Table 2). In contrast, six of nine litters interacting with their sires showed small weight losses (range 4.0–10.5 mg), and three of nine showed no change in weight. During the second week postpartum, six of nine litters interacting with females showed weight gains (range 2.8–22.0 mg), two a small loss, and one no change. All 10–11-day-old litters interacting with their sires lost weight (range 2.8–16.4 mg) with the exception of one litter which was observed to eat solid food during the observation and gained almost 7 mg by the end of testing.

37

activity (r 5 0.56, p < 0.02). Consistency across testing was found for many measures of nursing/huddling behavior as well. These include the latency to become quiescent (r 5 0.53, p < 0.03), duration of time spent hovering over the litter while active (r 5 0.55, p < 0.02) and in kyphosis (r 5 0.77, p < 0.0002), the duration of the longest sustained kyphotic posture (r 5 0.53, p < 0.03), the duration of individual bouts of kyphosis (r 5 0.52, p < 0.03), duration of quiescence in any posture (r 5 0.72, p < 0.0007), the frequency (r 5 0.49, p < 0.04) and duration (r 5 0.61, p < 0.007) of quiescence bouts, and the duration of time between these bouts (r 5 0.60, p < 0.008). The duration of kyphosis on test Days 10–11 PP was significantly correlated with the ratio of the subject and their litter weights after testing (r 5 0.53, p < 0.03), although this may, in part, be explained by the fact that most females were heavier than most males [49 6 3 g vs. 36 6 2 g; t(16) 5 3.4, p < 0.004). No behavioral measures were found to be correlated between females and their mates for either Test Days 3–4 or 10–11 PP. DISCUSSION

The present results demonstrate that female and male prairie voles display an identical repertoire of behaviors while in contact with pups. These behaviors included periods of heightened activity including licking the pups and self-grooming, but not frequent carrying of the young, interspersed with periods of relative quiescence, which for both sexes included the kyphotic posture. Males spent less time in contact with the pups than females, but when they were contacting pups, males spent more time licking and carrying them. Accordingly, males spent less time quiescently positioned over pups, including in the kyphotic posture, but more time hunched over them than females. In addition, whereas females did not interact differently with younger and older pups, males licked older pups less, carried them more, and displayed less kyphosis over them. No relationship was found between the behavior of females and males within individual pairs.

Behavioral Consistency Across Lactation and Within Pairs Behavior of male and female subjects was found to be quite consistent between the first and seconds weeks postpartum. Significant correlations between test weeks were found for numerous active behaviors including self-grooming (r 5 0.48, p < 0.05), digging/nesting (r 5 0.48, p < 0.05), and total

Methodological Considerations Our present observation of individual prairie vole parents interacting with their pups in the absence of their mates represents a natural type of parent–young interaction in this species. Prairie vole parents most often build and maintain a sin-

FIG. 3. Frequency histogram of duration of bouts of quiescence for female and male prairie voles on (A) Days 3–4 and (B) 10–11 postpartum during 45-min interactions with pups.

38

LONSTEIN AND DE VRIES

FIG. 4. Frequency histogram of duration of bouts of kyphosis for females and males prairie voles on (A) Days 3–4 and (B) days 10–11 postpartum during 45-min interactions with pups.

gle nest (15,30,43,46), but interactions between single parents and pups are common (50), and either the dam or sire is alone with the litter in the nest between 20% (43) and 55% (15) of the preweanling period. Furthermore, with the exception of licking of the pups, which increases when only one parent is in the nest, the parental behavior of male and female prairie voles does not change if they interact with pups alone or in the presence of their mates (18). The use of a relatively lengthy parent–litter separation prior to behavioral testing applied in the current study may have potentially altered some facets of parental behavior by increasing the motivation of the pups and parents to seek and maintain contact with each other (3,26). However, this procedure has proven useful in determining factors underlying the sensory and neuroanatomical regulation of maternal behavior in lactating rats because it increases the probability that the entire range of parent–young interactions are seen within a restricted testing period [e.g., (24)]. Additionally, mother–litter separation paradigms provide similar data for both lactating rats (26) and guinea pigs (19) compared with data obtained via undisturbed observation. Display of Active Parental Behaviors in Prairie Voles The absence of robust retrieval of prairie vole pups by their parents observed herein is consistent with earlier reports (15,18,30,32), as is the greater likelihood of sires carrying older pups compared with dams (15). The presence of teeth in prairie vole pups soon after birth (35) and their strong adherence to their dam’s nipples even when she is highly active (35,43,50) may reduce the need for retrieval or carrying behavior by their parents. In addition, a propensity for pups to seek and maintain physical contact with a parent may confine the young to the natal nest, because it is almost continuously occupied by at least one of the parents (30,32). Retrieval of pups may, therefore, be an unlikely event for prairie voles in natural settings, as well as for other biparental species that display infrequent carrying of the young in laboratory settings (8,17,28,49). This raises the question as to whether neural sites such as the medial preoptic area, which is particularly critical for retrieval behavior in species such as the rat that produce altricial young that are less capable of maintaining nipple attachment (31), play a central role in the parental behavior of prairie voles.

The greater licking of the pups in male prairie voles compared with females, particularly during the early postpartum period, may be related to a decreased tendency of the males to become quiescent in response to nonsuckling contact from the litter. Sex differences in parental licking behavior may also be related to differential attention paid to particular body regions of the pups (16). For example, the ingestion of water and electrolytes from pup urine regulates anogenital licking of pups in lactating rodents (9,16). Similarly, in male prairie voles anogenital licking may increase the activity of the central vasopressin projections implicated in their parental behavior (1,2). Mothers of other biparental species, however, have been reported to groom their pups more than fathers (8,18,29). This sex difference in licking, therefore, does not appear to be a general feature of biparental mammals, and may indicate neurobiological divergence across species in the proximate mechanisms controlling this behavior. Nursing/Huddling Behaviors in Prairie Voles Differences between males and females in the ventral somatosensory inputs they received from pups likely contributed to the observed differences in their parental behaviors. Tactile stimulation of the ventrum provided by suckling infants produces behavioral changes in many female mammals including a transition from heightened activity to prolonged periods of quiescence, which involves slow-wave sleep [e.g., (5,39,44)], and the assumption of distinct postures that expose the nipples and facilitate suckling by the young (24,39). Nonsuckling stimulation of the ventrum provided by either active or inactive pups does not trigger the same hormonal, behavioral, or neurobiological consequences (24,25,37–41). When suckling is prevented, lactating rats remain in physical contact with pups for as long as suckled females, but spend more time in nonkyphotic quiescent postures, actively hover over their litters longer, and spend more time displaying other pup-oriented and nonpup-oriented activities (24,25,27,38,39, 40). Reminiscent of these behavioral differences between suckled and nonsuckled lactating rats, males prairie voles spent more time in the hunched posture and more time licking and carrying pups than their suckled, lactating mates. However, in direct contrast to nonsuckled lactating rats, which exhibit very little kyphosis if any at all (38,39), all male prairie voles displayed this posture. In addition, bouts of quiescence, regardless of posture, had a latency, frequency, and duration

PARENTAL BEHAVIOR IN PAIR-BONDED PRAIRIE VOLES similar to suckled female voles. Furthermore, male prairie voles maintained less physical contact with pups than female prairie voles, as seen previously [(15,30); but see (46)], and did not actively hover over the litter longer than their mates. Whereas the hormones associated with pregnancy, parturition, and lactation may lead to enhanced ventral somatosensory processing necessary for kyphosis in lactating rats [see (37)], and possibly lactating voles, kyphosis in male prairie voles does not require such endocrine fluctuations because sexually and parentally naive males display kyphosis during their first exposure to pups (22). In the absence of lactation, the function of kyphosis in male prairie voles is probably thermoregulatory. The longer duration of time spent by female prairie voles in kyphosis and other quiescent positions than their mates nonetheless suggests that suckling influences the patterning of these behaviors. However, the presence of kyphosis in male prairie voles compared with the inability of parturient rats whose nipples were removed around the time of mating to display this posture but otherwise presumably received similar ventral stimulation by pups as male prairie voles (38), suggests an important distinction in the sensory control of kyphosis between rats and prairie voles. It is unclear whether this distinction is based on a difference between male and female prairie voles or between rats and prairie voles, because it is as yet unknown whether suckling is equally important for kyphosis in lactating prairie voles as it is in lactating rats. Kyphosis may be common in biparental male rodents because apparently similar postures have been described in males of other biparental species including spiny (Acomys cahirinus) (48) and California mice (Peromyscus californicus) (17). Changes in Parental Behaviors Over Time The changes in parental care across the postpartum period, including the decrease in parent–offspring contact that was observed in the present study, is a phenomenon seen in parental animals of numerous species (13,14,30,32,33,49), and is likely related to the onset of the pups’ ingestion of solid food and decreased need for thermoregulatory assistance with the growth of their own fur (46). Accordingly, most of the differences in parental behavior seen between Days 3–4 and 10–11 postpartum in the present study pertained to details of the subject’s nursing/huddling. Another factor that may have contributed to the decrease in the duration of kyphosis bouts and increase in time spent laying prone on the pups is the increase in litter mass. It was evident that subjects were simply physically unable to maintain the kyphotic posture over a large

39

mass of pups—which was typically over 90% of the weight of the sires and over 65% of the dams’ weight. The characteristics of older pups, however, are apparently more conducive for their parent’s ability to remain quiescent over them in postures other than kyphosis because the frequency of bouts of quiescence decreased while their duration increased during the second week postpartum. Increased intensity of suckling by older pups (3) cannot explain these changes, because they were observed in males as well as females. The most notable change in active parental behaviors over time was the increased carrying the young and heightened activity of males from the first to the second week postpartum. These results suggest that it is more difficult for them to maintain either active or quiescent contact with a larger litter mass. High levels of activity in 2-week-old pups may contribute to these behavioral changes in their sires because hungry pups may be less willing to remain in contact with males that provide neither the opportunity to suckle nor obtain milk.

CONCLUSIONS

The display of identical behavioral repertoires by pairbonded female and male prairie voles during interactions with pups demonstrates the similar efficacy of the different neuroendocrine and neurochemical mechanisms that are likely employed by the sexes to produce the onset and/or maintenance of parental behavior in this species. It is probable that the neural networks upon which these mechanisms act are similar for both sexes [e.g., (42)]. Comparisons between patterns of parent–offspring contact in male prairie voles and nonsuckled lactating rats suggest profound differences in the sensorimotor organization of nursing/huddling behaviors between these species. Qualitative differences between the behavior of female and male prairie voles interacting with pups are likely to be fundamentally linked to differences in the somatosensory inputs that they receive, thereby making the prairie vole a uniquely valuable model to study the somatosensory control of parental behaviors in rodents.

ACKNOWLEDGEMENTS

The authors would like to thank Ross E. Lonstein of the National Securities Clearing Corporation (New York) for creating the data acquisition software used in this study. This research was supported by National Institute of Mental Health Grant MH47538 to G. J. De Vries and postdoctoral NRSA HD08392 from the National Institute of Child Health and Human Development to J. S. Lonstein.

REFERENCES 1. Bamshad, M.; Novak, M. A.; De Vries, G. J.: Sex and species differences in the vasopressin innervation of sexually naive and parental prairie voles, Microtus ochrogaster and meadow voles, Microtus pennsylvanicus. J. Neuroendocrinol. 5:247–255; 1993. 2. Bamshad, M.; Novak, M. A.; De Vries, G. J.: Cohabitation alters vasopressin innervation and paternal behaviors in prairie voles (Microtus ochrogaster). Physiol. Behav. 56:751–758; 1994. 3. Brake, S. C.; Shair, H.; Hofer, M.: Exploiting the nursing niche: The infant’s sucking and feeding in the context of the mother– infant interaction. In: Blass, E. M., ed. Handbook of behavioral neurobiology, vol. 9: Developmental psychobiology and behavioral ecology. New York: Plenum; 1988:347–388. 4. Carter, C. S.; De Vries, A. C.; Getz, L. L.: Physiological substrates of mammalian monogamy: The prairie vole model. Neurosci. Biobehav. Rev. 19:303–314; 1995.

5. Cervantes, M.; Ruelas, R.; Alcala, R.: EEG signs of “relaxation behavior” during breast-feeding in a nursing woman. Arch. Med. Res. 23:123–127; 1992. 6. Dewsbury, D. A.; Baumgardner, D. J.; Evans, R. L.; Webster, D. G.: Sexual dimorphism for body mass in 13 taxa of muroid rodents under laboratory conditions. J. Mammal. 61:146–149; 1980. 7. Drewett, R. F.; Statham, C.; Wakerley, J. B.: A quantitative analysis of the feeding behavior of suckling rats. Anim. Behav. 22:907–913; 1974. 8. Elwood, R. W.: Paternal and maternal behaviour in the Monogolian gerbil. Anim. Behav. 23:766–772; 1975. 9. Friedman, M. I.; Bruno, J. P.; Alberts, J. R.: Physiological and behavioral consequences in rats of water recycling during lactation. J. Comp. Physiol. Psychol. 95:26–35; 1981. 10. Getz, L. L.; McGuire, B.: Communal nesting in prairie voles

40

11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

26. 27. 28. 29. 30.

31. 32.

LONSTEIN AND DE VRIES (Microtus ochrogaster): Formation, composition, and persistence of communal groups. Can. J. Zool. 75:525–534; 1997. Getz, L. L.; McGuire, B.; Pizzuto, T.; Hofmann, J. E.; Frase, B.: Social organization of the prairie vole (Microtus ochrogaster). J. Mammal. 74:44–58; 1993. Getz, L. L.; Simms, L. E.; McGuire, B.; Snarski, M. E.: Factors affecting life expectancy of the prairie voles, Microtus ochrogaster. Oikos 80:362–370; 1997. Grota, L. J.; Ader, R.: Continuous recording of maternal behavior in Rattus norvegicus. Anim. Behav. 17:722–729; 1969. Grota, L. J.; Ader, R.: Behavior of lactating rats in a dual-chambered maternity cage. Horm. Behav. 5:275–282; 1974. Gruder–Adams, S.; Getz, L. L.: Comparison of the mating system and paternal behavior in Microtus ochrogaster and M. pennsylvanicus. J. Mammal. 66:165–167; 1985. Gubernick, D. J.; Alberts, J. R.: Maternal licking of young: Resource exchange and proximate controls. Physiol. Behav. 31:593–601; 1983. Gubernick, D. J.; Alberts, J. R.: The biparental care system of the California mouse, Peromyscus californicus. J. Comp. Psychol. 101:169–177; 1987. Hartung, T. G.; Dewsbury, D. A.: Paternal behavior in six species of muroid rodents. Behav. Neural Biol. 26:466–478; 1979. Hennessy, M. B.; Jenkins, R.: A descriptive analysis of nursing behavior in the guinea pig (Cavia porcellus). J. Comp. Psychol. 108:23–28; 1994. Insel, T. R.; Wang, Z.; Ferris, C. F.: Patterns of brain vasopressin receptor distribution associated with social organization in microtine rodents. J. Neurosci. 14:5381–5392; 1994. Kleiman, D.: Monogamy in mammals. Q. Rev. Biol. 52:39–69; 1977. Lonstein, J. S.; De Vries, G. J.: Factors influencing parental behavior in virgin prairie voles. Atlanta, GA: Society for Behavioral Neuroendocrinology Abstracts; 1998. Lonstein, J. S.; De Vries, G. J.: Sex differences in the parental behavior in adult virgin prairie voles: Role of gonadal hormones and vasopressin. (under review). Lonstein, J. S.; Stern, J. M.: Role of the midbrain periaqueductal gray in maternal nurturance and aggression: c-fos and electrolytic lesions studies in lactating rats. J. Neurosci. 17:3364–3378; 1997. Lonstein, J. S.; Stern, J. M.: Somatosensory determinants of c-fos activity in the periaqueductal gray of lactating rats: Role of suckling, rooting and perioral stimulation from pups. Horm. Behav. 32:155–166; 1998. Lonstein, J. S.; Simmons, D. A.; Stern, J. M.: Functions of the periaqueductal gray in lactating rats: Kyphosis, lordosis, maternal aggression and fearfulness. Behav. Neurosci. (in press). Lonstein, J. S.; Simmons, D. A.; Swann, J. M.; Stern, J. M.: Forebrain expression of c-fos due to active maternal behaviour in lactating rats. Neuroscience 82:267–281; 1998. McCarty, R.; Southwick, C. H.: Paternal care and the development of behavior in the Southern grasshopper mouse, Onychomys torridus. Behav. Biol. 19:476–490; 1977. McCarty, R.; Southwick, C. H.: Patterns of parental care in two cricetid rodents, Onychomys torridus and Peromyscus leucopus. Anim. Behav. 25:945–948; 1977. McGuire, B.; Novak, M.: A comparison of maternal behaviour in the meadow vole (Microtus pennsylvanicus), prairie vole (M. ochrogaster) and pine vole (M. pinetorum). Anim. Behav. 32:1132–1141; 1984. Numan, M.: Medial preoptic area and maternal behavior in the female rat. J. Comp. Physiol. Psychol. 87:746–759; 1974. Oliveras, D.; Novak, M.: A comparison of paternal behaviour in

the meadow vole Microtus pennsylvanicus, the pine vole M. pinetorum and the prairie vole M. ochrogaster. Anim. Behav. 34:519– 526; 1986. 33. Reisbick, S.; Rosenblatt, J. S.; Mayer, A. D.: Decline of maternal behavior in the virgin and lactating rats. J. Comp. Physiol. Psychol. 89:722–732; 1975. 34. Roberts, R. L.; Williams, J. R.; Wang, A. K.; Carter, C. S.: Cooperative breeding and monogamy in prairie voles: Influence of the sire and geographic variation. Anim. Behav. 55:1131–1140; 1998. 35. Salo, A. L.; Shapiro, L. E.; Dewsbury, D. A.: Comparison of nipple attachment and incisor growth among four species of voles (Microtus). Dev. Psychobiol. 27:317–333; 1994. 36. Shapiro, L. E.; Leonard, C. M.; Sessions, C. E.; Dewsbury, D. A.; Insel, T. R.: Comparative neuroanatomy of the sexually dimorphic hypothalamus in monogamous and polygamous voles. Brain Res. 541:232–240; 1991. 36a.Solomon, N.G.: Comparison of parental behavior in male and female prairie voles (microtus ochrogaster). Can. J. Zool. 71:434– 437; 1993. 37. Stern, J. M.: Somatosensation and maternal care in Norway rats. In: Rosenblatt, J. S.; Snowden, C. T., eds. Parental care: Evolution, mechanisms, and adaptive significance. Advances in the study of behavior, vol. 25. New York: Academic; 1996:243–294. 38. Stern, J. M.; Dix, L.; Bellomo, C.; Thramann, C.: Ventral trunk somatosensory determinants of nursing behavior in Norway rats: 2. Role of nipple and surrounding sensations. Psychobiology 20:71–80; 1992. 39. Stern, J. M.; Johnson, S. K.: Ventral somatosensory determinants of nursing behavior in Norway rats. I. Effects of variations in the quality and quantity of pup stimuli. Physiol. Behav. 47:993–1011; 1990. 40. Stern, J. M.; Lonstein, J. S.: Nursing behavior is impaired in small nestboxes and with hyperthermic pups. Dev. Psychobiol. 29:101– 122; 1996. 41. Stern, J. M.; Siegel, H. I.: Prolactin release in lactating, primiparous and multiparous thelectomized and maternal virgin rats exposed to pup stimuli. Biol. Reprod. 19:177–182; 1978. 42. Sturgis, J. D.; Bridges, R. S.: N-methyl-DL-aspartic acid lesions of the medial preoptic area disrupts ongoing parental behavior in male rats. Phyiol. Behav. 62:305–310; 1997. 43. Thomas, J. A.; Birney, E. C.: Parental care and mating system of the prairie vole, Microtus ochrogaster. Behav. Ecol. Sociobiol. 5:171–186; 1979. 44. Voloschin, L. M.; Tremezzani, J. H.: Milk-ejection reflex linked to slow wave sleep in nursing rats. Endocrinology 105:1201–1207; 1979. 45. Wang, Z.; Ferris, C. F.; De Vries, G. J.: Role of septal vasopressin innervation in paternal behavior in prairie voles (Microtus ochrogaster). Proc. Natl. Acad. Sci. USA 91:400–404; 1994. 46. Wang, Z.; Novak, M.: Influence of the social environment on parental behavior and pup development of meadow voles (Microtus pennsylvanicus) and prairie voles (M. ochrogaster). J. Comp. Psychol. 106:163–171; 1992. 47. Wang, Z.; Novak, M.: Alloparental care and the influence of father presence on juvenile prairie voles, Microtus ochrogaster. Anim. Behav. 47:281–288; 1994. 48. Wheeler–Makin, J.; Porter, R. H.: Paternal behavior in the spiny mouse (Acomys cahirinus). Behav. Neural Biol. 41:135–151; 1984. 49. Wilson, S. C.: Contact-promoting behavior, social development, and relationship with parents in sibling juvenile degus (Octodon degus). Dev. Psychobiol. 15:257–268; 1982. 50. Wilson, S. C.: Parent–young contact in prairie and meadow voles. J. Mamml. 63:300–305; 1982.