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Off” Signals for Maternal Nest-Building in Rabbits?
Hormones and Behavior 33, 1–8 (1998) Article No. HB971425
Hormones and External Factors: Are They ‘‘On/Off’’ Signals for Maternal Nest-Building in Rabbits? G. Gonza´lez-Mariscal,1 E. Cuamatzi, and J. S. Rosenblatt* Centro de Investigacio´n en Reproduccio´n Animal, CINVESTAV-Universidad Auto´noma de Tlaxcala, Tlaxcala, Mexico; and *Department of Psychology, Rutgers University, Newark, New Jersey, 08903 Received August 4, 1997; revised August 21, 1997; accepted October 7, 1997
Estradiol, progesterone, and prolactin regulate digging, carrying straw, and plucking hair for maternal nest-building in rabbits. To explore whether external factors also modulate this process, we assessed whether shaved pregnant rabbits with straw nests would collect their own, male, or synthetic hair for nest-building. Pregnant (but not estrous) does collected and used all hair types, indicating that hair-plucking can be bypassed and a nest constructed with ‘‘alternative’’ hair. Unshaved pregnant does with straw nests also collected synthetic hair, indicating that this behavior is not triggered by the absence of maternal hair. Yet, if hair-plucking/nest-building had occurred, hair-collecting was prevented, suggesting that an internally triggered ‘‘drive’’ was turned off by perceiving a built nest. When given only straw or hair, shaved pregnant does collected and used the material provided, indicating that nest-building is internally driven and accomplished by using any available elements. When given both materials, increasingly more shaved does built straw nests across prepartum days 7 to 2. Straw-carrying declined thereafter, suggesting that perceiving a straw nest limits the collection of such material. Hair-collecting was postponed until prepartum day 1 to postpartum day 2, indicating that: (a) mothers distinguish between straw and hair and (b) hormonal factors regulate the sequential selection of straw and then hair and when the change from straw to hair occurs. Maternal behavior was normal at parturition and for the next 4 days in a similar proportion of does among all experimental groups. We conclude that hormones and external factors regulate nest-building by acting as ‘‘on/off/on’’ signals. q 1998 Academic Press
Maternal behavior in rabbits begins in late pregnancy as the doe constructs a nest that will hold the young 1
To whom correspondence should be addressed at CIRA, CINVESTAV-UAT. Apdo. Postal 62, Tlaxcala, Tlax. 90,000, Mexico. E-mail: [email protected].
(Denenberg, Sawin, Frommer, and Ross, 1958; Gonza´lez-Mariscal, DıB az-SaB nchez, Melo, Beyer, and Rosenblatt, 1994; Gonza´lez-Mariscal and Rosenblatt, 1996; Ross, Zarrow, Sawin, Denenberg, and Blumenfield, 1963; Zarrow, Farooq, Denenberg, Sawin, and Ross, 1963). Following parturition maternal care is restricted to a single daily nursing bout, lasting around 3 min (Cross, 1952; Drewett, Kendrick, Sanders, and Trew, 1982; Gonza´lez-Mariscal et al., 1994; Lincoln, 1974; Zarrow, Denenberg, and Anderson, 1965). Maternal nestbuilding is a complex process that, in nature, involves digging an underground burrow, collecting straw, shaping this material into a straw nest inside the burrow, and lining it with hair that the mother plucks from her ventrum and thighs (Ross et al., 1963). We recently quantified, under laboratory conditions, the expression of maternal nest-building in pregnant New Zealand white rabbits (Gonza´lez-Mariscal et al., 1994). As in nature, the various aspects of maternal nest-building were expressed in an orderly, predictable sequence under laboratory conditions. Thus, digging (into a cardboard substrate) appeared earliest (8 to 6 days prepartum) and its decline preceded the onset of straw-carrying (3 to 1 days prepartum). Hair-pulling did not appear until the day before parturition, i.e., after completion of the straw nest, and was still expressed in some does for up to 4 days postpartum. These results suggest that the adequate expression of maternal nest-building relies on the operation of complex internal and external regulatory mechanisms that determine the onset and offset of specific motor patterns. Among the internal factors, hormones play a decisive role in the regulation of nestbuilding (Anderson, Zarrow, Fuller and Denenberg, 1971; Gonza´lez-Mariscal et al., 1994; Gonza´lez-Mariscal, Melo, Jime´nez, Beyer, and Rosenblatt, 1996; Zarrow, Sawin, Ross, Denenberg, Crary, Wilson, and Farooq,
0018-506X/98 $25.00 Copyright q 1998 by Academic Press All rights of reproduction in any form reserved.
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1961; Zarrow et al., 1963; Zarrow, Gandelman, and Denenberg, 1971). Thus, digging occurs in both pregnant does (Gonza´lez-Mariscal et al., 1994) and ovariectomized (ovx; Gonza´lez-Mariscal et al., 1996) steroidtreated does under high levels of estradiol and progesterone, while straw-carrying occurs only as progesterone declines and estradiol is still present (i.e., in late pregnancy or following progesterone removal and continuation of estradiol in ovx does). Hair-plucking is expressed at the end, under high levels of estradiol and prolactin and a low concentration of progesterone. In contrast to the participation of internal factors, little is known about the modulation of nest-building by external factors. Zarrow et al. (1963) showed that mothers provided with straw but shaved to prevent accesss to their hair built just a straw nest and only 42% of them showed adequate maternal care. By contrast, mothers not provided with straw, but allowed to pluck their hair, built ‘‘hairy’’ maternal nests and were as successful in rearing their young as does who had access to both straw and hair. Taken together, the above findings raise the possibility that the orderly expression of each phase of the nest-building process may be: (a) the direct result of a specific hormonal milieu; (b) the immediate consequence of the expression and turn-off of the previous phase; or (c) the integrated output of hormones (internal factors), acting as ‘‘general coordinators,’’ plus the availability of elements in the environment (external factors). To distinguish among these three possibilities we designed specific experiments that assessed: (a) whether, shaved of their own hair, late pregnant mothers that had already built straw nests and were about to initiate hair-plucking could distinguish among different types of hair and express a preference for any one type; (b) whether the collection of ‘‘alternative’’ hair was dependent on the absence of the mother’s own hair (externally driven) or whether this behavior occurred even when the female could pluck her own hair (internally driven); (c) whether the responsiveness to hair was dependent on the previous construction of a straw nest or whether such phase could be bypassed and females would collect hair for nest-building regardless of the absence of a straw nest; (d) whether a preference for straw or hair for nestbuilding was revealed across successive days in late gestation when both materials were readily available to shaved females.
METHODS Animals New Zealand white female rabbits, bred in our colony, ages 16 – 35 months and having experienced at
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least one succesful parturition with adequate maternal care, were used. They were mated with sexually active males and housed in individual wire mesh cages (90 cm long 1 60 cm wide 1 40 cm high) inside the rabbit colony under controlled light (14 h light:10 h dark; lights off at 2100 h) and natural temperature conditions. They were given water and food (Purina rabbit pellets) ad libitum. A wooden box (50 cm long 1 30 cm wide 1 32 cm high) with a round (24 cm diameter) opening on one side was introduced into the cage of each female on day 25 of pregnancy, to be used for maternal nestbuilding. Animal care adhered to the Law for the Protection of Animals (Mexico).
Experiment 1: Collection and Use of Hair for Nest-Building by Preparturient Females That Had Built Straw Nests: Effect of Shaving On day 26 of pregnancy 100 g of straw was introduced into the cage of each female to allow the construction of the straw nest. Two days later, females (n Å 19; Groups 1, 2, and 3) were taken out of their home cages and held on their backs, and their ventrum and inner thighs were shaved with an electric razor. The shaved hair was collected into a plastic bag. To obtain a completely hairless skin, females were further shaved with a razor blade after a solution of neutral soap in lukewarm water was applied. Traces of soap were then removed with a cloth soaked in plain lukewarm water and the wet skin was patted dry with a clean cloth. Does were then put back into their home cages. Nonpregnant, estrous females (n Å 9; Group 4) were used to assess whether the display of hair-collecting was specific to pregnant does. They were housed and shaved, and their hair was collected as described above. Other pregnant does were left unshaved (n Å 13; Group 5), to determine whether the collection of hair was dependent on the absence of the mother’s own hair. On pregnancy days 29 – 30 (or 2 days after estrous does were shaved), a cardboard container (24.5 cm long by 11.3 cm wide by 5.4 cm high) was introduced into the home cage of each female but outside the wooden nest box. The cardboard container held the female’s own hair (n Å 7 pregnant, shaved; n Å 3 estrous), hair from an adult male (obtained as described for females; n Å 7 pregnant, shaved; n Å 3 estrous), or white synthetic hair (n Å 5 pregnant, shaved; n Å 13 pregnant, unshaved; n Å 3 estrous). An observer then recorded the female’s behavior (approaching the container, sniffing the hair) until the doe collected the hair or for 2 h if this did not occur. When hair-collecting was not observed on the first exposure day, the above procedure was repeated for up to 2 more days. The amount of material collected and
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used for nest-building was not quantified in this or in Experiment 2 (see below), because, in practically all cases when it occurred, females used up all the contents of the container. From pregnancy day 30 until parturition the home cage of every female was spot-checked throughout the day to determine the approximate time of delivery. To assess the relation between the construction of the nest and the display of maternal care we determined, shortly after parturition, the following parameters indicative of adequate maternal behavior (Denenberg, Petropolus, Sawin, and Ross, 1959): (a) the occurrence of placentophagia (inferred from the absence of placentas in or around the nest area); (b) the adequate licking of pups (suggested by the presence of live clean young inside the maternal nest); (c) the absence of cannibalism (indicated by the lack of mutilated pups); and (d) the display of nursing (indicated by the presence of milk inside the stomachs of the litter). Once these parameters were determined, pups were taken out of the nest, counted, returned to the nest, and left with their mother for the rest of the experiment. For the following 4 days we inspected once daily the stomach of pups to determine the presence of milk; this indicated that the mother had nursed them.
Experiment 2: Collection and Use of Straw or Hair for Nest-Building by Shaved Preparturient Females Presented with One or Both Materials On days 22 – 24 of pregnancy, females (n Å 30) were shaved as in Experiment 1, returned to their home cages, and left undisturbed until pregnancy day 26. On this day, straw (n Å 10; Group 6), the mother’s own hair (n Å 10; Group 7), or both materials (n Å 10; Group 8) were introduced into the home cage of each female as described earlier. The doe’s behavior (approaching the container, sniffing the straw or hair) was assessed until she collected the material(s) provided or for the following 2 h if this did not occur. If a female did not collect anything on the first exposure, the procedure was repeated for up to 3 more days in Groups 6 and 7. The collection of materials for nest-building in all females of Group 8 was assessed daily from gestation day 26 into postpartum day 2. All mothers of Groups 6, 7, and 8 were spot-checked throughout the day from pregnancy day 30 until delivery to determine the approximate time of parturition and the occurrence of normal maternal care at this time (see Experiment 1). Once a litter was noted, pups were taken out of the nest, counted, returned to the nest, and left with their mother for the rest of the experiment.
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Statistical Analysis A Kruskal – Wallis ANOVA was performed among all the groups in an experiment and, when pertinent, this was followed by a median test to compare, between two independent groups: (a) the latency to initiate the collection of straw or hair and (b) the first day when the collection of straw or hair occurred (Siegel and Castellan, 1988). Fisher’s exact probability test was used to compare, between two independent groups, the proportion of females that collected a material for nestbuilding (Siegel and Castellan, 1988).
RESULTS Experiment 1: Collection and Use of Hair for Nest-Building by Preparturient Females That Had Built Straw Nests: Effect of Shaving The percentage of shaved preparturient females that collected the type of hair tested (i.e., their own, male, or synthetic) was nearly the same among Groups 1, 2, and 3 (86, 86, and 100%, respectively; Table 1). A similar proportion (i.e., 85%) was observed in preparturient, unshaved females given synthetic hair (Group 5; Table 1). Some of these groups, however, differed in their latency to initiate hair-collecting. Thus, shaved pregnant does exposed to their own hair (Group 1) initiated its collection almost immediately after the container was introduced into their home cage. Shaved pregnant females exposed to synthetic hair (Group 3) showed slightly longer latencies to initiate its collection than females given their own hair (median values Å 2.0 vs 0.31 min, respectively; P õ 0.02). Unshaved pregnant does (Group 5) showed latencies to collect synthetic hair (median Å 0.23 min; P ú 0.05) that were similar to those exhibited by shaved females given the same material (Group 3). By contrast, shaved pregnant does presented with male hair (Group 2) took much longer to begin this activity (median Å 35.0 min; P õ 0.01 vs shaved, own hair). Moreover, before collecting male hair, females approached the cardboard container, sniffed the hair, and withdrew. This sequence was repeated on several occasions by all females that eventually collected the hair and incorporated it into the straw nest. By contrast, the collection of synthetic hair was rarely preceded by any olfactory inspection of the material. Estrous does (Group 4) did not show any interest in any type of hair across the 2 h of observation on any of the 3 days in which the test was performed. Rather, they were attracted to the cardboard box, started chewing on it, and let the hair fall onto the cage floor. The day when hair-collecting was first observed did
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TABLE 1 Collection and Use of Hair for Nest-Building by Preparturient Females That Had Built Straw Nests First day when hair-collecting occurreda Groups 1. 2. 3. 4. 5.
Incidence
Pregnant, shaved, own hair Pregnant, shaved, male hair Pregnant, shaved, synthetic hair Estrous, shaved, all hair types Pregnant, unshaved, synthetic hair
86% 86% 100% 0% 85%
(6/7) (6/7) (5/5) (0/9) (11/13)
Latency to begin haircollectingb (min)
Median
Range
Median
Range
00.5 01.0 0.0 — 01.0
03 to 0 04 to /2 01 to /2 — 03 to 0
0.31 35** 2.0* — 0.23
0.08 to 0.5 2.0 to 120 0.5 to 7.7 — 0.12 to 20.5
a
Relative to parturition Å day 0. Determined on the first day when the behavior occurred. ** P õ 0.01, *P õ 0.02 vs shaved, own hair. b
not significantly differ among the three groups of shaved pregnant females: it occurred from prepartum day 4 to postpartum day 2 (Table 1). The first day when unshaved pregnant does collected synthetic hair was also observed in the same period (prepartum day 3 to the day of parturition; Table 1). Interestingly, the two unshaved mothers that did not collect synthetic hair had already plucked their own hair and built a maternal nest before they were exposed to the synthetic material. On the other hand, the collection of synthetic hair did not in all cases prevent the subsequent plucking of the female’s own hair: 6 of the 11 does that collected synthetic hair also plucked their own hair on subsequent days. In all four groups of pregnant does, the motor pattern displayed to collect the hair was identical to that normally used for the collection of straw, i.e., lowering the head into the container, gathering the material with the mouth, and abruptly pulling the head backward (as for uprooting grass). This sequence of movements used to collect hair from the container is very different from the one normally used by a mother to pluck her own body hair: flexing her neck toward her body, grabbing the hair with her muzzle, and using a rotating movement of her head to uproot it. Yet, as with normal mothers, does in this experiment entered the nest box only until they had gathered ‘‘enough’’ hair in their muzzle. Then they incorporated the collected hair into the previously built straw nest. Interestingly, the six females that built straw nests outside the wooden box incorporated the collected hair into such a ‘‘misplaced’’ straw nest. The perception of hair in the container triggered in some females a variety of behaviors oriented toward maternal nest-building. These activities included: (a) the reconstruction of the straw nest, at either the same site as the original nest (n Å 1) or at a new site (n Å 1);
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(b) hair-plucking from the few reachable places on the female’s body that had small tufts of hair left, e.g., their feet (n Å 4; these females became irritated and started grunting after their strenuous efforts led to very little hair gathering); (c) simulation of hair-plucking from the ventrum and thighs (regions completely shaved of hair) by displaying the same stereotyped motor pattern females normally use to pluck their hair from these regions (n Å 1; see above). The proportion of females that displayed normal maternal behavior at parturition (i.e., ingestion of placentas, licking of pups, lack of cannibalism, and nursing) did not significantly differ among groups 1, 2, 3, and 5 (Table 2). That is, regardless of shaving or the type of hair provided, between 57 and 80% of mothers in these groups (all of which had previously built a straw nest) nursed their litters across postpartum days 1 – 4 and weaned more than 50% of their young on lactation day
TABLE 2 Effect of Material(s) Provided for Nest-Building on the Display of Maternal Behavior
Groups Shaved, given straw plus 1. Own hair 2. Male hair 3. Synthetic hair Unshaved, given straw plus 5. Synthetic hair Shaved, given 6. Only straw 7. Only own hair
Incidence of normal maternal behavior a
57% (4/7) 72% (5/7) 80% (4/5) 77% (10/13) 70% (7/10) 90% (9/10)
a Determined by: (a) the display of placentophagia and cleaning and nursing of pups at parturition and (b) the willingness to nurse across postpartum days 1 – 4 (see Methods).
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TABLE 3 Collection and Use of the Single Material (Straw or Hair) Provided for Nest-Building by Shaved Preparturient Females First day when collection occurreda Groups
Incidence
Median
Range
6. Only straw 7. Only own hair
90% (9/10) 90% (9/10)
04 05
06 to 0 06 to 01
a
DISCUSSION
Relative to parturition Å day 0.
30. Interestingly, these behaviors occurred as a ‘‘package,’’ i.e., when maternal care was normal at parturition, it remained so until weaning.
Experiment 2: Collection and Use of Straw or Hair for Nest-Building by Shaved Preparturient Females Presented with One or Both Materials In groups 6 and 7, 90% of shaved pregnant females given either straw or their own hair collected the single material provided for nest-building (Table 3). The time, relative to parturition, when such material was collected did not differ between the group given only straw (Group 6; median was prepartum day 4) and that given only its own hair (Group 7; median was prepartum day 5). In both groups some females initiated this behavior as early as prepartum day 6 whereas others began collecting the material on prepartum day 1 or the day of parturition. The proportion of does showing a normal maternal behavior at parturitition and into lactation day 4 was not significantly different between Group 6 and Group 7 (70 and 90%, respectively; P ú 0.05; Table 2). When straw and hair were presented simultaneously to shaved pregnant does (Group 8), straw-collecting was displayed earlier (median was prepartum day 4) than hair-collecting (median was prepartum day 1). However, great variability was evident across individuals regarding the first day of collection. In fact, the proportion of females displaying straw-collecting for the first time was almost uniformly spread out across prepartum days 7 to 1 (Fig. 1). However, when we determined the total proportion of does collecting straw on each day, a gradual increase from 10% on prepartum day 7 to 80% on prepartum day 2 became apparent (Fig. 2). By contrast, the incidence of hair-collecting peaked abruptly around the time of delivery: 70% of does collected hair for the first time between prepartum day 1 and the day of parturition and only 20% of them displayed this activity earlier (prepartum day 5; Fig. 1).
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Moreover, the total proportion of does collecting hair on a given day never exceeded 20% up to prepartum day 2 but rose to 40% on prepartum day 1 (Fig. 2). Furthermore, the collection of straw and hair by the same female on a given day was noted in only 30 and 10% of mothers on the day of parturition and on postpartum day 2, respectively (Fig. 2).
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Our results enable us to draw several conclusions concerning the natural sequence of behaviors involved in nest-building during late pregnancy and early lactation. We confirm that the hormones of gestation play an essential role in stimulating the collection and use of straw and hair for nest-building because estrous does (presumably lacking specific endocrine factors associated with pregnancy) never collected any type of hair. Whereas under normal conditions hair-plucking invariably precedes the use of this material for nest-building, if shaved of their own hair, late-pregnant does will collect and use their own hair or even a substitute (male or synthetic) from a container to line the straw nest. This indicates that the construction of the ‘‘hairy’’ nest does not depend on the previous display of hair-plucking. Indeed, few shaved does that collected and used the alternative hair displayed (or simulated) hair-pulling from body places with little or no hair and, when this occurred, it was invariably observed shortly after the hair container was introduced. Therefore, our results suggest that endocrine factors, common to all females in the same stage of gestation, promote the collection of hair. Accordingly, the incidence of this behavior (85 to 100% of females) and the time (relative to parturition) when it was first observed did not significantly differ among the four groups of pregnant does in Experiment 1. The collection and use of an external source of hair for nest-building was not dependent on the mother’s perception of the absence of her own hair: the proportion of does that collected synthetic hair was not significantly different between the shaved (100%) and the unshaved (85%) groups. Nonetheless, the fact that the two unshaved mothers that did not collect synthetic hair had already plucked their own hair and used it to line the straw nest may indicate that the perception of hair in the nest may inhibit an additional collection of this material. Under natural conditions, however, this situation is unlikely to occur because the availability of hair for nest-building is limited by when and how much of it becomes loose from the mother’s body. The hormonal factors that regulate hair-loosening are un-
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FIG. 1. First day when straw or own hair was collected and used for nest-building by shaved pregnant does (n Å 10) given these materials simultaneously across successive days in late pregnancy.
known. However, we found that the injection of estradiol benzoate plus progesterone (followed by progesterone withdrawal), while effectively inducing digging and straw-carrying in all ovariectomized does, provoked hair-pulling in only 40% of females (Gonza´lezMariscal et al., 1996). This finding indicates that another factor(s) participates in regulating this behavior and raises the question of whether the same or different
hormones act on the hair-loosening/hair-plucking/ hair-collecting processes. A major role of hormones is also evident in the stimulation of straw-carrying, a behavior initiated only as progesterone levels decline in both pregnant (Gonza´lezMariscal et al., 1994) and ovx, estrogen/progesteronetreated does (Gonza´lez-Mariscal et al., 1996). Nonetheless, the fact that despite a continuous supply of straw,
FIG. 2. Total proportions of shaved, pregnant does (n Å 10) that collected and used straw, own hair, or both for nest-building when these materials were given simultaneously across successive days in late pregnancy and early postpartum (PP).
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females at some point stopped collecting this material for nest-building indicates the onset of inhibitory mechanisms. The nature of such putative mechanisms is unknown but circumstantial observations in our laboratory indicate that they may involve the doe’s perception of having completed a straw nest. Thus, the accidental destruction/removal of a previously constructed nest provokes a state of general unrest (anxiety?) in preparturient does. Yet, if new material for nest-building (e.g., straw, paper strips) is provided at this time, these does will rapidly reconstruct a new maternal nest with such material and the hair they pluck (unpublished observations). On the other hand, the previous construction of a straw nest is not a prerequisite for the collection and use of hair because shaved does given hair as the sole material for nest-building collected it at a time when they would normally collect straw (Experiment 2). This may indicate that late-pregnant mothers have a motivation to collect materials for nest-building and that they will use any available elements to this end. However, when mothers are given the opportunity to choose, they show a remarkable capacity to distinguish between straw and hair (Experiment 2). That is, they show a preference for straw when they would normally be using straw and a preference for hair when they would normally use this material to line the nest. The proportion of does that selected straw for nestbuilding gradually increased across prepartum days 7 to 2, when 80% of does displayed this activity. This pattern resembles the increase in the amount of straw collected that we had observed across prepartum days 3 to 1 in unshaved preparturient does not given an external source of hair (Gonza´lez-Mariscal et al., 1994). Because in pregnant does this activity is expressed under high levels of estradiol and low levels of progesterone (Gonza´lez-Mariscal et al., 1994) and in ovx does it is observed following progesterone withdrawal and continuation of estradiol benzoate (Gonza´lez-Mariscal et al., 1996), it is tempting to speculate that this hormonal combination promotes not only straw-collecting but also the selection of straw over hair. Similarly, the choice of hair for nest-building was maximally expressed between prepartum day 1 and postpartum day 2. This pattern, again, coincides with our observation that the maximal incidence of hair-plucking occurs between prepartum day 1 and postpartum day 2 in females not given an external source of hair (Gonza´lezMariscal et al., 1994). As above, based on this coincidence we may speculate that the hormonal combination associated with the expression of hair-plucking in pregnant and in ovx does (namely, high levels of estradiol and prolactin and a low concentration of progesterone)
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is also involved in promoting the selection of hair over straw. According to this speculation, the collection of both materials would rarely occur, as was indeed the case: only 30% of does at parturition and 10% on postpartum day 2 collected straw and hair for nest-building. Nonetheless, these speculations must be tested in the future by administering specific hormonal combinations to ovx does and assessing their selection of materials for maternal nest-building. Interestingly, in canaries, nest-building also occurs as a two-stage process in which birds initially select only grass and, as egg-laying approaches, they then select feathers (Hinde and Steel, 1962). Whether this change in preference is a direct consequence of a specific hormonal stimulation or an indirect consequence of a peripheral effect (development of the brood patch) was not determined. However, females treated with only estrogen and those not about to lay eggs built only grass nests. Finally, a twostage model for maternal nest-building has been proposed for sows (Jensen, 1993), in which a preparation phase (including standing, nosing, and rooting), triggered by internal factors, is followed by a nesting phase (walking, carrying material, and arranging), dependent mainly on external stimuli. In short, though the evidence is scanty, our results coincide with the possibility that maternal nest-building may occur across species as a two-stage process upon which internal (hormonal) and external stimuli act. Moreover, based on our present findings we can select from the three possibilities stated in the Introduction to explain the orderly expression of each phase of the nest-building process. It seems that specific hormonal combinations are acting as ‘‘turn-on’’ signals that allow the expression of motor patterns not displayed by estrous females. By contrast, ‘‘turn-off’’ signals may come from the perception of specific external cues. This would offer the female the advantage of being able to adapt herself to changes in the environment and to reinitiate maternal nest-building across several days in the peripartum period. The capacity to use the available elements in the environment for maternal nest-building may aid rabbits in reproducing successfully in a variety of habitats with different types of vegetation. On the other hand, within the same environment, mother rabbits are known to select a variety of herbs and grasses that serve different functions in the nest (e.g., bactericides, fungicides) and that aid in the transmission of food preferences from mother to young (Bilko´, Altba¨cker, and Hudson, 1994; Hudson and Altba¨cker, 1992). Thus, the straw nest varies in its composition across habitats and perhaps even across seasons within the same environment. By contrast, the hairy nest is constant, providing the same kind of stimulation (olfactory, thermal, tactile), regardless of
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season or habitat. The relevance of this variable/constant microenvironment, represented by the straw nest and hair nest, respectively, for the development of pups should be assessed in the future. Nonetheless, because the incidence of normal maternal behavior was not significantly different among the groups receiving straw plus one hair type and those receiving a single material (straw or hair) for nest-building, our results indicate that, as long as mothers can build a nest with one material, they will interact normally with the litter at parturition and display maternal care that will allow the survival of their young. These findings contrast with those of Zarrow et al. (1963), who found that the incidence of normal maternal behavior was decreased in shaved does receiving only straw compared to mothers that plucked their own hair but lacked straw. Perhaps the small number or the breed of rabbits (Dutch-belted) used by Zarrow et al. (1963) may account for the differences observed.
ACKNOWLEDGMENTS This work was supported by Grant MH45891 04 from the National Institute of Mental Health (USA) and Grant 3294N from the National Council of Science and Technology (CONACYT; Mexico).
REFERENCES Anderson, C. O., Zarrow, M. X., Fuller, G. B., and Denenberg, V. H. (1971). Pituitary involvement in maternal nest-building in the rabbit. Horm. Behav. 2, 183 – 189. Bilko´, A., Altba¨cker, V., and Hudson, R. (1994). Transmission of food preference in the rabbit: The means of information transfer. Physiol. Behav. 56, 907 – 912. Cross, B. A. (1952). Nursing behavior and the milk ejection reflex in rabbits. J. Endocrinol. 8, 13 – 14. Denenberg, V. H., Sawin, P. G., Frommer, G. P., and Ross, S. (1958). Genetic, physiological and behavioural background of reproduction in the rabbit. IV. An analysis of maternal behaviour at successive parturitions. Behaviour 13, 131 – 142. Denenberg, V. H., Petropolus, S. F., Sawin, P. B., and Ross, S. (1959). Genetic, physiological and behavioural background of reproduc-
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tion in the rabbit. VI. Maternal behaviour with reference to scattered and cannibalized newborn and mortality. Behaviour 15, 71 – 76. Drewett, R. F., Kendrick, K. M., Sanders, D. J., and Trew, A. M. (1982). A quantitative analysis of the feeding behavior of suckling rabbits. Dev. Psychobiol. 15, 25 – 32. Gonza´lez-Mariscal, G., DıB az-SaB nchez, V., Melo, A. I., Beyer, C., and Rosenblatt, J. S. (1994). Maternal behavior in New Zealand white rabbits: Quantification of somatic events, motor patterns and steroid plasma levels. Physiol. Behav. 55, 1081 – 1089. Gonza´lez-Mariscal, G., Melo, A. I., Jime´nez, P., Beyer, C., and Rosenblatt, J. S. (1996). Estradiol, progesterone, and prolactin regulate maternal nest-building in rabbits. J. Neuroendocrinol. 8, 901 – 907. Gonza´lez-Mariscal, G., and Rosenblatt, J. S. (1996). Maternal behavior in rabbits: A historical and multidisciplinary perspective. In J. S. Rosenblatt, and C. T. Snowdon (Eds.), Parental Care: Evolution, Mechanisms, and Adaptive Significance, Advances in the Study of Behavior, Vol XXV, pp. 333 – 360. Academic Press, New York. Hinde, R. A., and Steel, E. A. (1962). Selection of nest material by female canaries. Anim. Behav. 10, 67 – 75. Hudson, R., and Altba¨cker, V. (1992). Development of feeding and food preference in the European rabbit: Environmental and maturational determinants. In B. G. Galef, M. Mainardi, and P. Valsecchi (Eds.), Ontogeny and Social Transmission of Food Preferences in Mammals: Basic and Applied Research, pp. 125 – 145. Harwood, London. Jensen, P. (1993). Nest-building in domestic sows: The role of external stimuli. Anim. Behav. 45, 351 – 358. Lincoln, D. W. (1974). Suckling: A time constant in the nursing behavior of the rabbit. Physiol. Behav. 13, 711 – 714. Ross, S., Denenberg, V. H., Sawin, P. B., and Meyer, P. (1956). Changes in nest building behaviour in multiparous rabbits. Br. J. Anim. Behav. 4, 69 – 74. Ross, S., Zarrow, M. X., Sawin, P. B., Denenberg, V. H., and Blumenfield, M. (1963). Maternal behaviour in the rabbit under semi-natural conditions. Anim. Behav. 11, 283 – 285. Sawin, P. B., Denenberg, V. H., Ross, S., Hafter, E., and Zarrow, M. X. (1960). Maternal behavior in the rabbit: Hair loosening during gestation. Am. J. Physiol. 198, 1099 – 1102. Siegel, S., and Castellan, N. J., Jr. (1988). Non-parametric Statistics for the Behavioral Sciences. McGraw – Hill, New York. Zarrow, M. X., Sawin, P. B., Ross, S., Denenberg, V. H., Crary, D., Wilson, E. D., and Farooq, A. (1961). Maternal behaviour in the rabbit: Evidence for an endocrine basis of maternal nest-building and additional data on maternal nest-building in the Dutch-belted race. J. Reprod Fertil. 2, 152 – 162. Zarrow, M. X., Farooq, A., Denenberg, V. H., Sawin, P. B., and Ross, S. (1963). Maternal behaviour in the rabbit: Endocrine control of maternal nest-building. J. Reprod. Fertil. 6, 375 – 383. Zarrow, M. X., Denenberg, V. H., and Anderson, C. O. (1965). Rabbit: Frequency of suckling in the pup. Science 150, 1835 – 1836. Zarrow, M. X., Gandelman, R., and Denenberg, V. H. (1971). Prolactin: Is it an essential hormone for maternal behavior in the mammal? Horm. Behav. 2, 343 – 354.
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Hormones and External Factors: Are They ‘‘On/Off’’ Signals for Maternal Nest-Building in Rabbits? G. Gonza´lez-Mariscal,1 E. Cuamatzi, and J. S. Rosenblatt* Centro de Investigacio´n en Reproduccio´n Animal, CINVESTAV-Universidad Auto´noma de Tlaxcala, Tlaxcala, Mexico; and *Department of Psychology, Rutgers University, Newark, New Jersey, 08903 Received August 4, 1997; revised August 21, 1997; accepted October 7, 1997
Estradiol, progesterone, and prolactin regulate digging, carrying straw, and plucking hair for maternal nest-building in rabbits. To explore whether external factors also modulate this process, we assessed whether shaved pregnant rabbits with straw nests would collect their own, male, or synthetic hair for nest-building. Pregnant (but not estrous) does collected and used all hair types, indicating that hair-plucking can be bypassed and a nest constructed with ‘‘alternative’’ hair. Unshaved pregnant does with straw nests also collected synthetic hair, indicating that this behavior is not triggered by the absence of maternal hair. Yet, if hair-plucking/nest-building had occurred, hair-collecting was prevented, suggesting that an internally triggered ‘‘drive’’ was turned off by perceiving a built nest. When given only straw or hair, shaved pregnant does collected and used the material provided, indicating that nest-building is internally driven and accomplished by using any available elements. When given both materials, increasingly more shaved does built straw nests across prepartum days 7 to 2. Straw-carrying declined thereafter, suggesting that perceiving a straw nest limits the collection of such material. Hair-collecting was postponed until prepartum day 1 to postpartum day 2, indicating that: (a) mothers distinguish between straw and hair and (b) hormonal factors regulate the sequential selection of straw and then hair and when the change from straw to hair occurs. Maternal behavior was normal at parturition and for the next 4 days in a similar proportion of does among all experimental groups. We conclude that hormones and external factors regulate nest-building by acting as ‘‘on/off/on’’ signals. q 1998 Academic Press
Maternal behavior in rabbits begins in late pregnancy as the doe constructs a nest that will hold the young 1
To whom correspondence should be addressed at CIRA, CINVESTAV-UAT. Apdo. Postal 62, Tlaxcala, Tlax. 90,000, Mexico. E-mail: [email protected].
(Denenberg, Sawin, Frommer, and Ross, 1958; Gonza´lez-Mariscal, DıB az-SaB nchez, Melo, Beyer, and Rosenblatt, 1994; Gonza´lez-Mariscal and Rosenblatt, 1996; Ross, Zarrow, Sawin, Denenberg, and Blumenfield, 1963; Zarrow, Farooq, Denenberg, Sawin, and Ross, 1963). Following parturition maternal care is restricted to a single daily nursing bout, lasting around 3 min (Cross, 1952; Drewett, Kendrick, Sanders, and Trew, 1982; Gonza´lez-Mariscal et al., 1994; Lincoln, 1974; Zarrow, Denenberg, and Anderson, 1965). Maternal nestbuilding is a complex process that, in nature, involves digging an underground burrow, collecting straw, shaping this material into a straw nest inside the burrow, and lining it with hair that the mother plucks from her ventrum and thighs (Ross et al., 1963). We recently quantified, under laboratory conditions, the expression of maternal nest-building in pregnant New Zealand white rabbits (Gonza´lez-Mariscal et al., 1994). As in nature, the various aspects of maternal nest-building were expressed in an orderly, predictable sequence under laboratory conditions. Thus, digging (into a cardboard substrate) appeared earliest (8 to 6 days prepartum) and its decline preceded the onset of straw-carrying (3 to 1 days prepartum). Hair-pulling did not appear until the day before parturition, i.e., after completion of the straw nest, and was still expressed in some does for up to 4 days postpartum. These results suggest that the adequate expression of maternal nest-building relies on the operation of complex internal and external regulatory mechanisms that determine the onset and offset of specific motor patterns. Among the internal factors, hormones play a decisive role in the regulation of nestbuilding (Anderson, Zarrow, Fuller and Denenberg, 1971; Gonza´lez-Mariscal et al., 1994; Gonza´lez-Mariscal, Melo, Jime´nez, Beyer, and Rosenblatt, 1996; Zarrow, Sawin, Ross, Denenberg, Crary, Wilson, and Farooq,
0018-506X/98 $25.00 Copyright q 1998 by Academic Press All rights of reproduction in any form reserved.
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1961; Zarrow et al., 1963; Zarrow, Gandelman, and Denenberg, 1971). Thus, digging occurs in both pregnant does (Gonza´lez-Mariscal et al., 1994) and ovariectomized (ovx; Gonza´lez-Mariscal et al., 1996) steroidtreated does under high levels of estradiol and progesterone, while straw-carrying occurs only as progesterone declines and estradiol is still present (i.e., in late pregnancy or following progesterone removal and continuation of estradiol in ovx does). Hair-plucking is expressed at the end, under high levels of estradiol and prolactin and a low concentration of progesterone. In contrast to the participation of internal factors, little is known about the modulation of nest-building by external factors. Zarrow et al. (1963) showed that mothers provided with straw but shaved to prevent accesss to their hair built just a straw nest and only 42% of them showed adequate maternal care. By contrast, mothers not provided with straw, but allowed to pluck their hair, built ‘‘hairy’’ maternal nests and were as successful in rearing their young as does who had access to both straw and hair. Taken together, the above findings raise the possibility that the orderly expression of each phase of the nest-building process may be: (a) the direct result of a specific hormonal milieu; (b) the immediate consequence of the expression and turn-off of the previous phase; or (c) the integrated output of hormones (internal factors), acting as ‘‘general coordinators,’’ plus the availability of elements in the environment (external factors). To distinguish among these three possibilities we designed specific experiments that assessed: (a) whether, shaved of their own hair, late pregnant mothers that had already built straw nests and were about to initiate hair-plucking could distinguish among different types of hair and express a preference for any one type; (b) whether the collection of ‘‘alternative’’ hair was dependent on the absence of the mother’s own hair (externally driven) or whether this behavior occurred even when the female could pluck her own hair (internally driven); (c) whether the responsiveness to hair was dependent on the previous construction of a straw nest or whether such phase could be bypassed and females would collect hair for nest-building regardless of the absence of a straw nest; (d) whether a preference for straw or hair for nestbuilding was revealed across successive days in late gestation when both materials were readily available to shaved females.
METHODS Animals New Zealand white female rabbits, bred in our colony, ages 16 – 35 months and having experienced at
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least one succesful parturition with adequate maternal care, were used. They were mated with sexually active males and housed in individual wire mesh cages (90 cm long 1 60 cm wide 1 40 cm high) inside the rabbit colony under controlled light (14 h light:10 h dark; lights off at 2100 h) and natural temperature conditions. They were given water and food (Purina rabbit pellets) ad libitum. A wooden box (50 cm long 1 30 cm wide 1 32 cm high) with a round (24 cm diameter) opening on one side was introduced into the cage of each female on day 25 of pregnancy, to be used for maternal nestbuilding. Animal care adhered to the Law for the Protection of Animals (Mexico).
Experiment 1: Collection and Use of Hair for Nest-Building by Preparturient Females That Had Built Straw Nests: Effect of Shaving On day 26 of pregnancy 100 g of straw was introduced into the cage of each female to allow the construction of the straw nest. Two days later, females (n Å 19; Groups 1, 2, and 3) were taken out of their home cages and held on their backs, and their ventrum and inner thighs were shaved with an electric razor. The shaved hair was collected into a plastic bag. To obtain a completely hairless skin, females were further shaved with a razor blade after a solution of neutral soap in lukewarm water was applied. Traces of soap were then removed with a cloth soaked in plain lukewarm water and the wet skin was patted dry with a clean cloth. Does were then put back into their home cages. Nonpregnant, estrous females (n Å 9; Group 4) were used to assess whether the display of hair-collecting was specific to pregnant does. They were housed and shaved, and their hair was collected as described above. Other pregnant does were left unshaved (n Å 13; Group 5), to determine whether the collection of hair was dependent on the absence of the mother’s own hair. On pregnancy days 29 – 30 (or 2 days after estrous does were shaved), a cardboard container (24.5 cm long by 11.3 cm wide by 5.4 cm high) was introduced into the home cage of each female but outside the wooden nest box. The cardboard container held the female’s own hair (n Å 7 pregnant, shaved; n Å 3 estrous), hair from an adult male (obtained as described for females; n Å 7 pregnant, shaved; n Å 3 estrous), or white synthetic hair (n Å 5 pregnant, shaved; n Å 13 pregnant, unshaved; n Å 3 estrous). An observer then recorded the female’s behavior (approaching the container, sniffing the hair) until the doe collected the hair or for 2 h if this did not occur. When hair-collecting was not observed on the first exposure day, the above procedure was repeated for up to 2 more days. The amount of material collected and
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used for nest-building was not quantified in this or in Experiment 2 (see below), because, in practically all cases when it occurred, females used up all the contents of the container. From pregnancy day 30 until parturition the home cage of every female was spot-checked throughout the day to determine the approximate time of delivery. To assess the relation between the construction of the nest and the display of maternal care we determined, shortly after parturition, the following parameters indicative of adequate maternal behavior (Denenberg, Petropolus, Sawin, and Ross, 1959): (a) the occurrence of placentophagia (inferred from the absence of placentas in or around the nest area); (b) the adequate licking of pups (suggested by the presence of live clean young inside the maternal nest); (c) the absence of cannibalism (indicated by the lack of mutilated pups); and (d) the display of nursing (indicated by the presence of milk inside the stomachs of the litter). Once these parameters were determined, pups were taken out of the nest, counted, returned to the nest, and left with their mother for the rest of the experiment. For the following 4 days we inspected once daily the stomach of pups to determine the presence of milk; this indicated that the mother had nursed them.
Experiment 2: Collection and Use of Straw or Hair for Nest-Building by Shaved Preparturient Females Presented with One or Both Materials On days 22 – 24 of pregnancy, females (n Å 30) were shaved as in Experiment 1, returned to their home cages, and left undisturbed until pregnancy day 26. On this day, straw (n Å 10; Group 6), the mother’s own hair (n Å 10; Group 7), or both materials (n Å 10; Group 8) were introduced into the home cage of each female as described earlier. The doe’s behavior (approaching the container, sniffing the straw or hair) was assessed until she collected the material(s) provided or for the following 2 h if this did not occur. If a female did not collect anything on the first exposure, the procedure was repeated for up to 3 more days in Groups 6 and 7. The collection of materials for nest-building in all females of Group 8 was assessed daily from gestation day 26 into postpartum day 2. All mothers of Groups 6, 7, and 8 were spot-checked throughout the day from pregnancy day 30 until delivery to determine the approximate time of parturition and the occurrence of normal maternal care at this time (see Experiment 1). Once a litter was noted, pups were taken out of the nest, counted, returned to the nest, and left with their mother for the rest of the experiment.
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Statistical Analysis A Kruskal – Wallis ANOVA was performed among all the groups in an experiment and, when pertinent, this was followed by a median test to compare, between two independent groups: (a) the latency to initiate the collection of straw or hair and (b) the first day when the collection of straw or hair occurred (Siegel and Castellan, 1988). Fisher’s exact probability test was used to compare, between two independent groups, the proportion of females that collected a material for nestbuilding (Siegel and Castellan, 1988).
RESULTS Experiment 1: Collection and Use of Hair for Nest-Building by Preparturient Females That Had Built Straw Nests: Effect of Shaving The percentage of shaved preparturient females that collected the type of hair tested (i.e., their own, male, or synthetic) was nearly the same among Groups 1, 2, and 3 (86, 86, and 100%, respectively; Table 1). A similar proportion (i.e., 85%) was observed in preparturient, unshaved females given synthetic hair (Group 5; Table 1). Some of these groups, however, differed in their latency to initiate hair-collecting. Thus, shaved pregnant does exposed to their own hair (Group 1) initiated its collection almost immediately after the container was introduced into their home cage. Shaved pregnant females exposed to synthetic hair (Group 3) showed slightly longer latencies to initiate its collection than females given their own hair (median values Å 2.0 vs 0.31 min, respectively; P õ 0.02). Unshaved pregnant does (Group 5) showed latencies to collect synthetic hair (median Å 0.23 min; P ú 0.05) that were similar to those exhibited by shaved females given the same material (Group 3). By contrast, shaved pregnant does presented with male hair (Group 2) took much longer to begin this activity (median Å 35.0 min; P õ 0.01 vs shaved, own hair). Moreover, before collecting male hair, females approached the cardboard container, sniffed the hair, and withdrew. This sequence was repeated on several occasions by all females that eventually collected the hair and incorporated it into the straw nest. By contrast, the collection of synthetic hair was rarely preceded by any olfactory inspection of the material. Estrous does (Group 4) did not show any interest in any type of hair across the 2 h of observation on any of the 3 days in which the test was performed. Rather, they were attracted to the cardboard box, started chewing on it, and let the hair fall onto the cage floor. The day when hair-collecting was first observed did
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TABLE 1 Collection and Use of Hair for Nest-Building by Preparturient Females That Had Built Straw Nests First day when hair-collecting occurreda Groups 1. 2. 3. 4. 5.
Incidence
Pregnant, shaved, own hair Pregnant, shaved, male hair Pregnant, shaved, synthetic hair Estrous, shaved, all hair types Pregnant, unshaved, synthetic hair
86% 86% 100% 0% 85%
(6/7) (6/7) (5/5) (0/9) (11/13)
Latency to begin haircollectingb (min)
Median
Range
Median
Range
00.5 01.0 0.0 — 01.0
03 to 0 04 to /2 01 to /2 — 03 to 0
0.31 35** 2.0* — 0.23
0.08 to 0.5 2.0 to 120 0.5 to 7.7 — 0.12 to 20.5
a
Relative to parturition Å day 0. Determined on the first day when the behavior occurred. ** P õ 0.01, *P õ 0.02 vs shaved, own hair. b
not significantly differ among the three groups of shaved pregnant females: it occurred from prepartum day 4 to postpartum day 2 (Table 1). The first day when unshaved pregnant does collected synthetic hair was also observed in the same period (prepartum day 3 to the day of parturition; Table 1). Interestingly, the two unshaved mothers that did not collect synthetic hair had already plucked their own hair and built a maternal nest before they were exposed to the synthetic material. On the other hand, the collection of synthetic hair did not in all cases prevent the subsequent plucking of the female’s own hair: 6 of the 11 does that collected synthetic hair also plucked their own hair on subsequent days. In all four groups of pregnant does, the motor pattern displayed to collect the hair was identical to that normally used for the collection of straw, i.e., lowering the head into the container, gathering the material with the mouth, and abruptly pulling the head backward (as for uprooting grass). This sequence of movements used to collect hair from the container is very different from the one normally used by a mother to pluck her own body hair: flexing her neck toward her body, grabbing the hair with her muzzle, and using a rotating movement of her head to uproot it. Yet, as with normal mothers, does in this experiment entered the nest box only until they had gathered ‘‘enough’’ hair in their muzzle. Then they incorporated the collected hair into the previously built straw nest. Interestingly, the six females that built straw nests outside the wooden box incorporated the collected hair into such a ‘‘misplaced’’ straw nest. The perception of hair in the container triggered in some females a variety of behaviors oriented toward maternal nest-building. These activities included: (a) the reconstruction of the straw nest, at either the same site as the original nest (n Å 1) or at a new site (n Å 1);
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(b) hair-plucking from the few reachable places on the female’s body that had small tufts of hair left, e.g., their feet (n Å 4; these females became irritated and started grunting after their strenuous efforts led to very little hair gathering); (c) simulation of hair-plucking from the ventrum and thighs (regions completely shaved of hair) by displaying the same stereotyped motor pattern females normally use to pluck their hair from these regions (n Å 1; see above). The proportion of females that displayed normal maternal behavior at parturition (i.e., ingestion of placentas, licking of pups, lack of cannibalism, and nursing) did not significantly differ among groups 1, 2, 3, and 5 (Table 2). That is, regardless of shaving or the type of hair provided, between 57 and 80% of mothers in these groups (all of which had previously built a straw nest) nursed their litters across postpartum days 1 – 4 and weaned more than 50% of their young on lactation day
TABLE 2 Effect of Material(s) Provided for Nest-Building on the Display of Maternal Behavior
Groups Shaved, given straw plus 1. Own hair 2. Male hair 3. Synthetic hair Unshaved, given straw plus 5. Synthetic hair Shaved, given 6. Only straw 7. Only own hair
Incidence of normal maternal behavior a
57% (4/7) 72% (5/7) 80% (4/5) 77% (10/13) 70% (7/10) 90% (9/10)
a Determined by: (a) the display of placentophagia and cleaning and nursing of pups at parturition and (b) the willingness to nurse across postpartum days 1 – 4 (see Methods).
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TABLE 3 Collection and Use of the Single Material (Straw or Hair) Provided for Nest-Building by Shaved Preparturient Females First day when collection occurreda Groups
Incidence
Median
Range
6. Only straw 7. Only own hair
90% (9/10) 90% (9/10)
04 05
06 to 0 06 to 01
a
DISCUSSION
Relative to parturition Å day 0.
30. Interestingly, these behaviors occurred as a ‘‘package,’’ i.e., when maternal care was normal at parturition, it remained so until weaning.
Experiment 2: Collection and Use of Straw or Hair for Nest-Building by Shaved Preparturient Females Presented with One or Both Materials In groups 6 and 7, 90% of shaved pregnant females given either straw or their own hair collected the single material provided for nest-building (Table 3). The time, relative to parturition, when such material was collected did not differ between the group given only straw (Group 6; median was prepartum day 4) and that given only its own hair (Group 7; median was prepartum day 5). In both groups some females initiated this behavior as early as prepartum day 6 whereas others began collecting the material on prepartum day 1 or the day of parturition. The proportion of does showing a normal maternal behavior at parturitition and into lactation day 4 was not significantly different between Group 6 and Group 7 (70 and 90%, respectively; P ú 0.05; Table 2). When straw and hair were presented simultaneously to shaved pregnant does (Group 8), straw-collecting was displayed earlier (median was prepartum day 4) than hair-collecting (median was prepartum day 1). However, great variability was evident across individuals regarding the first day of collection. In fact, the proportion of females displaying straw-collecting for the first time was almost uniformly spread out across prepartum days 7 to 1 (Fig. 1). However, when we determined the total proportion of does collecting straw on each day, a gradual increase from 10% on prepartum day 7 to 80% on prepartum day 2 became apparent (Fig. 2). By contrast, the incidence of hair-collecting peaked abruptly around the time of delivery: 70% of does collected hair for the first time between prepartum day 1 and the day of parturition and only 20% of them displayed this activity earlier (prepartum day 5; Fig. 1).
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Moreover, the total proportion of does collecting hair on a given day never exceeded 20% up to prepartum day 2 but rose to 40% on prepartum day 1 (Fig. 2). Furthermore, the collection of straw and hair by the same female on a given day was noted in only 30 and 10% of mothers on the day of parturition and on postpartum day 2, respectively (Fig. 2).
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Our results enable us to draw several conclusions concerning the natural sequence of behaviors involved in nest-building during late pregnancy and early lactation. We confirm that the hormones of gestation play an essential role in stimulating the collection and use of straw and hair for nest-building because estrous does (presumably lacking specific endocrine factors associated with pregnancy) never collected any type of hair. Whereas under normal conditions hair-plucking invariably precedes the use of this material for nest-building, if shaved of their own hair, late-pregnant does will collect and use their own hair or even a substitute (male or synthetic) from a container to line the straw nest. This indicates that the construction of the ‘‘hairy’’ nest does not depend on the previous display of hair-plucking. Indeed, few shaved does that collected and used the alternative hair displayed (or simulated) hair-pulling from body places with little or no hair and, when this occurred, it was invariably observed shortly after the hair container was introduced. Therefore, our results suggest that endocrine factors, common to all females in the same stage of gestation, promote the collection of hair. Accordingly, the incidence of this behavior (85 to 100% of females) and the time (relative to parturition) when it was first observed did not significantly differ among the four groups of pregnant does in Experiment 1. The collection and use of an external source of hair for nest-building was not dependent on the mother’s perception of the absence of her own hair: the proportion of does that collected synthetic hair was not significantly different between the shaved (100%) and the unshaved (85%) groups. Nonetheless, the fact that the two unshaved mothers that did not collect synthetic hair had already plucked their own hair and used it to line the straw nest may indicate that the perception of hair in the nest may inhibit an additional collection of this material. Under natural conditions, however, this situation is unlikely to occur because the availability of hair for nest-building is limited by when and how much of it becomes loose from the mother’s body. The hormonal factors that regulate hair-loosening are un-
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FIG. 1. First day when straw or own hair was collected and used for nest-building by shaved pregnant does (n Å 10) given these materials simultaneously across successive days in late pregnancy.
known. However, we found that the injection of estradiol benzoate plus progesterone (followed by progesterone withdrawal), while effectively inducing digging and straw-carrying in all ovariectomized does, provoked hair-pulling in only 40% of females (Gonza´lezMariscal et al., 1996). This finding indicates that another factor(s) participates in regulating this behavior and raises the question of whether the same or different
hormones act on the hair-loosening/hair-plucking/ hair-collecting processes. A major role of hormones is also evident in the stimulation of straw-carrying, a behavior initiated only as progesterone levels decline in both pregnant (Gonza´lezMariscal et al., 1994) and ovx, estrogen/progesteronetreated does (Gonza´lez-Mariscal et al., 1996). Nonetheless, the fact that despite a continuous supply of straw,
FIG. 2. Total proportions of shaved, pregnant does (n Å 10) that collected and used straw, own hair, or both for nest-building when these materials were given simultaneously across successive days in late pregnancy and early postpartum (PP).
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females at some point stopped collecting this material for nest-building indicates the onset of inhibitory mechanisms. The nature of such putative mechanisms is unknown but circumstantial observations in our laboratory indicate that they may involve the doe’s perception of having completed a straw nest. Thus, the accidental destruction/removal of a previously constructed nest provokes a state of general unrest (anxiety?) in preparturient does. Yet, if new material for nest-building (e.g., straw, paper strips) is provided at this time, these does will rapidly reconstruct a new maternal nest with such material and the hair they pluck (unpublished observations). On the other hand, the previous construction of a straw nest is not a prerequisite for the collection and use of hair because shaved does given hair as the sole material for nest-building collected it at a time when they would normally collect straw (Experiment 2). This may indicate that late-pregnant mothers have a motivation to collect materials for nest-building and that they will use any available elements to this end. However, when mothers are given the opportunity to choose, they show a remarkable capacity to distinguish between straw and hair (Experiment 2). That is, they show a preference for straw when they would normally be using straw and a preference for hair when they would normally use this material to line the nest. The proportion of does that selected straw for nestbuilding gradually increased across prepartum days 7 to 2, when 80% of does displayed this activity. This pattern resembles the increase in the amount of straw collected that we had observed across prepartum days 3 to 1 in unshaved preparturient does not given an external source of hair (Gonza´lez-Mariscal et al., 1994). Because in pregnant does this activity is expressed under high levels of estradiol and low levels of progesterone (Gonza´lez-Mariscal et al., 1994) and in ovx does it is observed following progesterone withdrawal and continuation of estradiol benzoate (Gonza´lez-Mariscal et al., 1996), it is tempting to speculate that this hormonal combination promotes not only straw-collecting but also the selection of straw over hair. Similarly, the choice of hair for nest-building was maximally expressed between prepartum day 1 and postpartum day 2. This pattern, again, coincides with our observation that the maximal incidence of hair-plucking occurs between prepartum day 1 and postpartum day 2 in females not given an external source of hair (Gonza´lezMariscal et al., 1994). As above, based on this coincidence we may speculate that the hormonal combination associated with the expression of hair-plucking in pregnant and in ovx does (namely, high levels of estradiol and prolactin and a low concentration of progesterone)
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is also involved in promoting the selection of hair over straw. According to this speculation, the collection of both materials would rarely occur, as was indeed the case: only 30% of does at parturition and 10% on postpartum day 2 collected straw and hair for nest-building. Nonetheless, these speculations must be tested in the future by administering specific hormonal combinations to ovx does and assessing their selection of materials for maternal nest-building. Interestingly, in canaries, nest-building also occurs as a two-stage process in which birds initially select only grass and, as egg-laying approaches, they then select feathers (Hinde and Steel, 1962). Whether this change in preference is a direct consequence of a specific hormonal stimulation or an indirect consequence of a peripheral effect (development of the brood patch) was not determined. However, females treated with only estrogen and those not about to lay eggs built only grass nests. Finally, a twostage model for maternal nest-building has been proposed for sows (Jensen, 1993), in which a preparation phase (including standing, nosing, and rooting), triggered by internal factors, is followed by a nesting phase (walking, carrying material, and arranging), dependent mainly on external stimuli. In short, though the evidence is scanty, our results coincide with the possibility that maternal nest-building may occur across species as a two-stage process upon which internal (hormonal) and external stimuli act. Moreover, based on our present findings we can select from the three possibilities stated in the Introduction to explain the orderly expression of each phase of the nest-building process. It seems that specific hormonal combinations are acting as ‘‘turn-on’’ signals that allow the expression of motor patterns not displayed by estrous females. By contrast, ‘‘turn-off’’ signals may come from the perception of specific external cues. This would offer the female the advantage of being able to adapt herself to changes in the environment and to reinitiate maternal nest-building across several days in the peripartum period. The capacity to use the available elements in the environment for maternal nest-building may aid rabbits in reproducing successfully in a variety of habitats with different types of vegetation. On the other hand, within the same environment, mother rabbits are known to select a variety of herbs and grasses that serve different functions in the nest (e.g., bactericides, fungicides) and that aid in the transmission of food preferences from mother to young (Bilko´, Altba¨cker, and Hudson, 1994; Hudson and Altba¨cker, 1992). Thus, the straw nest varies in its composition across habitats and perhaps even across seasons within the same environment. By contrast, the hairy nest is constant, providing the same kind of stimulation (olfactory, thermal, tactile), regardless of
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season or habitat. The relevance of this variable/constant microenvironment, represented by the straw nest and hair nest, respectively, for the development of pups should be assessed in the future. Nonetheless, because the incidence of normal maternal behavior was not significantly different among the groups receiving straw plus one hair type and those receiving a single material (straw or hair) for nest-building, our results indicate that, as long as mothers can build a nest with one material, they will interact normally with the litter at parturition and display maternal care that will allow the survival of their young. These findings contrast with those of Zarrow et al. (1963), who found that the incidence of normal maternal behavior was decreased in shaved does receiving only straw compared to mothers that plucked their own hair but lacked straw. Perhaps the small number or the breed of rabbits (Dutch-belted) used by Zarrow et al. (1963) may account for the differences observed.
ACKNOWLEDGMENTS This work was supported by Grant MH45891 04 from the National Institute of Mental Health (USA) and Grant 3294N from the National Council of Science and Technology (CONACYT; Mexico).
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