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Communal nesting and discriminative nursing by captive degus, Octodon degus
Animal Behaviour 78 (2009) 1183–1188
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Communal nesting and discriminative nursing by captive degus, Octodon degus Stephanie A. Jesseau a, *, Warren G. Holmes b,1, Theresa M. Lee c, 2 a
Department of Psychology, University of Nebraska Department of Psychology and Department of Biology and Institute of Cognitive and Decision Sciences, University of Oregon c Department of Psychology, University of Michigan b
a r t i c l e i n f o Article history: Received 5 December 2008 Initial acceptance 22 January 2009 Final acceptance 23 July 2009 Available online 25 September 2009 MS. number: A08-00776R Keywords: allomaternal care communal nesting communal nursing degu kin discrimination kin recognition lactation milk transfer Octodon degu radioactive label
When two or more females rear their young in a common nest or burrow (communal nesting), mothers may be challenged to direct care to their own offspring. In a laboratory study on degus, Octodon degus, a communally nesting South American caviomorph rodent, we used a radionuclide (phorphorus-32) to track milk transfer from mothers to their young in nests occupied by two mothers and their litters. Conesting pairs consisted of either two unrelated mothers or two sisters. We analysed faecal samples from 2-week-old and 4-week-old young to learn whether mothers would nurse discriminately, favouring their own offspring over their co-nesting partner’s offspring. Mothers housed in unrelated pairs nursed their own 2-week-old offspring preferentially (although not exclusively) compared with their co-nesting partner’s offspring, whereas mothers housed with a sister nursed indiscriminately, delivering roughly equal amounts of milk to their own offspring and their nieces and nephews. Faecal analyses from 4-week-old young revealed that mothers may nurse co-nesting young indiscriminately, transferring similar amounts of milk to both types of young, regardless of the relatedness of their co-nesting partner. We propose that discriminative nursing as a function of relatedness between co-nesting female degus may be an adaptation to communal nesting when mothers share a burrow that contains many young of different degrees of relatedness. Ó 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Lactation is energetically expensive for female mammals and is often several times more expensive than gestation (Gittleman & Thompson 1988; Clutton-Brock et al. 1989). Mothers are thus expected to nurse discriminatively, that is, to provide milk only to their own offspring under most social and ecological conditions (Roulin 2002). In rodents, the solitary nesting by females that occurs in most species simplifies identifying and caring exclusively for a mother’s own young because solitary nesting affords a mother many opportunities to learn her offsprings’ kin signatures (Beecher 1982) before young become mobile and begin to mix with other young (Holmes 1990; Holmes & Mateo 2007). In contrast, communal nesting, in which two or more females and their dependent young share a common nest or burrow, may make exclusive nursing of a mother’s own young difficult (Hayes 2000). Indeed, one of the often-cited costs of communal nesting for
* Corresponding author: S. A. Jesseau, 238 Burnett Hall, University of NebraskaLincoln, Lincoln, NE 68588-0308, U.S.A. E-mail address: [email protected] (S.A. Jesseau). 1 W. G. Holmes, 259 Straub Hall, 1227 University of Oregon, Eugene, OR 97403, U.S.A. 2 T. M. Lee, 1004 East Hall, 530 Church Street, Ann Arbor, MI 48109, U.S.A.
females is misdirected maternal care (Solomon 1993; Hayes 2000; Hayes & Solomon 2004). In some cases, communally nesting females may be unable to distinguish between their own offspring and those produced by a co-nesting female (hereafter ‘alien’ young). In communally nesting house mice, Mus musculus domesticus, for example, mothers nurse all young in their nest indiscriminately (Ko¨nig 1989), and in retrieval tests they fail to distinguish between their own and age-matched alien young (Manning et al. 1995). However, even if mothers can discriminate between their own and alien young, nursing may be indiscriminative because young often attempt to obtain milk from any available mother, not just their own. In lions, Panthera leo, for example, mothers can recognize their own offspring (Pusey & Packer 1994), but they may be unable to thwart alien suckling (i.e. milk theft) because the rearing environment includes so many alien young (Packer et al. 1992). According to inclusive fitness theory (Hamilton 1964), a mother could benefit by nursing alien young if they are closely related to her (e.g. nieces or nephews). In fact, when the nursing of alien young occurs, it often involves sisters that are nesting together (Wilkinson & Baker 1988; Hoogland et al. 1989; Jacquot & Vessey 1994). However, given the energetic cost of lactation and the importance of
0003-3472/$38.00 Ó 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.anbehav.2009.07.037
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milk to developing young, a mother sharing a nest with her sister might still benefit from being able to distinguish between her offspring and her nieces and nephews, and if mothers sometimes nest with distantly related or unrelated females, mother–offspring recognition abilities may be especially important. Degus are diurnal, South American caviomorph rodents (Hystricognathi: Octodontidae) that nest communally in nature (Fulk 1976; Ebensperger et al. 2004), and also in captivity even when multiple unused nest sites are available (Ebensperger et al. 2002). Microsatellite analyses from field data show that female degus typically share nests with close kin and occasionally with distantly related or unrelated females (Ebensperger et al. 2004; Jesseau 2004). In olfactory discrimination tests, mothers can discriminate between the odours of their own young and those of their co-nesting partners, even when partners are sisters (Jesseau et al. 2008). Although female degus do not always use these discrimination abilities when providing maternal care (Ebensperger et al. 2006), the observation that mothers can discriminate between the odours of their own young and those of their conesting partner suggests that mothers could behave nepotistically by preferentially nursing their own young. Thus, we sought to determine (1) whether degu mothers housed in pairs nurse discriminatively and (2) whether nursing behaviour differs depending on the relatedness between co-nesting mothers. Because an array of terms has been used in the ‘communal breeding’ literature (examples in Solomon & French 1997), we provide definitions for key terms used here. Communal nesting is the joint occupancy of a common nest or burrow by two or more adult females and their dependent young. Our definition does not imply that females cooperate to care for each other’s young, although they could. Nursing is the transfer of milk between a mother and a dependent young, regardless of which individual is primarily responsible for milk transfer even though some transfer could be due to milk theft, especially by older young (Pusey & Packer 1994). Communal nursing occurs when milk is transferred from a mother to her own offspring and also to alien young (Packer et al. 1992). Communal nursing can be (1) exclusive: nearly all of a mother’s milk is transferred to her own young, (2) preferential: a mother nurses one type of pup more than the other type (e.g. her own offspring more than alien young) or (3) indiscriminate: a mother nurses both types of young nearly equally.
Housing and Animals
METHODS
Radioactive Labelling and Analysis
Degu Natural History
To track milk delivery, we used the transfer of radioactively labelled milk from mother to pup. Quantifying milk intake by measuring suckling behaviour is unreliable in many mammals (Mendl & Paul 1989; Cameron 1998), including Octodon (Kleiman 1974). A study on human infants (Butte et al. 1988) showed that estimates of milk intake using a nonradioactive isotope (deuterium oxide) to trace milk from mother to baby were very similar to results obtained from test-weighing (weighing an infant before and after nursing). Radioactive labelling has been used to answer various questions in different species (Wolff & Holleman 1978; Tamarin et al. 1983), including whether mothers sometimes nurse communally (Hoogland et al. 1989). We therefore assessed milk transfer by injecting postpartum mothers with a radioactive label, and later measuring radioactivity in pups’ faeces (Bailey et al. 1973; Cameron 1998). A limitation of this technique in degus and perhaps in other species is that it is ineffective until pups produce enough faecal material to assay, which may preclude using the technique on very young pups. One mother from each co-nesting pair was randomly chosen to receive a radionuclide injection. One day after parturition, these
Degus are medium-sized (about 200 g) diurnal caviomorph rodents that inhabit shrubland areas of arid central Chile. Females typically produce one litter per year of five or six pups, and births within a population are synchronized (Woods & Boraker 1975; Fulk 1976). Pups are precocial (e.g. they are born fully furred with their eyes open) and are ambulatory immediately after birth (Wilson 1982). They nurse for about 4 weeks and mature sexually by the time they are about 6 months old (Reynolds & Wright 1979). Degus are active above ground during the day, and at night, they occupy underground burrow systems that typically include groups of two to six adult females, one or two adult males and dependent young (Fulk 1976; Ebensperger et al. 2004). Co-nesting females primarily include first- and second-degree kin (e.g. sisters, aunts), although co-nesting females are sometimes unrelated (Ebensperger et al. 2004; Jesseau 2004), which means that during rearing, co-nesting mothers can encounter their own offspring, other related offspring (e.g. nieces and nephews) and unrelated young.
The adult animals used in our study were 66 female degus in a colony at the University of Michigan (Lee 2004). Approximately half of the females were two generations from the field, while the other half were obtained from breeding stock from zoos (generations from the field unknown). Females ranged in age from 9 months to 3 years, and all but two had bred previously. Prior to this study, each female had lived with either one or two of her sisters since birth. To produce young, each female was housed alone with an unrelated adult male for a 3-week mating period. After the mating period, we removed each male and housed females in groups of six, consisting of either three pairs of sisters (4 groups) or two trios of sisters (7 groups). Until we could identify which females were pregnant, each group of six females occupied a 55 37 20 cm (large) plastic tub cage to mimic communal nesting in which females live together during gestation. Of the original 66 females, 44 were not used because they failed to produce a litter or did so when another litter was unavailable to create a co-nesting pair (see below). Approximately 3 weeks before the expected date of parturition, 22 group-housed females that were likely to be pregnant (determined by weight gain and abdominal palpation) were re-housed in pairs in 46 2621 cm (small) plastic tub cages. We were able to create six pairs of conesting sisters (‘sister pairs’) and five pairs of unrelated co-nesting females (‘unrelated pairs’). ‘Co-nesting mothers’ were two females, regardless of their relatedness to each other, that shared the same cage prior to and after the birth of their litters. Since co-nesting females could not be monitored continuously, 5 days before the expected date of parturition, we housed co-nesting females individually at night and sometimes during the day, which allowed us to specify mother–offspring relatedness when observers could not be present at parturition. Throughout the study, all animals had access to ad libitum food (Labdiet Prolab Laboratory Animal Diet, Gray Summit, MO, U.S.A.) and water, and corn cob bedding (Teklad 7097) was used as a substrate. The colony was maintained on a 12:12 h light:dark cycle, and temperature was held at 20 C. After the first mother in each co-nesting pair gave birth, we placed both mothers and pups in a single large tub cage (see above) where they lived through weaning. When pups were 1 day old, we marked them individually with numbered eartags (National Band and Tag Co., no. 1005-1, Newport, KY, U.S.A.).
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mothers were injected intraperitoneally with 15 microcuries (mCi) of Phosphorus-32 (P-32), a beta energy emitter (Tamarin et al. 1983; Hoogland et al. 1989) that would allow us to trace the transfer of milk from an injected mother to the pups that suckled from her. Of the 11 females that were injected, four gave birth before their conesting partner, six gave birth after their co-nesting partner, and one gave birth on the same night as the co-nesting partner. Litter sizes did not differ significantly between sister pairs (X SD number of pups per litter: injected females: 5.5 1.05 pups, range 4–7; noninjected females: 5.17 2.14 pups, range 2–8; paired t test: t5 ¼ 0.349, N ¼ 6, P ¼ 0.741) or between unrelated pairs (injected females: 4.8 1.79 pups, range 2–6; noninjected females: 4.0 2.0 pups, range 2–6; paired t test: t4 ¼ 1.0, N ¼ 5, P ¼ 0.374). The mean SE interbirth interval (i.e. the number of days between the birth of the first and second litters) was 3.6 2.6 days for all pairs of co-nesting mothers and did not differ significantly for sister pairs and unrelated pairs (t9 ¼ 1.25, P ¼ 0.2). We planned to track milk transfer just after mothers began to nurse, but pups produced no appreciable faecal samples until about 2 weeks of age. Therefore, our initial analysis reflects nursing behaviour over the first 14 days of pups’ lives. We collected faecal samples from pups and mothers when young were 2 and 4 weeks old (mothers finish nursing when young are about 4 weeks old) by placing each individual alone in a clean cage until a faecal sample was produced. Samples were immediately weighed and analysed for radioactivity with a Packard 1900 TR Liquid Scintillation Analyzer that detected the amount of beta energy emitted from samples. Pups usually produced samples within the first 15 min of isolation, and mothers always produced a sample in this amount of time. If no faecal sample was produced within 1 h, we returned pups to their original cages, and made another sampling attempt within 24 h. One sample (which consisted of one or more faecal pellets) was collected from each pup at 2-weeksold (X SD ¼ 0:036 0:043 g), and another at 4 weeks old (X SD ¼ 0:094 0:11 g). Ethical Note The levels of radiation that we used for this study were very low, and the degus did not show any behaviours that would indicate that the radiation was harmful. In addition, the longevity and future breeding success of individuals in this project were not affected by radiation exposure. This study was approved by the University of Michigan University Committee on Use and Care of Animals (IACUC protocol number 08497).
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radioactivity in samples from the injected and noninjected mothers (controlling for weight of sample, measured in cpm divided by sample weight). There was significantly more beta energy detected in the faeces of mothers injected with P-32 than in faeces of noninjected mothers when their pups were 2 weeks old (Mann–Whitney U test: U ¼ 3.64, N1 ¼ N2 ¼ 11, P ¼ 0.0003) and 4 weeks old (U ¼ 2.22, N1 ¼ N2 ¼ 11, P ¼ 0.03; Fig. 1). This means that injected mothers had absorbed the radioactive label and, presumably, would pass it to any pups that suckled from them. Note, however, that the amount of P-32 that we detected in mothers decreased significantly from the first measurement (when pups were 2 weeks old) to the second measurement (when pups were 4 weeks old) (U ¼ 2.73, N1 ¼ N2 ¼ 11, P ¼ 0.006), and mothers that were not injected had significantly higher amounts of beta energy in their faeces when their pups were 2 and 4 weeks old than did 57 ‘blank vials’ containing only nonradioactive liquid scintillation fluid (Mann–Whitney U test: Z ¼ 5.93, N1 ¼ 22, N2 ¼ 57, P < 0.0001; see Discussion). For pups, patterns of faecal radioactivity differed depending on whether co-nesting mothers were related. For unrelated co-nesting mothers, there was significantly more beta radiation detected in the faeces of litters from injected mothers (median interquartile range (IQR) ¼ 6447.3 2938.8 cpm) than in the faeces of litters from noninjected mothers (median IQR ¼ 3138.3 976.4 cpm) when pups were 2 weeks old (Wilcoxon signed-ranks tests: T ¼ 2.02, N ¼ 5 pairs of mothers, P ¼ 0.043; Fig. 2). That is, injected mothers appeared to nurse both their own and alien young, but they nursed their own offspring preferentially. When co-nesting mothers were sisters, however, there was no significant difference in beta energy detected in pups’ faeces of injected (median IQR ¼ 7653.2 8443.07 cpm) and noninjected (median IQR ¼ 6643.4 19054.3 cpm) mothers (T ¼ 0.52, N ¼ 6 pairs of mothers, P ¼ 0.6; Fig. 2). That is, injected mothers seemed to nurse indiscriminately, delivering similar amounts of P-32-labelled milk to their offspring and alien young. When pups were 4 weeks old, there was no difference in beta energy detected in pups’ faeces whether mothers were unrelated (T ¼ 0.67, N ¼ 5 pairs of mothers, P ¼ 0.5; Fig. 3) or sisters (T ¼ 1.57, N ¼ 6 pairs of mothers, P ¼ 0.12; Fig. 3). That is, both sisters and unrelated pairs of co-nesting mothers seemed to nurse nearly weaned young indiscriminately. DISCUSSION Our use of a radioactive label to track milk transfer revealed that communally nesting degu mothers appear to nurse young
Statistics
RESULTS To ensure that P-32 was absorbed by injected mothers, allowing us to trace radionuclides from mother to pups, we compared
14 000 Mothers’ faecal radioactivity (counts per min per g)
To control for the size of the faecal sample for both mothers and pups, we divided the amount of beta energy emitted (measured in counts per minute, or cpm) by the sample weight to obtain cpm per gram of faecal matter, and used this value for all analyses. To avoid ‘litter effects’ (the nonindependence of young from the same litter) that might be present when dealing with entire litters of multiparous species, we used the litter as the unit of analysis by obtaining a mean value for each litter (Zorrilla 1997). Wilcoxon signed-ranks tests were then used to compare the mean amount of radiation detected in the litters of injected and noninjected mothers from the same cage. Mann–Whitney U tests were used for all analyses pertaining only to adult females.
Noninjected
12 000
P-32 injected
10 000 *
8000 6000
*
4000 2000 0
* 2 weeks 4 weeks Age of mothers’ litters
Figure 1. Box plot (median, quartiles and 10th and 90th percentiles) of radioactivity detected in faeces of mothers that were or were not injected with a radioactive label. Faeces were collected when young were 2 and 4 weeks old. *P < 0.05.
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Pups’ faecal radioactivity (counts per min per g)
35 000 30 000
Noninjected P-32 injected
25 000 20 000 15 000 10 000
*
5000 0 Unrelated Sisters Relatedness of co-nesting mothers
Figure 2. Box plot (median, quartiles and 10th and 90th percentiles) of radioactivity detected in faeces of litters of 2-week-old young housed with co-nesting mothers, one of which was injected with a radioactive label. Co-nesting mothers were sisters or were unrelated. *P < 0.05.
discriminatively, although their nursing behaviour depended on pups’ ages and whether co-nesting mothers were sisters or were unrelated to each other. When young were 2 weeks old, radioactively injected mothers housed in unrelated pairs and exposed to both their own offspring and those of their nest partner (alien young) nursed both types of young, although they nursed their own offspring preferentially. In contrast, injected mothers housed in sister pairs appeared to nurse their own and alien young indiscriminately, delivering roughly equal amounts of milk to their offspring and their nieces and nephews (Fig. 2). When pups were about 4 weeks old and lactation was ending, injected mothers in both sister and unrelated pairs appeared to nurse their offspring and alien young indiscriminately (Fig. 3). Collectively, these results suggest that female degus can use a conditional strategy of maternal investment: nursing behaviour that favours their own kin in communal nests. Our results show that beta energy in the faeces of injected mothers decreased steadily over the course of the 4-week study (Fig. 1). This finding is important because it shows that beta radiation was gradually excreted in faeces and presumably in milk, ensuring that we could track its transfer across time. (If P-32 were metabolized very quickly or very slowly, decreased levels of radioactivity might have made tracking difficult.) This gradual
Pups’ faecal radioactivity (counts per min per g)
8000 7000
Noninjected
6000
P-32 injected
5000 4000 3000 2000 1000 0
Unrelated Sisters Relatedness of co-nesting mothers
Figure 3. Box plot (median, quartiles and 10th and 90th percentiles) of radioactivity detected in faeces of litters of 4-week-old young housed with co-nesting mothers, one of which was injected with a radioactive label. Co-nesting mothers were sisters or were unrelated.
release of the radionuclide makes it well suited for the study of milk transfer (also see Hoogland et al. 1989). Consistent, statistically significant differences in P-32 levels between injected and noninjected co-nesting mothers (Fig. 1) indicate that contamination of the noninjected mother with the radionuclide of the injected mother was minimal. Contamination was possible because degus engage in coprophagy (Kenagy et al. 1999) and ano-genital licking of pups (Wilson & Kleiman 1974; Reynolds & Wright 1979), and extensive contamination would have masked any effect of preferential nursing. In addition, we consider the use of a radioactive label to track milk transfer more reliable than observing nursing behaviour directly (Cameron 1998). During nonsystematic observations of conesting mothers and their litters, we saw only one instance of a mother actively rejecting a pup’s suckling attempt, and we routinely observed most or all pups in a cage huddled together under a single mother, apparently attempting to suckle. Ebensperger et al. (2002) also describe apparent suckling attempts by alien young in captive, group-nesting degus. Behavioural observations of nursing would therefore prove unreliable since there were often more pups attempting to suckle than a mother had teats. There are at least three proximate explanations for apparent indiscriminate nursing by injected mothers in sister pairs when pups were 2 weeks old (Fig. 2). First, mother–offspring recognition is routinely mediated by olfaction in rodents (Mateo 2003; Holmes & Mateo 2007) and mothers may be unable to distinguish between the odours of their offspring and those of their nieces and nephews. Olfactory kin labels are perceived as more similar when they come from close kin than when they come from distantly related or unrelated conspecifics (Solomon 1993; Mateo 2002; Todrank & Heth 2003), and discriminating among the odours of cousins may have been challenging for mothers in sister pairs. However, the ‘similar labels’ hypothesis is not consistent with our prior finding that degu mothers can discriminate between the odours of conesting cousins (Jesseau et al. 2008). That degu mothers can discriminate between cousins’ odours and yet nurse cousins indiscriminately serves as a reminder that the ability to discriminate does not always result in the preferential treatment of closely related kin over more distantly related kin (Holmes & Mateo 2007). Second, the energetic cost of lactation may have been inadequate to trigger discriminative nursing. Degu mothers experience the highest energetic cost of milk production during early lactation (Veloso & Bozinovic 2000), which was about 7 days before we began to collect pups’ faeces to infer nursing behaviour. Our ad libitum feeding regimen would have further minimized the energetic demands of lactation. When the energetic costs of lactation are low, nursing mothers may be unmotivated to nurse discriminatively (Hayes & Solomon 2004; Ebensperger et al. 2006). It is possible, for example, that co-nesting sisters in the wild or in a captive environment with food restriction might use this recognition ability and preferentially nurse their own pups. However, the ‘low-cost’ hypothesis does not seem to apply to unrelated pairs of mothers that, unlike sister pairs, did nurse discriminatively (Fig. 2). Finally, nursing decisions by degus may be influenced more by the identities of co-nesting partners than by the identities of conesting young. Mothers may follow a specific ‘familiarity’ rule of thumb that directs them to nurse indiscriminately whenever they nest with a sister or another closely related female. Females are ‘familiar’ with each other if they grow up together, having shared a common rearing environment prior to weaning. This rule of thumb could explain indiscriminate nursing by sister pairs and discriminative nursing by unrelated pairs since members of unrelated pairs did not interact with each other until they lived together in adulthood. In an experimental study of communal rearing in house mice, for example, pairs of females that lived together since
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birth produced more offspring as adults than females that were reared apart and then paired as adults, regardless of their relatedness to each other (Ko¨nig 1994). Even though degu mothers are capable of discriminating between the odours of their own offspring and those of nieces and nephews (Jesseau et al. 2008), females may have nursed indiscriminately because of the familiarity that resulted from growing up with their nesting partner. In contrast to sister pairs, injected mothers in unrelated pairs appeared to nurse their own offspring preferentially when pups were 2 weeks old, although they did not nurse their own offspring exclusively (Fig. 2). We propose that, in the face of a nutritional surplus (ad libitum feeding) and high pup density, it was not worthwhile for mothers to work as hard as would have been necessary (e.g. preventing all milk theft) to ensure exclusive nursing of their own offspring (Hayes & Solomon 2004). Despite abundant nutritional resources and high pup density, mothers still used their olfactory discrimination abilities (Jesseau et al. 2008) to direct their nursing behaviour preferentially towards their own offspring. By the time degu young were 4 weeks old, injected mothers, whether they were housed with a sister or a nonrelative, appeared to nurse indiscriminately. That is, we found no difference in faecal radioactivity between litters of co-nesting pups of injected and noninjected mothers (Fig. 3). There are several reasons why we may have obtained these results. First, the cumulative effect of coprophagy over the 4-week period may have masked the effects of discriminative nursing. Even if pups from noninjected mothers nursed only from their own mother, they could still display measurable amounts of radioactivity if their mothers consistently obtained the radioactive label by ingesting faeces from injected mothers or their pups. In addition, the resolving power of our radionuclide technique may have been inadequate to detect discriminative nursing by 4-week-old young. We injected mothers only once, 1 day after parturition, and radioactive levels declined noticeably in mothers after injection (Fig. 1), which may have prevented us from detecting differences in 4-week-old young. Despite these issues, it is possible that we obtained no difference when pups were 4 weeks old because females did not discriminate among pups at this time. Weaning in degus occurs around 4 weeks of age (Weir 1974), and young begin consuming solid food well before weaning (Reynolds & Wright 1979), so perhaps mothers with 4-week-old young were not motivated to nurse discriminatively because the cost of indiscriminate nursing was low. Degu mothers with 6-week-old young do not discriminate between the odours of their own and those of familiar alien young (Jesseau et al. 2008) and perhaps this lack of discrimination also explains indiscriminate nursing by mothers with 4-week-old young. Our discussion of disciminative nursing by degu mothers has thus far assumed that mothers are responsible for milk transfer to pups and that pups have little influence on this process, a questionable assumption given the precocial nature of degu pups (Wilson 1982). We had intended to observe mother–pup interactions directly, but this proved difficult because we routinely observed most or all pups in a cage huddled together under a single mother, which prevented us from observing the behaviour of individual pups. During nonsystematic observations of co-nesting mothers and their litters, we saw multiple pups attempting to suckle from a single mother and only one instance of a mother actively rejecting a pup’s suckling attempt. Thus, we believe that co-nesting mothers and their pups all played a role in milk transfer rather than mothers being solely responsible. The functional benefit of mother–offspring recognition has long been appreciated by animal behaviourists (Beach & Jaynes 1956; Cullen 1957; Davies & Carrick 1962), since direct interactions between mothers and their newborn young afford the learning
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opportunities that typically mediate mother–offspring recognition. Even in rearing environments that include multiple mothers and their offspring, kin recognition could be mediated by the prior association mechanism if mothers have opportunities to learn the identities of their young before litters or broods begin to intermix (Komdeur et al. 2004; Sharp et al. 2005). If social contexts during early development offer no reliable opportunities to learn kin signatures before various categories of kin and nonkin are encountered, a recognition mechanism such as self-referent phenotype matching might mediate mother–offspring recognition. Despite the existence of mechanisms that could mediate mother– offspring recognition in the face of ambiguous social contexts, recognition, and the subsequent discriminative care that it could allow, may not occur if the costs of discrimination outweigh the benefits. How mother–offspring recognition abilities are used to mediate discriminative nursing in communally nesting mammals is enigmatic because studies that address both recognition abilities and discriminative care are rare (Holmes & Mateo 2007). For instance, nursing of alien young is sometimes explained by suggesting that mothers cannot recognize their own offspring (the misdirected parental care hypothesis; reviewed in Roulin 2002). In some situations we know that this is not the case because other behavioural evidence shows that mothers can discriminate between their own and alien offspring despite engaging in alien nursing (examples in Packer et al. 1992). We suggest that to understand the relationship between mother–offspring recognition and communal nursing at the proximate level, one should study recognition behaviour independently of nursing behaviour. In earlier work (Jesseau et al. 2008), for example, we used an olfactory habituation–discrimination technique (Johnston 1993) to determine whether degu mothers housed in pairs could distinguish between the odours of their own offspring and those of their co-nesting partner’s offspring. We found that during lactation, mothers could make this discrimination, regardless of whether mothers were housed in sister pairs or in unrelated pairs. In the current study, mothers housed in unrelated pairs may have used their olfactory discrimination abilities to nurse their own offspring preferentially, and for mothers housed in sister pairs it seems unlikely that indiscriminate nursing was caused by ‘recognition errors’ given the results of Jesseau et al. (2008). In degus and perhaps other communally nesting mammals, mother–offspring recognition ability is just one of the proximate factors that influence how mothers care for young. We suggest that empirical studies of mother–offspring recognition coupled with studies of discriminative care by co-nesting mothers is the most effective route to understanding both kin recognition and discriminative care in species that nest communally. Acknowledgments We thank Dr Seema Bhatnagar for the use of laboratory space and radioactivity analyzers. Nate Nowak and Larry Vining offered technical assistance. Joe Miklos assisted with experimental design, data interpretation and radioactive materials safety. John Mitani provided helpful statistical advice. Dan Leger and two anonymous referees provided helpful comments on the manuscript, for which we are grateful. References Bailey, G. N. A., Linn, I. J. & Walker, P. J. 1973. Radioactive marking of small mammals. Mammal Review, 3, 11–23. Beach, F. A. & Jaynes, J. 1956. Studies of maternal retrieving in rats I. Recognition of young. Journal of Mammalogy, 37, 177–180. Beecher, M. D. 1982. Signature systems and kin recognition. American Zoologist, 22, 477–490.
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Communal nesting and discriminative nursing by captive degus, Octodon degus Stephanie A. Jesseau a, *, Warren G. Holmes b,1, Theresa M. Lee c, 2 a
Department of Psychology, University of Nebraska Department of Psychology and Department of Biology and Institute of Cognitive and Decision Sciences, University of Oregon c Department of Psychology, University of Michigan b
a r t i c l e i n f o Article history: Received 5 December 2008 Initial acceptance 22 January 2009 Final acceptance 23 July 2009 Available online 25 September 2009 MS. number: A08-00776R Keywords: allomaternal care communal nesting communal nursing degu kin discrimination kin recognition lactation milk transfer Octodon degu radioactive label
When two or more females rear their young in a common nest or burrow (communal nesting), mothers may be challenged to direct care to their own offspring. In a laboratory study on degus, Octodon degus, a communally nesting South American caviomorph rodent, we used a radionuclide (phorphorus-32) to track milk transfer from mothers to their young in nests occupied by two mothers and their litters. Conesting pairs consisted of either two unrelated mothers or two sisters. We analysed faecal samples from 2-week-old and 4-week-old young to learn whether mothers would nurse discriminately, favouring their own offspring over their co-nesting partner’s offspring. Mothers housed in unrelated pairs nursed their own 2-week-old offspring preferentially (although not exclusively) compared with their co-nesting partner’s offspring, whereas mothers housed with a sister nursed indiscriminately, delivering roughly equal amounts of milk to their own offspring and their nieces and nephews. Faecal analyses from 4-week-old young revealed that mothers may nurse co-nesting young indiscriminately, transferring similar amounts of milk to both types of young, regardless of the relatedness of their co-nesting partner. We propose that discriminative nursing as a function of relatedness between co-nesting female degus may be an adaptation to communal nesting when mothers share a burrow that contains many young of different degrees of relatedness. Ó 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved.
Lactation is energetically expensive for female mammals and is often several times more expensive than gestation (Gittleman & Thompson 1988; Clutton-Brock et al. 1989). Mothers are thus expected to nurse discriminatively, that is, to provide milk only to their own offspring under most social and ecological conditions (Roulin 2002). In rodents, the solitary nesting by females that occurs in most species simplifies identifying and caring exclusively for a mother’s own young because solitary nesting affords a mother many opportunities to learn her offsprings’ kin signatures (Beecher 1982) before young become mobile and begin to mix with other young (Holmes 1990; Holmes & Mateo 2007). In contrast, communal nesting, in which two or more females and their dependent young share a common nest or burrow, may make exclusive nursing of a mother’s own young difficult (Hayes 2000). Indeed, one of the often-cited costs of communal nesting for
* Corresponding author: S. A. Jesseau, 238 Burnett Hall, University of NebraskaLincoln, Lincoln, NE 68588-0308, U.S.A. E-mail address: [email protected] (S.A. Jesseau). 1 W. G. Holmes, 259 Straub Hall, 1227 University of Oregon, Eugene, OR 97403, U.S.A. 2 T. M. Lee, 1004 East Hall, 530 Church Street, Ann Arbor, MI 48109, U.S.A.
females is misdirected maternal care (Solomon 1993; Hayes 2000; Hayes & Solomon 2004). In some cases, communally nesting females may be unable to distinguish between their own offspring and those produced by a co-nesting female (hereafter ‘alien’ young). In communally nesting house mice, Mus musculus domesticus, for example, mothers nurse all young in their nest indiscriminately (Ko¨nig 1989), and in retrieval tests they fail to distinguish between their own and age-matched alien young (Manning et al. 1995). However, even if mothers can discriminate between their own and alien young, nursing may be indiscriminative because young often attempt to obtain milk from any available mother, not just their own. In lions, Panthera leo, for example, mothers can recognize their own offspring (Pusey & Packer 1994), but they may be unable to thwart alien suckling (i.e. milk theft) because the rearing environment includes so many alien young (Packer et al. 1992). According to inclusive fitness theory (Hamilton 1964), a mother could benefit by nursing alien young if they are closely related to her (e.g. nieces or nephews). In fact, when the nursing of alien young occurs, it often involves sisters that are nesting together (Wilkinson & Baker 1988; Hoogland et al. 1989; Jacquot & Vessey 1994). However, given the energetic cost of lactation and the importance of
0003-3472/$38.00 Ó 2009 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.anbehav.2009.07.037
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milk to developing young, a mother sharing a nest with her sister might still benefit from being able to distinguish between her offspring and her nieces and nephews, and if mothers sometimes nest with distantly related or unrelated females, mother–offspring recognition abilities may be especially important. Degus are diurnal, South American caviomorph rodents (Hystricognathi: Octodontidae) that nest communally in nature (Fulk 1976; Ebensperger et al. 2004), and also in captivity even when multiple unused nest sites are available (Ebensperger et al. 2002). Microsatellite analyses from field data show that female degus typically share nests with close kin and occasionally with distantly related or unrelated females (Ebensperger et al. 2004; Jesseau 2004). In olfactory discrimination tests, mothers can discriminate between the odours of their own young and those of their co-nesting partners, even when partners are sisters (Jesseau et al. 2008). Although female degus do not always use these discrimination abilities when providing maternal care (Ebensperger et al. 2006), the observation that mothers can discriminate between the odours of their own young and those of their conesting partner suggests that mothers could behave nepotistically by preferentially nursing their own young. Thus, we sought to determine (1) whether degu mothers housed in pairs nurse discriminatively and (2) whether nursing behaviour differs depending on the relatedness between co-nesting mothers. Because an array of terms has been used in the ‘communal breeding’ literature (examples in Solomon & French 1997), we provide definitions for key terms used here. Communal nesting is the joint occupancy of a common nest or burrow by two or more adult females and their dependent young. Our definition does not imply that females cooperate to care for each other’s young, although they could. Nursing is the transfer of milk between a mother and a dependent young, regardless of which individual is primarily responsible for milk transfer even though some transfer could be due to milk theft, especially by older young (Pusey & Packer 1994). Communal nursing occurs when milk is transferred from a mother to her own offspring and also to alien young (Packer et al. 1992). Communal nursing can be (1) exclusive: nearly all of a mother’s milk is transferred to her own young, (2) preferential: a mother nurses one type of pup more than the other type (e.g. her own offspring more than alien young) or (3) indiscriminate: a mother nurses both types of young nearly equally.
Housing and Animals
METHODS
Radioactive Labelling and Analysis
Degu Natural History
To track milk delivery, we used the transfer of radioactively labelled milk from mother to pup. Quantifying milk intake by measuring suckling behaviour is unreliable in many mammals (Mendl & Paul 1989; Cameron 1998), including Octodon (Kleiman 1974). A study on human infants (Butte et al. 1988) showed that estimates of milk intake using a nonradioactive isotope (deuterium oxide) to trace milk from mother to baby were very similar to results obtained from test-weighing (weighing an infant before and after nursing). Radioactive labelling has been used to answer various questions in different species (Wolff & Holleman 1978; Tamarin et al. 1983), including whether mothers sometimes nurse communally (Hoogland et al. 1989). We therefore assessed milk transfer by injecting postpartum mothers with a radioactive label, and later measuring radioactivity in pups’ faeces (Bailey et al. 1973; Cameron 1998). A limitation of this technique in degus and perhaps in other species is that it is ineffective until pups produce enough faecal material to assay, which may preclude using the technique on very young pups. One mother from each co-nesting pair was randomly chosen to receive a radionuclide injection. One day after parturition, these
Degus are medium-sized (about 200 g) diurnal caviomorph rodents that inhabit shrubland areas of arid central Chile. Females typically produce one litter per year of five or six pups, and births within a population are synchronized (Woods & Boraker 1975; Fulk 1976). Pups are precocial (e.g. they are born fully furred with their eyes open) and are ambulatory immediately after birth (Wilson 1982). They nurse for about 4 weeks and mature sexually by the time they are about 6 months old (Reynolds & Wright 1979). Degus are active above ground during the day, and at night, they occupy underground burrow systems that typically include groups of two to six adult females, one or two adult males and dependent young (Fulk 1976; Ebensperger et al. 2004). Co-nesting females primarily include first- and second-degree kin (e.g. sisters, aunts), although co-nesting females are sometimes unrelated (Ebensperger et al. 2004; Jesseau 2004), which means that during rearing, co-nesting mothers can encounter their own offspring, other related offspring (e.g. nieces and nephews) and unrelated young.
The adult animals used in our study were 66 female degus in a colony at the University of Michigan (Lee 2004). Approximately half of the females were two generations from the field, while the other half were obtained from breeding stock from zoos (generations from the field unknown). Females ranged in age from 9 months to 3 years, and all but two had bred previously. Prior to this study, each female had lived with either one or two of her sisters since birth. To produce young, each female was housed alone with an unrelated adult male for a 3-week mating period. After the mating period, we removed each male and housed females in groups of six, consisting of either three pairs of sisters (4 groups) or two trios of sisters (7 groups). Until we could identify which females were pregnant, each group of six females occupied a 55 37 20 cm (large) plastic tub cage to mimic communal nesting in which females live together during gestation. Of the original 66 females, 44 were not used because they failed to produce a litter or did so when another litter was unavailable to create a co-nesting pair (see below). Approximately 3 weeks before the expected date of parturition, 22 group-housed females that were likely to be pregnant (determined by weight gain and abdominal palpation) were re-housed in pairs in 46 2621 cm (small) plastic tub cages. We were able to create six pairs of conesting sisters (‘sister pairs’) and five pairs of unrelated co-nesting females (‘unrelated pairs’). ‘Co-nesting mothers’ were two females, regardless of their relatedness to each other, that shared the same cage prior to and after the birth of their litters. Since co-nesting females could not be monitored continuously, 5 days before the expected date of parturition, we housed co-nesting females individually at night and sometimes during the day, which allowed us to specify mother–offspring relatedness when observers could not be present at parturition. Throughout the study, all animals had access to ad libitum food (Labdiet Prolab Laboratory Animal Diet, Gray Summit, MO, U.S.A.) and water, and corn cob bedding (Teklad 7097) was used as a substrate. The colony was maintained on a 12:12 h light:dark cycle, and temperature was held at 20 C. After the first mother in each co-nesting pair gave birth, we placed both mothers and pups in a single large tub cage (see above) where they lived through weaning. When pups were 1 day old, we marked them individually with numbered eartags (National Band and Tag Co., no. 1005-1, Newport, KY, U.S.A.).
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mothers were injected intraperitoneally with 15 microcuries (mCi) of Phosphorus-32 (P-32), a beta energy emitter (Tamarin et al. 1983; Hoogland et al. 1989) that would allow us to trace the transfer of milk from an injected mother to the pups that suckled from her. Of the 11 females that were injected, four gave birth before their conesting partner, six gave birth after their co-nesting partner, and one gave birth on the same night as the co-nesting partner. Litter sizes did not differ significantly between sister pairs (X SD number of pups per litter: injected females: 5.5 1.05 pups, range 4–7; noninjected females: 5.17 2.14 pups, range 2–8; paired t test: t5 ¼ 0.349, N ¼ 6, P ¼ 0.741) or between unrelated pairs (injected females: 4.8 1.79 pups, range 2–6; noninjected females: 4.0 2.0 pups, range 2–6; paired t test: t4 ¼ 1.0, N ¼ 5, P ¼ 0.374). The mean SE interbirth interval (i.e. the number of days between the birth of the first and second litters) was 3.6 2.6 days for all pairs of co-nesting mothers and did not differ significantly for sister pairs and unrelated pairs (t9 ¼ 1.25, P ¼ 0.2). We planned to track milk transfer just after mothers began to nurse, but pups produced no appreciable faecal samples until about 2 weeks of age. Therefore, our initial analysis reflects nursing behaviour over the first 14 days of pups’ lives. We collected faecal samples from pups and mothers when young were 2 and 4 weeks old (mothers finish nursing when young are about 4 weeks old) by placing each individual alone in a clean cage until a faecal sample was produced. Samples were immediately weighed and analysed for radioactivity with a Packard 1900 TR Liquid Scintillation Analyzer that detected the amount of beta energy emitted from samples. Pups usually produced samples within the first 15 min of isolation, and mothers always produced a sample in this amount of time. If no faecal sample was produced within 1 h, we returned pups to their original cages, and made another sampling attempt within 24 h. One sample (which consisted of one or more faecal pellets) was collected from each pup at 2-weeksold (X SD ¼ 0:036 0:043 g), and another at 4 weeks old (X SD ¼ 0:094 0:11 g). Ethical Note The levels of radiation that we used for this study were very low, and the degus did not show any behaviours that would indicate that the radiation was harmful. In addition, the longevity and future breeding success of individuals in this project were not affected by radiation exposure. This study was approved by the University of Michigan University Committee on Use and Care of Animals (IACUC protocol number 08497).
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radioactivity in samples from the injected and noninjected mothers (controlling for weight of sample, measured in cpm divided by sample weight). There was significantly more beta energy detected in the faeces of mothers injected with P-32 than in faeces of noninjected mothers when their pups were 2 weeks old (Mann–Whitney U test: U ¼ 3.64, N1 ¼ N2 ¼ 11, P ¼ 0.0003) and 4 weeks old (U ¼ 2.22, N1 ¼ N2 ¼ 11, P ¼ 0.03; Fig. 1). This means that injected mothers had absorbed the radioactive label and, presumably, would pass it to any pups that suckled from them. Note, however, that the amount of P-32 that we detected in mothers decreased significantly from the first measurement (when pups were 2 weeks old) to the second measurement (when pups were 4 weeks old) (U ¼ 2.73, N1 ¼ N2 ¼ 11, P ¼ 0.006), and mothers that were not injected had significantly higher amounts of beta energy in their faeces when their pups were 2 and 4 weeks old than did 57 ‘blank vials’ containing only nonradioactive liquid scintillation fluid (Mann–Whitney U test: Z ¼ 5.93, N1 ¼ 22, N2 ¼ 57, P < 0.0001; see Discussion). For pups, patterns of faecal radioactivity differed depending on whether co-nesting mothers were related. For unrelated co-nesting mothers, there was significantly more beta radiation detected in the faeces of litters from injected mothers (median interquartile range (IQR) ¼ 6447.3 2938.8 cpm) than in the faeces of litters from noninjected mothers (median IQR ¼ 3138.3 976.4 cpm) when pups were 2 weeks old (Wilcoxon signed-ranks tests: T ¼ 2.02, N ¼ 5 pairs of mothers, P ¼ 0.043; Fig. 2). That is, injected mothers appeared to nurse both their own and alien young, but they nursed their own offspring preferentially. When co-nesting mothers were sisters, however, there was no significant difference in beta energy detected in pups’ faeces of injected (median IQR ¼ 7653.2 8443.07 cpm) and noninjected (median IQR ¼ 6643.4 19054.3 cpm) mothers (T ¼ 0.52, N ¼ 6 pairs of mothers, P ¼ 0.6; Fig. 2). That is, injected mothers seemed to nurse indiscriminately, delivering similar amounts of P-32-labelled milk to their offspring and alien young. When pups were 4 weeks old, there was no difference in beta energy detected in pups’ faeces whether mothers were unrelated (T ¼ 0.67, N ¼ 5 pairs of mothers, P ¼ 0.5; Fig. 3) or sisters (T ¼ 1.57, N ¼ 6 pairs of mothers, P ¼ 0.12; Fig. 3). That is, both sisters and unrelated pairs of co-nesting mothers seemed to nurse nearly weaned young indiscriminately. DISCUSSION Our use of a radioactive label to track milk transfer revealed that communally nesting degu mothers appear to nurse young
Statistics
RESULTS To ensure that P-32 was absorbed by injected mothers, allowing us to trace radionuclides from mother to pups, we compared
14 000 Mothers’ faecal radioactivity (counts per min per g)
To control for the size of the faecal sample for both mothers and pups, we divided the amount of beta energy emitted (measured in counts per minute, or cpm) by the sample weight to obtain cpm per gram of faecal matter, and used this value for all analyses. To avoid ‘litter effects’ (the nonindependence of young from the same litter) that might be present when dealing with entire litters of multiparous species, we used the litter as the unit of analysis by obtaining a mean value for each litter (Zorrilla 1997). Wilcoxon signed-ranks tests were then used to compare the mean amount of radiation detected in the litters of injected and noninjected mothers from the same cage. Mann–Whitney U tests were used for all analyses pertaining only to adult females.
Noninjected
12 000
P-32 injected
10 000 *
8000 6000
*
4000 2000 0
* 2 weeks 4 weeks Age of mothers’ litters
Figure 1. Box plot (median, quartiles and 10th and 90th percentiles) of radioactivity detected in faeces of mothers that were or were not injected with a radioactive label. Faeces were collected when young were 2 and 4 weeks old. *P < 0.05.
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Pups’ faecal radioactivity (counts per min per g)
35 000 30 000
Noninjected P-32 injected
25 000 20 000 15 000 10 000
*
5000 0 Unrelated Sisters Relatedness of co-nesting mothers
Figure 2. Box plot (median, quartiles and 10th and 90th percentiles) of radioactivity detected in faeces of litters of 2-week-old young housed with co-nesting mothers, one of which was injected with a radioactive label. Co-nesting mothers were sisters or were unrelated. *P < 0.05.
discriminatively, although their nursing behaviour depended on pups’ ages and whether co-nesting mothers were sisters or were unrelated to each other. When young were 2 weeks old, radioactively injected mothers housed in unrelated pairs and exposed to both their own offspring and those of their nest partner (alien young) nursed both types of young, although they nursed their own offspring preferentially. In contrast, injected mothers housed in sister pairs appeared to nurse their own and alien young indiscriminately, delivering roughly equal amounts of milk to their offspring and their nieces and nephews (Fig. 2). When pups were about 4 weeks old and lactation was ending, injected mothers in both sister and unrelated pairs appeared to nurse their offspring and alien young indiscriminately (Fig. 3). Collectively, these results suggest that female degus can use a conditional strategy of maternal investment: nursing behaviour that favours their own kin in communal nests. Our results show that beta energy in the faeces of injected mothers decreased steadily over the course of the 4-week study (Fig. 1). This finding is important because it shows that beta radiation was gradually excreted in faeces and presumably in milk, ensuring that we could track its transfer across time. (If P-32 were metabolized very quickly or very slowly, decreased levels of radioactivity might have made tracking difficult.) This gradual
Pups’ faecal radioactivity (counts per min per g)
8000 7000
Noninjected
6000
P-32 injected
5000 4000 3000 2000 1000 0
Unrelated Sisters Relatedness of co-nesting mothers
Figure 3. Box plot (median, quartiles and 10th and 90th percentiles) of radioactivity detected in faeces of litters of 4-week-old young housed with co-nesting mothers, one of which was injected with a radioactive label. Co-nesting mothers were sisters or were unrelated.
release of the radionuclide makes it well suited for the study of milk transfer (also see Hoogland et al. 1989). Consistent, statistically significant differences in P-32 levels between injected and noninjected co-nesting mothers (Fig. 1) indicate that contamination of the noninjected mother with the radionuclide of the injected mother was minimal. Contamination was possible because degus engage in coprophagy (Kenagy et al. 1999) and ano-genital licking of pups (Wilson & Kleiman 1974; Reynolds & Wright 1979), and extensive contamination would have masked any effect of preferential nursing. In addition, we consider the use of a radioactive label to track milk transfer more reliable than observing nursing behaviour directly (Cameron 1998). During nonsystematic observations of conesting mothers and their litters, we saw only one instance of a mother actively rejecting a pup’s suckling attempt, and we routinely observed most or all pups in a cage huddled together under a single mother, apparently attempting to suckle. Ebensperger et al. (2002) also describe apparent suckling attempts by alien young in captive, group-nesting degus. Behavioural observations of nursing would therefore prove unreliable since there were often more pups attempting to suckle than a mother had teats. There are at least three proximate explanations for apparent indiscriminate nursing by injected mothers in sister pairs when pups were 2 weeks old (Fig. 2). First, mother–offspring recognition is routinely mediated by olfaction in rodents (Mateo 2003; Holmes & Mateo 2007) and mothers may be unable to distinguish between the odours of their offspring and those of their nieces and nephews. Olfactory kin labels are perceived as more similar when they come from close kin than when they come from distantly related or unrelated conspecifics (Solomon 1993; Mateo 2002; Todrank & Heth 2003), and discriminating among the odours of cousins may have been challenging for mothers in sister pairs. However, the ‘similar labels’ hypothesis is not consistent with our prior finding that degu mothers can discriminate between the odours of conesting cousins (Jesseau et al. 2008). That degu mothers can discriminate between cousins’ odours and yet nurse cousins indiscriminately serves as a reminder that the ability to discriminate does not always result in the preferential treatment of closely related kin over more distantly related kin (Holmes & Mateo 2007). Second, the energetic cost of lactation may have been inadequate to trigger discriminative nursing. Degu mothers experience the highest energetic cost of milk production during early lactation (Veloso & Bozinovic 2000), which was about 7 days before we began to collect pups’ faeces to infer nursing behaviour. Our ad libitum feeding regimen would have further minimized the energetic demands of lactation. When the energetic costs of lactation are low, nursing mothers may be unmotivated to nurse discriminatively (Hayes & Solomon 2004; Ebensperger et al. 2006). It is possible, for example, that co-nesting sisters in the wild or in a captive environment with food restriction might use this recognition ability and preferentially nurse their own pups. However, the ‘low-cost’ hypothesis does not seem to apply to unrelated pairs of mothers that, unlike sister pairs, did nurse discriminatively (Fig. 2). Finally, nursing decisions by degus may be influenced more by the identities of co-nesting partners than by the identities of conesting young. Mothers may follow a specific ‘familiarity’ rule of thumb that directs them to nurse indiscriminately whenever they nest with a sister or another closely related female. Females are ‘familiar’ with each other if they grow up together, having shared a common rearing environment prior to weaning. This rule of thumb could explain indiscriminate nursing by sister pairs and discriminative nursing by unrelated pairs since members of unrelated pairs did not interact with each other until they lived together in adulthood. In an experimental study of communal rearing in house mice, for example, pairs of females that lived together since
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birth produced more offspring as adults than females that were reared apart and then paired as adults, regardless of their relatedness to each other (Ko¨nig 1994). Even though degu mothers are capable of discriminating between the odours of their own offspring and those of nieces and nephews (Jesseau et al. 2008), females may have nursed indiscriminately because of the familiarity that resulted from growing up with their nesting partner. In contrast to sister pairs, injected mothers in unrelated pairs appeared to nurse their own offspring preferentially when pups were 2 weeks old, although they did not nurse their own offspring exclusively (Fig. 2). We propose that, in the face of a nutritional surplus (ad libitum feeding) and high pup density, it was not worthwhile for mothers to work as hard as would have been necessary (e.g. preventing all milk theft) to ensure exclusive nursing of their own offspring (Hayes & Solomon 2004). Despite abundant nutritional resources and high pup density, mothers still used their olfactory discrimination abilities (Jesseau et al. 2008) to direct their nursing behaviour preferentially towards their own offspring. By the time degu young were 4 weeks old, injected mothers, whether they were housed with a sister or a nonrelative, appeared to nurse indiscriminately. That is, we found no difference in faecal radioactivity between litters of co-nesting pups of injected and noninjected mothers (Fig. 3). There are several reasons why we may have obtained these results. First, the cumulative effect of coprophagy over the 4-week period may have masked the effects of discriminative nursing. Even if pups from noninjected mothers nursed only from their own mother, they could still display measurable amounts of radioactivity if their mothers consistently obtained the radioactive label by ingesting faeces from injected mothers or their pups. In addition, the resolving power of our radionuclide technique may have been inadequate to detect discriminative nursing by 4-week-old young. We injected mothers only once, 1 day after parturition, and radioactive levels declined noticeably in mothers after injection (Fig. 1), which may have prevented us from detecting differences in 4-week-old young. Despite these issues, it is possible that we obtained no difference when pups were 4 weeks old because females did not discriminate among pups at this time. Weaning in degus occurs around 4 weeks of age (Weir 1974), and young begin consuming solid food well before weaning (Reynolds & Wright 1979), so perhaps mothers with 4-week-old young were not motivated to nurse discriminatively because the cost of indiscriminate nursing was low. Degu mothers with 6-week-old young do not discriminate between the odours of their own and those of familiar alien young (Jesseau et al. 2008) and perhaps this lack of discrimination also explains indiscriminate nursing by mothers with 4-week-old young. Our discussion of disciminative nursing by degu mothers has thus far assumed that mothers are responsible for milk transfer to pups and that pups have little influence on this process, a questionable assumption given the precocial nature of degu pups (Wilson 1982). We had intended to observe mother–pup interactions directly, but this proved difficult because we routinely observed most or all pups in a cage huddled together under a single mother, which prevented us from observing the behaviour of individual pups. During nonsystematic observations of co-nesting mothers and their litters, we saw multiple pups attempting to suckle from a single mother and only one instance of a mother actively rejecting a pup’s suckling attempt. Thus, we believe that co-nesting mothers and their pups all played a role in milk transfer rather than mothers being solely responsible. The functional benefit of mother–offspring recognition has long been appreciated by animal behaviourists (Beach & Jaynes 1956; Cullen 1957; Davies & Carrick 1962), since direct interactions between mothers and their newborn young afford the learning
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opportunities that typically mediate mother–offspring recognition. Even in rearing environments that include multiple mothers and their offspring, kin recognition could be mediated by the prior association mechanism if mothers have opportunities to learn the identities of their young before litters or broods begin to intermix (Komdeur et al. 2004; Sharp et al. 2005). If social contexts during early development offer no reliable opportunities to learn kin signatures before various categories of kin and nonkin are encountered, a recognition mechanism such as self-referent phenotype matching might mediate mother–offspring recognition. Despite the existence of mechanisms that could mediate mother– offspring recognition in the face of ambiguous social contexts, recognition, and the subsequent discriminative care that it could allow, may not occur if the costs of discrimination outweigh the benefits. How mother–offspring recognition abilities are used to mediate discriminative nursing in communally nesting mammals is enigmatic because studies that address both recognition abilities and discriminative care are rare (Holmes & Mateo 2007). For instance, nursing of alien young is sometimes explained by suggesting that mothers cannot recognize their own offspring (the misdirected parental care hypothesis; reviewed in Roulin 2002). In some situations we know that this is not the case because other behavioural evidence shows that mothers can discriminate between their own and alien offspring despite engaging in alien nursing (examples in Packer et al. 1992). We suggest that to understand the relationship between mother–offspring recognition and communal nursing at the proximate level, one should study recognition behaviour independently of nursing behaviour. In earlier work (Jesseau et al. 2008), for example, we used an olfactory habituation–discrimination technique (Johnston 1993) to determine whether degu mothers housed in pairs could distinguish between the odours of their own offspring and those of their co-nesting partner’s offspring. We found that during lactation, mothers could make this discrimination, regardless of whether mothers were housed in sister pairs or in unrelated pairs. In the current study, mothers housed in unrelated pairs may have used their olfactory discrimination abilities to nurse their own offspring preferentially, and for mothers housed in sister pairs it seems unlikely that indiscriminate nursing was caused by ‘recognition errors’ given the results of Jesseau et al. (2008). In degus and perhaps other communally nesting mammals, mother–offspring recognition ability is just one of the proximate factors that influence how mothers care for young. We suggest that empirical studies of mother–offspring recognition coupled with studies of discriminative care by co-nesting mothers is the most effective route to understanding both kin recognition and discriminative care in species that nest communally. Acknowledgments We thank Dr Seema Bhatnagar for the use of laboratory space and radioactivity analyzers. Nate Nowak and Larry Vining offered technical assistance. Joe Miklos assisted with experimental design, data interpretation and radioactive materials safety. John Mitani provided helpful statistical advice. Dan Leger and two anonymous referees provided helpful comments on the manuscript, for which we are grateful. References Bailey, G. N. A., Linn, I. J. & Walker, P. J. 1973. Radioactive marking of small mammals. Mammal Review, 3, 11–23. Beach, F. A. & Jaynes, J. 1956. Studies of maternal retrieving in rats I. Recognition of young. Journal of Mammalogy, 37, 177–180. Beecher, M. D. 1982. Signature systems and kin recognition. American Zoologist, 22, 477–490.
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