406 PRIMATES, 210): 406--415, July 1980

Alloparental Care and Kinship in Captive Social Groups of Vervet Monkeys ( Cercopithecus aethiops sabaeus)

CANDACE JOHNSON, University of California, Los Angeles CATHY KOERNER, MARTY ESTRIN, Sepulveda Veterans Administration Hospital

and DEANNA DUOOS, Behavioral Science Foundation

ABSTRACT. Estimates of alloparental behavior on the basis of kinship and maternal qualities of permissiveness/restrictiveness were examined in vervet mother-infant pairs. Amounts of alloparenting were examined for the subgroup categories female siblings, juvenile females, subadult nulliparous females, adult females, males, and yearlings. Analysis of the six subgroups for months 1 through 6 revealed that the most alloparenting occurred in month I with siblings caretaking more than any other subgroup. By month 2 subadult nulliparous females were the most frequent caretakers.

Correlations between kin alloparenting and maternal permissiveness/restrictiveness indicated that kinship was a better predictor of alloparenting than maternal qualities of permissiveness. Addi- tionally, examination of the data on maternal behaviors towards related and unrelated individuals revealed significant differences by months.

INTRODUCTION

The mother-infant bond is the most important social relationship in an individual's life. This primary bond both insures initial survival and sets the stage for future relationships. The ex- tended infancy period in primates has made this bond even stronger than other infra-primate species (HoRWICH & MANSKI, 1975). Increases in investment of care by the mother presuma- bly maximizes her chances of leaving surviving offspring who will subsequently be successful parents (HRDY, 1976). To the extent that caretaking and bond formation between individ- uals is a costly investment, the passing of infants to unskilled individuals other than the mother and taking a risk of infant survival is somewhat curious.

The term alloparent or helper is used to denote any individual other than the mother who cares for a young infant (xqVILSON, 1975). The characteristic is richly expressed in the primates having been recorded in lemurs, New World monkeys, Old World monkeys, and chimpan- zees (DUMOND, 1968; SPENCER-BOOTH, 1968; LANCASTER, 1971; STRUHSAKER, 1971; JOLLY, 1972; ROWELL, 1972; HRDY, 1976). The term aunting has also been used to refer to this caretaking behavior by young females (ROWELL, HINDE, & SPENCER-BOOTH, 1964). However, neither of these terms necessarily designate any genetic relatedness between caretaker and infant.

Current hypotheses have concerned themselves with inclusive fitness, i.e., the sum of an individual's own fitness plus the effects that their behavior has on the fitness of their relatives and vice versa. With the recent theoretical suggestions about the roles of kinship in the deter- mination of behaviors (HAMILTON, 1964), it has become increasingly important to know the extent to which such aunts or alloparents are related to their charges. On the basis of HAMIL- TON'S theories, the degree of relationship would be expected to be a rough predictor of the type of behavior that will be directed towards an infant.

Alloparental Care and Kinship in Vervet Monkeys 407

Evidence supports that young females who handle infants gain experience as mothers be- fore committing themselves to motherhood (WILSON, 1975; LANCASTER, 1971). HARLOW and HARLOW (1965), in support of necessary experience foryoung females to be good mothers, has shown that those females who themselves had no mothering or had only been raised with peers, became inadequate mothers with their first offspring. We may grant that both related and unrelated allomothers may benefit in this way, but is there potential penalty for mothers who tolerate alloparenting by unrelated individuals ? The mother could expect to lose fitness by turning her offspring over to the care of incompetent caretakers.

The allowance of kin alloparenting presupposes that the infant is not adversely effected by related females who may have had little practice, for mothers would be trading damage to an infant which shares one-half her genes for potential benefit to an infant who shares only one-fourth or less (HAMILTON, 1964; TRIVERS, 1971). The mother controls access to her in- fant. It would not be expected that she would allow another female to take her infant unless the benefits to the mother-infant pair offset the potential costs of this behavior.

It would seem that a critical reason for taking this risk would be for the benefit of kinship bonds. By permitting daughters and other close relatives to practice with their children, moth- ers can improve their inclusive fitness through proliferation of additional kin as these relatives bear and successfully rear their first offspring (KURLAND, 1977; WILSON, 1975).

Alternately, prediction of who would be allowed to practice alloparenting may be based up- on qualities in the parents' maternal behaviors. LANCASTER (1971) in her field study of vervet groups found no significant relationship between aunting and relatedness. She concluded that the degree of permissiveness of the mother was a better predictor of alloparenting than related- ness. Permissiveness/restrictiveness, as a quantified measure with vervets, was recently con- structed by the senior author (JOHNSON, GILBERT, • HERDT, 1979).

Two working hypotheses are conceivable. Female siblings would more likely be allowed by a mother to caretake a related infant than she would allow unrelated females. This should be especially true in the early months of the infant's life. If LANCASTER'S conclusion is the better explanation, then the index of maternal permissiveness, would show a negative correlation with alloparenting.

The data reported below are taken from ongoing research of maternal bonding and infant development among St. Kitts vervets (Cercopithecus aethiops sabaeus). This study aims to fur- ther document the alloparental practices of this species and to understand how these prac- tices are related to current theories of evolutionary biology.

METHODS

SUBJECTS

The subjects included six vervet mother-infant pairs (Cercopithecus aethiops sabaeus). Four infants (three males and one female) were born in Sepulveda, California, while two infants (females) were born in St. Kitts, West Indies. The composition of the groups in which the in- fants were born and raised was as follows (Sepulveda and St. Kitts, respectively): 6 female siblings (4, 2); 4 juvenile females (2, 2); 6 subadult nulliparous females (3, 3); 12 adult females (4, 8); 12 males (7, 5); and 8 yearlings (3, 5). Records were kept of which females gave birth to which infants. The sibling offspring of the same female of all the younger monkeys were known.

408 C. JOHNSON et al.

Each colony ranged in number from 24-26 individuals and were balanced by age and sex to approximate naturally occurring troops (McGUIRE, 1974). Any data on the infants which had female siblings are included in the formal analysis. Those infants who had no siblings or only a male sibling are discussed briefly.

HOUSING

The infants were born in captive social groups in outdoor enclosures. The St. Kitts colony was housed in a 1,000 m 2 enclosure. The enclosure contained a variety of perches and visual barriers, an open roof, and a grassy floor. The animals were fed local produce including pea- nuts, corn, breadfruit, sweet potatoes, and mangoes supplemented with high protein biscuits. Food was provided every morning at 1030 and in the evening at 1600. Water was available ad lib. The Sepulveda colony was housed in an outdoor enclosure consisting of two outer octagonal sections totaling 195 m 2 having chain link sides, open roof, and a grassy floor. Each section contained several wooden structures to provide shade and added surface area for climbing and sitting. A center section was covered to provide protection from wind and rain and contained 2 halves. The monkeys had access to this section at all times except during data collection. The animals received monkey chow and water ad lib.

OBSERVATION SCHEDULE

Each mother-infant pair was systematically observed from the time the infant was 2 weeks of age until it was 6 months old. Observations were conducted 5 hr a week for each mother- infant pair. Data from the Sepulveda colony were taken during one year of observations from December 1976 to January 1978. Data from the St. Kitts colony were collected for six months between June 1977 and December 1977. The same schedule and coding systems were used for both colonies.

CODING SYSTEM

The mother-infant pair was the primary sampling unit with the behaviors for both mother and infant recorded whenever they were within 1 m of each other. When the infant was sepa- rated from its mother by at least 1 m, the infant became the focal animal and the mother's behaviors were not recorded. Observations were made using a one-zero time sampling tech- nique. By this method a defined behavior was recorded once if it occurred one or more times during a 30-sec interval. The data thus represent the number of 30-sec observation periods in which the behaviors occurred.

BEHAVIORAL REPERTOIRE AND INTEROBSERVER RELIABILITY

Alloparental care was recorded when an individual showed any of the following parental behaviors: successful stealing of an infant f rom the mother; picking up of an infant when alone; embracing, pat-stroke-jiggling of an infant; carrying, restraining, and inspecting the infant while off the mother.

Maternal behaviors observed and identified as the index permissiveness/restrictiveness in- clude: (a) mother aggresses against others; (b) mother leaves others, takes infant; (c) mother restrains infant. A more positive score on these sub-indices indicates restrictive behavior of the mother while a lower score on the parts of the scale as a whole showed permissiveness on the part of the mother. These behaviors were scored for each mother-infant dyad and totaled to give a behavioral index such that each pair could be ranked with every mother-infant pair (JOHNSON, GILBERT, & HERDT, 1979).

Alloparental Care and Kinship in Vervet Monkeys 409

Six separate and distinct behaviors were also recorded for each pair. Maternal retrievals were recorded when a mother approached any individual in the groups who had her infant and took it from them or approached her infant when it was alone and picked it up. Maternal aggression was defined as all forms of direct aggression including staring, jerking of head or forequarters, slapping, chasing, the forming or joining into coalitions, and biting. Time spent in contact with infant was recorded when any physical body contact occurred between the infant and any other individual in the group other than the mother. Time in proximity to infant was recorded whenever any individual in the group was within its arm's reach of an infant. Two additional categories of time in proximity and contact with nonkin were recorded to show in particular how certain related individuals (female siblings) responded to nonkin infants in the group.

Phi correlation coefficients (SIEGEL, 1956) were calculated for each behavior for all observ- ers in the study. Interobserver reliability for the behavioral categories ranged from .85-.95.

RESULTS

The mean frequencies of the six subgroups (female siblings, juvenile females, subadult nulliparous females, adult females, males, and yearlings) were examined monthly for allo- parenting of the infants during the first six months. Corrected mean values were used, such that a subgroup with six members had the same weight as a subgroup with only one or two members. If a sibling was also considered a juvenile female, that individual was counted only for the sibling category with her own sibling and only counted as a juvenile female with all other infants. For example, an average score for the number of perieds an infant was allo- parented by a sibling was corrected if there were two sibs for infant A and only one for in- fant B. The resultant score provides a single value indicating the degree to which the average sibling, average subadult, etc., cared for the average infant in each group.

Differences among months and the six subgroups, and between the two captive social groups were tested using a mixed design (one between, two within) analysis of variance (MEYERS, 1972). The two groups (St. Kitts and Sepulveda) were the between variable while months and subgroups were the two within variables. The analysis indicated no significant effect due to groups (F ---- 6.56, df ---- 1,4, p =.063). The St. Kitts and Sepulveda animals showed similar amount of alloparenting behaviors; for this reason all data were pooled. The averages derived for each subgroup were based on the monthly totals of each behavior. The use of mean scores adjusted for any differences in total frequency that may have occurred be-

tween subjects. The result was a significant months effect (F = 6.80, df = 5,20, p<.001) and a significant

interaction between months and subgroups (F = 1.86, df = 25,100, p<.01). Figure 1 shows that the six subgroups considered together alloparented most in month 1 ; the next highest fre- quency occurred in month 2. Scheff6 post-hoe comparisons were used to examine the differ- ences between consecutive months. The significant decrease in alloparental care occurred be- tween months 2 and 3 (p<.05). After month 3 there were no significant differences between

months. To further investigate the relationships between months and demographic subgroups, each

subgroup was examined for caretaking behaviors during months 1 through 3. Figure 2 shows the amount of alloparental care observed for each of the six subgroups over the first three months.

410 C. JOHNSON et al.

I00

80

60 - Lt._J

~ 20

=E

0 I I 0 I 2

AGE OF INFANTS (months)

I ~ ' ~ k . - - - ~ - - - - ~ 3 4 5 6

Fig. 1. Mean number of intervals of alloparental behavior for all subgroup categories for months 1 through 6.

6o

iS so k-'Xl ~ r . ,

~ <~ 20

o

SUBGROUP CATEGORIES Fig. 2. Mean number of intervals of alloparental behavior for each subgroup category for months 1, 2, and 3.

In month 2 both the significant decrease in caretaking by female siblings and the significant increase in alloparental care by subadult nulliparous females were responsible for the interac- tion effect between months and subgroups. Post-hoe comparisons show that at month 1 fe- male siblings showed significantly more aUoparenting (p<.01) than any other category of individuals. However, by month 2, the female sibling subgroup showed significantly less

Alloparental Care and Kinship in Vervet Monkeys 411

Table 1. Spearman correlations between alloparental care for kin and nonkin and maternal permis- siveness]restrictiveness.

Overall Nonkin Kin Maternal permis- alloparental alloparental alloparental siveness/restrictive-

Behavioral indices care care care hess Overall

alloparental care - - +0.51 +0.87* --0.25 Nonkin

alloparental care - - --0.72* +0.22 Kin aUoparental

care - - +0.43 Maternal

permissiveness/ restrictiveness

*p<0.05.

caretaking (p<.O1) while the subadult nulliparous females showed a significant increase from month 1 to month 2 (p<.01). The juvenile females showed no significant increase from month 1 to month 2.

The males, whether or not they were siblings, were rarely observed alloparenting. For all males (during 3,329 periods) there occurred only 140 observations of alloparental behavior. A large number of these were contributed by a single subadult male who used the infants as a sexual releaser. During the six months of observations, male siblings were observed caretak- ing in only eight time periods.

Table I shows the Spearman rank order correlations used to compare the behavioral index of maternal permissiveness/restrictiveness, and the amounts of overall alloparental care, kin alloparental care, and nonkin alloparental care with each other. The results in Table 1 indicate no significant correlation between permissiveness~restrictiveness on the part of the mother and the amount of overall alloparental care received by the infant. There was also no significant re- lationship between permissiveness and kin alloparenting. When the relationship between overall alloparental care and kin alloparental care was examined, a significant positive cor- relation was found to exist. Further, there was a significant negative correlation between nonkin alloparental care and kin alloparental care but no significant correlation between non- kin and overall alloparental care.

Maternal behaviors, whether directed towards related individuals (female sibling) or other unrelated individuals, were also examined. If we assume that alloparents (including relatives) are basically acting selfishly to increase their own mothering skills, the mothers should be selective in who has access to herself and her infant. This should also be seen in the proximity and contact maintained by related versus unrelated individuals over months.

Table 2 shows the mean number of time intervals that each of four behaviors occurred for related versus unrelated individuals (kin versus nonkin respectively). Each behavior is shown for months 1 and 2; it was hypothesized that infants' mothers' behavior as well as other group members' behaviors would change during the early period of infant development. The number of maternal retrievals o f infants from siblings was significantly less than was the num- ber of retrievals from unrelated individuals (t = 3.597, df = 10, p<.01) . This was true for both months 1 and 2. The number of intervals in which maternal aggression occurred against a sibling (kin) was significantly less during month 1 than was maternal aggression towards nonkin (t = 2.921, d f = 10, p<.02). However, in month 2 maternal aggression against siblings

412 C. JOHNSON et al.

Table 2. Mean number of intervals of four behavioral measures in month I and month 2, for kin and nonkin groups.

Maternal retrievals Maternal aggressive Time in proximity Timein contact Behaviors of infants from: behavior against: to infants by: with infants by:

Month Month Month Month 1 2 1 2 1 2 1 2

Groups Nonkin 21.40 22.20 11.60 10.76 73.85 160.00 24.05 76.60

(6.19)* (6.89) (3.28) (6.68) (36.04) (26.10) ( 6 . 8 2 ) (26.42) Kin (siblings) 6.71 8.12 4.23 11.94 355.00 146.20 116.00 50.40

(2.14) (2.43) (1.41) (3.62) (110.59) (74.12) (34.37) (24.89) *Standard deviations are in parentheses.

increased, becoming close to the level demonstrated against unrelated animals (t ---- 2.620, d f ~ 5, p<.05).

Infants were found to have spent significantly more time in proximity with their older sib- lings than with unrelated individuals during month 1 (t = 2.341, df = 10, p<.05). The fre- quency of this behavior decreased during month 2 with significantly less time spent by the sibs near a sib infant (t = 2.598, df = 5, p<.05).

A separate analysis (not shown in tabular form) was performed to evaluate whether the fe- male siblings spent more or less time with other unrelated infants after month 1. More time in proximity was spent by the sibs with unrelated infants in month 2 than month 1 ( t= 2.741, df~ 10, p<.05) . A decrease from month 1 to month 2 was observed in the number of times the infants were touched or spent time in contact with an older female sibling (t=2.67, dJ=5, p<.05). This decrease in proximity and contact of young females with sibs was followed by a significant increase in the amount of time spent in contact with unrelated infants (t=2.72, df= 5, p <.05).

DISCUSSION

Analyses of the observations of vervet infants in the present study indicate that related- ness, at least when the relationship is as close as being siblings, is a good predictor of allo- parental care. It has been shown that during the first month of an infant's life, its mother has greater control of her other offspring (the infant's older female siblings) than over unrelated kidnapping females. It makes adaptive sense that instances of care for infants by their siblings in month 1 should be greater than by other unrelated caretakers during this early period of vulnerability.

LANCASTER's (1971) study had indicated that no strong orientation of juvenile females was shown towards their newborn siblings. She did state that a mildly anxious mother would tend to be more restrictive of her infant and even though other kinds of behavior such as play may not be inhibited by tension, it is possible that the mother's permissiveness would be. Therefore, LANCASTER saw the relative tension or permissiveness of the mother rather than relatedness of the caretaker as a more significant factor for predicting the amount of alloparenting. In contrast, the data of the present study indicate kinship to be more salient than the mater- nal quality of permissiveness. The differences between these two findings may be due in part to the relative habituation of the animals to the observers in the present study. LANCASTER stated that several of the mothers in her field study tolerated the presence of the observer

Alloparental Care and Kinship in Vervet Monkeys 413

better than all the others. Over a three year period our captive social groups had become well habituated to human observers.

In most species maternal supervision of caretakers has been observed where alloparental care has been reported (DuMoND, 1968; LAr~CASTER, 1971). At the first symptoms of distress, usually the vocalizing of her infant, the mother retrieves it. LANCASTER has suggested that such watchfulness on the part of the mother enhances the process of learning to mother by conditioning caretakers to keep infants quiet and content. From a fitness point of view both the sibling and the mother can benefit.

The vulnerability of the infant monkey decreases rapidly with age. For example, its hand and toe grip strengthens and the infant grows more robust after month 1 (KRIGE & LUCAS, 1975). This study shows that sibling caretaking occurs most while the infant is more vulner- able to exposure, may lack adequate food and water, or be endangered by possible abuse. This would be beneficial for the infant. In addition, the mother is allowed time-out from the re- sponsibilities of her motherhood (e.g., to forage or build up strength). Once the infant is eat- ing on its own and able to locomote in its environment (around month 2), the risk or cost is less to both the mother and infant. It is predictable that the other young females would be allowed access to the infant from this time forth.

The vervet mothers seem quite tolerant of alloparental behavior and especially of allo- parenting by siblings. In social groups of Japanese monkeys, KtrRLAND (1977) found that caretaking occurred primarily with unrelated individuals and that female siblings were in fact discouraged from caretaking their sibs. Although the present study of vervets indicates more kin alloparenting than KURLAND'S study, there were similar behaviors that occurred in the vervets which were also shown in the social group study of rhesus monkeys.

KURLAND's study of kin-selection showed low retrieval rates and higher aggression against sibling caretakers as compared to nonkin. Our data indicate that in month 1 there was a low retrieval rate as well as a low aggression rate. Later in the infant's development we begin to see the effect mentioned by KtJRLAND. In month 2 the female sibling subgroup described here- in became more aggressed against by their mothers. The siblings also spent less time in prox- imity with their sib infants. This may indicate that the mother is discouraging selfish allo- parenting by siblings and thereby forcing them to spend the rest of their learning experience with unrelated infants. These data fit KURLAND'S hypothesis of selfish caretaking.

Byhaving relatives care for her infant this strategyalso maximizes the benefits for the moth- er during the early months of the infant's development. Later when she allows aUoparenting by nonkin, she is able to strengthen her social bonds with other females in the group. The female sib who alloparents benefits by the gaining of experience from its mother, the forma- tion of strong kinship bonds, and bythe formation of bondswith others outside the kin group.

Because of different inclusive fitness strategies, conflict between parents and these older female offspring over the amount of parental investment is expected (TRIVERS, 1974). TRIVERS' parent-offspring model predicts conflict between mothers and older offspring with regards to the latter's selfish alloparenting of infant offspring. The mother, being equally related to both the new and older offspring, is selected to tolerate the older offspring's selfish alloparen- tal behavior toward younger offspring whenever the benefit outweighs the cost.

The older offspring is related to its younger sib by about a third on the average (KuRLAND, 1977). It maximizes its own inclusive fitness with respect to allopaxenting of younger siblings whenever benefit is greater than cost divided by 3. Therefore, older offspring will be selected to alloparent more than the mother will be selected to tolerate alloparenting. Maternal ag-

414 C. JOHNSON et al.

gression toward sibling alloparents was found to be relatively common during month 2.This indicates that there is disagreement between mother and older offspring about the latter's be- havior.

In those species in which the period of contact between a mother and her female offspring overlap with the birth of subsequent offspring, kin selection would predict that the daughters would be the preferred caretakers. This appears to hold true in vervets. When daughters are used as caretakers, the "cost" of their caretaking behavior is deductible for a close relative may profit. When the cost to the mother is too high, for example, from too much caretaking even by a female sibling, an effective evolutionary strategy must be the reduction of this con- tact.

A c k n o w l e d g e m e n t s . This project was in part supported by the Medical Research Service of the Vet- erans Administration and BRSG grant RR05756 awarded by the Biomedical Research Support Grant Program, Division of Research Resources, National Institute of Health. The research was con- ducted while the senior author was an NIMH postdoctoral fellow. The author extends thanks to MARK GILBERT for his assistance with the observations and B. HARRIS for his editorial comments on the manuscript.

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Alloparental Care and Kinship in Vervet Monkeys 415

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--Received April 16, 1979; Accepted October 5, 1979

Authors' Present Addresses: CANDACE JOHNSON, Department of Psychiatry, School of Medicine, Neuropsychiatric Institute, The Center for the Health Sciences, 760 Westwood Plaza, Los Angeles, California 90024, U.S.A.; CATHY KOERNER and MARTY ESTRIN, Department of Psychiatry/Biobehavioral Sciences, University of California, Los Angeles, Los Angeles, California 90024, U.S.A.; DEANNA DUOOS, Department of Biology, University of Cali- fornia, Davis, Davis, California, 95616, U.S.A.