Variable infant care contributions in cooperatively breeding groups of wild saddleback tamarins

Among non-human primates, alloparental infant care is most extensive in callitrichines, and is thought to be particularly costly for tamarins whose helpers may suffer increased energy expenditure, weight loss, and reduced feeding time and mobility. The costs and benefits of infant care likely vary among group members yet very few wild studies have investigated variable infant care contributions. We studied infant care over an 8-month period in four wild groups of saddleback tamarins in Bolivia to evaluate: (a) what forms of infant care are provided, by whom, and when, (b) how individuals adjust their behavior (activity, vigilance, height) while caring for infants, and (c) whether individuals differ in their infant care contributions. We found that infant carrying, food sharing, and grooming varied among groups, and immigrant males—those who joined the group after infants were conceived—participated less in infant care compared to resident males. Adult tamarins fed less, rested more, and increased vigilance while carrying infants. Although we did not detect changes in overall activity budgets between prepartum and postpartum periods, tamarins spent more time scanning their environments postpartum, potentially reflecting increased predation risk to both carriers and infants during this period. Our study provides the first quantitative data on the timing and amount of infant carrying, grooming, and food transfer contributed by all individuals within and among multiple wild groups, filling a critical knowledge gap about the factors affecting infant care, and highlighting evolutionary hypotheses for cooperative breeding in tamarins.     

Social dynamics and individual plasticity of infant care behavior in cooperatively breeding cotton‐top tamarins

Individual variation in infant caretaking behavior is prevalent among marmoset and tamarin monkeys. Although most group members participate in infant care, the timing and amount provided differs greatly. In this study, we quantified general trends in infant carrying behavior by using a longitudinal database that included 11 years of instantaneous scan observations following 80 births of cotton‐top tamarins. Using detailed focal observations on a subset of the same families (10 births) we identified influences that affected expression of infant care at the group and individual levels. Fathers were the primary carriers and paternal carry time gradually decreased with increasing infant age. Paternal carry time also decreased significantly with an increasing number of older sibling helpers. Most fathers began to carry on the first day postpartum. However, we report circumstances in which fathers delayed carrying until almost a month postpartum. Fathers retrieved infants the most, although adult brothers' rates of retrievals peaked and surpassed fathers' rates during week 4 postpartum. Fathers delayed rejection of infants until week 4, whereas mothers rejected infants immediately and throughout the eight weeks. Nonetheless, infants climbed onto their mothers more than onto any other family member. Mothers showed a high initial investment in carrying during the first two weeks, decreasing quickly thereafter. Maternal contributions to infant carrying remained low and relatively consistent regardless of group size. However, mothers dramatically increased their infant carrying behavior in families in which fathers were absent. Older siblings cared for infants more than did younger siblings, and brothers retrieved and carried infants more than did sisters. Individual expression of infant care changed to accommodate infant needs and changed according to varying social dynamics and circumstances across litters. Am. J. Primatol. 72:296–306, 2010. © 2009 Wiley‐Liss, Inc.     

Why do cotton‐top tamarin female helpers carry infants? A preliminary study

Shortly after giving birth, cotton-top tamarin mothers frequently attack the eldest female helpers [Snowdon et al., American Journal of Primatology 31:11-21, 1993]. Sometimes this aggression leads to the eviction of the eldest daughters from their natal groups 3-4 months after the birth of infants [Price & McGrew, Folia Primatologica 57:16-27, 1991]. We propose that daughters, during the act of carrying infants, may receive less aggression from mothers than when they are not carrying. On the other hand, given that mothers benefit from having others carry their infants, overall aggression received by female helpers from their mothers might be lower in those female helpers with a larger relative contribution to this activity. Four groups were observed during the first 9 weeks following the birth of infants, and aggression received was recorded for symmetrical as well as nonsymmetrical interactions. We found a positive correlation between contribution to infant carrying of female helpers and the overall aggression received from their mothers. Furthermore, the two eldest daughters with highest values of carrying contribution were evicted from their natal groups 3-4 months after the birth of infants. Although mothers do not appear to be more tolerant of female helpers that contribute the most to infant carrying, daughters do benefit from reduced aggression during the act of carrying, and remain in their natal group during the time period when infants must be carried.     

Further hormonal suppression of eldest daughter cotton-top tamarins following birth of infants

Reproductive suppression of females is found throughout the Callitrichids. However, in many species some evidence of ovarian activity is observed in subordinate females. Subordinate cotton-top tamarin females in our colony have never been observed to ovulate in the presence of a reproductive female. However, ovarian follicular development does occur, and measurable levels of urinary estrogen and luteinizing hormone are frequently found in subordinate females. We studied 11 female tamarins living in family groups with a reproductive female. Each of the 8 eldest daughters had measurable urinary estrogen and LH levels and showed a reduction of hormonal levels when new infants were born. The 3 younger daughters showed barely detectable hormonal levels that did not change. Following the birth of infants the eldest daughters scent marked less frequently, increased time in contact with and grooming group members other than the mother, but they were more often targets of aggression than immediately prior to infant births. The eldest daughters were somewhat less involved in care of new infants than expected, although they spent much time in proximity to those carrying the infants. These results suggest that the further reduction of hormonal levels in subordinate females after the birth of infants may function to prevent these females from competing with mothers during the post-partum estrus rather than recruiting the eldest daughters as helpers for infant care. © 1993 Wiley-Liss, Inc.     

Infant-care Behavior of Non-reproductive Helpers in a Communal-care Primate,the Cotton-top Tamarin (Saguinus oedipus)

Infant-care behavior patterns of captive cotton-top tamarins were examined to assess factors defining participation by non-reproductive helpers. The time spent carrying infants and characteristics of infant transfers were examined for 47 helpers in a total of 18 groups. We predicted that age, previous experience, and carrying opportunity would all affect participation of non-reproductive helpers. Our results confirm that carrying by non-reproductive helpers was related to age, with older helpers carrying more often. However, this difference declined with increasing infant age, suggesting that body size of the carrier was not the only factor determining participation in carrying. When observations were classified relative to type of interaction with infants, older juveniles were found to both investigate and harass infants more often than subadults or younger juveniles. There was no effect of gender on carrying. The carrying behavior of subadults was not affected by their previous experience; that is, subadults with no previous exposure to infants carried as often as those with previous experience. Inter-individual variation among helpers was high; within 11 twin litters of helpers, one animal usually carried significantly more than the other.     

The benefits of helpers: Effects of group and litter size on infant care in tamarins (Saguinus oedipus)

The Callitrichidae (marmosets and tamarins) typically give birth to twins, and infant care is shared by all group members. The potential benefits to callitrichids of having helpers were investigated in a study of 21 captive cotton-top tamarin (Saguinus oedipus) infants, living in groups with two to 12 older members. Time carried and suckled and amount of food received from other family members were recorded for each infant during the first 12 weeks of life. The results showed that infants in larger groups were carried more and received more food than those in smaller groups. Twin infants in larger families were less likely to be on the same carrier. Singleton infants were carried more than twins but did not receive more food. On average, individual caretakers in larger groups carried less and shared less food with infants than those in smaller groups. When parental contributions to care were analyzed, no effects of group size were found on mothers' contributions to carrying or food sharing, but fathers in larger groups both carried infants less and shared less food with them. There appear to be several benefits of a communal rearing system to cotton-top tamarins. 1) Infants may receive more care in larger families, thus increasing their chances of survival. 2) The burden of care is spread over several animals, reducing the costs to a given individual. 3) The parents, particularly fathers, may benefit most from reducing the costs of investing in the present litter and increasing their ability to invest in future litters.     

The Role of Non-reproductive Helpers in Infant Care in Captive Callithrix jacchus

Callitrichids have been proposed as communal breeders, with non-reproductive animals acting as helpers. Reports in the literature attest to the high variability in the behaviour of helpers. The present study assessed four proximate factors involved in helping in captive Callithrix jacchus: age, gender and experience of helpers and composition of their groups. Forty-eight helpers from 15 families from Natal (Brazil) and Reading (UK) colonies were observed during the first 4 weeks of life of new-born twins. Infant carrying, transfers and retrieval, and duration of carrying bouts were registered. Results showed that the presence of helpers decreased the costs of infant carrying, with a negatively significant correlation between group size and mean percentage of carrying. Age, but not gender, was a significant factor for infant carrying, transfers and retrieval, and mean duration of carrying bouts, with older helpers contributing more than younger ones. Singletons and oldest non-reproductive offspring carried infants more, and longer, than twins and offspring with older siblings present in the group. Experience decreased motivation and actual carrying time, but not to a significant level. Age, experience and group composition appeared important factors in the display of infant care by helpers in captive Callithrix jacchus.     

The Effect of Infant Body Mass on Carrier Travel Speed in Cotton-top Tamarins (Saguinus oedipus)

Infant carrying is common in primates and may be the second most costly activity related to reproduction, after lactation. In cooperative breeding groups of callitrichids, all group members carry and care for twin infants. Previous studies have described the costs of infant carrying in terms of body mass loss and reduced locomotor capability. However, infant carrying may also influence travel speed, an important potential cost because slower speed may handicap foraging, energetic budgets, and predator avoidance. We evaluated the impact of infant carrying on the travel speed of 27 adult and 9 subadult cotton-top tamarins (Saguinus oedipus) of both sexes in large outdoor enclosures. We compared carrier speed to speed when not carrying during the 10 weeks after nine births. Subadult tamarins, which have a lower body mass than adults do, moved faster than adults when not carrying. We found no difference between the mean speeds of subadults and adults while carrying. However, the speed of carriers decreased as infant mass increased, and the slope of this negative relationship was more pronounced in subadult carriers. For every 80 g of extra mass load (the body mass of newborn twins), adults reduced their speed by 6 % and subadults by 19 % relative to noncarrying speed. We also observed a reduction in speed while carrying two infants in adult tamarins as carrying time increased. Our results contribute to an understanding of the costs of infant carrying, and serve to emphasize the importance of cooperative breeding systems in coping with these costs.     

Variations in care for cottontop tamarin, Saguinus oedipus, infants as a function of parental experience and group size

We examined the effects of parental experience and group size on infant care and development. Ten cottontop tamarin families were followed across three consecutive births of offspring to examine differences in survival, quality of care, developmental maturation and physical development for infants in two experimental conditions: (1) in large groups with previous parenting experience and (2) in small groups with little or no parenting experience. Although we found no differences in infant mortality, parents treated infants differently between conditions. However, when we examined the cumulative care provided by parents and sibling helpers together across experimental conditions, there were no differences in the rates of retrieval or rejection of infants. Overall levels of infant transport and food transfers were similar between conditions, with fathers and sibling helpers contributing the bulk of care. Fathers in small groups carried infants and transferred food with infants much more than helpers but, as group size increased, helpers augmented this role. Infant development varied with the experience of the family both between conditions and within a family. The results indicate that infants receive similar levels of care regardless of group size and parenting experience, although the individual contributions of family members change with both factors. Parental experience with previous infants had effects on early infant development, but only for the first sets of infants. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.     

Costs of infant‐carrying in the cotton‐top tamarin (Saguinus oedipus)

Infant‐carrying behavior among callitrichids seems to be a costly activity. Costs have been related to the physical efforts of carrying the weight of very heavy infants and to the resulting reduction in foraging efficiency. However, the costs of carrying in terms of the physical consequences for carriers have not previously been assessed. In this study, we have regarded weight loss in infant carriers as a measure of costs. We studied five families of cotton‐top tamarins (Saguinus oedipus) during the first 9 weeks following the birth of infants. Captive‐breeding conditions were required so that body weight could be measured frequently. To avoid inflicting undue stress on the subject animals, we used a noninvasive method for weighing the tamarins. Differences in carrying contribution were found amongst fathers and male and female helpers, with female helpers contributing less. We have found that carrying infants in the cotton‐top tamarins is an activity that produces a weight loss. Fathers and male helpers go through a maximal body weight loss. While carrying, the tamarins also decrease food intake. However, no relationship was found between contribution to carrying and feeding time or in energetic intake during feeding observations. Thus, it seems that a direct relation doesn't exist between the observations of feeding and weight loss. Fathers increase their contribution to carrying during mothers' periovulatory periods. In this period, male helpers and especially fathers go through a maximal body weight loss. We found body weight losses of up to 11.3% in one subadult male and 9.1percnt; in a father during the fifth week. No changes occurred in food intake in fathers or other male helpers during this period. During periovulatory periods, mothers carried less frequently but did increase their food intake. They gained weight from the second week after birth onward, especially during the periovulatory period. It seems that the infant‐carrying behavior of fathers and male helpers may contribute to the improvement of the mothers' physical condition after birth and therefore may support a consecutive pregnancy. Am. J. Primatol. 48:99–111, 1999. © 1999 Wiley‐Liss, Inc.     

Infant Carrying Behavior in Callitrichid Primates: Callithrix and Leontopithecus

Care of offspring by individuals other than the mother is ubiquitous in callitrichid primates. In spite of its widespread occurrence, however, there is considerable taxonomic variation in the timing and intensity of parental effort by breeding males and nonbreeding juvenile group members. These differences may be attributable to generic and specifies differences in the costs of reproduction for females or in ecological constraints on travel and foraging. We present data on patterns of infant carrying in social groups of two taxa of callitrichid primates (Callithrix and Leontopithecus) throughout the first 3 months of infant life. We evaluated patterns of care in small groups (two or fewer juvenile or subadult helpers) and in large groups (three or more helpers in addition to the breeding adult male and female). Group size had little effect on levels of maternal carrying effort in either marmosets or lion tamarins, and mothers ceased carrying infants by 3 months of age. Carrying efforts by fathers were significantly reduced in groups with many helpers relative to small groups. Helpers carried at consistent rates during the second and third months of infant life in Leontopithecus, while in Callithrix, carrying by helpers peaked during the second month. These results suggest that if helpers reduce energetic demands on lactating females, the mechanism by which helpers reduce these burdens is independent of maternal carrying effort.     

Explaining variation in maternal care in a cooperatively breeding mammal

Because of the necessity of lactation, mammalian mothers must perform at least a minimum amount of infant care. In cooperatively breeding species, other group members aid in all other aspects of infant care. However, some mothers continue to carry and nurse their infants more than others. The golden lion tamarin, Leontopithecus rosalia, is a small, communally breeding primate in the family Callitrichidae. We studied hormonal, individual, historical and social factors hypothesized to contribute to variation in levels of maternal care. We used neonatal weight as a measure of prenatal care, and carrying and nursing as measures of postnatal care. Greater neonatal weight was associated with smaller litter size, lower prepartum levels of oestrogen conjugates, and higher prepartum cortisol levels. Higher rates of carrying during weeks 2 and 3 were associated with higher maternal weight, larger litter size and smaller numbers of helpers per infant. Higher rates of nursing in weeks 2 and 3 were predicted by smaller group size and provisioning of the mother. The most important factors affecting postnatal maternal care were maternal weight, group size, litter size and provisioning status of the mother. Thus, females that display higher levels of maternal care do so either because they have to (they have fewer helpers) or because they can (they are in better condition).     

Food Sharing in Lion Tamarins (<Emphasis Type="Italic">Leontopithecus chrysomelas</Emphasis>): Does Foraging Difficulty Affect Investment in Young by Breeders and Helpers?

Food sharing with immatures is an important and relatively well studied aspect of infant care in many cooperative species. A key point that has not yet been fully addressed, however, is how increasing the difficulty of obtaining food influences the willingness of breeders and helpers to provision immature offspring. We used captive golden headed lion tamarins (Leontopithecus chrysomelas) to examine how breeders and helpers differ in provisioning juvenile individuals according to the level of difficulty of obtaining food. The level of difficulty in obtaining food was varied by placing the food inside tubes that allowed access only by adults. When food acquisition became more difficult, food sharing with juveniles and breeding females increased significantly. Begging calls by breeding females and juveniles increased during the experimental condition, which probably led to increases in food sharing. Breeders and helpers did not differ in their contribution to provisioning when food was easily available, nor did they differ in their contribution when food was difficult to obtain. Breeding males in callitrichids have a prominent role in transferring food to offspring, but contrary to our expectations, they did not increase food transfer in the experimental condition. An unexpected result was the increased investment of the breeding female into her current offspring when the level of difficulty of obtaining food was higher. We suggest that breeding lion tamarin females are not as constrained by reproductive costs as breeding females of other callitrichids. Degree of reproductive skew is hypothesized as a factor affecting the contribution of breeders and helpers to offspring care in cooperative breeding mammals, though we suggest that more studies are needed to validate such a generalization.     

Effects of allocare-givers on fitness of infants and parents in callitrichid primates

The effects of callitrichid primate helpers (allocare-givers other than an infant's father) on the survival, reproduction or behavior of infants and parents are reviewed, using both published studies and data from free-ranging golden lion tamarins (Leontopithecus rosalia). Three lines of evidence suggest that helpers may increase their own inclusive fitness: (1) The number of adult males acting as helpers in free-ranging groups is correlated with the number of surviving infants in 3 callitrichid species. However, the lack of a negative correlation with number of infants dying suggests that activities other than direct infant care (e.g. territory defense) may be more important, especially in newly formed groups. (2) In 2 species, captive groups with helpers carry infants for longer periods of time than do groups without helpers. Whether such differences would translate into meaningful survival differences in free-ranging groups is unclear. (3) Helpers reduce the energetic burden of parents by reducing the amount of time they spend transporting or provisioning infants in at least 4 species. Reproductive males are more likely than reproductive females to benefit from the presence of helpers, reducing their investment in infant care activities as the number of helpers in the group increases. In free-ranging golden lion tamarins, the reproductive tenure of males, but not females, increases with the number of helpers in the group, suggesting that a reduction in energetic investment may translate into increased survival. 'Decisions' made by helpers to participate in infant transport are weighed against competing needs for foraging, vigilance, territory defense and, in some cases, prospecting for breeding opportunities. Given this complexity, a sophisticated model may be required to answer the question of how helpers 'decide' to participate in infant care versus other activities.     

The effects of lactation and infant care on adult energy budgets in wild siamangs (Symphalangus syndactylus)

In mammals with biparental care of offspring, males and females may bear substantial energetic costs of reproduction. Adult strategies to reduce energetic stress include changes in activity patterns, reduced basal metabolic rates, and storage of energy prior to a reproductive attempt. I quantified patterns of behavior in five groups of wild siamangs (Symphalangus syndactylus) to detect periods of high energetic investment by adults and to examine the relationships between infant care and adult activity patterns. For females, the estimated costs of lactation peaked at around infant age 4–6 months and were low by infant age 1 year, whereas the estimated costs of infant‐carrying peaked between ages 7 and 12 months, and approached zero by age 16 months. There was a transition from primarily female to male care in the second year of life in some groups. Females spent significantly less time feeding during lactation than during the later stages of infant care, suggesting that female siamangs do not use increased food intake to offset the costs of lactation. Female feeding time was highest between infant ages 16 and 21 months, a period of relatively low female investment in the current offspring that coincided with the period of highest male investment in infant care. This suggests that male care may reduce the costs of infant care for females in the later stages of a reproductive attempt. The female energy gain resulting from male care was likely invested in somatic maintenance and future reproduction, rather than the current offspring. Am J Phys Anthropol, 2009. © 2009 Wiley‐Liss, Inc.     

Contributions to infant care in captive cotton-top tamarins (Saguinus oedipus): The influence of age,sex, and reproductive status

Marmosets and tamarins are characterized by a reproductive strategy that includes twinning, and a communal rearing system in which infant care is shared among all group members, both breeders and nonbreeding helpers (often older offspring). In order to test some predictions about the extent to which different age-sex classes should invest in infants, contributions to infant carrying and food-sharing by all family members were measured in captive groups of cotton-top tamarins (Saquinus oedipus) ranging in size from 2 to 12 independent individuals. Fourteen litters were observed from birth to 12 weeks. Carrying by mothers decreased steadily over the study period, while carrying by fathers and other offspring increased for 3–5 weeks, then declined. Infants spent more time carried by siblings than by either parent, but parents did more carrying than individual siblings and, also, shared more food with infants. Older siblings contributed more care than younger siblings did. Adult sons carried infants more than adult daughters did, but immature daughters carried more than immature sons did. However, adult daughters actively offered food to infants more than any other class of helper did. These results were interpreted in the light of hypotheses concerning the reproductive and dispersal strategies of callitrichid species.     

Mothers, not fathers, determine the delayed onset of male carrying in Goeldi's monkey (Callimico goeldii)

In biparental species, females are thought to accept and even to seek male assistance in rearing of the young. In this study, we present data that suggests that Goeldi's monkey females (Callimico goeldii) actually reject male parental care for a certain period. In Callimico, which have biparental care, mothers carry the infant exclusively for the first three weeks despite the fact that fathers are interested in their infants directly after birth. Fathers initiated significantly more body contact with their mates when newborn infants were present and retrieved one-day-old infants that were experimentally presented to them. The onset of paternal carrying in our colony was 27.5 days compared to 11.5 days (medians) observed in the field. However, presentation of a live potential predator induced earlier onset of paternal carrying by about 10 days. Additional costs to maternal carrying such as foraging and predator avoidance are likely factors influencing the decision of the mother as to when to share the cost of infant carrying with the father. We conclude that Goeldi's monkey fathers start carrying their infants so late because they do not get them earlier from the mother. We suggest that one ultimate explanation for that delay might be increased infant mortality when infant transfer starts at an earlier stage.     

Long-term study of infant-carrying behavior in captive common marmosets (Callithrix jacchus): Effect of nonreproductive helpers on the parents' carrying performance

Infant-carrying behavior in four families ofCallithrix jacchus was investigated over a period covering six to eight litters. We evaluated the effect of the total number of helpers and the number of adult helpers on the parents' carrying performance for the total carrying period and for three age stages of the infants. The carrying performance of the parents reached an asymptote beginning with litters 3 to 5. In two groups, the carrying performance of the parents was significantly negatively correlated with group size. However, considerable differences existed within each group and between the groups. The mother was the primary caregiver in the early weeks of the infants' lives. In this phase the helpers' contribution to carrying was less than might be expected. The relief of the parents from infant-carrying was greatest when the infants were 4 to 7 weeks old. Breeding males benefited most and from every helper, while breeding females benefited most from adult helpers, i.e., the carrying performance of the breeding female was considerably reduced until the respective group had grown to 8 to 10 members (infants not included). This group size is considered to be ideal for maximal relief of the parents from infant carrying. An increasing number of adult nonreproductive helpers (>4 or 5) does not induce a further reduction of the parents' carrying performance. Infant-rearing experience of nonreproductive helpers seems to be more important for the parents' and, especially, the breeding female's relief from infant-carrying than the overall number of helpers is. The data also strongly evidence that a group member's participation in infant-carrying is influenced by housing conditions and the demographic history of the group.     

Individual and group effects on early caregiver-infant relationships in red-bellied tamarin monkeys

The relationship between changes in caregiver-caregiver and caregiver-infant behaviour was studied during the infants' first month in 10 family groups of red-bellied tamarins, Saguinus labiatus, with twin infants. With infants of a given age, fathers generally spent a higher proportion of time carrying per infant than did mothers, but in most cases these differences were not significant. Mothers and fathers carried significantly more than older siblings did at the beginning and end of the infants' first month. With older infants, caregivers started to rub off infants they were carrying, so that infants started to spend some time off all caregivers. Each caregiver initiated carrying by retrieving an infant from another caregiver, and sometimes retained an infant against a retrieve attempt by another caregiver in order to continue carrying it. Before infants started to spend some time off, time spent carrying by mothers, fathers and older siblings was positively correlated with the percentage of retrieve attempts by other caregivers that they retained against, and, more strongly, with the value of their own number of retains minus the number of retains by other caregivers against them. These results indicate that competition to care for infants has a direct effect on the early development of each caregiver-infant relationship within the family group.     

Developmental changes in food transfers in cotton-top tamarins (Saguinus oedipus)

We investigated the development of food transfer and independent feeding in cotton-top tamarin (Saguinus oedipus) families. We studied the relationship between infant-directed vocalizations and food transfers on the development of independent feeding in infants. We experimentally tested ten infants (eight twins and two singletons) three times a week for 17 weeks from before weaning through 20 weeks. Food transfers and vocalizations made during tests were recorded and analyzed to determine (1) the role of vocalizations (C- and D- chirps and D-chirp series) in food acquisition, (2) the relationship between food transfers and individual food acquisition, and (3) whether, owing to energetic costs of nursing and carrying twins, food is transferred to twins sooner than to singletons. Infants were more successful in acquiring food via begging when adults produced repeated vocalizations than when adults did not vocalize. Adults emitted more food-related vocalizations in rapid series when in the presence of infants, whereas during feeding in the absence of infants only single unit vocalizations were produced. Begging occurred frequently. Changes over infant age were not significant when all infants were included in analysis. However, when twin data were analyzed alone, begging success changed significantly over months, with successful begging peaking in month 3 (week 12). Begging success rate did not differ between twins and singletons, although twins fed independently sooner and at a higher rate than did singletons.