Male-biased Adult Sex Ratios and their Significance for Cooperative Breeding in Dhole,Cuon alpinus,Packs

Two free-ranging packs of dholes (Asiatic wild dog, Cuon alpinus) were monitored for a period of 6 yr (Sep. 1990-Sep. 1996) in the Mudumalai sanctuary, southern India. Demographic data on age structure, litter-size, sex ratio and age and sex specific dispersal were collected. Behavioural data on social interactions and reproductive behaviour among pack members were obtained to determine the frequencies of dominant and subordinate behaviours shown by male and female pack members and a measure of each male's reproductive access to females. Behaviours displayed by pack members at dens were recorded to determine whether any age- or sex-specific role specialization existed during pup care. Tenures for dominant males and females within the pack were calculated to ascertain the rate of breeding vacancies occurring within packs. Approximate levels of genetic relatedness within packs were determined by studying pedigrees. In most years one study pack had a male-biased adult sex ratio. This was caused by almost two-fold higher dispersal of adult females over adult males. A considerable variance existed in the percentage of sub-adults dispersing from the two packs. Differences existed in the frequencies of dominant and subordinate behaviours shown by males. For males, dominance ranks and ranks based on submissive behaviours were not correlated with frequencies of reproductive behaviours. Subordinate males also displayed reproductive behaviours. In packs, dominant males had lower tenures than dominant females indicating that among males breeding vacancies arose more quickly. The litter size was found to be negatively correlated with the age of the breeding female. There were no significant differences across individuals of varying age or sex classes in the display of pup care behaviours. Significant differences did exist among individual adults. Genetic relatedness among packs tended to vary temporally as a consequence of possible mating by subordinate animals and immigration of new males into the pack. In conclusion, it appears that males delay dispersal and cooperate within their natal packs because of the variety of reproductive strategies they could pursue within. A combination of ecological constraints and the difficulties of achieving breeding status within non-natal packs may make early dispersal and independent breeding less beneficial.     

How does parental role influence the activity and movements of breeding wolves?

We studied the activity and movements of a male and a female wolf during breeding in the Bieszczady Mountains, Poland. The female was less active and mobile and on average closer to the den than the male. The male was less active and mobile when the pack size was seven compared to a year when the pack consisted only of the breeding pair. We conclude that the roles played by breeding males and females rearing pups influence their activity, and that breeding males in larger packs move less during the nursing period because of help by other pack members.     

Population dynamics and food habits of the banded mongoose

A population of approximately 100 banded mongooses living in six packs was studied for over 2 years in Rwenzori National Park, Uganda. The packs were relatively stable, cohesive social units composed of approximately equal numbers of males and females and varied in size from six to thirty-five adults and subadults. Breeding was synchronized within the packs with several females producing their litters at approximately the same time. The packs bred up to four times per year. Mortality of animals over 6 months old was approximately 10% annually. Less than 50% of juveniles survived to the age of 3 months. The banded mongoose feeds primarily on invertebrates, particularly millipedes and beetles. Ants, crickets, termites and earwigs are also important elements of the diet. Vertebrate remains were found in 12% of the droppings analysed. Although the pack forages as a unit each individual finds its own food. The mean home range size of five of the packs was 80-4 ha; the two largest packs had ranges exceeding 1 km². The banded mongoose is strictly diurnal typically leaving a den in the hour following sunrise and returning a few minutes before sunset with a prolonged rest during the heat of the day. Dens were in termite mounds (usually in thicket clumps), erosion gullies, aardvark holes and occasionally in man-made structures. Most were used for only a few days and then abandoned but a few preferred dens were occupied for periods up to 54 days and re-used repeatedly. Interactions between packs were aggressive and appeared to function in spacing the packs. The result of an encounter depended upon pack size with larger packs dominating smaller ones; the area of occurrence did not appear to affect the outcome. Mating between packs was observed during aggressive encounters. Group life is of adaptive significance in protecting the individual from predators and in care of the young. The pack bunches around young mongooses when disturbed and also bunches to attack certain predators and competitors. One or more adults remain at the den to guard the young when the pack forages.     

THE BANDED MONGOOSE, MUNGOS MUNGO GMELIN

The banded mongoose is a social, diurnal, medium-sized carnivore classified in the family Viverridae. Packs of up to 32 animals were found to occupy a single, old termite mound for periods in excess of 2 months. Nine out of ten dens were in termite mounds, the other was in a gully. One den was excavated to show structure. The whole pack occupied the central chamber each night. One group of 32 consisted of 17 adults, 7 juveniles, and 8 babies too small to leave the den with the pack during the day. The animals were strictly diurnal, usually leaving the den about one hour after dawn; time of return was more variable, but was usually during the hour before dark. Senses of sight, smell and hearing were found to be very acute. During the day the packs covered large distances, foraging as they went. During the period of study, food (estimated by dung analysis) consisted almost entirely of Coleoptera (mainly dung beetles) and millipedes. The packs followed the trails of elephant and buffalo and obtained most of their food from their dung. Detailed behaviour is described. Mating was seen on three occasions and two males were seen to copulate with the same female. Family size was not determined, but eight young were seen to be suckled by three females indiscriminately. One mother looked after all eight young during the day when the pack was away. The animals used communal defaecating areas on and around the den, the faeces being scattered over the surface of the ground. Reactions to various animals, including predators, are described. When caught in the open the pack tended to bunch together acting as a single unit. If bushes were near they used them for shelter. Adaptations are discussed.     

Spatial organization in the Ethiopian wolf Canis simensis: large packs and small stable home ranges

The spatial organization of the rare Ethiopian wolf ( Canis simensis ) was studied in the Afroalpine heathlands of Bale Mountains National Park, southern Ethiopia, between 1988 and 1992. Nineteen animals were radio-tracked, 48 ear-tagged and 64 others recognized by coat patterns and observed directly. Dry season (October—March) home ranges of resident wolves covered between 2 and 15 km². The ranges of adult males were slightly larger than those of females, and subadults' home ranges were slightly smaller than those of adults. The population density of the wolves was correlated with prey biomass. In optimal habitat, wolves lived in packs of 3—13 adults (mean 5.9 wolves > 1year old) containing several close-kin males; adult sex ratio favoured males 1.88: 1 and combined pack home ranges averaged 6.0km². In an area of lower prey productivity, wolves lived in pairs or small groups (mean 2.7), adult sex ratio was 1:1 and home ranges averaged 13.4 km².
Home ranges overlapped extensively (mean 85%) between members of the same pack. Four to seven percent of the population was additionally composed of non-resident females, inhabiting larger ranges (mean 11.1 km²). Home ranges of neighbouring packs were largely discrete, forming a tessellating mosaic of packs occupying all available habitat. Pack home ranges were stable in time, drifting only during major pack readjustment after the disappearance of a pack or significant demographic changes. Ethiopian wolf home ranges were smaller than would be expected for a carnivore of its size and sociality, presumably as a result of the high rodent productivity of the Afroalpine ecosystem.     

Seasonality, sociality, and reproduction: Long-term stressors of ring-tailed lemurs (Lemur catta)

Fecal glucocorticoid (fGC) concentrations are reliable, non-invasive indices of physiological stress that provide insight into an animal's energetic and social demands. To better characterize the long-term stressors in adult members of a female-dominant, seasonally breeding species - the ring-tailed lemur (Lemur catta) - we first validated fecal samples against serum samples and then examined the relationship between fGC concentrations and seasonal, social, demographic, genetic, and reproductive variables. Between 1999 and 2006, we collected 1386 fecal samples from 32 adult, semi-free-ranging animals of both sexes. In males and non-pregnant, non-lactating females, fGC concentrations were significantly elevated during the breeding season, specifically during periods surrounding known conceptions. Moreover, group composition (e.g., multi-male versus one-male) significantly predicted the fGC concentrations of males and females in all reproductive states. In particular, the social instability introduced by intra-male competition likely created a stressor for all animals. We found no relationship, however, between fGC and the sex, age, or heterozygosity of animals. In reproducing females, fGC concentrations were significantly greater during lactation than during the pre-breeding period. During pregnancy, fGC concentrations were elevated in mid-ranking dams, relative to dominant or subordinate dams, and significantly greater during the third trimester than during the first or second trimesters. Thus, in the absence of nutritional stressors, social dominance was a relatively poor predictor of fGC in this female-dominant species. Instead, the animals were maximally challenged by their social circumstances and reproductive events—males by competition for mating opportunities and females by late-term gestation and lactation.     

Socially Adjusted Synchrony in the Activity Profiles of Common Marmosets in Light-Dark Conditions

Synchronized state of activity and rest might be attained by mechanisms of entrainment and masking. Most zeitgebers not only act to entrain but also to mask circadian rhythms. Although the light-dark (LD) cycle is the main zeitgeber of circadian rhythms in marmosets, social cues can act as weaker zeitgebers. Evidence on the effects of social entrainment in marmosets has been collected in isolated animals or in pairs where activity is not individually recorded. To characterize the synchronization between the daily activity profiles of individuals in groups under LD conditions, the motor activity of animals from five groups was continuously monitored using actiwatches for 15 days during the 5th, 8th, and 11th months of life of juveniles. Families consisting of twins (4 ♂♀/1 ♂♂) and their parents were maintained under controlled lighting (LD 12:12?h), temperature, and humidity conditions. Synchronization was evaluated through the synchrony between the circadian activity profiles obtained from the Pearson correlation index between possible pairs of activity profiles in the light and dark phases. We also calculated the phase-angle differences between the activity onset of one animal in relation to the activity onset of each animal in the group (ψ_on). A similar procedure was performed for activity offset (ψ_off). By visual analysis, the correlation between the activity profiles of individuals within each family was stronger than that of individuals from different families. A mixed-model analysis showed that within the group, the correlation was stronger between twins than between twins and their parents in all families, except for the family in which both juveniles were males. Because a twin is an important social partner for juveniles, a sibling is likely to have a stronger influence on its twin’s activity rhythm than other family members. Considering only the light phase, the second strongest correlation was observed between the activity profiles of the individuals in the reproductive pair. Regarding the parameters ψ_on and ψ_off, the juvenile/juvenile dyad had lower values than the other dyads, but these differences did not reach statistical significance in relation to all dyads. Comparing the results of the ψ_on and ψ_off, and correlation indices, we suggest that the latter could detect differences between the animals that were not observed in the results of the phase-angle differences. These differences could be related to changes that occur during the active phase but not only in a particular phase, such as the temporal changes during the activity phase that characterize unimodal or bimodal patterns. Based on the differences in the correlations between individuals subjected to the same LD routine, we suggest that social cues modulate the circadian activity profiles of marmosets as a result of interactions between the animals within each group. Future studies are necessary to characterize the mechanisms of synchronization that are involved in this social modulation. (Author correspondence: carolina@cb.ufrn.br)     

Mammalian activity-rest rhythms in Arctic continuous daylight

Activity - rest (circadian) rhythms were studied in two species of Arctic mammals living in Arctic continuous daylight with all human-induced regular environmental cues (zeitgebers) removed. The two Arctic species (porcupine and ground squirrel) lived outdoors in large enclosures while the Arctic summer sun circled overhead for 82 days. Would local animals maintained under natural continuous daylight demonstrate the Aschoff effect described in previously published laboratory experiments using continuous light, in which rats' circadian activity patterns changed systematically to a longer period, expressing a 26-hour day of activity and rest? The outdoor experiments reported here, however, showed that under natural continuous daylight, both species (porcupine and ground squirrel) had specific times of activity and rest on a nearly 24-hour scale, and their activity peaks did not come later each day. The daily rhythms of the two species were recorded using implanted physiological radio capsules, and from direct observation.     

Serum cortisol concentrations of single-housed and isosexually pair-housed adult rhesus macaques

Possible social distress was evaluated in 20 adult rhesus macaques housed in compatible isosexual pairs (5 female pairs, 5 male pairs) for the purpose of social environmental enrichment. Serum cortisol concentrations of paired animals were compared with serum cortisol concentrations of individually housed adult rhesus macaques of both sexes (5 females, 5 males). In both sexes, cortisol concentrations of paired animals (means 10 females = 19.5 +/- 2.9 micrograms/dl; means 10 males = 17.5 +/- 4.6 micrograms/dl) showed no significant difference (p always greater than 0.1) with those of single animals (means 5 females = 20.5 +/- 2.1 micrograms/dl; means 5 males = 15.9 +/- 2.6 micrograms/dl). Both in male and in female pairs, dominant partners had cortisol concentrations that were equivalent to those of their subordinate counterparts. It was concluded that neither female nor male adult rhesus macaques experience more distress when sharing a cage with a compatible partner of the same sex than when living alone.     

Contribution to the welfare of captive wolves (Canis lupus lupus): A behavioral comparison of six wolf packs

An interesting way to understand and eventually improve the well‐being of captive animals is to compare different living and social conditions, in order to analyze the behavioral differences between animals living in very restrictive conditions and animals enjoying more permissive ones. In the present study we performed 10 observations of six wolf enclosures with quite different living and social conditions. The rest/activity balance, behavioral diversity, and use of available space were used as welfare criteria. Results show that the proportion of time resting was higher in large, comfortable enclosures. In each park, animals used only a part of the available space, the proportion being lower in large enclosures. The behavioral diversity was little affected by the size of the enclosure, but highly related to the composition of the pack. The results underline the importance of spatial choice and social group management. Zoo Biol 22:33–44, 2003 © 2003 Wiley‐Liss, Inc.     

Differences in winter activity,courtship, and social behavior of two captive family groups of Mexican wolves (Canis lupus baileyi)

The purpose of this study was to determine differences in activity patterns and social behavior of two groups of endangered Mexican wolves maintained at two quite different facilities and to determine some of the variables that should be considered when making specific behavioral comparisons among wolves in this binational captive breeding program. Quantitative measurements of an Activity Index and social behaviors were obtained for three individuals in each pack. Within each age/sex category, activity, aggression, and play were more frequent in the pack at a zoo facility, compared to the pack at a field station facility. Frequency of courtship interactions and scent marking were significantly higher in the field station pack. The packs were similar in the frequency of active submission, but differed significantly in the pattern of this behavior. Given the large number of interacting variables and small number of individuals in this study, we recommend caution in generalizing results to other packs or facilities. Zoo Biol 16:435–443, 1997. © 1997 Wiley-Liss, Inc.     

Plasma concentrations of progesterone and testosterone in captive woolly opossums (Caluromys philander)

Plasma testosterone and progesterone concentrations were measured in captive woolly opossums, a didelphid marsupial originating from neotropical forests in French Guyana. Although not exposed to cyclic environmental conditions as in the field, both sexes exhibited spontaneous circannual changes in sexual hormones. Males showed synchronous variations in plasma testosterone characterized by significant elevated concentrations during April and September (8.6 +/- 1 ng/ml, N = 5) and lower levels from May to July (3.6 +/- 0.4 ng/ml). In females, synchronous periods of 2-3 successive oestrous cycles occurred. Between these periods, females remained acyclic. The oestrous cycle, determined by urogenital smears, lasted 28-45 days (n = 14) and included a 20-day spontaneous luteal phase in which progesterone concentrations reached 30-40 ng/ml plasma. Even though testosterone concentrations in paired males increased significantly in response to oestrous periods of the paired females, spontaneous circannual rhythms of sexual activity were not well synchronized between the sexes in captivity. When compared to field data, sexual activity of captive animals followed a pattern similar to that in wild animals, without any changes in males but with a delay of 3 months in females.     

Gender differences in activity patterns of American mink <Emphasis Type="Italic">Neovison vison</Emphasis> in Germany

Circannual and circadian activity patterns of American mink Neovison vison were studied in a fishpond area in northern Germany. Fourteen mink were radiotracked, nine of them (five males, four females) for at least 6 months, some of them even up to 21 months. Both sexes have reduced activity rates in winter months and showed a considerable increase of activity during mating season (March). In summer months—during the pup raising period—female activity continued to be high. Male mink, on the other hand, were less active with the exception of August and September. During this time, dispersal of juveniles takes place with resident males increasingly having to defend their territories. Circadian activity rhythms differed markedly between sexes. All investigated females exhibited a perennial diurnal pattern. Female activity during twilight and night was significantly lower than during daylight. Three investigated male mink showed a typical nocturnal course throughout the year. Daylight activity of these animals was very low, increased during dusk and dawn activity rates and stayed at a higher level during the night. Two males behaved arrhythmic; they did not prefer any time of the day for activity. The observed temporal strategies were discussed in context of intra-sexual territoriality, sexual dimorphism in body size and different environmental factors.     

Seasonal variation in sexual behavior of all-male rhesus troops

Free roaming groups of Rhesus monkeys show a clear annual cycle in the incidence of sexual behavior. One hypothesis explaining the control of this cycle is that seasonal variables act directly on females, altering their endocrine status. Aroused females then communicate their sexual receptivity to male group members. The sexual behavior of 35 male Rhesus was observed during the non-mating and the mating season. All animals were maintained in large outdoor enclosures. One group of males (N = 7) were housed with females and juveniles as part of a breeding group. A second group of males (N = 8) had visual contact with the members of the breeding group, and a third group of males (N = 20) had no contact with either of these groups. During the non-mating season no sexual behavior was observed. At the onset of the mating season, the males with females displayed evidence of hormonal arousal and became sexually active. The males with visual contact with the breeding group also evidenced hormonal arousal and began engaging in male-to-male sexual activity. The males in the third group showed no sign of sexual arousal. These findings support the hypothesis that seasonal influences act only on females; and demonstrate that females can communicate their endocrine status to males, even without direct physical contact.     

Group unity of chimpanzees elucidated by comparison of sex differences in short-range interactions in Mahale Mountains National Park,Tanzania

Chimpanzees (Pan troglodytes) form multi-male and multi-female unit groups with fission–fusion grouping patterns. Short-range interaction (SRI) plays an important role in the unity of these groups and in maintaining social bonds among members. This study evaluated three models of chimpanzee social structure that differed according to the emphasis each placed on social bonds between the sexes, i.e., the male-only, the bisexual, and the male-bonded unit-group model. I investigated differences in SRI between the sexes among group members in well-habituated wild chimpanzees in Mahale Mountains National Park, Tanzania. I followed six focal adult males and six females, and quantified their respective SRI with other chimpanzees. Except between subordinate males and adult females, adults in general engaged in SRI with about 60–90% of the individuals with whom they made visual contact each day, whether in large or small parties. Although the number of social grooming (SGR) partners was limited, male–male SGR networks were wider than were either male–female or female–female SGR networks among adults. The number of contact-seeking behavior (CSB) partners was also limited, but dominant males had more CSB partners. Adult females mainly interacted by pant-grunt greeting (PGG) with adult males, but tended to do so mainly with the highest-ranking male(s) within visual contact. These results indicated that the social bonds among adult males were essential to group unity. Because of clear male dominance, adult females established peaceful coexistence with all group members despite less frequent SRI with subordinate males by maintaining affiliative social bonds with dominant males, thereby supporting the male-bonded unit-group model. Adult females had many female SRI partners, but these interactions did not involve performing conspicuous behaviors, suggesting that females maintain social bonds with other females in ways that differ from how such bonds are maintained with and between adult males.     

Rank,relationships, and responses to intruders among adult male vervet monkeys

We examined the influences of dyadic relationships among captive adult male vervet monkeys (Cercopithecus aethiops sabaeus) on behavior directed toward caged “intruder” males placed inside subjects' enclosures. Subjects were all 9 adult male residents from three stable social groups, each of which contained 3 adult males, at least 3 adult females, and their immature offspring. Every male was observed in two 3-hour sessions, each time with one of the 2 other adult males from his group. Observation sessions consisted of six consecutive 30-min stages in which group composition and the presence of the intruder were manipulated. All groups exhibited a stable, linear male dominance hierarchy prior to and throughout the study. In each group, there was one pair of males, when together, in which each member exhibited higher rates of intruder-directed approach and aggressive behaviors than when either animal was paired with the third male of his social group. Such pairs were also distinguished by high levels of within-pair agonistic interactions. The higher-ranking member of each dyad was the most aggressive male toward the intruder in his social group, although only one of these animals was the dominant male of his group. Mutual facilitation of aggression against intruding males is interpreted as cooperative behavior benefitting both males by increasing the likelihood of repelling a potential competitor for resident females. Such cooperation provides further evidence in nonhuman primates for cohesive male-male dyads between animals whose social interactions are characterized by agonism. © 1993 Wiley-Liss, Inc.     

Sex-specific interactions between aggressive and sexual behavior in the rat: Effects of testosterone and progesterone

The influence of progesterone on sexual and aggressive behaviors during aggressive encounters was investigated in pairs of TP-treated male and female rats. Gonadectomized females, chronically injected with testosterone propionate (TP), showed low but consistent levels of feminine sexual behavior which alternated with aggression. Progesterone when given in addition to TP facilitated receptive and proceptive behaviors, but reduced levels of aggression. In TP-treated males, levels of aggression were the same as observed in TP-treated females. However, TP-treated males seldomly showed sexual behavior during aggressive encounters and additional treatment with progesterone did not affect their behavior. After the aggression tests, animals were tested in a social preference test in which an ovariectomized female cage mate and the opponent from the aggressive encounter served as incentives. Positive correlations between levels of aggression and social preference for an opponent were found in both sexes, although correlations only reached statistical significance when progesterone was given in addition to TP. These correlations were found in both sexes, despite the fact that group analysis revealed pronounced sex differences in social preference: males preferred to spend their time near ovariectomized female cage mates, whereas females divided their time equally among female cage mates and opponents.     

Embryonic duration and post-embryonic growth rates of the tropical freshwater shrimp Caridina nilotica (Decapoda:Atyidae) under laboratory and experimental field conditions

SUMMARY. Embryonic durations and post embryonic growth rates of Caridina nilotica were determined under laboratory conditions at constant temperatures near 18, 24 and 30°C. Embryonic durations and intermoult intervals were negative curvilinear functions of temperature. At a given temperature moulting frequency varied inversely with shrimp size and slight sexual differences were apparent. Moulting frequency of berried females was governed by the temperature-specific embryonic durations. Growth rates were determined from changes in carapace length (CL) of individual shrimps (laboratory) or batches of shrimps (field enclosures) over 1 month and these data were used to calculate temperature-specific life-long growth curves for males and females. Growth in body mass was estimated indirectly from the carapace length-mass relationship of C. nilotica. On average, males grew marginally faster than females during the first 2 months of life, but growth of males larger than CL= 4 mm was considerably depressed relative to that of females. Inflexions in growth rate, apparently related to the onset of sexual maturity, were apparent in both sexes. Under laboratory conditions, the growth rate of males increased with temperature, but temperature-related differences were not as marked in females. Notwithstanding the more rapid moulting rate at 30°C the growth rate of females was slightly slower at 30 than at 24°C as a result of marginally but significantly smaller per moult growth increments observed at 30°C in animals up to CL= 5.5 mm. Possible reasons for this depressed growth are discussed. Growth rates of animals in field enclosures in Lake Sibaya over 1 month in winter (20 ± 3°C) were generally comparable to those estimated for the 18°C laboratory experiments. Growth rates in enclosures containing tripled standing stocks were almost identical to those containing the naturally occurring biomass of animals, suggesting a non-limited environment at least during the time of the experiment.     

Estradiol deficiency during development modulates the expression of circadian and daily rhythms in male and female aromatase knockout mice

Gonadal steroids modify the phase, amplitude and period of circadian rhythms. To further resolve the role of estradiol, we examined daily patterns of activity, circadian free running period and behavioral responses to light pulses using aromatase deficient (ArKO) mice. These animals lack the enzyme necessary to produce estradiol. We hypothesized that circulating estrogens during development and adulthood modulate the amount of activity, the temporal relationship of activity patterns relative to a light:dark cycle, and the free running period. Intact and gonadectomized male and female ArKO and wildtype (WT) littermates were used. WT males, but not ArKO males, retained the ability to respond to steroid hormones; the time of activity onset, free running period in constant darkness, and total daily activity were significantly different in gonadectomized compared to intact males. In contrast, gonadectomy did not alter the expression of these variables in ArKO males. ArKO females had a longer free running period in constant darkness compared to WT females regardless of gonadal state. Ovariectomized ArKO females had a significantly delayed activity onset when compared to intact ArKO females and ovariectomized WT females, despite all 3 groups being estrogen deficient. Phase shifts in response to light pulses given at different times of the day revealed an interaction between genotype, sex, and circulating steroids. These results from ArKO animals strongly suggest an organizational effect of estradiol during a critical period of development on the expression of biological rhythms.     

Sex differences in leadership during group movement in mantled howler monkeys (Alouatta palliata)

Benefits of group life depend in large part on whether animals remain cohesive, which often requires collective decisions about where and when to move. During a group movement, the leader may be considered as the individual occupying the vanguard position of the group progression, when its movement evokes following by other group members. In nondespotic societies, individuals with greater incentives to move frequently are leaders. During 15 months of observations (1,712 contact hours), we investigated two mantled howler monkey (Alouatta palliata) groups at La Flor de Catemaco (Los Tuxtlas, Mexico) to examine whether sex and female reproductive state influenced leadership likelihood in two contexts: movements toward feeding trees; movements associated with loud calls, a group-defense behavior used by males of this genus. Females led and occupied forward positions during group movements toward feeding trees more often than adult males. Adult females led these movements more frequently when they were gestating than when they were lactating or cycling. There were no differences between sexes in the leadership of group movements associated with loud calls. Leadership by gestating females is perhaps the result of their higher nutritional/energetic needs when compared with cycling females, and of their greater mobility when compared with lactating females carrying dependent offspring. Female leadership during movements toward feeding trees may be a mechanism to optimize access to food resources in mantled howler monkeys.