Non-lactating Females Research Articles

Detection and identification of plasma progesterone metabolites in the female Florida manatee ( Trichechus manatus latirostris) using GC/MS/MS

Florida manatees ( Trichechus manatus latirostris) have relatively low peripheral concentrations of progesterone (P4). The objective of this study was to determine if these relatively low P4 concentrations are associated with a high ratio of progestin metabolites and to document metabolite concentrations from individual blood samples obtained from manatees during diestrus or pregnancy. Metabolites known to exist in elephants—terrestrial manatee relatives—were targeted. These included 5α-reduced progestins (5α-pregnane-3,20-dione [5α-DHP] and 3α-hydroxy-5α-pregnan-20-one [5α-P3-OH]) and 17α-hydroxyprogesterone (17α-OHP), which occurs in Asian elephants. An additional, inactive metabolite, 20α-hydroxyprogesterone (20α-OHP), indicative of P4 overproduction, was also targeted. Progesterone itself was the predominant progestin detected in pregnant and nonpregnant manatee plasma (n = 10) using gas chromatography–mass spectrometry with tandem quadrupole detectors (GC/MS/MS). Progesterone concentrations in pregnant females varied from early (moderate to high) through mid and late (low) pregnancy. Progesterone concentrations ranged from low to high in nonpregnant, nonlactating females. The most commonly detected metabolite was 5α-P3-OH (n = 7), which occurred in pregnant (lower limit of detection [LLOD] to high) and nonpregnant (trace to high) females. The 5α-DHP metabolite was also detected in pregnant (LLOD to moderate) and nonpregnant (low) females. The 17α-OHP metabolite was not detected in any tested female. The 20α-OHP metabolite was detected in one nonpregnant, nonlactating, captive female (LLOD). Metabolites were most prevalent during early pregnancy, concurrent with maximum P4 concentrations. Based on their concentrations in peripheral circulation, we inferred that these metabolites may have, opposite to elephants, a limited physiologic role during luteal, pregnant, and nonpregnant phases in the manatee.

Loose housing of small goat groups: Influence of visual cover and elevated levels on feeding, resting and agonistic behaviour

Frequent social conflicts among goats due to their strict rank relationships can pose problems in loose housing of goats by negatively influencing feeding and resting times of low-ranking group members. In this study, we tested whether enrichment of loose-housing pens with structural elements providing visual cover (partitions, lying niches) and elevated levels (platforms) is effective in reducing agonistic interactions and interruptions in feeding and resting behaviour in small groups of goats. The study involved 8 groups of goats consisting of 8–9 non-lactating females that differed in grouping age and the presence of horns in a two-by-two factorial design. We compared the feeding, resting and agonistic behaviour the goats showed in pens to which they were well habituated (original situation) with the behaviour they performed in these pens after enriching them with additional structural elements providing visual cover and elevated levels (enriched situation). Finally, the arrangement of the pens was restored to the original situation (restored situation). In each group, the behaviour of two low-ranking, two medium-ranking and two high-ranking goats was continuously recorded by video. Using linear mixed-effects models, variables of goat behaviour were analysed with situation, rank, grouping age and presence of horns as fixed explanatory variables. With most of the outcome variables, the effects of the housing situation interacted with those of the rank and/or the presence of horns. From the original to the enriched situation, the feeding bouts were generally longer. Furthermore, from the original to the enriched situation, fewer feeding bouts of low- and medium-ranking goats were interrupted by displacements, and fewer of these bouts were interrupted by medium- and high-ranking goats to initiate agonistic interactions. In addition, fewer resting bouts were interrupted by displacements received, and in medium- and high-ranking goats, the risk of an interruption whilst resting due to initiating an agonistic interaction decreased from the original to the enriched situation. The outcome variables’ values in the restored situation hardly ever returned to the values of the original situation which is likely to indicate a carry-over effect of the enriched to the restored situation. In conclusion, our results show that enriching pens for small groups of goats with structural elements providing visual cover and elevated levels positively affects feeding, resting and agonistic behaviour.

Characterization of seasonal reproduction patterns in female pichis Zaedyus pichiy (Xenarthra: Dasypodidae) estimated by fecal sex steroid metabolites and ovarian histology

Reproductive strategies vary considerably among species, but most studies have focused on a very limited number of mammalian species. Knowledge of the reproductive cycle and behavior is essential for developing and implementing in situ and ex situ conservation strategies for threatened and endangered species. This study aimed at characterizing the seasonal reproductive pattern of female pichis Zaedyus pichiy, a threatened small armadillo native to arid regions of Argentina and Chile, through direct observations, histological studies, and by measuring fecal immunoreactive estrogens, progestagens and glucocorticoids in 10 wild-born, captive pichis and in free-ranging individuals. Results suggest that pichis are seasonal breeders that give birth to one yearly litter of 1–2 offspring, which do not leave the burrow until they are weaned at approximately 37 days. Ovarian follicular growth seems to occur throughout the year. Fecal progestagen, estrogen and glucocorticoid concentrations were minimal during the first half of pregnancy, increased to peak concentrations of up to 3500, 200 and 200 ng/g dry feces, respectively, and decreased before parturition. Postpartum progestagen concentrations were greater in lactating females than females that aborted or did not raise their offspring ( p < 0.0001), which is probably related to an elevated corticosteroid synthesis that contributes to maintain lactation, given that fecal glucocorticoid concentrations were of similar pattern. Observations of a second pregnancy after late abortion or death of the newborn litter and sustained follicular growth during pregnancy and lactation suggest that female pichis can become receptive briefly after having lost their litter. Fecal estrogen and progestagen concentrations of non-pregnant, non-lactating females did not have a well-defined hormonal cyclic pattern, and corpora lutea were only observed in pregnant females.

Lactation modifies stress-induced immune changes in laboratory rats

Lactation and stressor exposure both influence the activity of the immune system, but the interaction of both factors on the immune defense is poorly understood. The aim was therefore to investigate in lactating Long–Evans rats the effect of social stress on aspects of cellular immunity in the blood and mesenteric lymph nodes (MLN). Acute social stress (2 h) was induced in lactating and non-lactating female intruders using a confrontation model that yielded into social defeat and increased plasma corticosterone concentrations. Stress as well as lactation had marked effects on the immune system. Acute social stress caused granulocytosis, reduced lymphocyte proliferation, and cytokine production in the blood, but had no significant effects in MLN. In the blood of lactating rats, increased numbers of granulocytes and enhanced phagocytosis, but decreased B cell numbers and reduced IL-2 production was observed. However, in MLN both lymphocyte proliferation and monocyte numbers were increased in lactating rats. The effect of stress on the immune measures was often similar in lactating and non-lactating females, but a few important differences were evident: Only non-lactating animals showed an increase in blood granulocyte numbers and a decrease in IL-2 production in response to stressor exposure. Thus, during lactation, a neuroendocrine status may exist which impedes stress-induced modulations at least of some immune parameters.

Reproductive energetics in free-living female chimpanzees (Pan troglodytes schweinfurthii)

Mammalian females generally carry the bulk of reproductive costs. They gestate for relatively long periods of time and provide the majority of parental care for dependent offspring. For this reason, many studies have examined how females deal with the energetic costs of reproduction. Here, we examine the influence of reproductive state on activity budgets, diet quality, and sociality in free-living female chimpanzees (Pan troglodytes) at Gombe National Park, Tanzania. After controlling for dominance rank, we found that pregnant and lactating females consumed higher quality foods than nonpregnant, nonlactating females. However, pregnant females also traveled less. This result did not reflect differences in sociality, as the pregnant female group sizes included in our analyses were comparable to those in other reproductive categories.

Mother–infant interactions in free-ranging rhesus macaques: Relationships between physiological and behavioral variables

Studies of mother–infant relationships in nonhuman primates have increasingly attempted to understand the neuroendocrine bases of interindividual variation in mothering styles and the mechanisms through which early exposure to variable mothering styles affects infant behavioral development. In this study of free-ranging rhesus macaques on Cayo Santiago, Puerto Rico, we aimed to: 1) compare lactating and nonlactating females to investigate whether lactation is associated with changes in plasma cortisol, prolactin and oxytocin, as well as changes in CSF levels of serotonin and dopamine metabolites (5-HIAA and HVA); 2) examine the extent to which interindividual variation in maternal physiology is associated with variation in maternal behavior; 3) examine the extent to which interindividual variation in infant physiology and behavior is accounted for by variation in maternal physiology and behavior. Lactating females had higher plasma concentrations of cortisol, prolactin, and oxytocin but lower CSF concentrations of HVA than nonlactating females. Variation in maternal rejection behavior was positively correlated with variation in maternal plasma cortisol levels and in CSF 5-HIAA levels while variation in the time spent nursing and grooming was associated with maternal plasma oxytocin levels. Infants who were protected more by their mothers had higher cortisol levels than those who were protected less, while infants who were rejected more had lower CSF 5-HIAA than infants who were rejected less. Since exposure to high levels of maternal protectiveness and rejection is known to affect the offspring's behavior and responsiveness to the environment later in life, our results are consistent with the hypothesis that these effects are mediated by long-term changes in the activity of the offspring's HPA axis and brain serotonergic system.

Variability in the diet of New Zealand sea lion (Phocarctos hookeri) at the Auckland Islands, New Zealand

We examined the stomach contents of 121 New Zealand (NZ) sea lions (Phocarctos hookeri) caught by the squid fishery during the summer/autumn 1997–2006 around the Auckland Islands (51°S, 166°E). Dietary variation was assessed among juveniles, lactating females, nonlactating females and males, and between areas on the Auckland Islands shelf. The digested fraction of the contents consisted mostly of opalfish (Hemerocoetes spp.) (50.1% by number [N], 4.7% by mass [M]), rattail (Coelorinchus spp.) (12.0%N, 2.4%M), arrow squid (Nototodarus sloani) (14.1%N, 17.9%M), octopus (Enteroctopus zealandicus) (2.1%N, 27.8%M), and red cod (Pseudophycis bachus) (3.8%N, 4.3%M). Opalfish was found in greater proportions in the stomachs of females (lactating: 58.1%N, nonlactating: 62.4%N) and juveniles (56.9%N) than males (14.5%N). Juveniles caught smaller opalfish and rattail than adults did. Over all classes, sea lions ate larger prey in the east than in the north of the Auckland Islands shelf. The common prey—arrow squid and rattail—constitute an abundant resource at the edges of the Auckland Islands shelf, where lactating NZ sea lions forage. Although these key areas are far from the rookeries and impacted by the squid fishery, they may provide the only reliable resource able to support the cost of benthic foraging behavior in the deepest diver of all otariids.

Temporal and sex differences in the blubber fatty acid profiles of the New Zealand sea lion Phocarctos hookeri

We determined the fatty acid (FA) composition of the blubber of 82 New Zealand (NZ) sea lions caught as squid fishery by-catch between the years 2000 and 2006 on the Auckland Islands (51° S, 166° E) shelf. A combination of univariate and multivariate analyses showed significant variation in the FA composition between sexes and years. Blubber FA compositions of some males differed significantly from that of females, whereas blubber FA compositions of lactating (LF) and non-lactating females (NLF) were similar. Significant annual FA variation was revealed between the pooled years 2005/2006 and the previous years and between 2000 and 2004. Part of these differences can be attributed to different diets. Indeed, FA variation between the sexes suggests that males feed on deeper species than females, which is consistent with the current knowledge on the different div- ing behaviours between male and female otariids. Concerning annual variation, NZ sea lions are generalist predators, thus their diet is expected to follow the trends of prey stock availability. Nonetheless, FA metabolism is likely to cause some of the FA variation seen between sexes and years, because the deposition and mobilization of FAs would vary according to the nutritional and reproductive states of the individuals.

Habitat choice of Grevy’s zebras (Equus grevyi) in Laikipia, Kenya

AbstractCharacterizing habitat choice is essential for endangered species conservation. For the endangered Grevy’s zebra (Equus grevyi), as with many widely ranging vertebrates, human activities may be an important factor affecting space use. Grevy’s zebras are grazing ungulates inhabiting the savannahs of central‐northern Kenya and Ethiopia. Past research on their social organization indicates that reproductive status shapes associations and movements. Here, we examine how habitat use varies across four reproductive classes: lactating and nonlactating females, bachelors and territorial males. We also test whether Grevy’s zebra avoid locations close to active livestock corrals, or bomas. We find that forage quality, forage quantity and habitat openness of locations used by Grevy’s zebra vary significantly depending on individual reproductive state. Lactating females and bachelors use areas with green, short grass and medium‐dense bush more frequently than nonlactating females or territorial males. We hypothesize that lactating females trade off forage quantity and safety to access nutrients in growing grass. Across reproductive classes, Grevy’s zebra choose locations further from active bomas than if they used the area randomly. Our results suggest that Grevy’s zebra may require a range of vegetation characteristics for different reproductive classes. Further, they may need areas free from competition or disturbance by livestock.

Hypothalamic and pituitary expression of ghrelin receptor message is increased during lactation

In the lactating rat there is a dramatic increase in food intake that peaks at around day 15 postpartum, a time when pups are near weaning age, yet still fully dependant on maternal nourishment. We examined whether the orexigenic hormone ghrelin plays a role in increasing food intake during lactation. To do this, we compared plasma levels ghrelin, as well as brain and pituitary expression of the growth hormone secretagogue receptor (GHS-R 1a) rats in one of three groups: (1) dams whose litters were removed the day after giving birth (non-lactating); (2) dams whose litters were removed on day 13 postpartum (litter removed), and dams allowed keeping their litters (lactating). On day 15 postpartum, all dams were decapitated and trunk blood collected for plasma analysis of active ghrelin levels. Also, brain and pituitaries were collected and snap frozen using liquid nitrogen and stored at -80 degrees C before mRNA extraction and RT-PCR analysis. Results show no differences in ghrelin concentrations between lactating and non-lactating rats. Hypothalamic and pituitary expression of GHS-R 1a, however, was significantly increased in lactating animals compared to non-lactating animals. Interestingly, litter removed dams had higher levels of plasma ghrelin concentrations than either lactating or non-lactating females. Furthermore, GHS-R mRNA expression in these animals remained elevated in the pituitary but not the hypothalamus. These data suggest that the hypothalamus and pituitary of lactating rats are more sensitive to the effects of ghrelin, and that hypothalamic sensitivity to ghrelin depends on the presence of a suckling litter.

Baboons

Baboons

Associação da medroxiprogesterona ao protocolo Ovsynch para inseminação artificial em tempo fixo de búfalas cíclicas (Bubalus bubalis) criadas na Amazônia Oriental

O presente estudo visou avaliar os efeitos da associação da medroxiprogesterona (análogo sintético da progesterona) ao protocolo Ovsynch sobre o crescimento folicular, a ovulação e a taxa de concepção de búfalas criadas na Amazônia Oriental (Tracuateua-PA). Vinte e sete fêmeas adultas (G1 n=14 e G2 n=13), cíclicas, sem bezerro ao pé e com ECC 3,5 foram submetidas a Ovsynch. Os animais do G2 receberam 60 mg de medroxiprogesterona entre D0 e D7 (D0=início do tratamento). A ultra-sonografia ovariana foi realizada nos D 0, 7, 9 e 10. O contingente de folículos pequenos diferiu no D7 (G1: 4,57±0,60 versus G2: 6,54±0,67; P=0,05). Tempo e tratamento influenciaram o diâmetro folicular no D7. O crescimento do folículo dominante entre D7 e D9 foi maior nos animais tratados (G1: 2,05±0,49 mm/dia versus 3,48±0,41 mm/dia; P&lt;0,05). Mais animais do G1 ovularam precocemente (35,71% versus 30,77%), porém isso não afetou as taxas de concepção (G1: 50,00% e G2: 30,77%; P&gt;0,05). Os achados sugerem que a medroxiprogesterona (1) aumenta recrutamento folicular e retarda o crescimento dos folículos com diâmetro maior que 5,0 mm entre D0 e D7; (2) sua retirada incrementa em 1,7 vezes o crescimento folicular do D7 ao D9; (3) pode contribuir para a ovulação de folículos maiores e, em tese, para maior formação de tecido luteínico; (4) não promove ovulação precoce após o Ovsynch; (5) não eleva as taxas de concepção após sincronização de fêmeas cíclicas e com bom escore corporal, devendo ser avaliada para uso em fêmeas acíclicas ou com ECC mais baixo.

Utilização de cloprostenol sódico e benzoato de estradiol em vacas de corte inseminadas em tempo-fixo ou com observação de cio

The aim of this study was to evaluate the use of a prostaglandin F2α analogue (PGF2α) and oestradiol benzo-ate (ODB) in estrus synchronization programs. A hundred and eighty five non-lactating females divided in 4 groups were used. G1 (44) and G2 (46) received two applications of PGF2α (D0 and D11) and G3 (26) and G4 (37) received one application after 5 days of estrous detection. G1 and G3 also received a 1 mg ODB application 48 hours after the PGF2α. The pregnancy rate at AI was 42,2% for G1, 47,8% for G2, 32,1% for G3 e 57,6% for G4 (p>0,05) and of 38,4% and 51,4% for animals submitted to FTAI and estrous detection, respectively (p>0,05). This results show that pregnancy rates of females fixed time inseminated didn’t differ of that inseminated after estrus detection. Key-words: Beef cattle, estrus synchronization, fixed time artificial insemination, prostaglandin.

Trade-offs in activity budget in an alpine ungulate: contrasting lactating and nonlactating females

Optimal time allocation to foraging behaviour may be constrained by intrinsic and extrinsic factors forcing animals to adopt compromises to meet their daily energetic needs. Our goal was to assess the influence of individual characteristics and extrinsic factors on the activity budget of marked adult female mountain goats, Oreamnos americanus. As lactation entails important energetic costs, we specifically aimed at contrasting activity budgets of lactating and nonlactating females. We determined female activity budgets using 10-min interval scan sampling between 2002 and 2005. Lactating females, females raising sons and females that were subordinate for their age spent more time foraging than nonlactating females, females raising daughters and dominant females, respectively. Although foraging is typically incompatible with scanning, increased time spent foraging was mainly performed at the expense of time spent lying rather than time spent in vigilance. Lactating females also increased time spent ruminating while lying compared with nonlactating females. All females traded lying time for increased foraging and ruminating times in early and late summer. Females spent less time foraging on warm days, and foraged more at dusk than during the rest of the day. Age and body mass did not influence female activity budgets. Our results indicate that lactating females partly compensated for the costs of lactation by trading lying for foraging time, and reorganized lying time to increase the proportion of time spent ruminating. This study emphasizes the fundamental behavioural trade-offs that female mammals must face to meet their daily energetic requirements and allows a better understanding of how these compromises vary through the lactation period.

Plasma cortisol responses to stress in lactating and nonlactating female rhesus macaques

Lactating female rats without their pups exhibit lower HPA responsiveness to stress than nonlactating females. However, responsiveness to stress is similar when lactating females are tested with their pups and the stressor involves a potential threat to the offspring. This study constitutes the first comparison of stress responsiveness in lactating and nonlactating female nonhuman primates. Subjects were 53 multiparous female free-ranging rhesus macaques. Approximately half of the females were lactating and half were nonpregnant/nonlactating. Blood samples were obtained after capture and after overnight housing in an individual cage. Lactating females were tested with their infants. Lactating females had significantly higher plasma cortisol levels than nonlactating females on both days. Having or not having an infant was also a better predictor of plasma cortisol levels among all females than their age, dominance rank, group of origin, time of day at which the sample was obtained, and time elapsed since beginning of the sampling procedure or since anesthesia. Plasma cortisol levels of lactating females were not significantly correlated with post-partum stage or with the cortisol levels of their infants. Capture, handling, and individual housing in a cage are powerful psychological stressors for free-ranging primates. We suggest that the higher plasma cortisol levels exhibited by lactating females reflect greater responsiveness to stress associated with perception of risks to infants. Hyporesponsiveness to stress may not be a general characteristic of lactation in all mammalian species, but a short-term effect of infant suckling that is most apparent with stressors unrelated to the offspring.

PLASMA BIOCHEMISTRY AND HEMATOLOGIC VALUES FOR WILD-CAUGHT FLYING FOXES (PTEROPUS GIGANTEUS) IN INDIA

Although bats of the genus Pteropus are important ecologically as pollinators and natural hosts for zoonotic pathogens, little is known about their basic physiology. Hematology and plasma biochemistries were determined from wild-caught flying foxes (Pteropus giganteus) in northern India (n=41). Mean lymphocyte differential count was higher for juveniles than adults. Mean platelet count was lower than previously reported. No hemoparasites were observed. No differences were observed between plasma biochemistry values of male and female bats, juveniles and adults, or lactating and nonlactating females. Variation in aspartate aminotransferase (AST) was seen based on body condition score. Blood urea nitrogen and cholesterol concentrations were lower in P. giganteus than other mammalian groups, but were consistent with those reported from other Pteropus species. Alanine aminotransferase and AST concentrations were higher than those reported for Pteropus vampyrus, a closely related species. This study provides basic physiologic information that can be used in future health and disease studies of Indian flying foxes.

Conservative maternal care in an iteroparous mammal: a resource allocation experiment

When resources are limited, life history theory predicts a trade-off between growth, reproduction and survival. In summer, lactating females of temperate large herbivores such as the white-tailed deer (Odocoileus virginianus) normally have access to abundant forage but also face the high energetic needs of lactation and recovery from winter mass loss. At high population density, however, females may face a trade-off between allocating resources for maintenance and for reproduction. To simulate the effects of increased intra-specific competition at high density, we measured for 2 years how an experimental food restriction of approximately 20% affected current reproduction and body mass changes of adult females and their fawns during the fawning and lactation periods. Fawn survival decreased 35%, and fawn growth decreased 26% in the food-restricted treatment. There was no effect of food restriction on female mass. Irrespective of treatment, however, lactating females gained 30 g/day less than non-lactating females, and females that had weaned a fawn the previous year gained 20 g/day less than females that had not. We conclude that when resources were scarce, females adopted a conservative strategy favouring their own survival, mass recovery and future reproductive potential over their current reproduction, probably to maximise their lifetime reproductive success.

Effects of Reproductive Status on Energy Intake, Ingestion Rates, and Dietary Composition of Female Cebus capucinus at Santa Rosa, Costa Rica

The limiting factor for fitness in female primates is the acquisition of high-quality food, i.e., food that is high in energy and nutrients, such as protein, fat, and carbohydrates. Reproductive status can influence female feeding patterns, e.g., lactating females in some primate species consume greater quantities of food and specific nutrients than do nonlactating females. We examined the energy intake, ingestion rate, and composition of the diet in female white-faced capuchins in 3 reproductive states: lactating, gestating, and cycling. We observed 12 reproductively active females for 7 mo and compared their energy intake, ingestion rates, and intake rates of nutrients: protein, fat, sugar, and fiber. Lactating females took in significantly more energy per hour while feeding than pregnant and cycling females did. Lactating females also ingested significantly more food items per hour, but they did not spend more time eating than other females did, and they did not differ in the composition of their diet as measured by insect consumption and proportion of protein. Pregnant and cycling females did not differ from each other in any of the measures. We predicted that as the infants aged and began to move independently to forage and play, their mothers’ energy intake rates would decrease in concert with decreased energy demands by the infant. We found a statistically significant relationship between these 2 factors, with infant age explaining 75.4% of variation in mothers’ energy intake, supporting previous studies that found lactation to be the most energetically expensive reproductive state.

Male harassment influences female movements and associations in Grevy's zebra (Equus grevyi)

In traditional models for social organization, female movements and association patterns track resource distribution, whereas males track females. More recently, this model has been expanded to include feedback effects of male behavior, especially sexual harassment, on female decisions. In Grevy’s zebra (Equus grevyi), males defend territories containing resources attractive to females, who form unstable groups. Past research has explained female behavior based on resource distribution and needs alone. Lactating females have been found to have restricted movements and fewer male associates than nonlactating females, a pattern we find in our study. This pattern has previously been attributed solely to the higher water needs of lactation. However, in our population, both lactating and nonlactating females are typically close to water. We test the hypothesis that male harassment also influences female ranging and associations with males. This effect is predicted to be greater for lactating females because harassment has higher costs to them. We find that lactating females experience higher harassment rates than those of nonlactating females. Lactating females tend to move faster during harassment periods, whereas nonlactating individuals do not. Lactating females experience lower harassment rates if they spend more time with a particular male, whereas nonlactating females’ harassment rates do not depend on their allocation of time to a primary male. We suggest that females concentrate their time with one male in order to reduce male harassment. Even in species such as Grevy’s zebra without strong male‐female bonds, social interactions may be a significant driver of female distribution. Key words: animal movement, associations, Equids, Grevy’s zebra, sexual harassment, social organization. [Behav Ecol]

Social relationships and reproductive state influence leadership roles in movements of plains zebra, Equus burchellii

In animal groups, collective movements emerge from individual interactions. Biologists seek to identify how characteristics of actors in these groups, and their relationships, influence the decision-making process. We distinguished two basic factors determining leadership in group choices: identity and state. We hypothesized that identity is more important to leadership in groups with stable relationships, which permit the development of habitual roles. In groups with fluid membership, particular individuals or subgroups are less likely to emerge as consistent leaders. Instead, we predicted that movement initiation in unstable groups depends on individual state at the time of the decision. We characterized how identity and reproductive state influenced leadership patterns in the movements of plains zebra. As in many other mammals, lactation in this species significantly alters water and energy needs. We investigated leadership in tightly knit harems and loosely bonded herds of multiple harems. Harem females tended to have habitual roles in the initiation of harem movement. In herds, however, we found no consistent leaders among harems. At both levels of social organization, lactation was a key determinant of leadership. In harems, lactating females were more likely to initiate movement than nonlactating females. In turn, harems containing lactating females were more likely to lead herd movements. Thus, we conclude that social relationships and reproductive state together shape the interactions that produce group behaviours. One benefit to lactating females of leading herd movements is preferential access to scarce water. Thus, leadership roles in group decisions may have fitness consequences.