Interbirth Intervals Research Articles

Erratum to: Development and behavior of wild infant-juvenile East Bornean orangutans (Pongo pygmaeus morio) in Danum Valley.

Orangutans have a long period of immaturity and the longest inter-birth interval (IBI) of all mammals, which can be explained by their solitary life style, preventing the mother from rearing two offspring simultaneously (solitary life hypothesis) [corrected]. We collected data on mother-offspring dyads living in a primary lowland forest in Danum Valley, East Borneo in an effort to examine the developmental and behavioral patterns of the subspecies Pongo pygmaeus morio. We analyzed developmental changes in mother-offspring distance, contact, and activity budgets in orangutans ranging from 1 to 7 years of age. The results indicated decreased resting and playing with increasing age, whereas feeding, traveling and social play all increased significantly. Mothers' feeding and traveling time were good predictors of their offspring's feeding and traveling activities. Mother-offspring contact lasted longer in resting contexts; contact during traveling was almost non-existent after 4 years of age. Comparisons with previously published data on the Sumatran species Pongo abelli revealed no fundamental differences in these behavioral measures. However, a shorter association time with the mother after behavioral independence is documented for this East Bornean population in comparison to Sumatran populations. These results are best explained by the solitary life hypothesis, in agreement with previous studies. We suggest that environmental constraints in Bornean forests, as well as a lower population density, should be considered when interpreting the differences between Sumatran and Bornean orangutans in both the period of association with mother and the IBI.

Substance, History, and Politics.

The aim of this article is to examine the relations between two approaches to the measurement of life history (LH) strategies: A traditional approach, termed here the biodemographic approach, measures developmental characteristics like birthweight, gestation length, interbirth intervals, pubertal timing, and sexual debut, and a psychological approach measures a suite of cognitive and behavioral traits such as altruism, sociosexual orientation, personality, mutualism, familial relationships, and religiosity. The biodemographic approach also tends not to invoke latent variables, whereas the psychological approach typically relies heavily upon them. Although a large body of literature supports both approaches, they are largely separate. This review examines the history and relations between biodemographic and psychological measures of LH, which remain murky at best. In doing so, we consider basic questions about the nature of LH strategies: What constitutes LH strategy (or perhaps more importantly, what does not constitute LH strategy)? What is gained or lost by including psychological measures in LH research? Must these measures remain independent or should they be used in conjunction as complementary tools to test tenets of LH theory? Although definitive answers will linger, we hope to catalyze an explicit discussion among LH researchers and to provoke novel research avenues that combine the strengths each approach brings to this burgeoning field.

Female Reproductive Parameters in Wild Geladas (Theropithecus gelada)

Accurate data on reproductive parameters are essential for evaluating the adaptive and mechanistic bases of variation within and between species. Here, we combine fecal ovarian hormone analysis with measurements of sex skin swellings and reproductive behavior to describe female reproduction in geladas living in the Simien Mountains National Park, Ethiopia. We collected 10 years of reproductive data from 197 females across 36 units, with hormone data from a subset of these females (60 females from 16 units). First, we use these data to calculate age at maturation, age at first birth, interbirth interval, menstrual cycle length, gestation length, and duration of postpartum amenorrhea. Second, we present composite profiles of ovarian hormones, i.e., estrogens, progestogens, and sex skin measures across reproductive stages. Third, we examine whether ovarian hormones, sex skin phenotypes, and/or reproductive behaviors reliably reflect fertility or pregnancy in female geladas. The reproductive parameters calculated from our data were similar to those in previous studies on geladas and within the range of those for closely-related baboon species. We were able to reliably identify fertility and pregnancy using fecal estrogens only. The tumescent sex skin phenotypes and reproductive behaviors associated with fertility were also observed across all reproductive stages. We suggest that their presence outside of fertility may be a by-product of the flexibility needed for one or more female counterstrategies to infanticide.

Male resource defence during intergroup aggression among tufted capuchin monkeys

In group-living species, conflicts between groups frequently involve multiple individuals participating in aggression towards the opposing group. Although defence is a collective action, individuals differ in their motivations. For males, the spatiotemporal distribution of resources should determine whether they defend food or mates. Although resource defence as a male mating strategy has been demonstrated primarily in unimale social systems, males in multimale groups generally directly defend access to females. I examined the factors affecting individual assessment of the costs and benefits of participation in aggressive intergroup encounters among tufted capuchin monkeys, Sapajus (Cebus apella) nigritus, at Iguazu National Park, Argentina. Through provisioning and playback experiments I show that male aggression during intergroup encounters in tufted capuchin monkeys is directly related to the presence of a high-quality food resource, whereas behaviours more consistent with direct mate defence were not apparent. Although males and females demonstrated concordant strategies, male responses were stronger than those of females, but no differences were observed between dominant and subordinate males in the presence of a defendable food resource. Resident females may be manipulating male–male competition within groups, using males as ‘hired guns’ to increase access to food resources, thereby increasing infant survivorship and decreasing interbirth intervals. In this population, intersexual conflict, which is often associated with infanticide and sexual coercion in nonhuman primates, has promoted cooperation between the sexes.

Factors Influencing the Reproductive Success of Female Black Howlers (Alouatta pigra)

Measuring reproductive success is necessary to determine an organism's fitness, and thus, to address evolutionary processes. In this study we determined the effects of social, ecological, and individual attributes on the reproductive success of black howlers (Alouatta pigra) measured through infant survival and interbirth intervals (IBIs). From 2006 to 2012 we studied 29 black howler (A. pigra) females living in 11 groups in Campeche (Mexico), and recorded 82 births. We recorded group size and composition and the sex of infants during weekly surveys. We calculated a food availability index based on rainfall levels and on the size of preferred food resources. Daughters had a 30 % higher probability of surviving than sons, and the survival of the latter was positively related to food availability. IBI decreased when the first infant in the interval died, and when considering only IBIs in which the first infant survived, IBIs were longer following the birth of a daughter. These results suggest that, even if the production of daughters reduces reproductive output due to longer IBIs, female black howlers may still accrue higher reproductive success through their daughters due to differences between sexes in survival.

Alloparenting is associated with reduced maternal lactation effort and faster weaning in wild chimpanzees

Alloparenting, when individuals other than the mother assist with infant care, can vary between and within populations and has potential fitness costs and benefits for individuals involved. We investigated the effects of alloparenting on the speed with which infants were weaned, a potential component of maternal fitness because of how it can affect inter-birth intervals, in wild chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Uganda. We also provide, to our knowledge, the first description of alloparenting in this population and present a novel measure of the contribution of milk to infant diets through faecal stable nitrogen isotopes (δ15N). Using 42 mother–infant pairs, we tested associations of two alloparenting dimensions, natal attraction (interest in infants) and infant handling (holding, carrying), to the proportion of time mothers spent feeding and to maternal lactation effort (mean nursing rates and mother–infant δ15N differences). Neither natal attraction nor infant handling was significantly associated with feeding time. Infant handling was inversely associated with both measures of lactation effort, although natal attraction showed no association. Alloparenting may benefit mothers by enabling females to invest in their next offspring sooner through accelerated weaning. Our findings emphasize the significance of alloparenting as a flexible component of female reproductive strategies in some species.

Birth Spacing of Pregnant Women in Nepal: A Community-Based Study.

Optimal birth spacing has health advantages for both mother and child. In developing countries, shorter birth intervals are common and associated with social, cultural, and economic factors, as well as a lack of family planning. This study investigated the first birth interval after marriage and preceding interbirth interval in Nepal. A community-based prospective cohort study was conducted in the Kaski district of Nepal. Information on birth spacing, demographic, and obstetric characteristics was obtained from 701 pregnant women using a structured questionnaire. Logistic regression analyses were performed to ascertain factors associated with short birth spacing. About 39% of primiparous women gave their first child birth within 1 year of marriage and 23% of multiparous women had short preceding interbirth intervals (<24 months). The average birth spacing among the multiparous group was 44.9 (SD 21.8) months. Overall, short birth spacing appeared to be inversely associated with advancing maternal age. For the multiparous group, Janajati and lower caste women, and those whose newborn was female, were more likely to have short birth spacing. The preceding interbirth interval was relatively long in the Kaski district of Nepal and tended to be associated with maternal age, caste, and sex of newborn infant. Optimal birth spacing programs should target Janajati and lower caste women, along with promotion of gender equality in society.

Reproductive performance of the largest captive Asian elephant (Elephas maximus) population in Sri Lanka

The Pinnawela Elephant Orphanage (PEO) in Sri Lanka maintains one of the largest captive Asian elephant (Elephas maximus maximus) populations in the world, with a total of 79 animals (45 females and 34 males) at present. It was established in 1975 as an orphanage for rescued wild calves, and commenced natural breeding under controlled conditions when they reached breeding age. This study summarizes reproductive data of 65 live births from 38 years of records at PEO. The age at first calving (n=31) was 14.6±0.7years, and the numbers of females giving birth two, three, four and five times were 21, 8, 4 and 2, with corresponding inter-birth intervals (IBI) of 4.9±0.3, 4.8±0.5, 7.9±1.9 and 5.7±0.5years, respectively. Females giving birth to males (5.7±2.2years) had longer IBIs compared to birthing female calves (4.7±1.1years). The average gestation for 18 pregnancies with known conception dates was 667±11days. The average birth weight was similar for male (83.1±4.6kg; n=14) and female (82.8±8.4kg; n=6) calves. Sex ratio for live births was 36 male:29 female and not different from 1:1; however, more males (10/14) were born after a second parity. Calf mortality and stillbirth rates were low: 7.6% and 4.4%, respectively. This study highlights the successful breeding program at the PEO, providing baseline reproductive data that can aid in improving breeding of other elephants managed under captive conditions.

Development and behavior of wild infant-juvenile East Bornean orangutans (Pongo pygmaeus morio) in Danum Valley.

Orangutans have a long period of immaturity and the longest inter-birth interval (IBI) of all mammals, which can be explained by their solitary life style, preventing the mother from rearing two offspring simultaneously (solitary life hypothesis) [corrected]. We collected data on mother-offspring dyads living in a primary lowland forest in Danum Valley, East Borneo in an effort to examine the developmental and behavioral patterns of the subspecies Pongo pygmaeus morio. We analyzed developmental changes in mother-offspring distance, contact, and activity budgets in orangutans ranging from 1 to 7years of age. The results indicated decreased resting and playing with increasing age, whereas feeding, traveling and social play all increased significantly. Mothers' feeding and traveling time were good predictors of their offspring's feeding and traveling activities. Mother-offspring contact lasted longer in resting contexts; contact during traveling was almost non-existent after 4years of age. Comparisons with previously published data on the Sumatran species Pongo abelli revealed no fundamental differences in these behavioral measures. However, a shorter association time with the mother after behavioral independence is documented for this East Bornean population in comparison to Sumatran populations. These results are best explained by the solitary life hypothesis, in agreement with previous studies. We suggest that environmental constraints in Bornean forests, as well as a lower population density, should be considered when interpreting the differences between Sumatran and Bornean orangutans in both the period of association with mother and the IBI.

Reproductive Responses to Economic Uncertainty : Fertility Decline in Post-Soviet Ust'-Avam, Siberia.

In the face of economic and political changes following the end of the Soviet Union, total fertility rates fell significantly across the post-Soviet world. In this study we examine the dramatic fertility transition in one community in which the total fertility rate fell from approximately five children per woman before 1993 to just over one child per woman a decade later. We apply hypotheses derived from evolutionary ecology and demography to the question of fertility transition in the post-Soviet period, focusing on an indigenous community (Ust'-Avam) in the Taimyr Region, northern Russia. We employ a mixed parametric accelerated failure-time model that allows comparison of age at first birth, interbirth interval, and reproductive postponement or cessation prior to and following 1993. We find that short-term reproductive delay alone does not explain the dramatic drop in fertility in Ust'-Avam. Age at first birth remains constant. Interbirth intervals increase moderately. The estimated fraction of women who have ceased or indefinitely postponed reproducing doubles (for parities 2 through 4) or triples (for nulliparous women). We caution against assuming that environmental harshness necessarily leads to earlier and more rapid reproduction. An evolutionary theory of fertility responses to acute environmental shocks remains relatively undeveloped. In such contexts it is possible that selection favors a conservative reproductive strategy while more information is learned about the new environment. When investigating fertility responses to environmental stressors we suggest researchers examine postponement and stopping behavior in addition to changes in age at first birth and interbirth interval.

Networks of Food Sharing Reveal the Functional Significance of Multilevel Sociality in Two Hunter-Gatherer Groups

Like many other mammalian and primate societies [1-4], humans are said to live in multilevel social groups, with individuals situated in a series of hierarchically structured sub-groups [5, 6]. Although this multilevelsocial organization has been described among contemporary hunter-gatherers [5], questions remain as to the benefits that individuals derive fromliving in such groups. Here, we show that food sharing among two populations of contemporary hunter-gatherers-the Palanan Agta (Philippines) and Mbendjele BaYaka (Republic of Congo)-reveals similar multilevel social structures, with individuals situated in households, within sharing clusters of 3-4 households, within the wider residential camps, which vary in size. We suggest that these groupings serve to facilitate inter-sexual provisioning, kin provisioning, and risk reduction reciprocity, three levels of cooperation argued to be fundamental in human societies [7, 8]. Humans have a suite of derived life history characteristics including a long childhood and short inter-birth intervals that make offspring energetically demanding [9] and have moved to a dietary niche that often involves the exploitation of difficult to acquire foods with highly variable return rates [10-12]. This means that human foragers face both day-to-day and more long-term energetic deficits that conspire to make humans energetically interdependent. We suggest that a multilevel social organization allows individuals access to both the food sharing partners required to buffer themselves against energetic shortfalls and the cooperative partners required for skill-based tasks such as cooperative foraging.

Non-human primates avoid the detrimental effects of prenatal androgen exposure in mixed-sex litters: combined demographic, behavioral, and genetic analyses.

Producing single versus multiple births has important life history trade-offs, including the potential benefits and risks of sharing a common in utero environment. Sex hormones can diffuse through amniotic fluid and fetal membranes, and females with male littermates risk exposure to high levels of fetal testosterone, which are shown to have masculinizing effects and negative fitness consequences in many mammals. Whereas most primates give birth to single offspring, several New World monkey and strepsirrhine species regularly give birth to small litters. We examined whether neonatal testosterone exposure might be detrimental to females in mixed-sex litters by compiling data from long-term breeding records for seven primate species (Saguinus oedipus; Varecia variegata, Varecia rubra, Microcebus murinis, Mirza coquereli, Cheirogaleus medius, Galago moholi). Litter sex ratios did not differ from the expected 1:2:1 (MM:MF:FF for twins) and 1:2:2:1 (MMM:MMF:MFF:FFF for triplets). Measures of reproductive success, including female survivorship, offspring-survivorship, and inter-birth interval, did not differ between females born in mixed-sex versus all-female litters, indicating that litter-producing non-human primates, unlike humans and rodents, show no signs of detrimental effects from androgen exposure in mixed sex litters. Although we found no evidence for CYP19A1 gene duplications-a hypothesized mechanism for coping with androgen exposure-aromatase protein evolution shows patterns of convergence among litter-producing taxa. That some primates have effectively found a way to circumvent a major cost of multiple births has implications for understanding variation in litter size and life history strategies across mammals.

Hormonal correlates of life history characteristics in wild female Colobus vellerosus.

Documenting primate life history characteristics is important because it provides information about traits that affect the timing and rate of reproduction in these long-lived species. This study describes the hormonal correlates of female reproductive events and quantifies for the first time key life history variables for Colobus vellerosus, using hormonal and observational data. This study also biologically validates that the reproductive events determined in the hormone profiles correspond to observed reproductive events for each female. We collected behavioural data on 18 females in our four study groups during 12months (May 2012-2013) at the Boabeng-Fiema Monkey Sanctuary, using 10-min continuous focal and ad libitum sampling. We concurrently collected faecal samples (n=1866) every 2-3days from these 18 females (prepubescent n=2, cycling n=2, lactating n=12, pregnant, n=7, and post-reproductive n=1) and extracted oestrogen (E2) and progesterone (P) metabolites in the field using solid-phase extraction cartridges. We created a hormone profile for each female by analyzing 1586 of our samples for E2 using radio-immuno assays, and P using enzyme-immunoassays at the Wisconsin National Primate Research Center. Mean ovarian cycle length was 24days±1 (n=2 cycles). Mean gestation length was 23weeks (range=21-25weeks, n=2 complete pregnancies). For females whose infants survived to nutritional independence, the mean inter-birth interval (IBI) was significantly longer than for females whose infants died prior to reaching nutritional independence (Mann-Whitney U Test; U=14.5, p=0.006; IBI surviving infants: 17.75 months, range=8-20.75 months, n=11 vs. IBI infant death: 11.89 months, range=8-18.5 months, n=9). The values for most life history traits reported in this study are similar to those documented in other similarly sized colobine species. Some values are on the lower end of the range for similarly sized colobines; C. vellerosus shows a cycle of 24days and gestation length of 5.75months vs. a range of 24-29days for cycle length and 5.25-7.5months for gestation length in other colobines. This may be due to C. vellerosus' smaller body size, or their limited access to higher quality food resources.

Nearby grandmother enhances calf survival and reproduction in Asian elephants.

Usually animals reproduce into old age, but a few species such as humans and killer whales can live decades after their last reproduction. The grandmother hypothesis proposes that such life-history evolved through older females switching to invest in their existing (grand)offspring, thereby increasing their inclusive fitness and selection for post-reproductive lifespan. However, positive grandmother effects are also found in non-menopausal taxa, but evidence of their associated fitness effects is rare and only a few tests of the hypothesis in such species exist. Here we investigate the grandmother effects in Asian elephants. Using a multigenerational demographic dataset on semi-captive elephants in Myanmar, we found that grandcalves from young mothers (<20 years) had 8 times lower mortality risk if the grandmother resided with her grandcalf compared to grandmothers residing elsewhere. Resident grandmothers also decreased their daughters’ inter-birth intervals by one year. In contrast to the hypothesis predictions, the grandmother’s own reproductive status did not modify such grandmother benefits. That elephant grandmothers increased their inclusive fitness by enhancing their daughter’s reproductive rate and success irrespective of their own reproductive status suggests that fitness-enhancing grandmaternal effects are widespread, and challenge the view that grandmother effects alone select for menopause coupled with long post-reproductive lifespan.

From rainforests to savannas and back: The impact of abiotic factors on non-human primate and hominin life histories

The small stature of modern human hunter–gatherers, and the developmental mechanisms by which this morphology is obtained, has been the subject of intense debate. Similarly, the causes for the unique modern human life history, which combines high reproductive rates with extended growth and long lifespans, have remained elusive. Here I explore the possible influence of some abiotic factors (temperature, rainfall, evapotranspiration) on life histories in non-human primates (gestation length, interbirth interval, longevity) and enquire whether there exist commonalities that could shed light on the evolution of hominin life histories and its variation among Homo sapiens. After accounting for the effects of brain size and phylogeny, life history variables showed only moderate trends with abiotic variables. In contrast, the results were statistically highly significant when multivariate statistics and path analyses were employed, particularly for gestation length. Life histories apparently respond to actual annual evapotranspiration (AET) and annual precipitation, and their effects are contrasted; a habitat openness/aridity index was found a poor predictor however. This points towards a complex relationship between abiotic variables and primate biology. Rather than responding to any one environmental variable, it is tentatively concluded that primate energetics will respond to both, primary productivity of the habitat and environmental predictability (seasonality), which will then -in turn-modulate the pace with which primates reproduce and grow up. Against this backdrop the “unique” modern human life history pattern is, in fact, unsurprising: it probably has its origin in the ecological setting in which hominins evolved.

What do men want? Re-examining whether men benefit from higher fertility than is optimal for women.

Several empirical observations suggest that when women have more autonomy over their reproductive decisions, fertility is lower. Some evolutionary theorists have interpreted this as evidence for sexual conflicts of interest, arguing that higher fertility is more adaptive for men than women. We suggest the assumptions underlying these arguments are problematic: assuming that women suffer higher costs of reproduction than men neglects the (different) costs of reproduction for men; the assumption that men can repartner is often false. We use simple models to illustrate that (i) menorwomen can prefer longer interbirth intervals (IBIs), (ii) if men can only partner with wives sequentially they may favour shorter IBIs than women, but such a strategy would only be optimal for a few men who can repartner. This suggests that an evolved universal male preference for higher fertility than women prefer is implausible and is unlikely to fully account for the empirical data. This further implies that if women have more reproductive autonomy, populations should grow, not decline. More precise theoretical explanations with clearly stated assumptions, and data that better address both ultimate fitness consequences and proximate psychological motivations, are needed to understand under which conditions sexual conflict over reproductive timing should arise.

The Effects of Wealth on Male Reproduction among Monogamous Hunter-Fisher-Trappers in Northern Siberia

Variability in men’s reproductive success (RS) is partly attributable to the ability of successful men to influence resource flows relevant to the mate choice and reproduction of women. This study explores the effects of variability in resource flows on men’s RS in an indigenous foraging/mixed-economy community in northern Siberia where monogamous marriage norms predominate. A series of material, embodied, and relational wealth indicators are tested as predictors of men’s age-adjusted RS and age at first birth. Material wealth related to hunting, embodied wealth as represented by hunting skill, and relational wealth as represented by numbers of kin are the most consistent predictors of men’s RS. In this monogamous population, the wives of men with more hunting capital and of men rated as better hunters have shorter interbirth intervals, and hunters show strong producer priority. These findings and ethnographic observations appear more consistent with a provisioning model than with a signaling-for-mates model.

Interbirth intervals are associated with age of the mother, but not with infant mortality in Indian rhinoceroses.

Rhinoceroses are among the most endangered mammals in the world. Despite a recent increase in numbers in most wild populations, poaching or political instability may exterminate large populations very quickly. Therefore, captive or ex situ rhinoceros populations can play an important role in their conservation. Previous studies identified infant mortality and interbirth intervals among the main parameters affecting the viability and survival of rhinoceros populations. In our study, we tested the recently suggested prediction that in captive Indian rhinoceros Rhinoceros unicornis, longer interbirth intervals may result in higher infant mortality. We also examined the factors that are the main predictors of infant mortality and interbith intervals using the studbook data on Indian rhinoceros born in zoos worldwide as well as data from Dudhwa National Park, India, where rhinoceroses were successfully reintroduced. We found no association between interbirth intervals and infant mortality. In both populations, the main predictor of infant mortality was mother’s parity, with higher mortality in calves born to primiparous mothers. In addition, we found that the interbirth intervals were shorter in zoos than in Dudhwa and that they increased with increase in age of the mother, which was the only factor affecting interbirth interval in both populations. Our results show that the same factors affect both parameters in both populations and thus illustrate that the reproduction and infant survival of Indian rhinoceros in zoos reflect the natural pattern. Furthermore, we suggest that in captivity, the interbirth intervals could be slightly prolonged to approach the situation in the wild.

Sophisticated sleep improves our brains: Our advanced cognitive and social skills might derive from the evolution of improved sleep quality; today, sleep therapy could help with mental health issues and learning.

In the past decades, sleep researchers have been quietly accumulating knowledge about the physiology of sleep. Without any real health application for this knowledge, however, the public and the press have slept blissfully through most of their breakthroughs. Yet, recent findings about the evolution of sleep in humans and the effects of sleep disruption on human health seem to have finally woken the public up. A recent paper from David Samson and Charles Nunn, evolutionary anthropologists from Duke University in the USA, puts forward the hypothesis that humans have evolved more efficient sleep patterns than other primates and that this is because “[e]arly humans experienced selective pressure to fulfill sleep needs in the shortest time possible” [1]. This efficient slumber, the authors argue, might have contributed to setting our ancestors on the evolutionary path toward human civilization. The New York Times , among other newspapers, covered the publication, which follows in the wake of research indicating that human sleep patterns have stayed largely stable during the course of human evolution, from hunter‐gatherers to modern city dwellers [2]. Both of these insights provide evolutionary context for the health impact of too little sleep, with both epidemiological and clinical research showing that sleep disruption has a negative effect on health and well‐being. More recently, however, there is evidence that sleep has distinctly positive therapeutic potential to treat a variety of phobias and other psychotic conditions. The evolutionary study that attracted media attention raises interesting questions about the co‐evolution of sleep and human cognitive faculties. Samson and Nunn suggest that sleep became subject to strong selective pressures—“including increased predation risk […] threats from intergroup conflict, and benefits arising from increased social interaction” [1]—when our ancestors moved down from the trees and onto the savannah. As a result, …

Male Infanticide in the Golden Snub-Nosed Monkey (Rhinopithecus roxellana), a Seasonally Breeding Primate

In nonhuman primates, infanticide by adult males can occur when the leader male is ousted from a one-male, multifemale group, or when male dominance rank changes within a multimale, multifemale group. According to the sexual selection hypothesis, this behavior may be adaptive if perpetrators increase their reproductive success by killing unrelated, unweaned infants, thus shortening the interbirth interval of the mother, and then siring her next infant. Under an alternative hypothesis, infanticide is a byproduct of aggressive male–male competition and these predictions do not hold. Direct observations of the context surrounding infanticide in free-ranging primate populations that allow a test of these predictions are rare. Here, we document four cases of male infanticide and report paternity data for a group of golden snub-nosed monkeys (Rhinopithecus roxellana) at Shennongjia, China. Three cases of infanticide by new leader males supported the predictions of the sexual selection hypothesis, while another provides partial support for the sexual selection hypothesis, but can also be explained via a nonadaptive hypothesis. In this latter case, a male from an all-male group killed an infant during an aggressive episode that appeared to be accidental, as it took place 7 mo before a male takeover happened, and the perpetrator did not obtain any reproductive advantage. We conclude that most male infanticide events in golden snub-nosed monkeys are consistent with the adaptive selection sexual hypothesis.