Communal burrowing in the hystricognath rodent, Octodon degus: a benefit of sociality?

We examined the hypothesis that a main benefit of group-living in the semifossorial rodent, Octodon degus (Rodentia: Octodontidae), is to decrease individual cost of burrow construction. We contrasted the digging behavior of groups of three same-sex, adult-sized individuals with that of solitary degus. The behavior of singles and trios was recorded inside a large terrarium partially filled with natural soil and under controlled conditions of food, light, and temperature. The observation that degus in groups do not decrease their burrowing time or frequency of digging compared with solitary diggers does not support the hypothesis that communal burrowing is a primary cause of degu sociality. On the other hand, the observation that degus in groups removed significantly more soil per capita than solitary digging degus, and that grouped individuals coordinated their digging – group members burrowed mostly in the same sites and formed digging chains –, suggests that social burrowing may potentially reduce the cost of burrow construction in the long term. We suggest that such long-term benefits will be a consequence rather than a cause of degu group-living. Received: 9 December 1999 / Received in revised form: 2 January 2000 / Accepted: 8 February 2000     

Ecology,feeding competition and social structure in baboons

Predictions of the model of van Schaik (1989) of female-bonding in primates are tested by systematically comparing the ecology, level of within-group contest competition for food (WGC), and patterns of social behaviour found in two contrasting baboon populations. Significant differences were found in food distribution (percentage of the diet from clumped sources), feeding supplant rates and grooming patterns. In accord with the model, the tendencies of females to affiliate and form coalitions with one another, and to be philopatric, were strongest where ecological conditions promoted WGC. Group fission in the population with strong WGC was “horizontal” with respect to female dominance rank, and associated with female-female aggression during a period of elevated feeding competition. In contrast, where WGC was low, females’ grooming was focused on adult males rather than other females. Recent evidence suggests that group fission here is initiated by males, tends to result in the formation of one-male groups, and is not related to feeding competition but to male-male competition for mates. An ecological model of baboon social structure is presented which incorporates the effects of female-female competition, male-male competition, and predation pressure. The model potentially accounts for wide variability in group size, group structure and social relationships within the genus Papio. Socio-ecological convergence between common baboons and hamadryas baboons, however, may be limited in some respects by phylogenetic inertia. Received: 22 April 1994/Accepted after revision: 9 December 1995     

Effects of More Frequent and Prolonged El Niño Events on Life-History Parameters of the Degu, a Long-Lived and Slow-Reproducing Rodent

Abstract:  Global climate change (GCC) can have profound effects on species whose ecology is governed primarily by climatic factors. The ecology of small mammals inhabiting semiarid Chile is strongly affected by the El Niño Southern Oscillation (ENSO). During La Niña events in this area, dry conditions prevail and species may disappear from the thorn-scrub habitat. Conversely, El Niño events bring high rainfall, and associated pulses of food trigger small-mammal population increases. We used capture–mark–recapture to study responses of the degu ( Octodon degus ), a dominant small mammal, to variation in rainfall over 18 years. In response to a recent trend toward wetter conditions, degus reached record-high densities and maintained more stable numbers in the area. Underlying mechanisms involved variation in adult survival, juvenile persistence, and fecundity linked to rainfall changes during consecutive years (i.e., rainfall phases). During prolonged droughts, degus had low survival and produced fewer offspring, with low persistence. Following high rainfall, these parameters reversed; consecutive wet years resulted in further increases. Weak declines in fecundity and adult survival and high persistence of juveniles explained delayed responses to deteriorating conditions in initial dry years. If GCC leads to increased frequency of El Niño events, we anticipate greater numerical dominance of degus in semiarid Chile and possible range expansion. Furthermore, degus have strong impacts on other small mammal and some plant species, are important prey species, and are agricultural pests and disease reservoirs. Hence, GCC has the potential to dramatically influence their ecology in northern Chile and to have cascading effects on other components of this system.     

Reproductive consequences of variation in the composition of howler monkey (Alouatta spp.) groups

At least three general categories of environmental pressure - predation, resource distribution, and demographics - shape the costs and benefits of group-living for animals. Among the demographic factors that influence individual survival and reproduction, the composition of social groups can play an important role. Census data drawn from 26 populations of howler monkeys (Alouatta spp.) were used to determine if the composition of groups explained variation in their reproductive performance. Each group's reproductive performance was estimated by calculating the difference between the observed number of immatures and the number expected from its population average. Of four group structure variables tested, only one - the residual of the adult and subadult sex ratio - was a consistent correlate of reproductive performance across the howler monkey populations. Groups with a greater proportion of adult and subadult males contained more juveniles than expected from the population average. I propose that the survival or retention of immatures within howler monkey groups depends in part on the behavior of resident males. Of particular importance, the relative proportions of resident males and females were more informative than the absolute number of males or females. On this basis, I evaluate the possible role of males in protection from predation, conspecific aggression, and resource competition. The techniques used here can also be used to forecast major changes in demographic structure within populations.     

Foraging in a variable environment: weather patterns and the behavioral ecology of baboons

We investigated the long-standing premise in behavioral ecology that the environment affects behavior and demography. We did this by evaluating the extent to which year-to-year variability in the behavioral ecology of a nonhuman primate population could be modeled from meteorological patterns. Data on activity profiles and home range use for baboons (Papio cynocephalus) in Amboseli, Kenya, were obtained over a 10-year period for three social groups: two completely wild-foraging ones, and a third that supplemented its diet with refuse from a nearby tourist lodge. The relationships across years among activity budgeting, travel distance, group size, and measures of temperature and rainfall patterns differed among the social groups. Although meteorological variation generally correlated with behavioral variation in the completely wild-foraging groups, different weather variables and direction of relationships resulted for each group. In addition, different relationships among variables were found before and after home-range shifts. The food-enhanced group spent half as much time foraging as did the other groups and therefore could be used to evaluate the relative extent to which foraging time was a limiting factor for resting and social time. Under their relaxed ecological conditions, the food-enhanced animals increased resting time much more than social time. These findings, combined with supplementary information on the population, lead us to suggest that baboons use a suite of interrelated responses to ecological variability that includes not only changes in activity budgets, but also home-range shifts, changes in the length of the active period, and changes in group size through fissions. Moreover, our results imply that group differences as well as interpopulational and interspecific differences in behavioral ecology provide significant sources of variability. Therefore, social groups rather than populations may be the appropriate unit of analysis for understanding the behavioral ecology of baboons and other highly social primates. The different patterns we observed among groups may have fitness consequences for the individuals in those groups and thereby affect population structure over time. Received: 18 February 1995/Accepted after revision: 6 January 1996     

Within-group spatial position in ring-tailed coatis: balancing predation, feeding competition, and social competition

A variety of factors can influence an individual’s choice of within-group spatial position. For terrestrial social animals, predation, feeding success, and social competition are thought to be three of the most important variables. The relative importance of these three factors was investigated in groups of ring-tailed coatis (Nasua nasua) in Iguazú, Argentina. Different age/sex classes responded differently to these three variables. Coatis were found in close proximity to their own age/sex class more often than random, and three out of four age/sex classes were found to exhibit within-group spatial position preferences which differed from random. Juveniles were located more often at the front edge and were rarely found at the back of the group. Juveniles appeared to choose spatial locations based on feeding success and not predation avoidance. Since juveniles are the most susceptible to predation and presumably have less prior knowledge of food source location, these results have important implications in relation to predator-sensitive foraging and models of democratic group leadership. Subadults were subordinate to adult females, and their relationships were characterized by high levels of aggression. This aggression was especially common during the first half of the coati year (Nov–April), and subadults were more peripheralized during this time period. Subadults likely chose spatial positions to avoid aggression and were actively excluded from the center of the group by adult females. In the Iguazú coati groups, it appeared that food acquisition and social agonism were the major determinants driving spatial choice, while predation played little or no role. This paper demonstrates that within-group spatial structure can be a complex process shaped by differences in body size and nutritional requirements, food patch size and depletion rate, and social dominance status. How and why these factors interact is important to understanding the costs and benefits of sociality and emergent properties of animal group formation.     

The influence of group size on natal dispersal in the communally rearing and semifossorial rodent, Octodon degus

In social or group living species, members of groups are expected to be affected differentially by competition through the effect of group size (i.e., the “social competition hypothesis”). This hypothesis predicts an increase in the probability of dispersal with increasing size of social groups. At a more mechanistic level and based on the known effects of competition on stress hormone levels, a positive relationship between group size and glucocorticoids of juveniles should be observed. We used a demographic approach to test these predictions on a natural population of the communally rearing and semifossorial rodent—Octodon degus. Burrow systems provide degus with places to rear offspring and to evade stressful thermal conditions and predators. Thus, we predicted dispersal to increase with increasing number of degus per main burrow system used, a measure of habitat saturation in degus. The probability of dispersal increased with increasing number of degus per main burrow system used. Mean fecal metabolites of cortisol in offspring increased, yet not statistically significantly, with the number of juveniles in groups. These results were consistent with a scenario in which competition drives natal dispersal in juveniles in social degus. In particular, competition would be the consequence of high degu abundance in relation to the abundance of burrow systems available at the time of offspring emergence.     

Ecological constraints on group size: an analysis of spider monkey and chimpanzee subgroups

The social organization of spider monkeys (Ateles geoffroyi) and chimpanzees (Pan troglodytes) appear remarkably similar. In this paper, field studies of these two species were used to (1) test a model of ecological constraints on animal group size which suggests that group size is a function of travel costs and (2) assess ecological and social factors underlying the social organization of these two species. Spider monkeys were studied over a 6-year period in Santa Rosa National Park, Costa Rica, and chimpanzees were studied for 6 years in Kibale National Park, Uganda. Adults of both species spent their time in small subgroups that frequently changed size and composition. Thus, unlike most primate species, spider monkeys and chimpanzees were not always in a spatially cohesive social group; each individual had the option of associating in subgroups of a different size or composition. Both species relied on ripe fruit from trees that could be depleted through their feeding activity. However, spider monkey food resources tended to occur at higher densities, were more common, less temporally variable, and did not reach the low levels experienced by chimpanzees. Analyses of the relationship between subgroup size and the density and distribution of their food resources suggested that travel costs limit subgroup size. However, these ecological factors did not influence all age/sex classes equally. For example, the number of adult males in a subgroup was a function of food density and travel costs. However, this was not the case for female chimpanzees, suggesting that the benefits of being in a subgroup for females did not exceed the costs, even when ecological conditions appeared to minimize subgroup foraging costs. Therefore, it seems likely that social strategies influenced the relationship between food resource variables and subgroup size.     

Access to mutualistic endosymbiotic microbes: an underappreciated benefit of group living

A central question in behavioral ecology has been why animals live in groups. Previous theories about the evolution of sociality focused on the potential benefits of decreased risk of predation, increased foraging or feeding efficiency, and mutual aid in defending resources and/or rearing offspring. This paper argues that access to mutualistic endosymbiotic microbes is an underappreciated benefit of group living and sets out to reinvigorate Troyer’s hypothesis that the need to obtain cellulolytic microbes from conspecifics influenced the evolution of social behavior in herbivores and to extend it to nonherbivores. This extension is necessary because the benefits of endosymbionts are not limited to nutrition; endosymbionts also help protect their hosts from pathogens. When hosts must obtain endosymbionts from conspecifics, they are forced to interact. Thus, complex forms of sociality may be more likely to evolve when hosts must repeatedly obtain endosymbionts from conspecifics than when endosymbionts can be obtained either directly from the environment, are vertically transmitted, or when repeated inoculations are not necessary. Observations from a variety of taxa are consistent with the ideas that individuals benefit from group living by gaining access to endosymbionts and the complexity of social behavior is associated with the mode of acquisition of endosymbionts. Ways to test this theory include (a) experiments designed to examine the effects of endosymbionts on host fitness and how endosymbionts are obtained and (b) using phylogenetic analyses to examine endosymbiont–host coevolution with the goal of determining the relationship between the mode of endosymbiont acquisition and host sociality.     

Sociogenetic responses to ecological variation in the ant Myrmica punctiventris are context dependent

Models of social evolution predict a strong relationship between ecological factors and sociogenetic organization in social insects (e.g. queen number, nestmate relatedness and population structure). Despite a large body of coherent theory, empirical support for these predictions is weak. Here we report the results of an experiment that manipulated two ecological parameters, food and nestsite availability, thought to be important for a population of the forest ant Myrmica punctiventris. Earlier work had shown that the sociogenetic structure varies between two populations of this species, and an ecological experiment in one of the populations (in Vermont) revealed that food supplementation had the strongest effects on nestmate relatedness. We repeated the experiment in the second population (in New York) and obtained strikingly different results. We show that nestsite supplementation had the strongest effect in the New York population, and that adding both food and nestsites affected nestmate relatedness in a direction opposite to that reported from the Vermont study. These results show that the ecological context is critically important for understanding the determinants of colony structure in ants. Furthermore, comparison of our data with that of a previous study shows that social organization in New York is temporally unstable. Thus, not only do ecological factors strongly influence social organization, but their influence can vary over time. Our study underscores the need for detailed information on the natural history and ecology of social species.     

Habitat-specific variation and performance trade-offs in shell armature of New Zealand mudsnails

Studies documenting phenotypic variation among populations show that ecological performance in one activity is sometimes traded off against another. Identifying environment-specific costs and benefits associated with performance trade-offs is fundamental to knowing how conflicting selection pressures shape phenotype-environment matching in populations. We studied phenotypic variation in shell armature (spininess) of the New Zealand mudsnail, Potamopyrgus antipodarum (Gray), and explored how this variability relates to performance trade-offs between flow resistance and predator deterrence. Smooth- and spiny-shell morphotypes exist in populations in New Zealand streams and lakes, but the patterns and correlates of spatial variation of these phenotypes, and the possible hydrodynamical constraints and antipredatory benefits associated with spiny shell armature, are unknown. Samples from 11 rivers and nine lakes on the South Island showed that, on average, nearly 70% of snails in streams were smooth-shelled, whereas >80% of snails in lakes were spiny, suggesting dissimilar selective pressures between habitats. A laboratory flume experiment revealed that spines collected seston (i.e., suspended algae) at current speeds <40 cm/s, making spiny morphs more prone to flow-induced dislodgment than smooth morphs. However, a fish feeding experiment showed that one benefit of spines on shells was a decrease in predation risk from the common bully (Gobiomorphus cotidianus), a widespread predator of mudsnails in both streams and lakes. All snails egested by bullies were dead, further suggesting that these fishes may exert strong lethal effects on mudsnail populations in nature. Spine expression in lakes also appeared to be temperature related. We conclude that functional trade-offs between risk of flow-induced dislodgment and risk of fish predation affect shell armature frequencies of Potamopyrgus in freshwater habitats.     

NEATS: A Network Economics Approach to Trophic Systems

The main principle of the economic approach to a trophic system we propose here lies in assuming that there is a transfer of food along a path between a prey and a predator if, for the predator, the benefits are greater than costs of predation on this path. Conversely, if the costs exceed the benefits, there are no flows. This trade-off, considered all along the food chains of an ecosystem, together with ecological processes (assimilation, somatic maintenance) results in a model coupling mass balance equations (biological constraints) and complementarity principles (Walras’ law). Here is the core of the Network Economics Approach to Trophic Systems (NEATS).     

The optimal balance of defence investment strategies in clonal colonies of social aphids

Social aphid species provide ideal systems to study the ecological influences upon the evolution of sociality because they consist of discrete colonies which are entirely clonal and therefore devoid of any genetic conflict over altruistic behaviour. Although selfishness can be discounted as an obstacle preventing the evolution of altruistic defenders, the vast majority of aphid species are not social. To examine the key life-history and ecological characteristics that interact to facilitate social evolution, we designed a matrix population model based on the natural history of one of the unique aphid species with soldiers, Pemphigus spyrothecae. In addressing the life-history factors, our special interest was to examine the optimal trade-off faced by colonies that can increase their defence investment by producing defenders at birth and/or increasing the duration of the defensive stage. The level and period of exposure to predation and a declining colony birth rate were key factors that selected for social defence. The model demonstrated that, in species which have soldiers that can facultatively develop to make a direct contribution to colony fitness, temporal extension of the soldier stage is a key mechanism of increasing defence investment. This extension is particularly favoured when predation is high and the lifetime of a colony is long. An increase in production of defenders at birth was favoured when mortality due to predation was strongly biased towards defenders. The model suggests that, in species which have the defensive flexibility of choosing whether soldiers remain as such, there is little requirement for flexibility in the morph allocations made at birth. All these predictions were found to be fully compatible with the available empirical data.     

The influence of experience on risk taking: results from a common-garden experiment on populations of Eurasian perch

Predation is often thought of as an unforgiving and strong selective force, quickly selecting against maladaptive behaviour in the prey. It is argued that experience is likely to have low influence on the phenotypic response to predation, as failing to react correctly to a predator may mean death to the prey and no second chance to learn and correct the behaviour. Individuals from different populations of Eurasian perch are known to differ in risk-taking behaviour. Variation in predation pressure has been suggested as a key factor causing these differences, but little is known about the underlying mechanism by which predation generates risk-taking phenotypes in perch. We compared the degree of boldness between two natural populations of Eurasian perch, living under different predation regimes, and the same populations hatched and reared under identical conditions, free from predation. By this common-garden approach, we sought to investigate patterns in the influence of inheritance and experience on boldness phenotype. The wild fish differed in risk taking, with fish from the low predation-risk population acting bolder than fish from the high-risk environment. In the reared fish, both populations behaved equally bold. Only the fish originating from the high predation population showed different behaviour when comparing wild and reared ecotypes. Our results suggest that experience has an important impact on the response to predators and that geographic variation in risk taking between populations of Eurasian perch to a high degree is shaped by adjustments to the current environment. Habituation had an effect of risk-taking behaviour over the experimental period, but consistent differences between individuals were also found. Furthermore, we also show, by the estimation of variance components, that the behaviour we observe is affected by a range of random effects, such as aquaria and group membership, that in concert shapes the behaviour of an individual perch.     

Interdemic variation in mixed-species association patterns: common diurnal primates of Kibale National Park, Uganda

We used interdemic variation in the tendency to form mixed-species groups to examine the costs and benefits of association among the primates of Kibale National Park, Uganda. A year-long survey of six sites revealed that the amount of time that the five common diurnal primates [red colobus (Procolobus tephrosceles), black-and-white colobus (Colobus guereza), redtail monkeys (Cercopithecus ascanius), blue monkeys (Cercopithecus mitis), and grey-cheeked mangabeys (Lophocebus albigena)] spent in mixed-species groups varied dramatically among sites. In many cases, the proportion of time that species associated was positively related to their densities. By using detailed behavioral observations of redtail monkeys and red colobus made over 4 years (2660 h) at four sites, we were able to reject the null hypothesis that associations occur by chance for only one of four sites. However, a correlative approach exploring the costs and benefits of association suggests that ecological variables do influence association patterns. We found that redtail monkeys and red colobus overlapped in diet (19.2% of their foraging effort) and traveled further when in mixed-species groups than when alone. Having demonstrated this, we examined the applicability of the ecological constraints model for predicting the proportion of the time spent in mixed-species groups based on food availability. For this analysis we concentrated on red colobus from the site with 35 months of observation and demonstrated that their tendency to be in mixed- species groups was related to food availability. We used two methods to examine if mixed-species associations function to decrease predation risk. First, chimpanzees are known to prey heavily on red colobus, but rarely kill other primates. The time red colobus spent in mixed-species groups was correlated to chimpanzee density, but it was not for the other monkey species, suggesting that mixed-species groups serve to decrease predation risk. Second, when red colobus groups contain more infants and are presumably at the greatest risk of predation, they form mixed-species groups most often. These results demonstrate that the costs and benefits of mixed-species associations vary dramatically over small spatial and temporal scales. If such variation is generally the case, then studies conducted at different locations or different times could easily highlight the importance of difference selective agents in favoring mixed-species associations. Received: 10 February 1999 / Received in revised form: 16 September 1999 / Accepted: 2 October 1999     

Personality trait differences between mainland and island populations in the common frog (Rana temporaria)

Understanding and predicting species range expansions is an important challenge in modern ecology because of rapidly changing environments. Recent studies have revealed that consistent within-species variation in behavior (i.e., animal personality) can be imperative for dispersal success, a key process in range expansion. Here we investigate how habitat isolation can mediate differentiation of personality traits between recently founded island populations and the main population. We performed laboratory studies of boldness and exploration across life stages (tadpoles and froglets) using four isolated island populations and four mainland populations of the common frog (Rana temporaria). Both tadpoles and froglets from isolated populations were bolder and more exploratory than conspecifics from the mainland. Although the pattern can be influenced by possible differences in predation pressure, we suggest that this behavioral differentiation might be the result of a disperser-dependent founder effect brought on by an isolation-driven environmental filtering of animal personalities. These findings can have important implications for both species persistence in the face of climate change (i.e., range expansions) and ecological invasions as well as for explaining rapid speciation in isolated patches.     

Effects of Harvest of Nontimber Forest Products and Ecological Differences between Sites on the Demography of African Mahogany

Abstract: The demographic impacts of harvesting nontimber forest products (NTFP) have been increasingly studied because of reports of potentially unsustainable harvest. Nevertheless, our understanding of how plant demographic response to harvest is altered by variation in ecological conditions, which is critical for developing realistic sustainable‐use plans, is limited. We built matrix population models to test whether and how variation in ecological conditions affects population responses to harvest. In particular, we examined the effect of bark and foliage harvest on the demography of populations of African mahogany (Khaya senegalensis) in two contrasting ecological regions of Benin, West Africa. K. senegalensis bark and foliage harvest significantly reduced its stochastic population growth rates, but ecological differences between regions had a greater effect on population growth rates than did harvest. The effect of harvest on population growth rates (Δλ) was slightly stronger in the moist than in the drier region. Life‐table response experiments revealed that the mechanism by which harvesting reduced λ differed between ecological regions. Lowered stasis (persistence) of larger life stages lead to a reduction in λ in the drier region, whereas lowered growth of all life stages lowered λ in moist region. Potential strategies to increase population growth rates should include decreasing the proportion of individuals harvested, promoting harvester‐owned plantations of African mahogany, and increasing survival and growth by promoting no‐fire zones in gallery forests. Our results show how population responses to harvest of NTFP may be altered by ecological differences across sites and emphasize the importance of monitoring populations over the climatic range in which they occur to develop more realistic recommendations for conservation.     

The evolution of female social relationships in nonhuman primates

Considerable interspecific variation in female social relationships occurs in gregarious primates, particularly with regard to agonism and cooperation between females and to the quality of female relationships with males. This variation exists alongside variation in female philopatry and dispersal. Socioecological theories have tried to explain variation in female-female social relationships from an evolutionary perspective focused on ecological factors, notably predation and food distribution. According to the current “ecological model”, predation risk forces females of most diurnal primate species to live in groups; the strength of the contest component of competition for resources within and between groups then largely determines social relationships between females. Social relationships among gregarious females are here characterized as Dispersal-Egalitarian, Resident-Nepotistic, Resident-Nepotistic-Tolerant, or Resident-Egalitarian. This ecological model has successfully explained differences in the occurrence of formal submission signals, decided dominance relationships, coalitions and female philopatry. Group size and female rank generally affect female reproduction success as the model predicts, and studies of closely related species in different ecological circumstances underscore the importance of the model. Some cases, however, can only be explained when we extend the model to incorporate the effects of infanticide risk and habitat saturation. We review evidence in support of the ecological model and test the power of alternative models that invoke between-group competition, forced female philopatry, demographic female recruitment, male interventions into female aggression, and male harassment. Not one of these models can replace the ecological model, which already encompasses the between-group competition. Currently the best model, which explains several phenomena that the ecological model does not, is a “socioecological model” based on the combined importance of ecological factors, habitat saturation and infanticide avoidance. We note some points of similarity and divergence with other mammalian taxa; these remain to be explored in detail. Received: 30 September 1996 / Accepted after revision: 20 July 1997     

Competitive regimes in forest-dwelling Hanuman langur females (Semnopithecus entellus)

Van Schaik’s socioecological model predicts interrelations among food distribution, competitive regimes, and female social relationships. To test the internal consistency of the model, feeding competition was examined in three differently sized groups of a forest-dwelling population of Hanuman langurs (Semnopithecus entellus). The nutritional condition of females was used as a direct indicator of feeding competition and related to the seasonal variation in resource distribution and abundance. Female dominance hierarchies were characterized by displacements. Dominance hierarchies were significantly linear and relatively stable, but less so with increasing group size. Physical condition correlated with dominance rank and high-ranking females were in the best condition, indicating within-group contest competition. The strength of this relationship became less pronounced with increasing group size. The females of the medium-sized group were in the best physical condition indicating between-group contest plus within-group scramble competition. Closer examination revealed variable costs and benefits of group foraging with a predominance of within-group scramble competition when food was more abundant. The results support some basic predictions of the model. Limiting food abundance was bound to ubiquitous within-group scramble competition. The use of clumped resources translated into differences in net energy gain based on dominance. In contrast to the predictions, group-size-related costs and benefits were related to food abundance instead of food distribution. As predicted, within-group contest competition was linked to a linear dominance hierarchy. The absence of nepotism and coalitions in Hanuman langurs may be attributed to dominance hierarchies that are unstable through time, probably minimizing fitness gain via kin support. Received: 25 May 1999 / Received in revised form: 18 February 2000 / Accepted: 25 February 2000     

Benefits and costs of earwig (Forficula auricularia) family life

The evolution of parental care and family group formation critically depends on offspring survival benefits and parental fecundity costs of care under given ecological conditions. Investigations of the functional significance of care in insect species that exhibit facultative parental care have been relatively rare but may be of particular interest for better understanding of benefit and cost schedules at an early evolutionary stage. In this study, aspects of benefits and costs of care were addressed in the sub-social European earwig (Forficula auricularia; Dermaptera: Forficulidae) by manipulating the presence of tending mothers and brood size in a fully crossed experimental design. Larvae growing in broods tended by their mother or of reduced size showed a higher survival probability than larvae growing in untended or large broods, as predicted if maternal care is beneficial and shaped by a trade-off between number and quality of offspring. Analysis of patterns of food consumption and developmental time further suggested that the benefit of maternal attendance is mediated by the maternal provisioning of food, while the quality–quantity trade-off seemed to be driven by sibling rivalry. Further, tending mothers delayed the production of a second clutch, indicating a potential cost of care in terms of lifetime fecundity. This study experimentally shows benefits and potential costs of maternal care and family group formation in the European earwig. More detailed behavioural experiments will be required to fully understand how behavioural interactions among family members mediate these reproductive outcomes.