Showing papers in "Behavioral Ecology and Sociobiology in 2003"

The bottlenose dolphin community of Doubtful Sound features a large proportion of long-lasting associations

Abstract: More than 12 studies of different bottlenose dolphin populations, spanning from tropical to cold temperate waters, have shown that the species typically lives in societies in which relationships among individuals are predominantly fluid. In all cases dolphins lived in small groups characterised by fluid and dynamic interactions and some degree of dispersal from the natal group by both sexes. We describe a small, closed population of bottlenose dolphins living at the southern extreme of the species' range. Individuals live in large, mixed-sex groups in which no permanent emigration/immigration has been observed over the past 7 years. All members within the community are relatively closely associated (average half-weight index>0.4). Both male–male and female–female networks of preferred associates are present, as are long-lasting associations across sexes. The community structure is temporally stable, compared to other bottlenose dolphin populations, and constant companionship seems to be prevalent in the temporal association pattern. Such high degrees of stability are unprecedented in studies of bottlenose dolphins and may be related to the ecological constraints of Doubtful Sound. Fjords are low-productivity systems in which survival may easily require a greater level of co-operation, and hence group stability. These conditions are also present in other cetacean populations forming stable groups. We therefore hypothesise that ecological constraints are important factors shaping social interactions within cetacean societies.

Female sticklebacks Gasterosteus aculeatus use self-reference to optimize MHC allele number during mate selection

Abstract: Theoretical and empirical studies suggest that an optimal resistance to pathogens and parasites requires an optimal number of MHC alleles per individual. Here we argue that three-spined sticklebacks (Gasterosteus aculeatus) achieve this goal by applying a strategy that involves a self-referential process. According to this model, females complement their own set of alleles with a more or less diverse set of male alleles such that the combined diversity reaches an optimum. In previous experiments, we have identified allele counting as a major mate-choice strategy in large populations. The self-referential allele-counting hypothesis predicts that MHC-based mate choice favors dissimilar MHC alleles in small populations facing the risk of inbreeding. Therefore, we conducted an experiment that simulated a small effective population size with low MHC class-II diversity. Our experiments are based on the analysis of MHC class-IIB alleles that explain a major part of the overall MHC diversity in sticklebacks, as determined by mathematical modeling. The results show that females preferred males with dissimilar alleles. Our present and the previous studies (which we reanalyzed with respect to our new predictions) show that irrespective of high or low population diversity faced by female sticklebacks for their mate choice, they use information about their own and their potential mate's MHC polymorphism for optimal complementation of their own set of alleles. From combining the data of the previous and the present experiment we found that female sticklebacks try to achieve an optimum number of MHC class-IIB alleles for their offspring through mate choice. The chosen MHC diversity is close to the most frequent diversity found naturally in individual fish, which in addition have the lowest parasite burden.

Prey dynamics affect foraging by a pelagic predator (Stenella longirostris) over a range of spatial and temporal scales

Abstract: Studies have shown that pelagic predators do not overlap with their prey at small scales. However, we hypothesized that spinner dolphin foraging would be affected by the spatio-temporal dynamics of their prey at both small and large scales. A modified echosounder was used to simultaneously measure the abundance of dolphins and their prey as a function of space and time off three Hawaiian islands. Spinner dolphin abundance closely matched the abundance patterns in the boundary community both horizontally and vertically. As hypothesized, spinner dolphins followed the diel horizontal migration of their prey, rather than feeding offshore the entire night. Spinner dolphins also followed the vertical migrations of their prey and exploited the vertical areas within the boundary layer that had the highest prey density. Cooperative foraging by pairs of dolphins within large groups was evident. The geometric and density characteristics of prey patches containing dolphins indicate that dolphins may alter the characteristics of prey patches through this cooperative foraging. The overlap of Hawaiian spinner dolphins and their prey at many temporal and spatial scales, ranging from several minutes to an entire night and 20 m to several kilometers, indicates that the availability of truly synoptic data may fundamentally alter our conclusions about pelagic predator-prey interactions.

Day length, latitude and behavioural (in)flexibility in baboons (Papio cynocephalus ursinus )

Abstract: Annual cycles in day length are an important consideration in any analysis of seasonal behaviour patterns, since they determine the period within which obligate diurnal or nocturnal animals must conduct all of their essential activities. As a consequence, seasonal variation in day length may represent an ecological constraint on behaviour, since short winter days restrict the length of the time available for foraging in diurnal species (with long summer days, and thus short nights, a potential constraint for nocturnal species). This paper examines monthly variation in activity patterns over a 4-year study of chacma baboons (Papio cynocephalus ursinus) at De Hoop Nature Reserve, South Africa. Time spent feeding, moving, grooming and resting are all significant positive functions of day length, even before chance events such as disease epidemics and climatically mediated home range shifts have been accounted for. These results provide strong support for the idea that day length acts as an ecological constraint by limiting the number of daylight hours and thus restricting the active period at certain times of year. Day length variation also has important implications across populations. Interpopulation variation in resting time, and non-foraging activity in general, is a positive function of latitude, with long summer days at temperate latitudes apparently producing an excess of time that cannot profitably be devoted to additional foraging or social activity. However, it is the short winter days that are probably of greatest importance, since diurnal animals must still fulfil their foraging requirements despite the restricted number of daylight hours and elevated thermoregulatory requirements at this time of year. Ultimately this serves to restrict the maximum ecologically tolerable group sizes of baboon populations with increasing distance from the equator. Seasonal variation in day length is thus an important ecological constraint on animal behaviour that has important implications both within and between populations, and future studies at non-equatorial latitudes must clearly be mindful of its importance.

Female dispersal and reproductive success in wild western lowland gorillas ( Gorilla gorilla gorilla )

Abstract: This paper presents data on the dispersal patterns and reproductive success of western lowland gorilla females from a long-term study at Mbeli Bai in the Nouabale-Ndoki National Park, Republic of Congo. We find that female natal and secondary transfer is common. Female immigration rates are negatively related to group size, and emigration rates are positively related to group size, with the net result that larger groups are losing females and smaller groups are gaining females. Furthermore, females transferring between known groups show a preference for significantly smaller groups. However, there is no effect of group size on female reproductive success. Male protection and male quality are considered important in determining female transfer decisions. The case for infanticide is argued and females exhibit strategies that appear to minimise the probability of infanticide following the death of the silverback. Exclusively single-male groups and group formation through female acquisition by solitary males may bias female transfer to lone silverbacks and small groups. The effects of group size on female dispersal and reproductive success are not wholly consistent with an argument for increased foraging costs, and group size effects are more parsimoniously explained by demographic factors. Male protection from intra-group aggression is the most likely factor underlying grouping patterns across gorilla taxa, but differences in population structure and male reproductive strategies may account for inter-specific variation. We stress the need for intra-specific comparisons and more complete data sets on western lowland gorilla feeding behaviour.

Loud calls as indicators of dominance in male baboons (Papio cynocephalus ursinus)

Abstract: Adult male baboons (Papio cynocephalus) give loud two-syllable 'wahoo' calls during dawn choruses, interactions between groups, when chasing females, and in aggressive interactions with other males. These 'contest' wahoos are acoustically different from 'alarm' wahoos given to predators. In a study of free-ranging baboons in the Okavango Delta, Botswana, we found no significant correlations between the acoustic features of wahoos and adult male size; however, acoustic features were correlated with male dominance rank, age, and calling bout length. Here we show that other measures of calling behavior also appear to function as honest indicators of stamina and competitive ability. High-ranking males were more likely than middle- or low-ranking males to participate in wahoo bouts. They called at significantly higher rates, and their bouts were longer and contained more calls. All males were significantly more likely to participate in wahoo bouts with another male if their opponent's rank was similar to their own. Bouts involving males of similar ranks were longer, contained more wahoos, and involved calling at higher rates, than other bouts. In contests between males of similar ranks, the subordinate and dominant were equally likely to end the bout, whereas in contests involving males of disparate ranks, subordinates were significantly more likely to end the bout. Bouts involving males of similar rank were significantly more likely than others to escalate and result in physical fighting.

A modelling framework for understanding social insect foraging

Abstract: The foraging of an insect society is a complex process involving large numbers of individuals collecting food from many different sources. Differential equation models have shown how quite simple communication mechanisms can produce complex and functional group-level foraging patterns. In this paper we review previous differential equation models for pheromone trails, honey bee dances and other methods of communication used during foraging. We develop a general framework for modelling social insect foraging systems that incorporates each of the previous models. This framework identifies the different behaviours that insects undertake while foraging, along with generalised rate functions that determine how the insects switch between behaviours. We describe how to tailor our framework to specific insect societies, by incorporating the details of specific behavioural mechanisms into appropriate expressions for rates of discovery of, recruitment to, and retirement from food sources. Our framework thus provides an experimental tool for improved understanding of the foraging behaviour of particular species, as well as a system for meaningful comparisons of foraging behaviour across species. We end this article by linking our framework to inclusive fitness theory. We demonstrate how understanding of the proximate mechanisms involved in social insect foraging ultimately furthers understanding, not only of how insect societies function, but also of how these mechanisms are used to optimise colony fitness and survival.

Choosing a home: how the scouts in a honey bee swarm perceive the completion of their group decision making

Abstract: This study considers the mystery of how the scout bees in a honey bee swarm know when they have completed their group decision making regarding the swarm's new nest site. More specifically, we investigated how the scouts sense when it is appropriate for them to begin producing the worker piping signals that stimulate their swarm-mates to prepare for the flight to their new home. We tested two hypotheses: "consensus sensing," the scouts noting when all the bees performing waggle dances are advertising just one site; and "quorum sensing," the scouts noting when one site is being visited by a sufficiently large number of scouts. Our test involved monitoring four swarms as they discovered, recruited to, and chose between two nest boxes and their scouts started producing piping signals. We found that a consensus among the dancers was neither necessary nor sufficient for the start of worker piping, which indicates that the consensus sensing hypothesis is false. We also found that a buildup of 10–15 or more bees at one of the nest boxes was consistently associated with the start of worker piping, which indicates that the quorum sensing hypothesis may be true. In considering why the scout bees rely on reaching a quorum rather than a consensus as their cue of when to start preparing for liftoff, we suggest that quorum sensing may provide a better balance between accuracy and speed in decision making. In short, the bees appear to begin preparations for liftoff as soon as enough of the scout bees, but not all of them, have approved of one of the potential nest sites.

Influences of moonlight, ambient temperature, and food availability on the diurnal and nocturnal activity of owl monkeys (Aotus azarai)

Abstract: The study of activity rhythms, their potential zeitgebers and masking factors among free-ranging primates has received relatively little attention in the past. Most primates are diurnal, a few of them nocturnal, and even fewer are cathemeral. Owl monkeys (Aotus azarai azarai) regularly show diurnal, as well as nocturnal, activity in the Argentinean and Paraguayan Chaco. The goal of this study was to examine how changes in activity patterns in owl monkeys of Formosa, Argentina are related to daily, monthly, and seasonal changes in temperature, light and food availability . During 1 year, I collected activity data from five groups followed continuously from dawn to dusk, dusk to dawn or uninterruptedly during 24 or 36 h for approximately 1,500 h. I kept hourly and daily records of temperature and light conditions, and I gathered monthly information on the density, distribution and abundance of food resources available to the monkeys. I found that the area of study is highly seasonal, and characterized by significant fluctuations in rainfall, temperature, photoperiod, and food availability. Owl monkeys had on average 5 h of activity during the day and 4 h during the night. The amount of diurnal activity remained fairly constant through the year despite seasonal changes in exogenous factors. Owl monkeys did not show changes in their activity patterns that could be attributed to changes in food availability. Nocturnal activity increased as the amount of moonlight increased, whereas diurnal activity decreased following a full-moon night. Ambient temperature was a good predictor of activity only when the moon was full. These results argue convincingly for an interaction between ambient temperature and moonlight in determining the observed activity pattern. It is then highly advisable that any evaluation of diurnal activity in cathemeral animals be analyzed controlling for the possible effects of moonlight during the previous night.

Testosterone, dominance signalling and immunosuppression in the house sparrow, Passer domesticus

Abstract: The immunocompetence handicap hypothesis (ICHH) suggests that dominance signals are costly because their development is controlled by testosterone, which is immunosuppressive. Signal control therefore links an increased disease risk with a high quality signal. The chest bib of the house sparrow, Passer domesticus, is a signal known to be related to dominance and under control of testosterone levels. We experimentally manipulated testosterone in male sparrows during the breeding season and again independently during the post-breeding period to test whether variation in levels of testosterone could cause variation in levels of immunocompetence. There was no effect of testosterone manipulation on the cell-mediated response of birds to phytohaemagglutinin injection, nor did testosterone levels appear to affect either white blood cell ratios or red blood cell counts. In contrast, both breeding season and post-breeding season testosterone levels had significant effects upon the humoral response of the birds to sheep red blood cell injections. However, whilst testosterone during the breeding season appeared to act immunosuppressively, the role of post-breeding levels is less clear. In concordance with a previous study, there was an indication that corticosterone is involved in mediating the immunosuppressive effects of testosterone. The strength of the secondary humoral response and the cell-mediated response were negatively related suggesting the possibility of a trade-off between the different arms of the immune system. These results provide some support for the ICHH as a mechanism promoting the evolution of costly badges of status, although the results question whether the immunosuppressive cost can be mediated by testosterone at the time of badge development.

On the move around the clock: correlates and determinants of cathemeral activity in wild redfronted lemurs (Eulemur fulvus rufus)

Abstract: Whether animals are active at night or during the day has profound consequences for many aspects of their behavioral ecology. Because of ecological and physiological trade-offs, most animals, including primates, are either strictly nocturnal or diurnal. However, a few primate species exhibit cathemeral activity, i.e., their activity is irregularly distributed throughout the 24-h cycle. Details and determinants of this unusual activity pattern are poorly understood because long-term 24-h observations are not feasible in the field. We therefore used small data loggers to record the activity of cathemeral redfronted lemurs (Eulemur fulvus rufus) from several neighboring groups quantitatively and continuously over a complete annual cycle in order to evaluate various proposed proximate and ultimate determinants of cathemeral activity. Activity data were examined for variation as a function of ambient temperature, time of day, lunar phase, and season. We found that cathemeral activity occurred year-round and that, on average, 3.5 times more activity occurred during the day. Total and diurnal activity increased during the long days of the austral summer. Nocturnal activity increased during the longer nights of the cool dry season. Irrespective of season, lunar phase had a significant effect on the distribution of activity across the 24-h cycle, with most nocturnal activity recorded during parts of the night with greatest brightness. These data indicate that light availability is the primary proximate determinant for the patterning of cathemeral activity. Several lines of evidence suggest that cathemerality in lemurs has evolved from nocturnal ancestors and that it represents a transitory state on the way to the diurnal niche.

Achromatic color variation in black-capped chickadees, Poecile atricapilla: black and white signals of sex and rank

Abstract: Sexual dichromatism and phenotypic variation in elaborate male traits are common products of sexual selection. The spectral properties of carotenoid and structurally-based plumage colors and the patch sizes of melanin-based plumage colors have received considerable attention as sexual signals in birds. However, the importance of variation in achromatic plumage colors (white, gray and black) remains virtually unexplored, despite their widespread occurrence. We investigated a potential signal function of the achromatic black and white plumage of black-capped chickadees (Poecile atricapilla). We captured and color-banded 178 free-living chickadees and assessed winter flock dominance hierarchies by tabulating pairwise interactions at feeders. We recaptured 73 of these birds and measured plumage coloration for six body regions using a reflectance spectrometer and the area of melanin-based plumage patches from standardized photographs. We found extensive individual variation in chickadee plumage traits and considerable sexual dichromatism. Male black-capped chickadees have significantly brighter white plumage than females, larger black patches, and greater plumage contrast between adjacent white and black plumage regions. We also found rank differences in the plumage reflectance of males; high-ranking males, who are preferred by females as both social and extra-pair partners, exhibit significantly darker black plumage and grow their feathers more rapidly than low-ranking males. This variation among individuals reveals a potential signal function for achromatic plumage coloration in birds.

Innate antipredator responses of Arctic charr ( Salvelinus alpinus ) depend on predator species and their diet

Abstract: The ability to discriminate between more dangerous and less dangerous predators can have serious fitness advantages for fish juveniles. This is especially true for hatchery-reared fish young used for stocking, because their post-release mortality is often much higher than that of wild-born conspecifics. We tested whether two coexisting fish predators and their different diets induce innate behavioral responses in predator-naive Arctic charr (Salvelinus alpinus) young originating from an endangered hatchery-bred population used for re-introductions. We predicted the antipredator responses of charr to be stronger towards chemical cues of brown trout (Salmo trutta) and pikeperch (Stizostedion lucioperca) than towards odorless control water. More pronounced antipredator behavior was predicted in treatments with predators fed on charr than when their diet consisted of another sympatric salmonid, European grayling (Thymallus thymallus), or when they were food-deprived. The Arctic charr young showed strong antipredator responses in all brown trout treatments, whereas odors of the less likely predator pikeperch were avoided with conspecific diet only. Freezing was the most sensitive antipredator behavior, as it was completely absent in control treatments. We found considerable individual variation in the amount and strength of antipredator responses. Although almost half of the charr failed to show antipredator behavior towards the piscivores, those with the innate ability showed highly sensitive recognition of predator odors. Our results indicate that the innate antipredator behavior of the juvenile fish is already finely tuned to respond specifically to chemical cues from different fish predators and even their diets.

Social environment during molt and the expression of melanin-based plumage pigmentation in male house sparrows (Passer domesticus)

Abstract: Evolutionary biologists have shown much recent interest in the costliness and signal content of colorful plumage displays in birds. Although many studies suggest that both carotenoid- and structurally-based plumage colors are condition-dependent indicators of health and nutritional state at the time ornamental feathers are grown, there is little experimental evidence supporting the idea that melanin pigmentation is a reliable signal of condition during molt. Instead, melanin-based ornamental coloration often reveals the competitive ability and dominance of individuals throughout the year. However, this work does not indicate which proximate environmental factors shape the expression of melanin pigmentation at the time of feather growth. Because of the link between melanin coloration and the social environment, it is possible that the development of brightly colored plumage may be associated with aggressive social interactions during feather molt. Here, we show that melanin-based ornamental coloration in male house sparrows (Passer domesticus) is correlated with the degree to which individuals interact aggressively with conspecifics during molt. Males that were dominant (beta, but not alpha) within captive social groups during molt grew larger badges than subordinates. Groups of males that had higher rates of aggression during molt grew larger badges than less aggressive triads. To our knowledge, this is the first demonstration that melanin pigmentation and plumage-based status badges are related to the competitive history of individuals during feather development. By coupling badge size directly with aggressive experiences during molt, birds can use their status signal to honestly indicate their likelihood of winning agonistic encounters throughout the year.

Bumblebees as model organisms to study male sexual selection in social insects

Abstract: Social-insect males are often regarded as being merely short-lived "flying sperm containers", which ignores their potential influence on females and paternity patterns as found in other animals. Consequently, social-insect males have received only marginal attention and sexual selection has almost never been studied in these species. Here I present a review of the mating biology of bumblebees (Bombus spp.), which are the best-studied social insects to date. I follow a male's pathway from his birth until he successfully contributes to the next generation, and show that males have evolved adaptations and behaviors to influence paternity patterns at various stages of their life, which are similar to those exhibited by males of non-social insects. By comparing the available bumblebee data with more sparse studies of male reproductive behavior in other social Hymenoptera, I argue that such male adaptations may indeed be widespread in social insects. I suggest that current paradigms on sexual selection should be challenged by using social insects as model systems, because they offer unique features, and a solid theoretical background in which clear predictions can be made and appropriate experimental tests of them can be designed.

Consensus building during nest-site selection in honey bee swarms: the expiration of dissent

Abstract: This study addresses a question that lies at the heart of understanding how the scouts in a honey bee swarm achieve unanimity in their dances, and so reach agreement in their choice of a future nest site: what causes the scouts that perform dances for the non-chosen sites to stop dancing for these sites? One possibility is that a scout stops dancing for a non-chosen site only after she follows a lively dance for another site, such as the site that is ultimately chosen. This hypothesis is contradicted by the finding that 23 out of 27 scouts (in 6 swarms) that danced initially for a non-chosen site stopped their dancing before they followed a dance for another site. Evidently, a scout that supports initially one of the non-chosen sites is likely to withdraw her support for this site even before she learns about another site. What causes her to do so? Close examination of the behavior of scouts revealed that they reduce the strength of their dancing (waggle runs/return to the swarm) for a given site over consecutive returns to the swarm. On average, the pattern of this reduction in dancing is strikingly linear, which suggests that it arises from an internal, neurophysiological process that automatically drives down a scout's motivation to dance for a site. Other results suggest that scouts from inferior sites start their dancing less strongly, and so cease their dancing more rapidly, than do scouts from superior sites. If so, then during the consensus-building process of the scouts, it is the support (the dancing) for inferior sites that is most likely to die out while it is the support for a superior site that is most likely to prevail.

Carotenoid-based status signalling in red-shouldered widowbirds (Euplectes axillaris): epaulet size and redness affect captive and territorial competition

Abstract: In breeding plumage, the African male red- shouldered widowbirds (Euplectes axillaris) are black except for red carotenoid-based epaulets ('shoulder patches'), similar to the well-studied American red- winged blackbirds (Agelaius phoeniceus). To experimen- tally test the signal function of the red epaulets in male red-shouldered widowbirds, we manipulated epaulet colour and size (within natural variation), and observed the effects in two competitive contexts. First, in captivity, unfamiliar males with epaulets of different size or redness were staged in dyadic contests over food. Only epaulet manipulations significantly affected contest outcome; red epaulet males out-competed orange and control males, which in turn dominated blackened (no epaulet) males. Epaulet size manipulations had particularly strong effects; enlarged epaulet males vigorously defeated smaller epaulet males within colour treatments, whereas the independent effect of 'redness' (i.e., within size treat- ments) was much weaker. Second, in a field experiment, epaulets were manipulated prior to territory establish- ment. Males with enlarged epaulets acquired territories, whereas most males with blackened or reduced signals failed to obtain territories. Furthermore, among the territorial males, birds with enlarged epaulets defended larger territories and spent less time in boundary signalling, yet they received fewer intrusions from conspecifics. In contrast, epaulet manipulations had no effect on either female visitation rates or subsequent female settlement. These results concur with recent studies and suggest that the carotenoid-based epaulets of red-shouldered widowbirds are status signals, sexually selected through male contest competition for territories.

Size of sexual swellings reflects ovarian function in chimpanzees ( Pan troglodytes )

Abstract: While many theories exist on the evolution of the sexual swelling signal in female primates, the significance of the very large and variable swellings found in some catarrhine primates is still debated. As suggested by Pagel [(1994) Anim Behav 47:1333–1341], these exaggerated and costly signals have the potential to provide information about differential female reproductive quality to males. While Domb [(2000) Department of Anthropology, Harvard University, Cambridge] and Domb and Pagel [(2001) Nature 410:204–206] propose that swellings serve to reliably indicate between-female fitness differences, we hypothesize that they may indicate current reproductive cycle quality, within or between females. This study examines the between-cycle relationship between ovarian function, gauged via fecal steroid analysis, and absolute size of sexual swellings, yielded through photogrammetric methods. Fourteen cycles from eight captive, group-living chimpanzees (Pan troglodytes) exhibited considerable variability both in ovarian steroid levels and maximum sexual swelling size. Nearly all measures of a female's maximal swelling in a cycle correlated with early follicular and early luteal estradiol averages and the duration of luteal progesterone elevation. Swelling area was also correlated with late follicular estradiol. Given past evidence that such measures of ovarian function predict conceptive success, these data strongly suggest that swellings could convey information to competing males about the fecundability of a potential mate.

Operational sex ratio and alternative reproductive behaviours in the European bitterling, Rhodeus sericeus

Abstract: We investigated the effects of male population density and male-biased operational sex ratio (OSR) with constant and limited resource density on male mating tactics shown by a freshwater fish, the European bitterling, Rhodeus sericeus. This species spawns inside living unionid mussels. Large males defended territories and were aggressive towards conspecifics under equal sex ratios. They also monopolised pair spawnings with females, releasing 98% of all sperm clouds during mating. However, the mating tactic changed at high male density where large males ceased to be territorial and instead competed with groups of smaller males to release sperm when females spawned. Large, medium and small males now obtained 61%, 33%, and 6% of sperm releases respectively, thereby reducing the opportunity for sexual selection by half. Females spawned at equal rates in the two densities of males, despite lower courtship at high density. These results run counter to the usual expectation that an increasingly male-biased OSR should lead to higher variance in male mating success. Instead, the use of alternative reproductive behaviours by males can lead to lower resource competition and mating variance at high male densities.

A model for leveling coalitions among primate males: toward a theory of egalitarianism

Abstract: We present a simple model of within-group leveling coalitions among male primates. The model assumes that the value of the coalition is the sum of the payoffs of its members, that the individual’s payoff is monotonically decreasing with its rank and that coalitions do not cause rank changes. It predicts that mainly mid- to low rankers engage in leveling coalitions, and that most coalition partners are of adjacent ranks. These predictions agree reasonably well with observations in nature. The model also makes the novel predictions that leveling coalitions are found where male mating competition has only a moderate contest component, and that male dominance ranks will become poorly differentiated where leveling coalitions are frequent. Both these predictions are consistent with observations on groups of macaques and baboons. The model also may account for leveling coalitions among egalitarian human foragers, without making additional assumptions about special human capabilities.

Countermarking by male pygmy lorises (Nycticebus pygmaeus): do females use odor cues to select mates with high competitive ability?

Abstract: Competitive countermarking occurs when animals compete to ensure that their scent marks are in the top-most position and more recently deposited than those of rivals. Because it takes a great deal of time and energy, and perhaps dominant or territorial status, to patrol the area and rapidly countermark rivals' marks, assessors may use the presence of countermarks as a reliable cue of the signaler's competitive ability. Selection on assessors, therefore, should favor the evolution of mechanisms to determine which scent is the countermark. Sexual selection theory predicts that females select high-quality males with whom to mate; therefore, females should mate preferentially with countermarking males. We examined the role of countermarking in intra-male competition and female mate choice in the pygmy loris. In experiment 1, we found that males deposit significantly more urine when countermarking other males' urine, but countermark female urine no more than control stimuli. In experiment 2, we exposed females to the urine of two males for several weeks. One male always deposited his urine first, whereas the second male always deposited his urine later and on top of the first male's urine. When reaching peak estrus, females were given a simultaneous choice test between the two males, and showed a significant preference for the countermarking male with regard to: (1) her location and orientation to the male, (2) her chemosensory interest in the male and his cage, and (3) her affiliative socio-sexual behaviors. These results are consistent with predictions from the competitive countermarking hypothesis, and demonstrate that chemosignals play a profound role in governing female reproductive behavior in the species.

To breed or not to breed—food competition and other factors involved in female breeding decisions in the pair-living nocturnal fork-marked lemur ( Phaner furcifer )

Abstract: Differential access to food resources is thought to be the main determinant of differences in female reproductive success but is poorly studied in both pair-living and nocturnal primates. The modes of food competition within and between families were investigated following the principles proposed by the “ecological model” using 3 years of field data from seven fork-marked lemur (Phaner furcifer) families. The major food resources were identified from year-round feeding observations and the strength and mode of competition were inferred from differences in physical condition. The most important food resource of fork-marked lemurs were tree exudates which occurred in small, defendable food patches, characterized by fast depletion and rapid renewal. These characteristics led to strong within-group contest and scramble competition, which were found to yield a positive dominance effect and a negative group-size effect on female net energy gain. Differential physical condition, however, did not translate directly into differential reproductive success. Low female fertility was best predicted by large family size associated with delayed dispersal by previous offspring. Although there is no obvious benefit from full-grown offspring in their territory, adults tolerate delayed natal dispersal, probably because dispersal poses extraordinary costs for the offspring. These costs are likely to accrue from decreased foraging efficiency in unfamiliar habitats because exudate feeding requires very rigid feeding itineraries. In conclusion, the presented evidence for group-size effects on reproductive success in pair-living females opens a new area for research on the costs and benefits of delayed dispersal and female reproductive decisions.

Variation in nesting patterns affecting nest temperatures in two populations of painted turtles (Chrysemys picta) with temperature-dependent sex determination

Abstract: Mechanisms maintaining sex ratios in populations with temperature-dependent sex determination (TSD) remain elusive Although geographic variation in embryonic sex determination (ie, pivotal temperature) has been widely investigated in reptiles exhibiting TSD, no previous studies have directly addressed geographic variation in maternal behavior affecting nest thermal conditions I evaluated patterns of nest-site selection and its effects on thermal and hydric nest conditions for a population of painted turtles (Chrysemys picta bellii) exhibiting TSD in New Mexico These results are compared to data collected from a well-studied, conspecific population experiencing relatively cooler climatic conditions in Illinois Since canopy vegetation cover reduces nest temperatures in Illinois, I expected females in New Mexico to nest under high amounts of canopy vegetation cover However, females from New Mexico placed nests under significantly less canopy vegetation cover, but closer to standing water, than did females from Illinois Experimental nests in New Mexico demonstrated that increased canopy vegetation cover and soil moisture reduced nest temperatures By nesting close to standing water rather than under canopy vegetation cover, females in New Mexico nested in habitats more closely associated with maximizing moisture around nests rather than reducing nest temperatures through shading Mean July nest temperatures were similar for both populations Since nest hydric conditions affect hatching success and hatchling size in C picta, nesting patterns in New Mexico may primarily reflect selection for microhabitats affecting offspring survivorship or size

The role of sexual harassment in cave and surface dwelling populations of the Atlantic molly, Poecilia mexicana (Poeciliidae, Teleostei)

Abstract: Sexual coercion is a common behaviour for males trying to compensate for being inferior in male competition and/or female choice. We measured the cost of male sexual harassment for females as reduced feeding time in three populations of the Atlantic molly (Poecilia mexicana). Of these populations, one originated from a typical river habitat, another from a lightless cave chamber and the third one from a small, milky creek outside the cave. We gave hungry females an opportunity to feed in the presence of either another female or a male. We found the following: (1) male sexual harassment represents a cost in the river dwelling population and both small and large males significantly reduce female feeding time; and (2) sexual harassment was not detected in the other two populations (cave and cave entrance). In the cave molly, small males are at a disadvantage in female choice, but predation by an aquatic heteropteran selects against large male body size.

Differences in olfactory sensitivity and behavioral responses among honey bees bred for hygienic behavior

Abstract: Honey bees, Apis mellifera L., bred for hygienic behavior uncap and remove diseased and mite-infested brood. We hypothesized that within a colony bred for hygienic behavior, there would be differences in olfactory sensitivity among bees of the same age. We predicted that bees that initiate the behavior by perforating and uncapping brood would have greater olfactory sensitivity to the odor of the diseased brood, and would be better able to discriminate between odors of healthy and diseased brood, compared to bees that complete the behavior by removing the uncapped brood from the cells. Electroantennogram recordings of 15- to 21-day-old bees from three colonies demonstrated that bees collected while uncapping dead brood had significantly greater olfactory sensitivity to all concentrations of diseased brood odor compared to bees collected while removing brood. Proboscis-extension reflex discrimination conditioning demonstrated that 15- to 21-day-old bees collected while uncapping discriminated significantly better and generalized significantly less between the odors of diseased and healthy brood compared to bees collected while removing, when the odor of diseased brood was rewarded, but not when the odor of healthy brood was rewarded. Bees collected while uncapping brood that had been pierced with a pin had significantly less olfactory sensitivity than bees collected while uncapping freeze-killed brood, most likely because the pierced brood had greater stimulus intensity. Initiation of hygienic behavior depends on the olfactory sensitivity of the bee and stimulus intensity of the abnormal brood. Differential olfactory sensitivity and responsiveness among hygienic bees could lead to the apparent partitioning of the behavior into uncapping and removing components.

Responses of female lizards, Lacerta monticola, to males' chemical cues reflect their mating preference for older males

Abstract: Female mate choice based on visual traits appears to be rare in lizards. Field observations suggest that females of the lizard Lacerta monticola preferred to mate with larger/older males. Although older males are usually green and larger, and younger males brown and smaller, there is some overlap in size and coloration between age classes. Thus, visual cues may not always be reliable indicators of a male's age. We hypothesized that female mate-choice preferences may be based on males' pheromones, which might transmit information about characteristics such as age. In a laboratory experiment, we analyzed the effect of age of males on attractiveness of their scents to females. When we offered scents of two males of different age, females associated preferentially with scents of older males. This suggested that females were able to assess the age of males by chemical signals alone, and that females preferred to be in areas scent-marked by older males. Thus, females may increase their opportunities to mate with males of high quality, or may avoid harassment by sneaking young males. This result agreed with field observations on females mating with old males, and rejection of advances by young males. Our results also suggested that female preference for older males may depend on their own body size. Large females showed a strong preference for older males, whereas smaller females were not so selective. This, together with males' preference for large females, might lead to size-assortative matings. We suggest that the quality and/or quantity of male pheromones could communicate to the female heritable male genetic quality (i.e. age) and thereby serve as the basis of adaptive female choice in lizards.

Bite rates in Rocky Mountain bighorn sheep ( Ovis canadensis ): effects of season, age, sex and reproductive status

Abstract: We investigated the effects of vegetation biomass, crude protein content of consumed forage, age, sex and reproductive status on bite rates in Rocky Mountain bighorn sheep. We expected higher bite rates and vigilance in lactating females with young and higher bite rates in young growing individuals, than in non-reproducing females or rams. Lactating ewes had higher bite rates than yeld ewes and than subadult or adult rams. Subadult rams had higher bite rates than adult rams or yeld ewes. On recently burned grassland in spring, however, rams had a higher bite rate than adult females, while the contrary was true on control plots and on the burned plots in autumn. Bite rates declined for both ewes and rams from April to September and varied from year to year. While rams of different ages had significantly different bite rates, there was no effect of age on bite rates for ewes. There was no correlation between bite rates and available total biomass or biomass of live vegetation, or the numbers of steps taken while foraging for either ewes or rams. Adult rams had a lower vigilance rate than adult ewes, and vigilance decreased with increasing bite rates for all sheep. Bite rates in bighorn sheep vary greatly according to age, season and vegetation structure. An increase in bite rates during the forage growing season may compensate for higher energy demands during lactation and growth. There is a potential trade-off between foraging and vigilance as vigilance decreased with increasing bite rates.

Fitness consequences of sexual cannibalism in female Argiope bruennichi

Abstract: The evolution of sexual cannibalism as the most extreme form of nuptial feeding is still poorly understood. Although increasing evidence suggests that female aggressiveness is related to other aspects of foraging behaviour, it is not clear whether the nutritional value of a male is sufficient to provide an adaptive significance for sexual cannibalism. A widely cited though rarely tested explanation is based on a paternal investment model, and predicts that consumption of a male results in increased female fecundity. The available evidence is either correlational or restricted to species with relatively large and potentially nutritious males, and different studies have come to different conclusions. Here we present a test of the paternal investment hypothesis using the very cannibalistic and highly size-dimorphic spider Argiope bruennichi. After a preset schedule, we had females consume none, one or two males independent of the female's cannibalistic behaviour. Consumption of male bodies did not result in any detectable fitness benefit for the female: neither the number of clutches, nor clutch size or hatching success were affected by consumption of males. The frequency of cannibalism was around 80%, independent of the female mating status. We did not observe male complicity, but cannibalism was associated with prolonged copulation. This suggests a sexually selected benefit of cannibalism for males. We conclude that the paternal investment hypothesis does not explain the existence of sexual cannibalism in A. bruennichi and probably not in other spider species with a pronounced sexual size dimorphism.

Correlates of horn and antler shape in bovids and cervids

Abstract: Using analyses that control for phylogeny, we examine whether the shapes of horns and antlers in ungulates are related to style of fighting, environmental factors, mating and social systems, or are simply arbitrary. Many of the predictions that relate horn shape to species' fighting tactics and to their mating and social systems are supported. Bovids with tips facing inwards are likely to wrestle with their horns, be monogamous and solitary, whereas those with tips facing out tend to be polygynous and live in large groups. Smooth horns are used for stabbing, and are found in females of polygynous species living in large groups. In female bovids, twisted horns are found in large species and are used in wrestling whereas, in male bovids, straight horns are found in solitary species. Finally, deer with more than five antler tines tend to be large and to fight by fencing. There was little support for shape of horns and antlers being related to environmental factors, nor were shapes arbitrary as might be expected if they had arisen through female choice. Body size had little effect on these results. In general, monogamous solitary bovids have straight inwardly facing horns whereas polygynous group-living species demonstrate a wide variety of horn shapes.

Carotenoid-derived ornaments reflect parental quality in male and female yellow-eyed penguins (Megadyptes antipodes)

Abstract: The handicap principle suggests that ornamental traits that function as honest signals in mate selection must be costly to be effective. We evaluated in the sexually monochromatic yellow-eyed penguin (Megadyptes antipodes) whether the carotenoid-derived plumage and eye coloration predicts parental quality and whether males and females within pairs mate assortatively in relation to these carotenoid-derived ornaments. In addition, we investigated whether age or body condition was related to the coloration of the ornamental traits. In yellow-eyed penguins, parental quality of males and females was predicted by eye and head plumage coloration. Even when we controlled for gender- and age-specific differences, eye and head plumage coloration reflected honestly parental quality. Males and females mated assortatively in relation to these ornamental traits. While age influenced coloration of both the eye and head plumage, body condition was related only to the saturation of plumage coloration. These results provide evidence that the carotenoid-derived ornaments in yellow-eyed penguins reflect the parental abilities of birds and, therefore, may be costly signals. Potentially, female and male yellow-eyed penguins could use eye and plumage coloration as an indirect cue in assessing age and quality of individual birds during mate choice. This is only the second study to examine plumage coloration in relation to sexual selection in penguins, while conspicuous ornamental traits in other species of penguin beg the question whether they also play a role in sexual selection.