Male Life History Research Articles

Increased performance of offspring sired by parasitic males in bluegill sunfish

Males of many species are characterized by alternative mating tactics. In bluegill sunfish (Lepomis macrochirus), some males delay maturation and become ‘‘parentals’’ while other males mature precociously and become ‘‘cuckolders.’’ Parentals use an overt, territorial mating tactic, defending a nest and courting females. Cuckolders instead use a sneaking tactic to parasitize parentals. It has previously been shown that parentals that are heavily cuckolded provide less care to their young, yet females do not appear to discriminate against cuckolders, and they may actually release more eggs when a cuckolder is present than when spawning only with a parental. Here I examined growth rate of fry of known paternity through the yolk-sac stage of development using complementary laboratory and field studies to assess a potential indirect benefit for females that mate with cuckolders. Comparison of maternal half-siblings sired in vitro shows that cuckolder offspring grow faster and to a larger size than parental offspring while feeding endogenously on their yolk sac. Because both food resource and maternal genes are equivalent across treatments, these data indicate a genetic difference in growth between the two male life histories. In the field, fry from nests that have proportionately more cuckolder offspring are larger when they emerge from the nest. This increased size can lead to threefold higher survivorship for cuckolder offspring than parental offspring from Hydra canadensis predation, a major predator of bluegill fry. These results are discussed in the context of mate choice for direct and indirect benefits and in the context of the evolution of alternative mating tactics. Key words: bluegill sunfish, good genes, growth rate, indirect benefits, Lepomis macrochirus, mate choice, mating systems. [Behav Ecol 15:327–331 (2004)]

Male life history and the unusual adult sex ratios of redfronted lemur, Eulemur fulvus rufus, groups

In group-living species, theoretical considerations indicate the existence of a fundamental conflict of interest between the sexes over the adult sex ratio within groups. Females may derive certain benefits from living with many males. Males, in contrast, should generally try to monopolize access to a group of females. Which sex ultimately controls adult group sex ratio is poorly known. We examined this conflict between the sexes in redfronted lemurs, Malagasy primates characterized by an unusual lack of female-biased adult sex ratios. Using various demographic and behavioural data from several groups collected over 6 years, we examined (1) the proximate determinants of this unusual sex ratio, (2) the temporal distribution of female fertile phases within groups as a determinant of male monopolization potential, (3) sources of between-group variation in the adult sex ratio, and (4) possible social benefits of the relatively high number of males for both sexes. Birth and mortality rates were not sex biased and males migrated considerably more frequently than females, providing no proximate explanation for the unusual sex ratio. However, certain life history traits (fast maturation, short interbirth intervals) may ultimately play a role because they act to facilitate joint group transfers of male coalitions. Despite a relatively small female group size and an associated high monopolization potential, female oestrous synchrony may prevent the formation of single-male groups. Reduced male group size seems to be the main predictor of take-over rate, and, thus, infanticide risk, suggesting that both sexes may benefit from the high number of coresident males, thereby considerably defusing the conflict of interest between the sexes.

Body condition and physiological changes in male green turtles during breeding

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 276:281-288 (2004) - doi:10.3354/meps276281 Body condition and physiological changes in male green turtles during breeding Tim S. Jessop1,*, Mark Hamann2, Colin J. Limpus3 1Center for Reproduction of Endangered Species, Zoological Society of San Diego, San Diego, California 92112, USA 2Queensland Parks and Wildlife Service, PO Box 1735, Bundaberg, Queensland 4670, Australia 3Department of Environment, PO Box 155, Brisbane, Queensland 4002, Australia *Email: tjessop@sandiegozoo.org ABSTRACT: Investigations were made into the body condition, energy metabolite and endocrinal changes of male green turtles Chelonia mydas prior to and during their vernal breeding period in the southern Great Barrier Reef. Prior to migration, breeding males exhibited a higher body condition index than non-breeding males. However, during the migratory reproductive period, breeding males lost significant body condition. Concurrent with these physical changes, breeding males showed a reduction in plasma triglycerides and an increased level of total protein towards the mid- to late breeding period. The plasma steroids corticosterone and testosterone increased and decreased, respectively, during the migratory/breeding phase. The pattern of change in body condition and physiology allude to a high-activity fasting period during the migratory/breeding phase of the male green turtle¹s life history. These marked physiological events during breeding suggest a proximate basis for terminating seasonal reproductive events, and a potential basis for phenotypic variation in male reproductive tactics. KEY WORDS: Chelonia mydas · Male green turtle · Reproduction · Energy requirements · Physiological changes Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 276. Online publication date: August 02, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Worldwide Variation in Life-Span Sexual Dimorphism and Sex-Specific Environmental Mortality Rates

In all human populations mean life span of women generally exceeds that of men, but the extent of this sexual dimorphism varies across different regions of the world. Our purpose here is to study, using global demographic and environmental data, the general tendency of this variation and local deviations from it. We used data on male and female life history traits and environmental conditions for 227 countries and autonomous territories; for each country or territory the life-span dimorphism was defined as the difference between mean life spans of women and men. The general tendency is an increase of life-span dimorphism with increasing average male-female life span; this tendency can be explained using a demographic model based on the Makeham-Gompertz equation. Roughly, the life-span dimorphism increases with the average life span because of an increase in the duration of expressing sex- and age-dependent mortality described by the second (exponential) term of the Makeham-Gompertz equation. Thus we investigated the differences in male and female environmental mortality described by the first term of the Makeham-Gompertz equation fitted to the data. The general pattern that resulted was an increase in male mortality at the highest and lowest latitudes. One plausible explanation is that specific factors tied to extreme latitudes influence males more strongly than females. In particular, alcohol consumption increases with increasing latitude and, on the contrary, infection pressures increase with decreasing latitude. This finding agrees with other observations, such as an increase in male mortality excess in Europe and Christian countries and an increase in female mortality excess in Asia and Muslim countries. An increase in the excess of female mortality may also be due to increased maternal mortality caused by an increase in fertility. However, this relation is not linear: In regions with the highest fertility (e.g., in Africa) the excess of female mortality is smaller than in regions with relatively lower fertility (e.g., in Asia). A possible explanation of this phenomenon is an evolutionary adaptation of women to the pressures of extremely high fertility by means of some reduction of their maternal mortality.

Effects of age, density and sex ratio on reproductive effort in male reindeer (Rangifer tarandus)

AbstractIn sexually dimorphic ungulates, male reproductive success depends on fighting with other males for access to females during a brief rutting season. Large body size is necessary for success in intrasexual competition, and a few large‐sized males are often able to monopolize access to female groups. Earlier studies have reported that reproductive effort increases with age until prime‐age is reached, and one study that population density lowered effort in (older) males. No study has directly assessed whether there is within‐age‐class variation in effort resulting from varying levels of intra‐male competition. It is reported here the weight loss during the rutting season of 54 individual male reindeer Rangifer tarandus coming from eight herds with varying density (3.3–6.0 deer/km2) and sex ratio (4–28% males). In agreement with earlier studies, reproductive effort was lower for young (1‐ to 2‐year‐old) than for prime‐aged (3‐ to 5‐year‐old) males both on an absolute and relative scale. Among 1‐year‐old males (n=33), effort was lower as sex ratio became closer to even, but density during the rutting season had no effect. This suggests that yearling males take a more active role when prime‐aged males are absent. In addition to the insight into male ungulate life history, understanding male rutting behaviour may also have implications for population dynamics.

High sexual signalling rates of young individuals predict extended life span in male Mediterranean fruit flies.

In a laboratory study, we monitored the lifetime sexual signalling (advertisement) of wild male Mediterranean fruit flies, and we tested the hypothesis that high lifetime intensity of sexual signalling indicates high survival probabilities. Almost all males exhibited signalling and individual signalling rates were highly variable from the beginning of the adults' maturity and throughout their life span (average life span 62.3 days). Sexual signalling rates after day 10 (peak maturity) were consistently high until about 1 week before death. There was a positive relationship between daily signalling rates and life span, and an increase in signalling level by one unit over all times was associated with an approximately 50% decrease in mortality rate. Signalling rates early in adult life (day 6-20) were higher in the longest-lived than in the shortest-lived flies. These results support the hypothesis that intense sexual signalling indicates longer life span. We discuss the importance of age-specific behavioural studies for understanding the evolution of male life histories.

Teenage contraception and pregnancy from the perspective of young low-income fathers in a slum area in Rio de Janeiro

The study focuses on how teenage fatherhood affects the life histories of young low-income males. Field work included individual interviews of young fathers living in a shantytown in Rio de Janeiro city. The argument is that an approach from the male standpoint affords access to other levels in a web of meanings and relations that refer back to the logic of contraception, the formation of couples, and active fatherhood. The argument also highlights the links between type of relationship and partnership, use and non-use of contraceptive methods, and the meanings of fatherhood for young low-income males. It highlights that entering actively into fatherhood helps boys consolidate their image as "mature", "responsible", "adult" men.

Costs of Mate Guarding and Opportunistic Mating Among Wild Male Japanese Macaques

A trade-off relationship between mating and feeding effort is important when considering reproductive strategies of long-lived species. I compared the influence of male sexual activities, female mate-choice behaviors and the daily activity budget on male mating success among males in a group of wild Japanese macaques (Macaca fuscata yakui) on Yakushima Island. The 1st-ranking male, which had immigrated into the troop at this rank, more frequently approached peri-ovulatory females, spent more time grooming peri-ovulatory females and in mounting series and spent less time feeding than subordinate males did. The 1st-ranking male attained the highest mating success as a result of his high expenditure of time and energy in sexual behaviors directed toward peri-ovulatory females. Mating success of subordinate males did not relate to the amount of sexual effort, but instead to the frequency of female approaches, female rush toward males and the number of peri-ovulatory females within the group. The pattern of intermale competition shifted from nearly contest competition to scramble competition as the number of peri-ovulatory females in the group increased. Feeding time of subordinate males did not vary between the days when they copulated and the days when they did not. The findings demonstrate that mate guarding in the 1st-ranking male is a high-cost mating tactic, while opportunistic mating in subordinate males is a low-cost mating tactic. The differences in male mating tactics are probably related to male life history and to the formation of groups with a high socionomic sex ratio.

Heritability and Y-chromosome influence in the jack male life history of chinook salmon (Oncorhynchus tshawytscha).

Jacking in chinook salmon (Oncorhynchus tshawytscha) is an alternative reproductive strategy in which males sexually mature at least 1 year before other members of their year class. We characterize the genetic component of this reproductive strategy using two approaches; hormonal phenotypic sex manipulation, and a half-sib breeding experiment. We 'masculinized' chinook salmon larvae with testosterone, reared them to first maturation, identified jacks and immature males based on phenotype, and genotyped all fish as male ('XY') or female ('XX') using PCR-based Y-chromosome markers. The XY males had a much higher incidence of jacking than the XX males (30.8% vs 9.9%). There was no difference in body weight, gonad weight, and plasma concentrations of testosterone and 17beta-estradiol between the two jack genotypes, although XY jacks did have a higher gonadosomatic index (GSI) than XX jacks. In the second experiment, we bred chinook salmon in two modified half-sib mating designs, and scored the number of jacks and immature fish at first maturation. Heritability of jacking was estimated using two ANOVA models: dams nested within sires, and sires nested within dams with one-half of the half-sib families common to the two models. The sire component of the additive genetic variance yielded a high heritability estimate and was significantly higher than the dam component (h(2)(sire) = 0.62 +/- 0.21; h(2)(dam) = -0.14 +/- 0.12). Our experiments both indicated a strong sex-linked component (Y-chromosome) to jacking in chinook salmon, although evidence for at least some autosomal contribution was also observed.

Life in the slow lane? Demography and life histories of male and female sifaka (Propithecus verreauxi verreauxi)

AbstractA 16‐year study of wild, unprovisioned sifaka Propithecus verreauxi verreauxi at Beza Mahafaly in south‐west Madagascar provides estimates of age‐specific fertility, mortality, and dispersal in a population of 426 marked animals, and longitudinal records of individual life histories. Sifaka females give birth for the first time later and live longer, for their size, than mammals in other orders; they also give birth later and continue reproducing longer, for their size, than other primates. Theory postulates that these features, commonly referred to as bet‐hedging, evolve in unpredictable environments in association with widely varying infant survival and a trade‐off between reproductive effort and adult survival. The climate of south‐west Madagascar is highly unpredictable compared to almost all other regions in the tropics with similar average rainfall, and we argue that sifaka females are bet‐hedgers par excellence. Male sifaka, in contrast, become reproductively active at an earlier age than females, and are less likely to have long lives than females. The atypical direction of this asymmetry between males and females reflects a ‘slowing down’ of female life histories rather than a ‘speeding up’ of male life histories. Two other unusual features of sifaka biology and behaviour may be linked to the unpredictability of Madagascar's climate: intense local competition between females, and a sex ratio at birth strongly biased in favour of males in most years. In drought years, reproductive females must cope with suddenly intensified resource constraints. This, in turn, may strongly limit the number of ‘breeding slots’ available over the long‐term for females.

Primate males: causes and consequences of variation in group composition

Preface Part I. Introduction: 1. The socioecology of primate males: history and theory Peter M. Kappeler Part II. Comparative Perspectives on Male-Female Association: 2. Multi-male breeding groups in birds: ecological causes and social conflicts Nick B. Davies 3. Males in macropod society Peter Jarman 4. Social counterstrategies against infanticide by males in primates and other mammals Carel van Schaik Part III. Variation in Male Numbers: Taxon-Level Analyses: 5. Causes and consequences of unusual sex ratios in lemurs Peter M. Kappeler 6. The number of adult males in callitrichine groups and its implications for callitrichine social evolution Eckhard W. Heymann 7. From binding brotherhoods to short-term sovereignty: the betwixting dilemma of male Cebidae Karen B. Strier 8. The number of males in guenon groups Marina Cords 9. Socioecology of baboons: the interaction of male and female strategies Robert A. Barton 10. Variation in adult sex ratios of red colobus monkey social groups: implications for interspecific comparisons Thomas T. Struhsaker 11. The number of males in langur groups: monopolizability of females or demographic process? Elisabeth H. M. Sterck and Jan A. R. A. M. van Hooff 12. Costs and benefits of the one-male, age-graded and all-male phase in wild Thomas langur groups Romy Steenbeek, Elisabeth H. M. Sterck, Han de Vries and Jan A. R. A. M. van Hooff 13. Male dispersal and mating season influxes in Hanuman langurs living in multi-male groups Carola Borries 14. Rethinking monogamy: the gibbon case Volker Sommer and Ulrich Reichard 15. Causes and consequences of variation in male mountain gorilla life histories and group membership David P. Watts Part IV. Behavioural Aspects of Male Coexistence: 16. Relationships among nonhuman primate males: a deductive framework Jan A. R. A. M. van Hooff 17. Collective benefits, free-riders and male extragroup conflict Charles L. Nunn 18. Dominance, egalitarianism and stalemate: an experimental approach to male-male competition in Barbary macaques Signe Preuschoft and Andreas Paul Part V. Evolutionary Determinants and Consequences: 19. The evolution of male philopatry in neotropical monkeys Theresa Pope 20. Models of outcome and process: predicting the number of males in primate groups Jeanne Altmann 21. Why are male chimpanzees more gregarious than mothers? A scramble competition hypothesis Richard W. Wrangham 22. Male mating strategies: a modeling approach Robin I. M. Dunbar Part VI. Conclusions: 23. Understanding male primates Michael E. Pereira, Timothy H. Clutton-Brock and Peter M. Kappeler References Index.

Birth season testosterone levels in male Verreaux's sifaka, Propithecus verreauxi : insights into socio-demographic factors mediating seasonal testicular function

Previous research suggests that although reproduction and testicular function in wild sifaka (Propithecus verreauxi) are highly seasonal, birth season elevations in fecal testosterone (T) in transferring males indicate that social factors may be as important as climatic factors in regulating reproductive function in sifaka. This paper examines the relationship of male dispersal and social status to the patterning of birth season aggression and testicular function in P. verreauxi at Beza Mahafaly Special Reserve, Madagascar. Behavioral and hormonal data were collected from a total of 38 adult males, 15 residing in seven stable groups and 23 living in eight unstable groups, yielding 186 fecal samples and 493 focal animal hours of observation. The results suggest that birth season elevations in fecal testosterone are the consequence of social disruption resulting from male movements between groups and the particular responses of individual males to dispersal events. Hormonal responses to dispersal events appear to precede behavioral responses and occasionally register reactions not concomitantly evident in the behavioral response, and may be predictive of future events. Hormonal reactions occurred primarily in resident alpha males, suggesting that they differ in their assessment of destabilizing influences, perhaps due to different reproductive opportunities and/or investment. These findings are important for the new insights they provide into the role of androgens in mediating male dispersal, life history, and reproductive strategies, and suggest that investigations of androgen-behavior interactions in free-ranging populations can be a powerful new tool for assessing the contextual and motivational basis of social behavior.

Reproductive ecology and life history of the human male.

Until recently, the reproductive ecology of human males has not been extensively investigated, primarily as a result of the need for a theoretical framework based on the reproductive constraints and energetics of mammalian males. More specifically, male reproductive ecology has necessitated an integrative interpretation of clinical and anthropological data based on the premise that the evolution of human male life histories has involved selection for physiological mechanisms aimed at optimizing trade-offs between survivorship and reproductive effort. This paper attempts to address this gap in our understanding by presenting the current state of male reproductive ecology, including physiologic data from clinical and anthropological investigations as well as recent theoretical developments. Recent investigations outlining population variation in reproductive endocrine function are discussed within the context of potential sources of variation, including energetic expediture, caloric intake, and developmental canalization during adolescence. Additional summaries of male senescence and behavior are presented to provide a complete overview of male life history. Implications of recent anthropological data on contemporary health issues such as prostate cancer and the development of a male contraceptive are also discussed. Finally, several theories are presented that may contribute to our understanding of the evolution of male life histories and reproductive ecology, including theoretical suggestions involving the role of competition, mate choosiness, and potential constraints on male insemination ability, as well as a theory suggesting that male reproductive ecology may be best understood by analyzing energetic allocation decisions between differing somatic tissues that may be indicative of the competing needs for optimizing survivorship and reproductive effort. Directions for future research are finally considered.

Reproductive ecology and life history of the human male

Until recently, the reproductive ecology of human males has not been extensively investigated, primarily as a result of the need for a theoretical framework based on the reproductive constraints and energetics of mammalian males. More specifically, male reproductive ecology has necessitated an integrative interpretation of clinical and anthropological data based on the premise that the evolution of human male life histories has involved selection for physiological mechanisms aimed at optimizing trade-offs between survivorship and reproductive effort. This paper attempts to address this gap in our understanding by presenting the current state of male reproductive ecology, including physiologic data from clinical and anthropological investigations as well as recent theoretical developments. Recent investigations outlining population variation in reproductive endocrine function are discussed within the context of potential sources of variation, including energetic expediture, caloric intake, and developmental canalization during adolescence. Additional summaries of male senescence and behavior are presented to provide a complete overview of male life history. Implications of recent anthropological data on contemporary health issues such as prostate cancer and the development of a male contraceptive are also discussed. Finally, several theories are presented that may contribute to our understanding of the evolution of male life histories and reproductive ecology, including theoretical suggestions involving the role of competition, mate choosiness, and potential constraints on male insemination ability, as well as a theory suggesting that male reproductive ecology may be best understood by analyzing energetic allocation decisions between differing somatic tissues that may be indicative of the competing needs for optimizing survivorship and reproductive effort. Directions for future research are finally considered.

Changes in expression and honesty of sexual signalling over the reproductive lifetime of sticklebacks.

Fitness costs of signalling are essential in order for reliable sexual signalling to prevail when the interests of the sexes conflict. This means that signalling can be subjected to a life history trade-off between present and future signalling effort. Here, I show that three-spined stickleback males (Gasterosteus aculeatus), who have a single breeding season during which they breed repeatedly, change their red nuptial coloration over the season depending on their body size at the start of breeding. Large males that completed several breeding cycles increased their red coloration over the season, whereas small males, who completed only a few cycles, did not. The increase in coloration was accompanied by an increase in parental success when males were energy constrained, but not when they had access to an unlimited food supply. Red coloration was thus an honest signal of male parental ability despite changes in signal expression when both signalling and parental care were costly and the investments in them changed simultaneously over the reproductive lifetime. However, the honesty of the signal varied over a lifetime. At the penultimate cycle, bright males cannibalized some of their eggs, probably to increase survival to the last cycle, whereas males cared for their offspring independent of coloration at the ultimate cycle.

SEXUAL DIMORPHISM WITH FEMALE DEMOGRAPHIC DOMINANCE: AGE, SIZE, AND SEX RATIO AT MATURATION

Under the influence of natural and sexual selection, males and females will often differ in the costs and benefits of achieving some particular body size; we might therefore expect to find them growing at different rates for different times, and dying on different schedules. The analysis of sexual dimorphism presented here assumes that life histories are optimized over three key variables: activity level (or “effort”) during the juvenile stage, duration of the juvenile stage, and size at maturation. Reproduction is taken to be semelparous (or monocarpic), reproductive success is size dependent, and females are demographically dominant. Juvenile male and female life histories are assumed to be “structurally identical” (i.e., governed by identical functions of activity, size, and development time). Under these conditions, sexually dimorphic life histories readily arise when features of juvenile behavior and development are adjusted to maximize r in response to gender-specific selection pressures on adults. ...

Sexually transmitted diseases in polygynous mating systems: prevalence and impact on reproductive success.

Studies of disease in relation to animal mating systems have focused on sexual selection and the evolution of sexual reproduction. Relatively little work has examined other aspects of ecological and evolutionary relationships between host social and sexual behaviour, and dynamics and prevalence of infectious diseases; this is particularly evident with respect to sexually transmitted diseases (STDs). Here, we use a simulation approach to investigate rates of STD spread in host mating systems ranging from permanent monogamy to serial polygyny or polyandry and complete promiscuity. The model assumes that one sex (female) is differentially attracted to the other, such that groups of varying size are formed within which mating and disease transmission occur. The results show that equilibrium disease levels are generally higher in females than males and are a function of variance in male mating success and the likelihood of a female switching groups between mating seasons. Moreover, initial rates of disease spread (determining whether an STD establishes in a population) depend on patterns of host movement between groups, variance in male mating success and host life history (e.g. mortality rates). Male reproductive success can be reduced substantially by a sterilizing STD and this reduction is greater in males that are more 'attractive' to females. In contrast, females that associate with more attractive males have lower absolute fitness than females associating with less attractive males. Thus, the potential for STDs to act as a constraint on directional selection processes leading to polygyny (or polyandry) is likely to depend on the details of mate choice and group dynamics.

Sex-ratio optimization with helpers at the nest

In many cooperatively breeding animals, offspring produced earlier in life assist their parents in raising subsequent broods. Such helping behaviour is often confined to offspring of one sex. Sex-allocation theory predicts that parents overproduce offspring of the helping sex, but the expected degree of sex-ratio bias was thought to depend on specific details of female and male life histories, hampering empirical tests of the theory. Here we demonstrate the following two theories. (i) If all parents produce the same sex ratio, the evolutionarily stable sex ratio obeys a very simple rule that is valid for a general class of life histories. The rule predicts that the expected sex-ratio bias depends on the product of only two parameters which are relatively easily measured: the average number of helping offspring per nest and the relative contribution to offspring production per helper. (ii) If the benefit of helping varies between parents, and parents facultatively adjust the sex ratio accordingly, then the population sex ratio is not necessarily biased towards the helping sex. For example, in line with empirical evidence, if helpers are produced under favourable conditions and parents do not adjust their clutch size to the number of helpers, then a surplus of the non-helping sex is expected.

Sources of Variation in Physiological Phenotypes and Their Evolutionary Significance

SYNOPSIS. We offer the thesis that environmental physiologists and evolutionary biologists can find fertile common ground in the study of how individual variation in physiological phenotypes originates and develops. The sources of such individual variation are often complex; the consequences affect how natural selection will act on a suite of traits, of which some may seem, at first glance, far removed from the usual domain of environmental physiology. We illustrate our thesis in two ways. First, we offer two examples drawn from studies of thermal tolerance in the poeciliid fish Heterandria formosa. We show how fitness variation can be a complex function of the gestational temperature and thermal tolerance and how these effects can produce environmentally induced variation among populations in thermal tolerance that mimics a pattern of adaptive variation. Second, we review two case studies that illuminate how environmental effects on a multivariate phenotype can channel the action of natural selection. The phenotypic plasticity of male life history in Poecilia latipinna in response to temperature embraces a spectrum of traits; the effects of each one upon fitness will influence the ability of selection to mold the response of any one of them to temperature. The phenotypic covariances in thermal tolerance and life-history traits in Heterandria formosa differ slightly between populations from different parts of the species range, apparently because of differences between them in thermal sensitivity; this difference insures that the multivariate nature of selection will be correspondingly different in those different populations.

Primate socioecology: new insights from males.

Primate males have only recently returned to the center stage of socioecological research. This review surveys new studies that examine variation in the behavior of adult males and their role in social evolution. It is shown that group size, composition, and social behavior are determined not only by resource distribution, predation risk, and other ecological factors, but that life history traits and social factors, especially those related to sexual coercion, can have equally profound consequences for social systems. This general point is illustrated by examining male behavior at three levels: the evolution of permanent associations between males and females, the causes and consequences of variation in the number of males between group-living species, and the determinants of social relationships within and between the sexes. Direct and indirect evidence reviewed in connection with all three questions indicates that the risk of infanticide has been a pervasive force in primate social evolution. Several areas are identified for future research on male life histories that should contribute to a better understanding of male reproductive strategies and corresponding female counter-strategies.