Readers are referred to Andersson (1994), Trivers (1972), Otte (1979), Pizzari and Bonduriansky (2010), Westneat and Fox (2010, Sect. V), Davies et al. (2012), and Darwin (2004) for comprehensive reading on sexual selection theory, including extensions of it and recent advances in that field. In brief, Darwin (2004) recognized that some animal characteristics appeared to compromise rather than promote survival, thereby appearing to function counter to natural selection. Reasoning that traits deleterious to survival might be favored if sufficient benefits accrued to reproduction, Darwin proposed two mechanisms of “sexual selection,” “intrasexual” and “intersexual” selection , driven by same-sex and between-sex competition, respectively (Darwin 2004; this brief, Sect. 3.3). West-Eberhard (1979; see Crook 1972) noted that both mechanisms entail intersexual competition for mates. Darwin (2004) was impressed that exaggerated structures (e.g., horns, antlers, colorful features) employed as sexual signals and displays were likely to expose types to predation , and these same, and other traits, may be costly to survival by increasing a type’s vulnerability to parasitism, including “social parasitism” (“natural enemies”; Chesson 2000; this brief, Sect. 8.4), and increased intra- and interspecific competition.

The evolution of mating systems by sexual selection is well established (Crook 1972; Emlen and Oring 1977; Clutton-Brock and Harvey 1978; Clutton-Brock 1989; Davies et al. 2012), but intra- and intersexual competition as drivers of social evolution have received less focus in the mammalian literature (but see Nelson et al 2013; Clutton-Brock et al. 2006; and, for birds, Cornwallis et al. 2010). It is assumed herein that same-sex competition for mates and “mate choice” would follow the same rules as competition for other resources (nutrients, space) whereby facilitation of a conspecific’s reproduction may represent a type’s optimal response (Chap. 2) following Hamilton’s rule . Traits of rivals and mates will vary, yielding types differentially successful in combat, display, and fertilization (males) or implantation (females). The intensity of competition for mates is expected to increase with increased population density and, under some conditions, reproductive groups of one or both sex may be favored. Following the scenarios addressed in Chap. 6 of this brief, groups may form as a result of interindividual interactions within populations “mapped” onto clumped resources, including mates, with the dynamics of those interactions bounded by the parameters of Hamilton’s rule (Chaps. 1 and 2). Where types vary in sexual traits and where the spatiotemporal dispersion of mates is clumped, sexual selection may favor sociality within and/or between sexes, and associated traits may become “fixed” in a population.

8.1 The Energetics of Sexual Allocation

Schoener (1971; see Gittleman and Thompson 1988; Bergman et al. 2001) demonstrated theoretically that males are expected to be time-minimizers and females, energy-maximizers (see Sect. 3.1). These different life-history strategies are biased by initial reproductive allocations or energetic investments (Selman et al. 2012; Trivers 1972; see Proulx 1999). Queller and Strassmann (2010) pointed out that, compared to female insects, vertebrate females, particularly birds and mammals, invest heavily in reproduction, showing that taxonomic differences obtain (also see Selman et al. 2012; this brief, Sect. 5.4, Synopsis). The latter states may represent a “life insurance” strategy for one or both parents to protect reproductive investment in heterogenous regimes. Female mammals, as well, allocate nutrients to secondary sexual characteristics, such as pendulous breasts and other fatty deposits, presumably as characters facilitating mate attraction (see Jones 2007). Costly allocation of energy to reproduction and mating suggests a trade-off between efficiency and flexibility for female mammals (but see Trebatická et al. 2007; this brief, Chap. 2). Unlike most birds, characterized by biparental care, resource dispersion associated with most mammals’ regimes has promoted “sexual segregation” and polygyny , apparently derived from promiscuous aggregations, as the primary sociosexual structures (Chap. 3). Apparently, the latter scenarios, combined with benefits of sexual segregation for males (a time-minimizing strategy?) favored female types predisposed to assume primary and obligate care of offspring. Female mammals, then, may have been under severe selective pressures to conserve resources (Clutton-Brock et al. 1989; Nagy et al. 1999), leading to the adoption of energy-conserving features associated with some social tactics and strategies (e.g., “coyness,” selectivity, mate choice, “allomothering”).

The aforementioned conditions and patterns are expected to enhance the basic asymmetry of male and female reproductive optima whereby females benefit most from control of fertilization, males from control of insemination (Alexander et al. 1997) The latter character state (“social selection”) is predisposed to favor the evolution of signaling as an equalizing mechanism. Given the energetic constraints attendant to the female mammal, energy-saving counterstrategies to their fundamental reproductive conflict with potential mates (“sexual conflict”; Chapman et al. 2003) , judicious mate “choice” and mate competition mechanisms (“intersexual” selection; see West-Eberhard 1979; Jones 2003), are bound to be critical counterstrategies to male persuasion, coercion, force, and control (“parasitism” by males; Davies et al. 2012). It may not be hyperbole to suggest that female mammals can ill afford to make a mistake in their choice of mates, predisposing them to select males with extreme (genetically correlated) traits, inducing positive feedback loops (Fisherian “runaway selection,” Sect. 8.6). Schoener’s (1971) and Trivers’ (1972) formulations, then, encompass all conditions in which female mammals make “decisions” regarding reproductive allocation, including decisions to join (group formation) or remain in groups, linking natural selection to sexual selection and the evolution of mammalian sociality. A caveat to the study of reproductive strategies in female mammals must be evolution in heterogeneous regimes, conditions likely to have stressed “fitness budgets” as well as energy allocation tactics and strategies by increasing margins of error via decreasing likelihoods of accuracy.

Where female dispersion is determined by food dispersion, and if male dispersion is “mapped” onto dispersion of females (see Proulx 1999; this brief Chap. 6), male and female mammals do not, per se, compete for food but over mate “choice” (mate selectivity, “intersexual competition”) . Males will exhibit mate “choice” to the degree that, for one reason or another, potential mates are unavailable (e.g., due to kinship, lack of female receptivity, lack or failure of male attraction, “social parasitism” or other forms of exploitation). Females are likely to exhibit female–female competition (“intrasexual selection”) where attractive, available males are in short supply. Mechanisms of mate choice might have favored the evolution of mammalian social actions as mechanisms to manage competition for resources or for mates, conditions more likely to arise for energy-stressed females (female types with high l* values relative to other female group members).

As polygyny and “sexual segregation” are the norms among mammals (Chaps. 3, 4 and 5), males in most populations are obligated to search for females, a response that may be costly in time that, at the extreme, may have favored male–female coresidence (monogamy, “harems,” or multimale–multifemale assemblies, Sects. 3.3 and 8.5). This strategy favors species recognition, high dispersal or colonization ability, and highly developed sensory perception (Ewer 1968; Wilson 1975). Since females are often spatiotemporally clumped in polygynous mammal species, promoting male search strategies, females may benefit from group living to facilitate their location by males (Sect. 8.6). On the other hand, in some mammalian taxa, females exhibit mate search strategies (e.g., in lekking species), and empirical studies demonstrate the high-energetic, including nutritional, expense of female mate search (pronghorns, Antilocapra americana, Byers et al. 2005, 2006; but see Trebatická et al. 2007). Mammalian taxa, such as pronghorns, in which females search for mates (lek-like), but where males do not display at a breeding site may represent an evolutionary precursor to “true” leks (Chap. 3). The costs of searching to females, combined with the already high “reproductive load” of female mammals, strongly suggest that, where this sex searches for mates, heterogeneous conditions render attractive, available mates difficult to locate via increased spatiotemporal unpredictability. Where females search for mates as members of groups, they may experience an energy savings, this possibility as well as the previous topics are in need of systematic investigation.

As proposed, features associated with mate attraction may favor the evolution of female groups. A variety of signals and displays are ubiquitous among polygynous, including lekking, mammalian males, such as tusks, horns, antlers, colorful pelage, pendulous, large, or colorful testes and penises, and large body size (Ralls 1977). Female mammals may also exhibit structures to attract males, such as genital engorgement or exaggerated coloration to advertise fertility or receptivity (e.g., Jones 1985, 1997a). These signals and displays may function as appeasement to other females and may intensify male–male competition, facilitating mate assessment of male traits where females mate multiply. In addition, females of some species emit vocalizations before, during, and/or after assessment and/or copulation (rats, Rattus norvegicus; Thomas and Barfield 1985), audible responses that may attract social parasites (conspecifics) or predators (“natural enemies”; Chesson 2000), and that may heighten female–female competition for mates (“intrasexual competition,” e.g., interference competition, “copying,” “eavesdropping”) . Sexual signals and displays observed in mammals will ultimately be defined by formal statements of “information theory” (Frank 2012; Proulx 2001).

The evolution of group living may benefit females if groups serve as “information centers” about female competitors and reproductive males in a population (Kerth and Reckardt 2003; Jerison 1983). Indeed, for these potential mates, benefits from advertisement may be highest where types cluster spatiotemporally (aggregations, Chaps. 3 and 6), “hotspots” beneficial to males as a time-minimizing strategy or to females as an energy maximization trajectory. For each sex, these different metabolic effects associated with “hotspots” are likely to decrease costs from mate search, broadcasting, and unpredictability as well as ignorance about competitors. Other features that may enhance attraction and proximity to the opposite sex are “nuptial feeding” and chemical marking (“urine ceremonies”; Schilder 1990) by males, and directed responses by females signaling changes in receptivity. Groups in a patch may vary in intensity of competition for mates and/or for competition for nutrients convertible to offspring, effects expected to influence relative benefits and costs to individuals from cooperation and/or altruism among kin on the one hand, and nonkin on the other. As mammalian females bear very high costs from allocation to reproduction (Clutton-Brock et al. 1989), this sex is expected to be most sensitive to variations in competitive regimes within and between patches.

8.2 “Sexual Conflict” Between Mammalian Males and Group-Living Females: Ecology Interacts with Traits

Where limiting resources and females are distributed unevenly, some males will control many more females than others, as found among most large mammals (Clutton-Brock 1989; this brief, Sect. 3) . Accurate identification of mates is essential to each sex, and sexual selection has modified species identification and communication systems, acting differently on males and females (Clutton-Brock and Huchard 2013; Otte 1974, 1975). In most conditions, male mammals dominate females because: (1) body size of reproductive males is usually larger than that of reproductive females, (2) reproductive competition is more intense among males compared to females, (3) mammalian males living in groups are generally unrelated (e.g., in multimale–multifemale reproductive units), and (4) in the same conditions, males are generally able to increase their reproductive output more than females are able to. In other words, compared to females in the same patch, variance in reproductive success is expected to be higher in males (Trivers 1972). Additionally, females should prefer to control the timing of fertilization while males should prefer to control insemination (Alexander et al. 1997), creating conditions whereby different intersexual “fitness optima” reflect conflicts of interest, effects that should enhance likelihood of energy-stressed mammalian females exhibiting cooperation and/or altruism to others of their sex in a patch (Silk et al. 2003) .

8.3 The Eco-Ethology of Male to Female Aggression

Among mammals, some environments have a high potential for male to female aggression (Estes 1992), reducing pressures on the differential fitness optima of each sex (“sexual conflict”; Rice 2000; Holland and Rice 1999; Chapman et al. 2003; Aloise King et al. 2013) . Female mammals may be vulnerable to male persuasion, coercion, force, and parasitism, as well as to coordination and control by males, because high maternal investment predisposes females to phenotypes designed for efficient execution of maternal roles (physiological characteristics, mammaries, mate selectivity; Clutton-Brock et al. 1989). Although the ethological perspective holds that ritualized signals and displays function to decrease likelihood of aggression among conspecifics, the costs of producing these ritualized, nonstereotyped, or learned characteristics may significantly stress females’ energy reserves (see Bonduriansky 2013). This condition pertains particularly to female mammals, obligated to and limited by extremely costly maternal allocation tactics and strategies (high “reproductive load”).

For the aforementioned reasons, male and female mammals engage in an ongoing coevolutionary “arms race,” imposing greater reproductive costs on each or “holding their own” in such a competitive “chase” (Chapman et al. 2003; Holland and Rice 1999). After Estes (1992), Ewer (1968), Clutton-Brock (1977), Chapman and Feldhamer (1982), Wasser (1983), Anderson and Jones (1984), and Mosser and Packer (2009), males appear to have won this race in some taxa (Agouti; Northern elephant seals, Mirounga angustirostris; walrus, Odobenus rosmarus; Hamadryas baboons, Papio hamadryas; chimpanzees, Pan troglodytes; lions, Panthera leo; domestic cats, Felis catus). In other taxa, females have apparently won (lemurs, Lemuridae; bonobo, Pan paniscus; mantled howler monkey, Alouatta palliata; coati, Nasua narica; African elephant, Loxodonta africana; reindeer, Rangifer tarandus), including species in which females are dominant to males. In a few species, intersexual relations have been characterized as “egalitarian” (striped mice, Rhabdomys pumilio; fox, Lycaon; muriquis, Brachyteles aracnoides), while in others, intersexual influence and “power” generally vary by context (Hawaiian monk seal, Monachus schauinslandi; squirrel monkeys, Saimiri spp.; most socially “monogamous” mammals involving single-male, single-female coresidence; “hierarchical” humans, Homo sapiens). The previous patterns may be influenced by alternative reproductive strategies employed by females and, particularly, males (P. C. Lee, personal communication; see Fig. 2.1) .

Aggression, including coerced or forced copulation by males to females (“rape,” “traumatic insemination,” humans, Thornhill and Thornhill 1983; male orangutans, Pongo pygmaeus; see Brooks and Jennions 1999) is likely to be favored by selection where females of polygynous mammalian species (“harem,” “age-graded”; red deer, Cervus elaphus; hartebeest, Alcelaphus caama) do not copulate outside their receptive period. Polygynous human systems represent one exception to this pattern. Where more than one female cycle concurrently in polygynous taxa and where females breed seasonally (pinnipeds), a female-biased “operational sex ratio” (OSR, relative occurrence of sexually active males to reproductive females) will obtain, a condition expected to (1) increase costs to males from attempts to monopolize cycling females, (2) restrict a polygynous male’s temporal window for copulation and successful fertilization and, (3) generate intense male–male competition between polygynous males, between males governing relatively small and relatively large female groups, and between males without a group of females to coordinate and control .

In the aforementioned conditions, aggression by males may be favored if heritability (a population parameter measured as proportion of differences between types attributed to differences in alleles) reaches some threshold value relative to ecological and demographic factors, in particular, large and structured populations, as well as intensity of selection. The latter phenomena are directly related to genotype × environment interactions (“norms of reaction”) and to intensities of competition within populations (within and between groups), for example, by variations in the effects of traits associated with male to female aggression . For example, in mammals, high levels of male to female aggression are associated with nonstereotyped (nonritualized) behavioral phenotypes (pinnipeds; ground squirrel, Citellus armatus; humans), high population density (pinnipeds; humans), breeding on land rather than in water (pinnipeds), a “catholic” (broad niche or opportunistic) diet (pinnipeds, humans), unstable male dominance hierarchies rather than resource or female defense (lions, Northern elephant seals, some bats), multiple mating by females (ubiquitous), polygynandry, “queuing” associated with hierarchical (“multilevel”) group structures (humans, some cetaceans), and/or very lengthy periods of female pregnancy, lactation or maternal care (chimpanzees, humans). The previous tactics and strategies will promote or sustain a low r in groups, inhibiting the evolution of aggressive and reproductive restraint favoring the evolution of sociality. These conditions should favor, instead, intense within-group reproductive competition increasing likelihood of coexistence among unrelated types.

In a few taxa (Agouti, humans), social monogamy is associated with high levels of male agonism during courtship. Furthermore, male to female aggression is relatively common where females remain in their natal groups (most mammals), uncommon among patrilocal taxa (most atelids and apes, except chimpanzees with high rates of aggression (Wrangham and Peterson 1996) and humans exhibiting bisexual dispersal from natal groups (Hill et al. 2011). In some conditions, female dispersal may have been an adaptive counterstrategy to male coercion and force, an operation likely to maintain low r within groups if female dispersal is random by genotype. Among primates, for example, female dispersal is associated with energetically costly, more evenly dispersed, plant forage, particularly mature leaves, while matrilocal societies and male to female aggression are associated with nutritionally poor, clumped, ephemeral, fruit resources (but see anomalous spider monkeys, Ateles, and chimpanzees) .

In some cases (promiscuity “polyandry,” “cryptic female choice”) females are not readily monopolized by males (atelids, bonobos), decreasing effectiveness of male to female aggression, lowering the strength of sexual selection , and decreasing mean r in groups. Evolution of the latter female strategies are dependent upon the prior evolution of mechanisms for conflict management. Low levels of male to female aggression occur where females are dominant to males (Ralls 1976, 1977) and where females exert strong “choice” of mates (“leks”; polygynandry), suggesting the occurrence of effective mechanisms to manage or reduce “sexual conflict” or of spatial dispersions of limiting resources disfavoring agonistic interactions. On the other hand, sexual preferences by mammalian females may incur significant aggressive costs from nonpreferred males (Hamadryas baboons, Northern elephant seals). Additionally, mammalian males in several genera harass and coerce females with some frequency (Halichoerus, Papio), suggesting that phylogeny, in addition to ecology, needs to be considered as a correlate of male to female aggression promoting group life and associated traits. Regardless of the differential contributions of ecology and phylogeny to patterns of male to female aggression, a trade-off exists for both sexes whereby “female emancipation” (Jones and Cortés-Ortiz 1998) increases differences in expressed, active, and effective reproductive “optima” between the sexes.

Except for cases in which selection has preadapted reproductive mammalian males for subordinance to females (Ralls 1976), increasing the overlap of reproductive optima between the sexes, male to female aggression may have promoted the evolution of group life by enforcing female subordinance to males. This scenario may account, in part, for many examples of “sexual segregation” among mammals (Clutton-Brock et al. 1987) if “solitary” group structure emancipates reproductive females and males from direct negative consequences of coresidence occasioned by recurrent interindividual interactions driven by “sexual conflict.” The analyses in this section suggest the testable proposition that, where mammalian males and females coreside, some stable degree of overlap in reproductive optima between the sexes must obtain resulting from shared kinship (“nested” bottlenose dolphins, Tursiops; Wiszniewski et al. 2010) or “shared reproductive fates” (cooperatively breeding kit foxes, Vulpes macrotis mutica; “nested” humans) .

8.4 A Simple Model of Male to Female Aggression in Mammals

Male to female aggression (persuasion coercion, force) may be modeled as male parasitism of a reproductive member of the opposite sex whereby a male exploits a female for reproductive advantage (“social parasitism”; Jones 1997b, 2005; Emerson 1958; Wheeler 1906; Wilson 1971; Michener 1974; Hölldobler and Wilson 1990) , and Davies et al. (2012) showed that male (parasite) to female (host) parasitism is a sexually selected trait. “Social parasitism” may be considered one type of Chesson’s (2000) category, “natural enemies,” and the latter factors may favor facilitation as a means of conflict management in some conditions. Quantitative modeling puts social parasitism in perspective. Consider a male aggressor, the Sender, exploiting the time–energy budget of a reproductive female (a Receiver). Following May and Anderson (1990, in Moore 2002), Moore pointed out that fitness of a parasite (here, an adult male aggressor) can be measured as reproductive rate (R 0), a density-dependent value (Gill 1974). May and Anderson’s equation formalizes virulence (rate of deleterious effects of male to female aggression) by way of a measure of cost to a female’s fitness (increased intensity of intra- and intersexual interactions). May and Anderson’s equation can be modified for male parasitism of females such that

$${{R}_{0}}=y(N)/(a+b+v),$$

where y is transmission rate (= “virulence,” in the present case, reproductive costs imposed upon females by males), N is population density of reproductive females, a is rate of cost to reproductive females, b is rate of cost to reproductive females from all but virulence (“opportunity costs”), and v is a host’s (a female Receiver’s) recovery rate (a female’s ability to completely or partially escape) from deleterious reproductive effects of a parasite’s (aggressor male) responses (e.g., by increasing future reproductive rate or exploiting a mutation for an effective counterstrategy to male parasitism, such as increasing defensive networks with other females). May and Anderson’s formula might be employed to predict conditions under which benefits from male parasitism decrease (e.g., where virulence, transmission, and recovery rate are independent; Moore 2002).

Females (hosts) may effect counterstrategies to male persuasion, coercion, and force, though males may control virulence at a level sufficient to coordinate and control hosts but not virulent enough to induce female counterstrategies (“immune response”). Such a state will limit a coevolutionary “chase” (“arms race”) between the sexes (Rice 2000; Holland and Rice 1999), increasing the value to female hosts of associating in defensive networks (groups) with other females and with nonaggressive males, mitigating sexual conflict and its attendant costs. It is important to keep in mind that social parasitism , like other exploitative strategies, cannot induce altruism unless Hamilton’s rule is satisfied.

Furthermore, if females are more canalized than males (Jones 2012), mammalian females may not display sufficient genetic heterogeneity to counter male parasitism, and increasing virulence will be costly to both sexes where female reproductive rate is significantly compromised via morbidity or mortality. Thus, in addition to affecting inclusive fitness of mates, male parasitism of females has the potential to affect growth rate of groups and mean fitness of populations, a condition that may increase or decrease intra- and interspecific competition with consequences for variations in intragroup levels of competition affecting differential benefits and costs from joining networks of kin or nonkin (West et al. 2002; De Bruyn 1980). Finally, the effectiveness of parasitic strategies by males may, in some conditions, depend upon the ability of reproductive females to discriminate parasitic from nonparasitic males, another topic requiring theoretical and empirical investigation. In sum, any increases in the reproductive load of females will depress their reproductive rates and outputs of offspring. This condition, beyond some threshold value for each type, could bias females to indirect reproduction (sociality) .

Progress in these areas of research should be significantly promoted by Bourke’s (2011) proposition that shared reproductive interests (e.g., between kin) and/or shared “reproductive fates” (e.g., between mates) should stabilize the evolution of groups and sociality, effects that may facilitate the evolution of cooperation and/or altruism among females as defensive and/or energy-maximizing strategies. Shared interests between mammalian reproductives may be forced upon prospective mates by male parasitism inducing a variety of subordinate traits in females (e.g., a large repertoire of submissive behaviors). Male parasitism of females via aggression may have originally favored increased maternal investment (i.e., costly gestation and lactation relative to body size), emancipating males by time savings and investments in male–male competition for monopolization of mates. For mammalian males, costly time minimization strategies may have been induced by environmental heterogeneity , the source of stressors (stimuli depressing reproductive rates) widely applicable to contexts in which mammals evolved (Jones 2009; Southwood 1977).

The possibility that male parasitism induced costly reproduction in mammalian females is suggestive of an evolutionary “arms race” between the sexes, though increased maternal investment by female mammals may have enhanced reproductive interests (“shared fates”) between mates. Michener (1974) classified social parasites as “natural enemies,” one of Chesson’s (2000) mechanisms for the management of competition . The latter author employed “natural enemies” when discussing interspecific competition. However, Wilson’s (1971) treatment shows that social parasitism is, as well, characteristic of intraspecific relations, in particular, closely associated types, including members of the same lineage. The foregoing ideas might be amenable to comparative tests, including mammalian females’ vulnerability to parasitism from other sources (offspring, other females; see Lewis and Pusey 1997; Jones 2005; Galef 1991).

8.5 Managing Conflict Where More than One Males Coreside with Reproductive Females

More than one reproductive males cohabiting in stable groups with reproductive females are virtually limited to mammals (Wilson 1975; Brown 1975; this brief, Sects. 3.3, 3.5, Chap. 4), and most empirical reports of these structures remain descriptive rather than theoretical, hypothetico-deductive, or empirical, including experimental (but see Jones 1982). A paucity of studies is available to describe degrees of relatedness, intrasexual competition , or tendencies for these males to exhibit mate “choice.” Additionally, systematic research on the stability of “fission–fusion” dynamics, frequently characterizing multimale–multifemale and “nested” reproductive groups, has not been conducted. In both multimale–multifemale and “nested” societies, males demonstrate hierarchies, coalitions, and alliances, but mammalian males rarely, if ever, demonstrate altruism, achievable only via shared genes among relatives.

Recent reports on polygynandrous lions (Mosser and Packer 2009) and hierarchically organized bottleneck dolphins (Wiszniewski et al. 2012) suggest that, in some conditions, defense of reproductive females may explain benefits to related or unrelated males. The latter reports indicated, as well, that (up to some limit) larger group sizes are associated with greater reproductive benefits to males (though not necessarily to females?). Discussing eusocial bathyergids, Lewis and Pusey (1997; also see Horwich et al. 2001) reported that higher infant mortality was associated with larger groups, a trend that, if common among mammals, would oppose Allee effects (Allee 1931) whereby female reproductive success increases with an increase in group size. Compared to sociality among females, the scientific literature on sociality among mammalian males is limited, a topic in need of systematic study, particularly, variations in tactics and strategies for the management of competition attendant to reproductive conflicts of interest as well as differential behaviors and network characteristics of related and unrelated reproductive males. Male dominance hierarchies are ubiquitous in multimale–multifemale assemblies, and a type’s condition- and spatiotemporally dependent dominance rank should be decomposable into traits covarying with fitness in fluctuating environments. In theory, these traits covary, as well, with measures of sexual selection (e.g., male–male displacements, copulation rates, rates of signaling and displaying). Controlled experiments in seminatural arenas are needed to separate the effects of female traits on a male’s traits, such as different fitness optima between the sexes, patterns of female choice, quality of care for a male’s offspring, and “polyandry.” Finally, male dominance hierarchies manage intrasexual competition among males whereby each type struggles for “maximum viability” and the relatively lowest levels of l*within.

8.6 A Final Note on Females: Potentials and Constraints

For mammalian females, energy savings drives the selection of traits (Schoener 1971; see Russell et al. 2003), a thermal regulatory process maintaining usable heat within limits propitious to optimal maintenance, survival, reproduction, and growth (Gittleman and Thompson 1988; McNab 1980). As females are “energy-maximizers,” sexually selected signals and displays may represent a significant cost to inclusive fitness that, in the same conditions, males, “time-minimizers,” may be in a better position to afford (see Clutton-Brock et al. 1989). Mammalian males can significantly influence population parameters by controlling reproductive careers of females. Such influence can be enhanced by ecological, by tactical and strategic decision-making (male herding behavior, infanticide, “sneaking”), or by females themselves (passive or “cryptic” “female choice,” providing information to males about reproductive state, facilitation of male intromission, repelling adult female or juvenile interference). Whatever the precise environmental components determining the reproductive strategies of mammalian females, their life-history “decisions” are expected to be a function of life in (thermally) heterogeneous regimes (Geisel 1976; Schaffer 1974). In theory, female traits and environmental filters are measurable using taxon-independent criteria permitting quantitative analyses within and between species and within and between “patches” (Fig. 8.1).

Fig. 8.1
figure 1

Anesthetized adult female Peruvian (“black”) spider monkey, Ateles chamek (Primates, Atelidae) exhibiting mimicry of male scrotum (also see wildebeest, Connochaetes taurinus), entailing a large, pendulous clitoris. This clitoris is an example of a “trait” that can be defined as “a physical, biochemical, morphological, physiological, phenological, or behavioral feature measurable at the individual level, from the cell to the whole-organism level” (Carnicer et al. 2012). When features of types are identified as (independent) standardized measurements, quantitative treatments can be conducted within and between taxa. This case of (aggressive? defensive?) morphological mimicry may represent an exaggerated (defensive? sexual? aggressive?) display favored by selection in response to intraspecific, intrasexual, or intersexual competition for food or other limiting resources (see Stankowich and Caro 2009). Across mammals, genital hypertrophy (e.g., female mantled howler monkeys) may be an evolutionary precursor to scrotal mimicry that may be ancestral to peniform, erectile clitorises (spotted hyena, Crocuta crocuta), and each state may induce “rapid” evolution of traits in other types (conspecific or contraspecific) affected by the display if “arms races” are operating. These displays may also function as species recognition devices (see Eibl-Eibesfeldt 1970). In mantled howler monkeys, also atelids, variations in vulval color, volume of vaginal excretions, and morphology may distinguish subspecies (C. B. Jones, personal observation; see Jones 1985, 1997a). This photograph was taken at Lago Caiman, Noel Kempff National Park, Bolivia, by © Rob Wallace

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There is a critical need to investigate the reaction norms of female mammals and the strength of selection pressures on female traits (e.g., intrasexual selection; Clutton-Brock et al. 2006). Qvarnström (2001) discussed reports showing that male traits attractive to females might vary in their effectiveness across changing conditions (spatial and temporal) and that females may gain reproductive advantages via tactics and strategies (genetically correlated traits) other than by favoring “good genes.” These results using insects as subjects provide testable hypotheses for research projects targeting male and female strategies, and the extent to which male and female fitness optima vary by condition has received little attention in studies of mammals (Sect. 5.3). Also, females often reside in groups in polygynous and polygynandrous as well as primitively eusocial mammals, condition-dependent structures reflecting tolerance and shared interests (e.g., energy savings) in particular environmental regimes. The potential for patterns of female groupings, and other female responses (e.g., allomaternal care, hygienic grooming, adoption, “interference” competition) to influence, if not manage, competitive relations among males is virtually unstudied; however, the extent to which females in polygynandrous assemblages exhibit “female emancipation” (Andersson 2005) is noteworthy. As Andersson’s (2005) paper suggests, “female emancipation” may be identified wherever females mate multiply, a virtually ubiquitous trait of female mammals almost certainly retained from the ancestral “promiscuous” toolkit.

Holland and Rice (1999) removed effects of sexual selection in one experimental population, finding that males’ virulent traits and their deleterious effects on females and on population growth rates “diminished” (sexual segregation or monogamy?) compared to a control population permitted to evolve with selection by sex unimpeded. Although toxicity of male sperm and female resistance decreased in the experimental condition in the previous study (see Gomendio and Roldan 1993), studying coevolution between “male ejaculates and female reproductive biology,” showed that sexual selection may entail benefits as well as costs for mammalian females. A detailed understanding of intrinsic and extrinsic constraints on female life-history tactics and strategies requires further study in mammals. However, high “reproductive load” is expected to burden female mammals as a result of uncommonly high zygote and maternal allocation strategies (Trivers 1972), including attendant thermal requirements (Gittleman and Thompson 1988). It would seem that, for the aforementioned and other reasons, female mammals are predisposed to “social neglect,” highlighting the advantages of closely monitoring (“record keeping”) their interactions with members of their group, possibly, to gain benefits for the lowest possible cost. Females experiencing “social neglect,” no doubt run the risk of a “social trap” whereby they may be, ceteris paribus, destined to one or more “helper” (dependent) roles.