Social Organization and Foraging in Emballonurid Bats: III. Mating Systems

Social Organization and Foraging in Emballonurid Bats: III. Mating Systems Author(s): J. W. Bradbury and S. L. Vehrencamp Source: Behavioral Ecology and Sociobiology , 1977, Vol. 2, No. 1 (1977), pp. 1-17 Published by: Springer Stable URL: https://www.jstor.org/stable/4599115 REFERENCES Linked references are available on JSTOR for this article: https://www.jstor.org/stable/4599115?seq=1&cid=pdfreference#references_tab_contents You may need to log in to JSTOR to access the linked references. JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact [email protected]. Your use of the JSTOR archive indicates your acceptance of the Terms & Conditions of Use, available at https://about.jstor.org/terms Springer is collaborating with JSTOR to digitize, preserve and extend access to Behavioral Ecology and Sociobiology This content downloaded from 24.49.174.161 on Sat, 01 May 2021 14:38:44 UTC All use subject to https://about.jstor.org/terms Behav. Ecol. Sociobiol. 2, 1-17 (1977) Behavioral Ecology and Sociobiology ? by Springer-Verlag 1977 Social Organization and Foraging in Emballonurid Bats III. Mating Systems J.W. Bradbury and S.L. Vehrencamp Department of Biology, C-016, University of California at San Diego, La Jolla, California 92093, USA Received June 12, 1976 Summary. 1. A general model of mating system evolution in mammals is developed, which takes into account the different male strategies of resource defense, female group defense, and male mating aggregations. The critical environmental variables determining differential defensibility of females and resources are identified by generalizing the resource defense model of Orians (1969). The model is then applied to available data on African antelopes (Jarman, 1974) to establish a set of hypothetical relations between certain patterns of habitat use and mating structures. The resulting relations are only likely to apply to species in which food determines female dispersion and in which any resource defense exhibited by males is directed towards food supplies. 2. The relations developed for antelopes are then compared to recently published data on mating systems in five neotropical emballonurid bats (Bradbury and Vehrencamp, 1976 a). 3. Antelopes and the bats are found to share the following features. Species living in wet and stable forests tend to be fine-grained socially and to have groups consisting of monogamous pairs or nested male-female territories. Species in more seasonal habitats show an inverse relation between the size stability of groups and the duration of use of a given foraging site. As the model predicts, in both groups resource defense occurs where groups are least stable and female defense where groups are most stable. Also as the model predicts, the numbers of females accessible to each male and the number of reproductive males per group can be anticipated in each of the two taxa wherever sufficient data for the critical variables are available. 4. Antelopes and bats differ in the following ways. Whereas body size is a good predictor of antelope habitat use and social dispersions, it is a poor predictor for emballonurid patterns. Similarly, although the numbers of females per male generally increase with group size in antelopes, this correlation does not hold for the bats in this study. These differences lead to the conclusion that application of the general model cannot be simplified This content downloaded from 24.49.174.161 on Sat, 01 May 2021 14:38:44 UTC All use subject to https://about.jstor.org/terms 2 J.W. Bradbury and S.L. Vehrencamp by measurement of a few variables such as body size or group size, but instead will generally require actual measurements of the critical resource dispersion parameters in the field. Introduction The form of most animal social systems can be characterized by specifying the social dispersion (i.e., group size and spacing), the mating system, and the patterns of juvenile dispersal and/or retention. Several authors have recently suggested that selection operates initially on social dispersion and that subsequent evolution of mating systems can only occur within the options allowed by that social dispersion (Jarman, 1974; Alexander, 1974). A similar notion implicitly underlies those models of mating system evolution which rely on resource distributions as environmental determinants (e.g., Orians, 1969). In a prior paper, we analyzed field data on several species of neotropical emballonurid bats and argued that social dispersion in these species was primarily determined by a passive mapping of the animals onto their food dispersion (Bradbury and Vehrencamp, 1976b). In this paper, we wish to utilize the same data, (Bradbury and Vehrencamp, 1976a), to see (1) to what degree prior determination of social dispersion limits mating options in these species, and (2) whether current models of mating system evolution are sufficient to explain the observed diversity of emballonurid mating structures. Methods of data collection can be found in Bradbury and Vehrencamp (1976a). Form and Process in Mating System Evolution It will be important to our analysis of emballonurid mating patterns to outline briefly the types of mating systems one might expect to find in mammals, and the ways in which environmental contexts have been invoked as determinants of one type or the other. A "classical" taxonomy of mammalian mating systems would distinguish between monogamous pairs, harems, groups of females tended by several adult males (hereafter called "multi-male groups"), and leks. We feel that a more useful system can be based upon the strategies available to males. We distinguish three broad categories: 1. Resource Defense. Males establish territories containing some resource (food, water, nesting or roosting sites, etc.), required by females. Exclusion of other males and admission of females only on condition of biased mating access are two ways that such territorial males might gain in fitness. Examples include many of the savannah antelopes of Africa (Jarman, 1974). 2. Female Defense. If it is advantageous to form stable groups as an anti-predator device, to increase foraging efficiency, or to share in parental duties, males may be able to exclude or dominate other males and thereby enhance their own mating success. Mammalian examples include some baboons and ungulates such as the eland and African buffalo (Kummer, 1968; Jarman, 1974). This content downloaded from 24.49.174.161 on Sat, 01 May 2021 14:38:44 UTC All use subject to https://about.jstor.org/terms Mating Systems in Emballonurid Bats 3 3. Male Mating Aggregations. Males may aggregate at traditional or speciesspecific sites and be visited by females only for mating. If females are able to approach the aggregation and make a choice of mate, we may call the aggregation a "lek". An example would be the Uganda kob (Buechner and Schloeth, 1965). If an approaching female is mobbed by males and no mating choice is possible, we may call the aggregation a "mating swarm" after the corresponding behavior in flying insects (Downes, 1969). The latter behavior appears to occur in some temperature bats prior to hibernation (Fenton, 1969). The important difference between this system and a more classical listing is that it redirects the focus away from the form of the mating system towards the conditions which have given rise to these forms. This shift is intentional because it helps to clarify the fact that the same mating structure may arise by several different routes. For example, harems may arise either by male defense of a resource to which groups of females come (e.g., antelopes), or by the appending of an adult male to an existing female group and defense of the group from other males (e.g., Burchell's zebras). It is also the case that within any one strategy, say defense of female groups, different mating forms may arise, e.g., harems vs. multi-male groups, depending upon factors such as the size and the defensibility of the female groups. A concept which is central to all theories of mating system evolution is "defensibility" [or "defendability" as originally coined by Brown (1964)]. Defensibility refers to the ratio between the benefits of controlling a territory or group of females, (as measured by an increase in survival or reproductive success through enhanced access to resources or mates), and the costs of doing so, (as measured by time and energy spent in defense, and risks incurred as a result). We expect defense to be adaptive when the benefits exceed the costs. It is now generally accepted that for a fixed cost, the amount of resource or number of mates defended must be greater than some minimum value before benefits exceed costs, but also less than some maximum value at which competition is too low to warrant defense (Schoener, 1971; Carpenter and MacMillen, 1976). In other words, defense is most likely when the local density of resource or females is intermediate in value. As a corollary, the resource defended must be exploitable long enough, or the group of females suitably stable in composition, for the defender to obtain sufficient benefits. A defensible resource is thus one that is available for more than some minimum amount of time and which occurs within a limited range of local densities; a defensible group of females is one which is moderately stable in composition and within some limited range of group sizes. It is also generally accepted that for a fixed benefit, the costs of defense increase with the area (or perimeter) to be patrolled and with the density of potential invaders (Holmes, 1970; Schoener, 1971; Gill and Wolf, 1975). Both defended area and invader densities are positively correlated with costs of defense. For a fixed ceiling on available time or energy for defense, it therefore follows that as invader density increases, the defender must reduce the size of the area defended. While this is generally so, there are exceptions which are critical to our discussion. In general, there are two types of territorial defense which can be distinguished in animals (Fretwell and Lucas, 1969; FretThis content downloaded from 24.49.174.161 on Sat, 01 May 2021 14:38:44 UTC All use subject to https://about.jstor.org/terms 4 J.W. Bradbury and S.L. Vehrencamp well, 1972). Consider the common case in which territories are being established in a habitat with locally varying densities of some crucial resource. Suppose that potential settlers are relatively able to assess these local differences. Initially, all settlers will establish territories in the optimal portions of the habitat with each new invader forcing compression of the territories of earlier settlers. Eventually, further compression of existing territories in the best sites results in an invader obtaining less total resource than if he establishes a larger territory in a less rich site. Settlement then begins to occur in the next most optimal site and so on. As long as settlers and invaders are equally able to seize and hold territory, a "free" distribution of territories will result, in which territory size and local resource density are inversely related and all territories will contain the same total amount of resource. Free territorial distributions are apparently not uncommon in birds (Stenger, 1958; Cody and Cody, 1972a, b; Gargett, 1975). If, on the other hand, settlers are better able to defend territories than invaders can invade them, compression of territories in the better sites will halt at a lower settlement density than in the free case. Invaders will then be forced into the less rich sites earlier, and the total amount of resource in territories on the good sites will exceed that in territories on the poor sites. This unequal partitioning of resources is called a "despotic" distribution. A despotic instead of a free distribution will arise whenever the costs to a defender of repelling each invader are not constant over all territory sizes, but instead decrease as territory size is compressed. It can also arise whenever, instead of holding the total amount of costs allocated for defense constant over all territory sizes, an animal is willing to increase these allocations as an inverse function of territory size. Despotic territorial distributions have been reported for a variety of bird species (Brown, 1969; Krebs, 1971; Klomp, 1972). In some of the cited examples, there is evidence that settlement at low invader densities is free, while that at higher densities is despotic. Some populations always experience high invader densities, typically show despotic behavior from the start, and exhibit "floater" sub-populations which are totally prevented from breeding by territorial holders. The importance of the concept of defensibility in mating system evolution is easily demonstrated using the model of Verner (1964) as modified by Orians (1969) for the environmental determination of monogamy vs. polygyny. In this model, all males are presumed to adopt a strategy of defending a resource which a female requires. Orians then shows graphically that whenever the partitioning of the resources by territorial males is sufficiently unequal, females may do better to mate polygamously with an already mated male on a good territory than to mate monogamously with an unmated one on a poor territory. The model is interpreted by superposing a fixed grid of territories onto an underlying resource distribution. Where the resource is unevenly distributed in space, male territories will vary in contained resource, and polygyny can result; where the resource is more uniformly distributed, all male territories are similar and monogamy is the result. Following the suggestions of Verner and Willson (1966), Orians then notes that certain habitats, such as marshes, do show the expected correlation between high local variability in resource densities and the occurrence of polygyny. This content downloaded from 24.49.174.161 on Sat, 01 May 2021 14:38:44 UTC All use subject to https://about.jstor.org/terms Mating Systems in Emballonurid Bats 5 While this model has had considerable popularity and success in some specific applications (e.g., Holm, 1973), its use as a predictor of mating systems is limited by the necessity to justify an implicit assumption. The nature of this assumption becomes clear when we note that the Orians model is really only a special case of the Fretwell and Lucas (1969) paradigm for territorial settlement. Seen in this way, monogamy is the result of male and female settlement histories which are similarly free or similarly despotic; polygyny is the result of the special case in which male settlement is more despotic than female settlement. More importantly, the presence of polygyny in one habitat and monogamy in another can only be predicted when we can also predict that male territorial settlement will be sufficiently more despotic in the first habitat. We are thus faced, in applying the Orians model, with identifying what makes one resource dispersion more defensible than another. The basic model circumvents this issue by taking the male distribution as a given, and showing how a subsequent "free" settlement pattern by females will result in monogamy or polygamy depending upon that initial male pattern. The identification by Orians of marshes and other locally variable habitats as being particularly polygyny-prone is in fact an attempt to address the issue of site-specific defensibility. As noted above, the superposition of a fixed territorial grid onto a resource distribution will result in greater variance in the amount of resource per territory as the point-to-point variation in local resource density increases. If one assumes that a similarly sized grid is superimposed upon two habitats having the same total amount of resource but differing in local "patchiness", we could predict, using the model, that polygyny is more likely in the most patchy site. The problem is the assumption of similar territorial grids in the two contexts. When is it justified to assume that two species or two populations will have similar grid sizes? Why can invader males in the more patchy situation not force subdivision of the richer patches until all males have equal amounts of resource? The model thus only works if one assumes that both populations experience territorial compression down to some minimal value at which territorial defense becomes entirely despotic, and that this minimal value is similar for the two populations. The first assumption probably has considerable validity; the second is more likely when related species with similar morphologies and habits are being compared than when very divergent species are compared. We might even consider the possibility that two very divergent species have very different minimal territory sizes. Imposing the territorial grid of each on the same resource distribution would lead to polygamy in that with the larger grid size and monogamy in that with the smaller one. Thus either differences in grid size or differences in resource distribution could lead to differences in mating system. We wish to formalize these comments as follows. Suppose that for a given species in a given habitat, there is a minimal territory size which has a mean value A. (A will vary somewhat in the population as a function of size, age, and experience of males; we shall just consider the average value). Suppose that males adopt a strategy of resource defense, and that females distribute themselves freely according to local abundances of resource. Let the average density of resource within the territory of male i be Ri. If a male has positioned This content downloaded from 24.49.174.161 on Sat, 01 May 2021 14:38:44 UTC All use subject to https://about.jstor.org/terms 6 J.W. Bradbury and S.L. Vehrencamp his territory over part or all of a rich patch of resource, Ri is large; if he settles over a less rich patch or between patches, Ri will be small. If B is the amount of resource required per female, then the number of females in the territory of male i is (ARi/B-1) if the male utilizes the resource, or (ARi/B) if he does not. Note that a high variance in the numbers of females per male among the males with females is not a necessary consequence of the model. A large and uniform patch of resource might be settled by a few territorial males all with territory sizes of A and all with the same amount of defended resource. While these males would have similar harem sizes, partitioning is still despotic since other males have been excluded from the patch. It follows that to use the Orians model, one is obliged to consider whether compression in the species being considered has reached a limiting value, and whether the A and B values are similar for the species or not. Where A and B are known to differ among species, it still may be possible to predict mating systems from Ri values if the differences in A and B only augment the differences in Ri. Where nothing is known about A or B, or where the degree of compression cannot be evaluated, the model will not be useful. Because unmated but mature males occur in all of the species we shall consider, it seems likely that compression to a critical and despotic level has occurred. We shall therefore presume that each of the populations discussed below is operating at its own value of A. We shall not presume without evidence that values of A and B are identical for different species. In general, one is obliged to estimate these values wherever possible. If a population is known to be despotic, (as evidenced by the existence of a floater population), an estimate of A for that population in that context is the average size of territories observed. Where food is the primary resource being defended, an index proportional to B is the body weight of females to the 3/4 power (McNab, 1963). Where these estimates are available, and where Ri values are known, it should be possible to determine which of two species is most likely to be polygynous or which is likely to have the larger harem size even though the absolute values of ARi/B are not known. We shall be most concerned with the maximal values of Ri given a certain A and B since these set the upper limits on the numbers of mates that a male can have. The preceding section indentifies a set of specific variables which can be measured and used to predict whether males will be monogamous or polygynous given a resource-oriented strategy. It would seem obvious that we need a similar paradigm for female-defense strategies. Suppose again that competition results in compression of the area defensible by a male down to some critical value, A, at which further compression is unlikely. (Note that while area defensible is important in female defense, it is not the object of it. The difference is particularly important when males dominate as opposed to exclude other males in the group.) If the density of females within a male's defensible area is denoted by Di, then the expected number of females per male will be ADi. By analogy with the previous discussion, species having similar values of A will differ in mating system depending upon the values of Di. The latter will differ depending upon the overall dispersion of females in the habitat. While female dispersion varies continuously, for convenience we recognize two general cases. In the This content downloaded from 24.49.174.161 on Sat, 01 May 2021 14:38:44 UTC All use subject to https://about.jstor.org/terms Mating Systems in Emballonurid Bats 7 first, females are solitarily dispersed either to optimize foraging or to reduce predation through crypsis (Treisman, 1975a, b; Jarman, 1974). Female group size, which we denote by P, is 1. There are three sub-cases depending upon whether maximal values of ADi are less than, equal to, or greater than one. ADi < 1 is unlikely since males cannot breed under these circumstances. ADi= 1 implies that males and females are similarly distributed and most likely live in monogamous pairs. It will occur whenever A or Di or both are small. AD,> 1 implies that the defended areas of males contain a number of solitary females. This "nested" system apparently occurs in a number of rodents, primates, and ungulates (Brown, 1966; Dubost, 1970; Charles-Dominique, 1972). When P> 1, the second general case, then females live in groups which move as units throughout the suitable habitat. Again we reject AD,< 1 as unlikely. When AD= 1, then we expect to find groups which have equal numbers of males and females and most likely consist of monogamous pairs. An example appears to be the large rodent, Dolichotis patagonum (Genest and Dubost, 1974). When AD> 1, we may get several mating structures depending upon the relation between AD, and P. If ADi=P, we expect to find either one-male harems, as occur in zebras (Klingel, 1972) and Hamadryas baboons (Kummer, 1968), or multi-male troops with strong male reproductive dominance hierarchies as in savannah baboons (Hall and DeVore, 1965). In multi-male groups where one male does most of the mating, the presence of other males appears to be due either to the retention of male offspring (Eisenberg et al., 1972), or the need for cooperative multi-male defense when predators are common and large in size (Denham, 1971). If ADi<P, then we expect several adult males to accompany the females and all to perform some of the mating. This occurs in a variety of large-sized African bovids (Jarman, 1974). We note that AD,<P either because the group of females is so large that it covers an area larger than A, or because even though P is small, the females occur at large individual distances and hence Di is small. We have now discussed the critical variables which must be measured to anticipate the mating forms given either resource-oriented or female-oriented male strategies. To complete a predictive theory of mating systems, we need to add two related components. The first is to identify those habitats and those habitat variables which predispose males to adopt either resource defense or female defense or neither. On the basis of our prior considerations, we presume that a habitat must meet the following three necessary conditions before a male should preferentially adopt resource defense: (1) resources are energetically defensible; (2) they are locally available for sufficiently long that a male can extract benefits; and (3) females and resources are so distributed that ARi/B>ADi. Similarly, a habitat will most likely lead to female defense if it meets the following three necessary conditions: (1) female groups or parts of groups are energetically defensible; (2) group size and/or composition are sufficiently stable for males to gain benefits; and (3) AR/ B<ADi for all limiting resources. (When neither resources nor females are defensible, or when neither is sufficiently stable in time, we believe males are forced to adopt the "default" strategy of male mating aggregations. Evidence for this view will be presented in a subsequent publication). Correlations between habitats which meet one This content downloaded from 24.49.174.161 on Sat, 01 May 2021 14:38:44 UTC All use subject to https://about.jstor.org/terms 8 J.W. Bradbury and S.L. Vehrencamp set of conditions or another and male strategies will indicate whether the conditions are sufficient as well as necessary. The second task is to combine estimated values of A, B, Ri, Di, and P to predict the final form of the mating system: monogamous pairs, permanent harems, multi-male groups with all males mating, multi-male groups with dominance, etc. We cannot in this paper develop a general synopsis of mating systems and habitat contexts. Instead, we shall concentrate on the particular subset of species in which food constitutes the best defensible resource for males and in which female dispersion is determined primarily by food dispersion. In other words, we shall focus on species for which both Ri and Di are direct functions of the food supply. This restriction appears to be satisfied for most of the African bovids recently reviewed by Jarman (1974). These animals appear to show male defense of either females or food supplies, and at least the upper limits. on group size and group spacing can be predicted with confidence from knowledge of their food habits and food dispersions. All antelopes tend to favor food sources which are high in protein relative to fiber content. Where diverse plant organs are available as food, antelopes will select those organs with highest protein contents; where diverse organs are not available, antelopes will seek out those plants whose phenology is at a stage (e.g., releafing, flowering, etc.), leading to high protein contents. These patterns are tied to habitats as follows. Species of antelopes which live in wet and stable forests encounter high plant species diversities and low seasonal synchrony in phenological activity. They thus can specialize both on specific organs and on particular stages of phenological activity. Their food is essentially "fine-grained" in dispersion: that is, it occurs in small patches of low richness, (measured in animals feedable), and both simultaneous and successively available patches occur at high densities (cf. Bradbury and Vehrencamp, 1976b for terminology). As Jarman (1974) has shown, such species tend to be socially "finegrained" as well: that is, they occur in small groups and have small annual home ranges. Antelopes in more seasonal habitats experience a rather different food dispersion. Typically, the vegetation consists of large patches of lower plant species diversity and greater local phenological synchrony than occurs in forest. Often the habitat consists of a mosaic of such large patches: hilltop and sumps in rolling catena, patches of trees separated by patches of grassland, etc. While patch size is generally larger than in forest, it varies from small clumps of bushes to enormous homogeneous expanses of grasses. Nearly all antelopes in such habitats show seasonal movements between patches of vegetation as they become seasonally active phenologically. However, the species differ in the frequency and in the range of these movements. Antelopes of intermediate size appear to be moderately specific about what plant parts are eaten, but less specific about the plant species and the degree of phenological activity. As a consequence of the latter two biases, they tend to remain within a given patch for longer and are more willing to move from one vegetation type into an adjacent but very different type than are larger bovids (Bell, 1970; Jarman, 1974). The ability to use adjacent patches in the mosaic and to remain longer on any given patch leads to lower annual home ranges than in larger species. This content downloaded from 24.49.174.161 on Sat, 01 May 2021 14:38:44 UTC All use subject to https://about.jstor.org/terms Mating Systems in Emballonurid Bats 15 despite considerable effort by several field workers (Lopez-Forment, 1976; Bradbury and Vehrencamp, 1976a). It is extremely difficult to mark enough individuals in the large colonies of this species to determine whether either females or resources are regularly defended. It is clear from both studies that if males are defending resources, they are doing so only occasionally and for short periods. Most individuals in a given colony appear to use several foraging sites each night and to change foraging locations frequently during the year. Group size, as well, appears to be highly labile in this species. Some of this fluctuation is apparently due to differential habitat use and differential propensities to make seasonal migrations in the two sexes (Lopez-Forment, 1976). The available data thus suggest that both group stability and the duration of use of foraging sites are low in this species. These facts, plus the observations of increases in the proportions of males and in male calling activity in large colonies during the short annual copulation period, all suggest that male mating aggregations may be the most likely mating system for these bats. Seasonal mating "swarms" are known to occur in a variety of temperate insectivorous bats and seem to be associated with mating during periods of high populational flux (as the bats migrate to hibernacula), and dwindling levels of available food. These conditions are analogous to what we have outlined above for B. plicata. This prediction will become testable as we establish more completely marked populations of these bats. It is worth summarizing the degree to which the antelope trends have or have not been confirmed in the emballonurid study. On the plus side, it does seem to be the case in both groups that fine social dispersions, and/or small groups, are primarily found in wet and stable forests and tend to consist of monogamous units. It also appears that in seasonal habitats, stability of group sizes and duration of patch use are inversely related, and this leads to predictable biases towards adoption of female or resource defense strategies. However, in B. plicata, this relation fails. Whether this is due to the much coarser social dispersion of this species or to its much more seasonal and arid habitat is not known. Lastly, where data are available, both the numbers of females per male and the numbers of reproductive males per group are directly related to values of A, Di, Ri/B and P. On the debit side, it is not true, as it is in antelopes, that smaller-bodied species of bats tend to live in the more stable and diversified habitats. It is also not true that social dispersion becomes more coarse as a function of body size. The failure of this antelope correlation in the bats, even though the latter show a significant relation between local densities of insect prey and body size, is due to the fact that patch size is totally unrelated to body size. Thus patch richness, which is the product of local prey densities and patch size, and which determines group size, tends to have no relation to body size. Similarly, foraging ranges, which must be limited in part by the distance a bat can fly, are more dependent upon wing shapes than they are on body sizes at least within the studied species (Bradbury and Vehrencamp, 1976a). A final trend in antelopes is an increase in Di as P increases. Again, because of the play-off in local prey densities and patch size in determining patch richness and group size, the correlation between these variables for bats is close to zero. The differences noted above between the two groups do not contradict This content downloaded from 24.49.174.161 on Sat, 01 May 2021 14:38:44 UTC All use subject to https://about.jstor.org/terms 16 J.W. Bradbury and S.L. Vehrencamp the basic premise in both studies: that is, that social dispersion is primarily determined by the food supply and that selection for mating structures occurs within the bounds set by a given female dispersion. There is no evidence in either study that mating systems drive social dispersions. What is brought into question by the differences between the two groups of mammals is the notion that such simple variables as body size and habitat type will ever serve as good predictors of mating systems even within taxa having a similar dependence on a common resource. Instead, it appears that for each group of animals, field data must be provided to determine how the habitat is experienced by the different species. More specifically, one must know (1) the average size of resource patches; (2) the typical local densities of resource within an active patch; (3) how far an animal must travel between successively available patches and how long a given patch is used; (4) how consistent is the richness of successively available patches and what are the costs to an animal of changing groups when variation is high; and (5) the size of A for each species and whether settlement densities force compression to this value. Where two related species using the same or similar habitats show some divergence in resource preferences, any or all of the values of these parameters may differ and the two species can be expected to have different social dispersions and mating systems as a result. In short, there seem to be no short-cuts to direct measurement of resource dispersions if we wish to predict mating systems in mammals. However, once these are known, social dispersion ought to be predictable and mating systems derived as a result. Acknowledgments. The authors wish to thank Dr. Peter Marler, Dr. Donald Griffin, Dr. Mike Baker and our other colleagues at Rockefeller University for reading earlier drafts of this manuscript and offering many helpful comments. This work was supported by NSF Grant GB-30478 to the first author. References Alexander, R.D.: The evolution of social behavior. Ann. Rev. Ecol. Syst. 5, 325-383 (1974) Bell, R.H.V.: The use of the herb layer by grazing ungulates in the Serengeti. In: Animal populations in relation to their food resources (ed. A. Watson), pp. 111-123. Oxford: Blackwell Scientific Publications 1970 Bradbury, J., Emmons, L.: Social organization of some Trinidad bats. I. Emballonuridae. Z. Tierpsych. 36, 137-183 (1974) Bradbury, J., Vehrencamp, S.: Social organization and foraging in emballonurid bats. I. Field Studies. Behav. Ecol. Sociobiol. 1, 337-381 (1976a) Bradbury, J., Vehrencamp, S.: Social organization and foraging in emballonurid bats. II. A model for the determination of group size. Behav. Ecol. Sociobiol. 1, 383-404 (1976b) Brown, J.L.: The evolution of diversity in avian territorial systems. Wilson Bull. 76, 160-169 (1964) Brown, J.L.: Territorial behavior and population regulation in birds. Wilson Bull. 81, 293-329 (1969) Brown, L.E.: Home range and movement of small mammals. Symp. Zool. Soc. London 18, 111-142 (1966) Buechner, H.K., Schloeth, R.: Ceremonial mating behavior in Uganda Kob (Adenota kob thomasii). Z. Tierpsych. 22, 209-225 (1965) Carpenter, F.L., MacMillen, R.E.: Threshold model of feeding territoriality and test with a Hawaiian honeycreeper. Science 194, 639-642 (1976) Charles-Dominique, P.: Ecologie et vie sociale de Galago demidovii. Z. Tierpsych., Beih. 9, 7-41 (1972) This content downloaded from 24.49.174.161 on Sat, 01 May 2021 14:38:44 UTC All use subject to https://about.jstor.org/terms Mating Systems in Emballonurid Bats 17 Cody, M.L., Cody, C.J.: Territory size, clutch size and food in populations of wrens. Condor 74, 473-477 (1972a) Cody, M.L., Cody, B.J.C.: Areal versus lineal territories in the wren, Troglodytes troglodytes. Condor 74, 477-478 (1972b) Denham, W.W.: Energy relations and some basic properties of primate social organization. Amer. Anthropologist 73, 77-95 (1971) Downes, J.A.: The swarming and mating flight of Diptera. Ann. Rev. Entomol. 14, 271-298 (1969) Dubost, G.: L'organisation spatiale et sociale de Muntiacus reevese Ogdby 1839 en semi-liberte. Mammalia 34, 331-355 (1970) Eisenberg, J.F., Muckenhirn, N.A., Rudran, R.: The relation between ecology and social structure in primates. Science 176, 863-874 (1972) Estes, R.D.: The comparative behavior of Grant's and Thompson's gazelles. J. Mammal. 48, 189-209 (1967) Estes, R.D.: Territorial behavior of the wildebeest. Z. Tierpsych. 26, 284-370 (1969) Fenton, M.B.: Summer activity of Myotis lucifugus at hibernacula in Ontario, Canada. Canad. J. Zool. 47, 597-602 (1969) Fretwell, S.D.: Populations in a seasonal environment. Monograph /5: Monographs in population biology. Princeton: Princeton Univ. Press 1972 Fretwell, S.D., Lucas, H.L.: On territorial behavior and other factors influencing habitat distribution in birds. I. Theoretical Development. Acta biotheor. 19, 16-36 (1969) Gargett, V.: The spacing of black eagles in the Matopos, Rhodesia. Ostrich 46, 1-44 (1975) Genest, H., Dubost, G.: Pair-living in the mara (Dolichotis patagonum) Mammalia 38, 155-162 (1974) Gill, F.B., Wolf, L.L.: Economics of feeding territoriality in the golden-winged sunbird. Ecology 56, 333-345 (1975) Hall, K.R.L., DeVore, I.: Baboon social behavior. In: Primate behavior. (ed. I. DeVore), pp. 53-110. New York: Holt, Rinehart and Winston 1965 Holm, C.H.: Breeding sex ratios, territoriality, and reproductive success in the red-winged blackbird (Agelaius phoeniceus). Ecology 54, 356-365 (1973) Holmes, R.T.: Differences in population density, territoriality, and food supply of dunlin on arctic and sub-arctic tundra. In: Animal populations in relation to their resources (ed. A. Watson), pp. 303-319. Oxford: Blackwell Press 1970 Jarman, P.J.: The social organization of antelope in relation to their ecology. Behavior 48, 215-267 (1974) Klingel, H.: Social behavior of African equidae. Zoologica Africana 7, 175-186 (1972) Klomp, H.: Regulation of the size of bird populations by means of territorial behavior. Netherl. J. Zool. 22, 456-488 (1972) Krebs, J.R.: Territory and breeding density in the great tit, Parus major. Ecology 52, 2-22 (1971) Kummer, H.: Social organization of Hamadryas baboons. Chicago: Univ. Chicago Press 1968 Lopez-Forment, W.: Some ecological aspects of the bat Balantiopteryx plicata plicata, Peters, 1867 (Chiroptera: Emballonuridae) in Mexico. M.S. Thesis, Cornell Univ. Ithaca, New York (1976) McNab, B.K.: Bioenergetics and the determination of home range size. Amer. Naturalist 97, 133-140 (1963) Orians, G.H.: On the evolution of mating systems in birds and mammals. Amer. Naturalist 103, 589-603 (1969) Schoener, T.W.: Models of optimal size for solitary predators. Amer. Naturalist 103, 277-313 (1969) Schoener, T.W.: Theory of feeding strategies. Ann. Rev. Ecol. Syst. 2, 369-404 (1971) Stenger, J.: Food habits and available food of ovenbirds in relation to territory size. Auk 75, 335-346 (1958) Tannenbaum, B.: Reproductive strategies in the white-lined bat. Ph.D. Thesis, Cornell Univ. Ithaca, New York (1975) Treisman, M.: Predation and the evolution of gregariousness. I. Models for concealment and evasion. Anim. Behav. 23, 779-800 (1975a) Treisman, M.: Predation and the evolution of gregariousness. II. An economic model for predatorprey interaction. Anim. Behav. 23, 801-825 (1975b) Verner, J.: Evolution of polygamy in the long-billed marsh wren. Ecology 18, 252-261 (1964) Verner, J., Willson, M.F.: The influence of habitats on mating systems of North American passerine birds. Ecology 47, 143-147 (1966) This content downloaded from 24.49.174.161 on Sat, 01 May 2021 14:38:44 UTC All use subject to https://about.jstor.org/terms