Larger Harems Research Articles

Male condition and group heterogeneity predict extra-group paternity in a Neotropical bat

Extra-group paternity, in which offspring are sired by a male outside the breeding group, may alter the distribution of reproductive success in a population, thus affecting the opportunity for sexual selection. Both inter- and intraspecific studies have focused largely on mating systems in which females choose their social mates, and less is known about extra-group paternity in polygynous systems in which social mates are largely dictated by male-male competition. In this study, we examine the frequency and distribution of extra-group paternity in a harem-forming bat, Phyllostomus hastatus. We find that despite aggressive harem defense, males are unable to fully monopolize reproduction within their harem and over 70% of harems contain extra-group offspring. Harem males in better body condition suffered less paternity loss, but we found no effect of male age or body size. Even though the age and size of individual females did not predict offspring paternity, we found a significant effect of age heterogeneity within the group. Harems composed of differently aged females were more likely to contain extra-group offspring. Our results not only provide evidence for the role of male defense in preventing extra-group paternity but also suggest that social group composition has consequences for male reproductive success. In polygynous societies, the ability to monopolize mating is critical to reproductive success. As the group size increases, defense often becomes more difficult, resulting in increased rates of extra-group paternity. We find that among greater spear-nosed bats, extra-group offspring occur in most harems, but the overall rate of extra-group paternity is relatively low despite their large harems (15–30 females). Variation in the rate of extra-group paternity between harems is explained by male body condition (i.e., relative mass) but not harem size. Additionally, the rate of extra-group paternity is not explained entirely by male attributes, as we find that age heterogeneity within the female group is a significant predictor of the extra-group paternity rate. Our results support the hypothesis that not only the physical condition of a male influences his ability to monopolize reproduction but also the group composition influences the female mating behavior.

Male Resource Defence Behaviour Strengthens Harem Size in Promiscuously Mating Fruit Bats

Mating success in group forming animals largely depend on the ability of same-sex competitors (usually males) to monopolize local resources that are spatially limited and are of importance to their sexual partners. Across taxa, this resource defence behaviour (RDB) is predicted to maximize male reproductive success. Although RDB is widely observed in polygynous societies, its significance as an alternative male mating strategy among randomly mating individuals (i.e., promiscuous) is a less explored topic in ethology, especially within Chiroptera. In this study, we elucidate the relationship between RDB (measured here as male tent defence behaviour) and the harem size associated with adult males of Cynopterus sphinx. From 12 independent resident male removal experiments, we found that inter-male aggressive combats often exclusively occur while defending foliage tents in mating seasons. Further, our harem census data of usurper males (during usurpation) and resident males (post release), suggests a significant bias among females to preferentially roost with the resident males. This preferential association between harem females and the resident males that almost always recovers the foliage tent from the usurpers, suggests that C. sphinx harem males use RDB as an alternative mating strategy to monopolize roosting resources and maintain large harems. However, further research is required to identify the direct effects of RDB on individual male reproductive success.

Zachsia zenkewitschi (Teredinidae), a Rare and Unusual Seagrass Boring Bivalve Revisited and Redescribed

The sea-grass borer Zachsia zenkewitschi belongs to a group of economically and ecologically important bivalves, commonly referred to as shipworms. The sole recognized representative of the genus Zachsia, this species displays an unusual life history and reproductive strategy that is now understood to include: environmental sex determination of free swimming larvae, extreme sexual and size dimorphism between males and females, internal fertilization, maintenance of often large harems of male dwarfs within a specialized cavity of the female mantle, and complex maternal care of larvae in specialized brood pouches within the gill. It is also the only shipworm species known to burrow in sea grass rhizomes rather than terrestrial wood. Although Z. zenkewitschi is rare and little studied, understanding of its biology and anatomy has evolved substantially, rendering some aspects of its original description inaccurate. Moreover, no existing type specimens are known for this species. In light of these facts, we designate a neotype from among specimens recently collected at the type location, and undertake a re-description of this species, accounting for recent reinterpretation of its life history and functional anatomy.

Early life expenditure in sexual competition is associated with increased reproductive senescence in male red deer.

The evolutionary theories of senescence predict that investment in reproduction in early life should come at the cost of reduced somatic maintenance, and thus earlier or more rapid senescence. There is now growing support for such trade-offs in wild vertebrates, but these exclusively come from females. Here, we test this prediction in male red deer (Cervus elaphus) using detailed longitudinal data collected over a 40-year field study. We show that males which had larger harems and thereby allocated more resources to reproduction during early adulthood experienced higher rates of senescence in both harem size and rut duration. Males that carried antlers with more points during early life did not show more pronounced declines in reproductive traits in later life. Overall, we demonstrate that sexual competition shapes male reproductive senescence in wild red deer populations and provide rare empirical support for the disposable soma theory of ageing in males of polygynous vertebrate species.

Reproductive Consequences of Male Arrival Order in the Bark Beetle, Ips grandicollis

For group-living animals the choice of whether to join aggregations or initiate their own is influenced by potential benefits such as group protection and reduced energetic expenditure, as well as costs such as competition for food and mates. The bark beetle Ips grandicollis is an invasive pest species that colonises recently felled timber in Australian pine (Pinus spp.) plantations. Male beetles initiate colonies by burrowing under the bark of trees and emitting an aggregation pheromone which attracts conspecifics, including a harem of females with whom they mate. We predicted that males that initiated colonies, or who arrived early, would have larger harems than later arrivals (due to decreased competition for females). However, we found the opposite effect with early-arriving males actually associated with fewer females than later arriving males, although this may have resulted from some females leaving harems as they get older. We conclude that pioneering does not improve male likelihood of attracting females in Ips grandicollis, at least initially, but it may provide advantages for offspring when competing for food during development.

The red deer rut revisited: female excursions but no evidence females move to mate with preferred males

In polygynous mammals, most variance in male reproductive success is expected to result from competition between males, and the role of female behavior remains poorly understood. Contests between red deer males during the annual rut are one of the most famous examples of male–male competition. However, anecdotal evidence suggests females in estrus make substantial movements, changing harems, and potentially disrupting the outcome of male contests. In other polygynous mammals, such movements have been interpreted as evidence of female mating preferences. Here, we use 34 years of detailed observational data on wild red deer to show that 43% of estrous females are found in different harems between successive observations and that 64% of such females make substantial movements (up to 4 km) when this occurs. Approximately 45% of these movements result in the male into whose harem a female moved fathering her offspring. We then test whether females move nonrandomly with respect to male phenotype, consistent with the hypothesis that females move to mate with preferred males. Although in general, females were more likely to be found in larger harems and the harems of younger males after harem changes, these effects were not specific to estrous females. Further, estrous females were not more likely to be found in the harems of less related males. We therefore find little support for the idea that estrous females move between harems to mate with a preferred male; as a result, the reasons females make such extraordinary movements when in estrus remain unclear.

Matrisibs, Patrisibs, and the Evolution of Imprinting on Autosomes and Sex Chromosomes

The conflict theory of genomic imprinting argues that parent-of-origin effects on allelic expression evolve as a consequence of conflict between maternally and paternally derived genomes. I derive explicit population-genetic models of this theory when individuals in a cohort with an arbitrary and variable number of sires and dams interact. I show that the evolution of imprinting is governed by the reciprocal of the harmonic mean number of fathers but the reciprocal of the arithmetic mean number of mothers per cohort. Thus, a few monandrous females in a polyandrous population decrease the strength of the genetic conflict and the opportunity for conflict-driven paternal imprinting. In contrast, in populations in which few males control large harems, rare males with small harems do not have such a disproportionate effect on genetic conflicts and maternal imprinting. Additionally, I demonstrate that under the conflict theory, selection for imprinted expression on paternally derived X chromosomes is much weaker than it is on maternally derived X chromosomes or autosomes.

Variance in the male reproductive success of western gorillas: acquiring females is just the beginning

Variance in male reproductive success is expected to be high in sexually dimorphic mammals, even when it is modulated by the costs and benefits of group living. Here, we investigate the variance in reproductive success of male western gorillas (Gorilla gorilla), a highly dimorphic primate with long-term male–female associations, using 12.5 years of data collected at Mbeli Bai in northern Congo. Access to mates and offspring survival were both major sources of variance in male reproductive success. Males with larger harems had lower offspring mortality with no apparent reduction in female fertility or observed tenure length, so the size of harems did not seem to be limited by female feeding competition or by the risk of takeovers and infanticide by outsider males. The lower mortality in larger harems may reflect improved vigilance against predators, and females may cluster around males that enhance offspring survival. Thus, this study illustrates how a detailed analysis of the components of male reproductive success can shed light on the interrelated social and ecological aspects that affect it.

Harem size and oviposition behaviour in a polygynous bark beetle

Abstract. 1. Harem polygyny can have fitness benefits and costs on females. In bark beetles of the genus Ips the latter may include within‐harem competition between larvae. However, earlier competition between females for male care and mating opportunities may also influence oviposition behaviour. There has been relatively little investigation into the relationship between harem size and initial egg output. The present study investigated this relationship in the bark beetle Ips grandicollis.2. The measure of egg output used was the number of eggs in the gallery with the most eggs in each harem. Mean (±SE) harem size of 242 observed harems was 3.25 ± 0.10. A curvilinear relationship was found between egg output and harem size, with females in smaller harems (one to four females) laying more eggs with increased harem size. However, females in larger harems (five to seven females) laid fewer eggs as harem size increased. The optimal harem size (in terms of number of eggs laid) was close to four females.3. We found no evidence from a behavioural assay that females could preferentially choose unmated males over mated males with harems of two females. Additionally, the distribution of harem sizes suggests that females distribute themselves among males randomly.4. The results suggest that harem size has effects on female reproduction that extend beyond larval competition and influence patterns of oviposition. The mechanism that determines why egg laying is greatest at intermediate levels is unknown. There is no evidence that smaller harems belong to lower quality males, but females may adjust egg‐laying behaviour in large harems as a result of reduced male attendance or anticipated larval competition.

Sexual selection explains sex-specific growth plasticity and positive allometry for sexual size dimorphism in a reef fish

In 1950, Rensch noted that in clades where males are the larger sex, sexual size dimorphism (SSD) tends to be more pronounced in larger species. This fundamental allometric relationship is now known as 'Rensch's rule'. While most researchers attribute Rensch's rule to sexual selection for male size, experimental evidence is lacking. Here, we suggest that ultimate hypotheses for Rensch's rule should also apply to groups of individuals and that individual trait plasticity can be used to test those hypotheses experimentally. Specifically, we show that in the sex-changing fish Parapercis cylindrica, larger males have larger harems with larger females, and that SSD increases with harem size. Thus, sexual selection for male body size is the ultimate cause of sexual size allometry. In addition, we experimentally illustrate a positive relationship between polygyny potential and individual growth rate during sex change from female to male. Thus, sexual selection is the ultimate cause of variation in growth rate, and variation in growth rate is the proximate cause of sexual size allometry. Taken together, our results provide compelling evidence in support of the sexual selection hypothesis for Rensch's rule and highlight the potential importance of individual growth modification in the shaping of morphological patterns in Nature.

Aggression and Tolerance by Dominant Males ofArtibeus jamaicensis: Strategies to Maximize Fitness in Harem Groups

Strategies of males in harem-forming mammals may change depending on 2 variables, female group size and consequent ability of the harem male to monopolize copulations, and the relative costs and benefits of tolerating other males. We studied harems of 4–20 females associated with a dominant male in small groups of Artibeus jamaicensis (<14 females), and with dominant and subordinate males in large groups (>14 females). Dominant males displayed defensive behavior toward satellite males when they intruded into the roosting site. Small groups received the highest number of visits by satellite males, and dominant males did not display total defense of females. During the breeding season, females roosted in highly compacted clusters and dominant males were more active in their defensive behavior. Subordinate males were generally tolerated in harem groups and their presence reduced the number of adult male visits. Some young in large harem groups were sired by subordinate males, resulting in a genetic benefit for both dominant and subordinate males. Dominant males had the highest fitness in the large harem groups by sharing paternity with related subordinate males and by rejecting unrelated intruders.

Costs of large communal clutches for male and female Greater RheasRhea americana

The breeding system of the Greater RheaRhea americanais almost unique among birds as it combines harem polygyny and sequential polyandry, with communal egg‐laying and uniparental male care. In this species, large communal clutches (more than 30 eggs) are rare and have a lower hatching success than smaller clutches. Here we analyse the proximate causes of hatching failures and the costs of large communal clutches (and therefore the costs of extensive polygyny) for males and females. We evaluated if length of the nesting period, egg viability, egg losses during incubation and male parental activity at the nest were affected by clutch size. We also evaluated if chicks hatched from large clutches have a lower survival during the first 2 months after hatching. Large clutches had longer nesting period and lower hatching success, mainly as a result of bacterial contamination of the eggs and increased hatching asynchrony. In addition, large clutches tended to lose more eggs as a result of accidental breakage or predation. Male activity at the nest and chick survival were not related to clutch size. Low hatching success, nest predation risk and energetic costs associated with large clutches penalize females that join large harems and males that accept additional eggs into the nest.

Queen-Queen Competition by Precocious Male Production in Multiqueen Ant Colonies

Arriving earlier in the breeding area than his rivals may be beneficial for a male when females mate only once or during a short time span. The timing of a male's entrance is usually determined by the male himself, e.g., through returning early from his winter quarters or through accelerated larval development . Here, we document a surprisingly simple way of "first come, first served" in a species with local mate competition. In multiqueen colonies of a Cardiocondyla ant, mother queens make sure that their own sons are the first to monopolize mating with a large harem of female sexuals by producing extremely long-lived males early in colony life. Whereas queens in newly founded single-queen colonies started to produce male and female sexuals only several weeks after the eclosion of their first worker offspring, queens in multiqueen colonies precociously reared sons long before the first female sexuals and even before the emergence of their first workers. These early males killed all later emerging males in the nest and mated with all female sexuals subsequently produced. Our data document that the patterns of growth and productivity of insect colonies are surprisingly flexible and can be turned upside down under appropriate selection pressures.

An Empirical Test of a Possible Solution to “the Central Theoretical Problem of Human Sociobiology”

KANAZAWA (2004) suggests that there is a negative association between social class and reproduction because lower-class individuals, who tend to have lower general intelligence, have greater difficulty employing evolutionarily-novel modern contraception. I derive three hypotheses from KANAZAWA's theory: 1) There are no class differences in the number of desired children; 2) The effect of sexual activity on reproduction is weaker among the more intelligent; and 3) The interaction between sexual activity and intelligence is stronger among men. The analyses of the U.S. General Social Surveys support all three hypotheses. Modern evolutionary psychology predicts that, because women prefer to mate with men with greater resources and higher status, such men attain greater reproductive success (BUSS 1994). Throughout human history, wealthy and powerful men of high status have had a greater number of mates and produced more children than poor and powerless men of low status (BETZIG 1986). In ancient civilizations, kings, emperors and sultans maintained large harems, consisting of hundreds and thousands of virgins, and local chiefs and noblemen kept several wives or concubines, while countless poor men in the countryside died mateless and childless (BETZIG 1993). And these wealthy and powerful men of high status often left a large number of descendants. In contrast, a strong positive correlation between social class and reproduction does not exist in the contemporary society. Marriage and successful reproduction are no longer the privilege of the wealthy and powerful only, and men's reproductive success is not related to social class. If anything, the correlation appears negative; in contemporary industrial societies like the United States, poor families tend to have more children than wealthy families. This is in contradiction to the prediction of evolutionary psychology.

REPRODUCTIVE BEHAVIOR IN FEMALE IBERIAN RED DEER: EFFECTS OF AGGREGATION AND DISPERSION OF FOOD

Resource dispersion influences mammalian mating systems by affecting space use by females and in turn the strategies employed by males. Other elements related to mating interactions, however, also may affect female decisions, and it usually becomes difficult to discriminate between them. Here we present the results of experimental food supplementation during rut in the red deer (Cervus elaphus hispanicus). We provided food at 2 levels of spatial dispersion in order to test whether changes in the distribution of food within a short period of time affect spatial behavior of females or other features of the mating system. Food clumping produced an increase in local density of females using an area and in the size of female groups. It also produced an increase in number of adult males, male–male interactions, and male harassment toward females. Despite the overall rate of harassment being higher with food clumping, females received a lower per capita rate of harassment because they gathered into much larger harems. Variance in harem size increased with higher levels of food clumping. Our data show that females were highly sensitive to food distribution when joining harems, which stresses the relative importance of direct benefits (food and sexual harassment avoidance) in molding the conditions under which selection operates during rut in Iberian red deer.

Queuing for harem access in colonies of the greater sac-winged bat

Colonies of the greater sac-winged bat, Saccopteryx bilineata, consist of nonterritorial males and males that defend harem territories in which females roost. The traits of nonterritorial males that define their success in harem take-overs are so far unknown. We predicted that the time nonterritorial males spent in the colony and their proximity to harems would be important factors. We temporarily removed harem holders from a colony of 60 greater sac-winged bats on consecutive days and observed which of the nonterritorial males took over the harem. To test for consistency of results, we repeated the experiments in some of the territories. In a second set of experiments, we removed both the harem holder and the corresponding usurper from the first experiment. On average, usurping males preferred territories with a large harem over territories with a small harem and they belonged to groups of males who spent the daytime near the corresponding harems. Usurpers in the second experiment were present in the colony for a shorter time than usurpers in the first experiment. Overall, the results support the hypothesis that nonterritorial males form overlapping peripheral groups for harems and that they live in a hierarchy according to their tenure in the group. Group membership and hierarchy seemed to be the factors that determined whether a male occupied a vacant harem. Hence, instead of floating, the pattern of harem succession in greater sac-winged bats is best described by queuing of nonterritorial males for access to a harem. Copyright 2003 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

Genetic mating system and the significance of harem associations in the bat Saccopteryx bilineata.

We analysed the polygynous mating system of the bat Saccopteryx bilineata using behaviour observations and genetic data on 11 microsatellite DNA loci. Basic social units in S. bilineata are harem groups that consist of single males and up to eight females. Colonies comprise several harem groups, and the composition of colonies and harems is often stable over several reproductive seasons. The combination of parentage exclusion and likelihood-based parentage assignment in this study produced detailed parentage information for a large colony of S. bilineata. Reproduction occurred mostly within the colony (17% extra-colony paternity), but social associations in harems within the colony did not represent reproductive units (70% extra-harem paternity). The latter finding was consistent over three reproductive seasons. Spatial association of the roosting sites of males and females could not explain parentage patterns in the colony. Even though intra-harem paternity was less frequent than expected, it contributed significantly to reproduction of harem males. On average, the number of offspring sired by a male with females in his harem territory increased significantly with harem size, which corresponds to the higher energetic investment that is related to the maintenance of large harems. However, extra-harem paternity was not correlated with a male's harem size or intra-harem reproductive success. This suggests that individual preferences of females rather than male traits associated with the ability to defend large harems are most likely to cause the detected differences between social association and genetic mating system.

Measures of breeding inequality: a case study in southern elephant seals

Inequality in distribution of resources is a key aspect of evolutionary biology particularly in relation to distribution of mates and copulations. Notwithstanding its important role, inequality is not easily defined, and its measurement is complicated by theoretical and methodological issues. Although the formal treatment of inequality has been mostly limited to the evolution of lek mating system, a methodologically correct approach to measurement of inequality is generally valid for the study of any kind of mating system. In this paper, we analyze inequality in a large set of southern elephant seal (Mirounga leonina) harems. The observed distribution of fertilizations was significantly different from both the expected distribution with equal shares of resources and the expected distribution with equal propensities to acquire resources. We calculate and compare various measures of inequality, observing a wide variation particularly among unbounded and bounded indices. We check the effect of choosing a specific measure of inequality by considering the effect of two aspects of harem socionomy, the number of females in the harem (i.e., the total amount of resources to be shared) and the number of males associated with the harem (i.e., the number of competitors). The choice of a specific measure of inequality had a strong impact on the results obtained and should be considered a critical step in every study of functional and evolutionary correlates of inequality. Unbounded indices showed a strong relationship with both harem size and number of males, while no effect was evident in the analysis of bounded indices. This demonstrates that, in this species, the despotism of the mating system remains high even in large harems and with many competitors, i.e., the worst conditions for monopolization.

Individual variation in perfume blending in male greater sac-winged bats

Male greater sac-winged bats, Saccopteryx bilineata, use hovering flights to court females in their harem territory. While hovering, males fan a fragrant perfume from sac-like organs in the wing membrane towards the females. Each afternoon, males renew the perfumes of their wing sacs during a stereotypical and time-consuming behavioural sequence, which includes blending secretions from genital and gular glands. I investigated whether male perfume-blending behaviour varies between the mating and the nonmating season, and whether successful males, those with large harems, have different patterns of perfume blending than less successful males. I measured variation in perfume-blending behaviour in 21 adult males. The pattern of perfume blending was not significantly different between the mating and the nonmating season. However, the time at which perfume blending began and terminated differed between seasons. During the mating season, males spent about the same time cleaning their wing sacs during phase I of perfume blending, irrespective of the number of females in their harem. During phase II, males with large harems spent significantly less time refilling and blending perfume than males with small harems. In addition, males with large harems took up fewer droplets of secretion from the genital region. Overall, male–male interactions were rare during perfume blending, and the decrease in duration in phase II was not attributable to more disturbance in large harems. There was also no evidence that males prevented each other from transferring secretions into their wing sacs. Copyright 2002 The Association for the Study of Animal Behaviour. Published by Elsevier Science Ltd. All rights reserved.

Pup mortality in southern elephant seals at Marion Island

Pre- and early post-weaning pup mortality of southern elephant seals (Mirounga leonina) at Marion Island from 1990 through 1999 ranged from 1.6% to 7.3% and averaged 3.8%. Mortality of pups after weaning before their first trip to sea accounted for only 12% of the total mortality. We found no relationship between population size and percentage pup mortality, indicating that pup survival is independent of seal density, at least at the densities of breeding seals that prevailed. Indeed, pup mortality was greatest in the smallest harems, apparently owing to a greater number of younger, less experienced mothers. Small harems were generally also found on less suitable beaches than larger harems and this could have contributed to pup injury as a cause of pup mortality on these beaches. Mother–pup separation and injury caused by beachmasters is likely to be responsible for pup mortality in the larger harems. The low rates of pup mortality observed in this study obviate it being a major population regulating agent at Marion Island.