Determinants of reproductive success in voles: space use in relation to food and litter size manipulation

Spacing behaviour of female mammals is suggested to depend on the distribution and abundance of food. In addition, food limitation has been found to constrain the reproductive success of females. However, whether females maximize their reproductive success by adjusting space use in relation to current food availability and reproductive effort (e.g. litter size) has not been experimentally studied. We examined these questions by manipulating simultaneously food resources (control vs. food supplementation) and litter sizes (control vs. plus two pups) of territorial female bank voles (Clethrionomys glareolus) in large outdoor enclosures. Females with supplementary food had smaller home ranges (foraging area) and home range overlaps than control females, whereas litter size manipulation had no effect on space use. In contrast, the size of territory (exclusive area) was not affected by food supplementation or litter size manipulation. As we have previously shown elsewhere, extra food increases the reproductive success of bank vole females in terms of size and proportion of weaned offspring. According to the present data, greater overlap of female home ranges had a negative effect on reproductive success of females, particularly on survival of offspring. We conclude that higher food availability increases the reproductive success of bank vole females, and this effect may be mediated through lower vulnerability of offspring to direct killing and/or detrimental effects from other females in the population. Moreover, it seems that when density of conspecifics is controlled for, home range sizes of females, but not territoriality, is related to food resources in Clethrionomys voles.     

Discrepancy between factors affecting nestling growth and survival and maternal success in Common Grackles

ABSTRACT Multiple factors potentially affect nestling survival and maternal reproductive success. However, little is known about the relative importance of different factors when operating simultaneously or whether the same factors are important for nestlings and their mothers. We determined the effect of hatching asynchrony, individual egg size, mean egg size, nestling sex, and clutch initiation date on the survival of individual nestlings and on maternal reproductive success in Common Grackles (Quiscalus quiscula) from 2004 to 2006 in central Illinois. Factors most important to maternal success differed from those important for individual nestling growth and survival. Hatching asynchrony had the greatest within‐nest influence on the fate of nestlings; the earlier a nestling hatched relative to siblings, the greater its mass and likelihood of fledging. Clutch size had the greatest influence on maternal reproductive success, with females with larger clutches fledging more young. Thus, both nestling survival and maternal success were largely determined by a single, albeit different, factor. A possible explanation for the apparent unimportance of most factors we measured in determining maternal success is that we did not consider variation among females. Individual variation in maternal attributes such as condition, size, age, experience, or mate quality may result in females tailoring clutch attributes (i.e., egg size, sex, and degree of hatching asynchrony) in ways that allow them to maximize their reproductive success. The discordance between factors that benefited mothers versus their offspring illustrates the importance of considering the maternal consequences of any factor that appears to affect offspring survival. Factors that increase the mass and survival of some offspring may not result in increased maternal reproductive success.     

Reproductive phenology of two co‐occurring Neotropical mountain grasslands

Aim

Climate tends to explain phenological variations in tropical ecosystems. However, water availability and nutrient content in soil strongly affect plant communities, especially those on old, climatically buffered, infertile landscapes (OCBILs), and may impact these ecosystems’ plant reproductive phenology over time. Here, we compare the reproductive phenology of sandy and stony tropical grasslands, two co‐occurring herbaceous communities of the campo rupestreOCBILs. We asked whether flowering, fruiting and dispersal are seasonal in both grasslands, and whether these phenophases differ due to variations in soil properties. We also asked whether the phenological strategies and the number of flowers and fruits differ between these two grasslands as soil conditions vary.

Location

Serra do Cipó, Minas Gerais, Brazil.

Methods

The phenology of herbaceous species of sandy and stony grasslands was monitored monthly over two consecutive years.

Results

Plants on sandy and stony grasslands flowered and fruited throughout the year. We did not find a distinct seasonal pattern at the community level of either studied grassland. However, flowering, fruiting and seed dissemination occurred in stony grasslands mainly during the rainy season, while sandy grassland species flowered in both seasons and fruited and disseminated seed mainly during the dry season, as observed in other savanna vegetation types in the Cerrado. Flower and fruit production was higher in sandy grasslands than in stony grasslands, which may be linked to higher water retention in sandy grassland soils. In both communities, species of Cyperaceae, Eriocaulaceae and Xyridaceae contributed most to overall production, whereas Poaceae and Velloziaceae, two important families in campo rupestre, barely participated in the reproductive phenology during our 2‐yr survey.

Conclusions

Despite a strong seasonal climate, there was no reproductive seasonal pattern at the community level in campo rupestre. This first investigation of Neotropical grassland phenology indicates that the differences in soil content may constrain the grassland reproductive phenology and restrict reproduction of stony grassland species to the most favourable season. Further studies of grassland phenology are necessary to disentangle the relative importance of soil, climate and other triggers, especially fire.     

Size Assortative Mating and Reproductive Success of the Funnel-Web Spider, Agelena limbata (Araneae; Agelenidae)

Field observations and laboratory experiments were conducted to assess the relation between male size and reproductive success in the funnel-web spider, Agelena limbata Thorell (Agelenidae), in 2 years. In this species, the body size of males is similar to that of females. In the field, size assortative mating occurred in both years. In 1 year, partial correlation coefficient analysis indicates that male cephalothorax width is a beter predictor of the copulated female cephalothorax width than of the date of pairing. In laboratory experiments, females tended to reject courting males that were smaller in relative body size, and males that were larger in relative body size had greater copulation success. Consequently female rejection of smaller courting males has some contribution to size assortative mating. Since larger females deposited more numerous eggs in the field, larger males are expected to have a higher reproductive success.     

Sexual selection and mating patterns in a mammal with female-biased sexual size dimorphism

In mammals, species with highly male-biased sexual size dimorphismtend to have high variance in male reproductive success. However,little information is available on patterns of sexual selection,variation in male and female reproductive success, and bodysize and mating success in species with female-biased size dimorphism.We used parentage data from microsatellite DNA loci to examinethese issues in the yellow-pine chipmunk (Tamias amoenus), asmall ground squirrel with female-biased sexual size dimorphism.Chipmunks were monitored over 3 years in the Kananaskis Valley,Alberta, Canada. We found evidence of high levels of multiplepaternity within litters. Variation in male and female reproductivesuccess was equal, and the opportunity for sexual selectionwas only marginally higher in males than females. Male and femalereproductive success both depended on mating success. We foundno evidence that the number of genetic mates a male had dependedon body size. Our results are consistent with a promiscuousmating system in which males and female mate with multiple partners.Low variation in male reproductive success may be a generalfeature of mammalian species in which females are larger thanmales.     

Group size and composition influence male and female reproductive success in black howler monkeys (Alouatta pigra)

It has been argued that grouping patterns might influence the reproductive performance of individuals. Increasing group size results in greater travel costs and competition over depletable food resources, which could lead to reduced individual reproductive success. However, in groups with an increasing number of males, female reproductive success is predicted to augment because larger male groups might better protect immatures from infanticidal attacks. In contrast, male reproductive success is predicted to decrease with number of males in a group because fertilization cannot be shared between males. In this paper, we test these predictions on the Mesoamerican black howler monkey (Alouatta pigra) with data on group size and composition for 120 groups from eight populations of black howler monkeys existing in eight protected forests in Mexico and Guatemala. Male and female reproductive success were calculated as a deviation of the observed number of infants (or immatures) from the expected number of infants (or immatures), relative to the number of males and females in a group. Results indicate that both male and female reproductive success decreased with group size. Male reproductive success decreased with an increasing number of males in a group and with increasing proportion of males relative to females in a group. Decreased female reproductive success was associated with increasing number of females in a group, and female reproductive success had a tendency to increase with increasing number of males in a group. These results suggest that in black howler monkeys, living in larger groups might negatively affect the reproductive success of each member. Our findings are similar to those reported for a population of a sister species, Alouatta palliata, living in larger groups.     

Maternal investment in reproduction and its consequences in leatherback turtles

Maternal investment in reproduction by oviparous non-avian reptiles is usually limited to pre-ovipositional allocations to the number and size of eggs and clutches, thus making these species good subjects for testing hypotheses of reproductive optimality models. Because leatherback turtles (Dermochelys coriacea) stand out among oviparous amniotes by having the highest clutch frequency and producing the largest mass of eggs per reproductive season, we quantified maternal investment of 146 female leatherbacks over four nesting seasons (2001–2004) and found high inter- and intra-female variation in several reproductive characteristics. Estimated clutch frequency [coefficient of variation (CV) = 31%] and clutch size (CV = 26%) varied more among females than did egg mass (CV = 9%) and hatchling mass (CV = 7%). Moreover, clutch size had an approximately threefold higher effect on clutch mass than did egg mass. These results generally support predictions of reproductive optimality models in which species that lay several, large clutches per reproductive season should exhibit low variation in egg size and instead maximize egg number (clutch frequency and/or size). The number of hatchlings emerging per nest was positively correlated with clutch size, but fraction of eggs in a clutch yielding hatchlings (emergence success) was not correlated with clutch size and varied highly among females. In addition, seasonal fecundity and seasonal hatchling production increased with the frequency and the size of clutches (in order of effect size). Our results demonstrate that female leatherbacks exhibit high phenotypic variation in reproductive traits, possibly in response to environmental variability and/or resulting from genotypic variability within the population. Furthermore, high seasonal and lifetime fecundity of leatherbacks probably reflect compensation for high and unpredictable mortality during early life history stages in this species.     

Female-biased sexual size dimorphism in the yellow-pine chipmunk (Tamias amoenus): sex-specific patterns of annual reproductive success and survival

Sexual size dimorphism is ultimately the result of independent, sex-specific selection on body size. In mammals, male-biased sexual size dimorphism is the predominant pattern, and it is usually attributed to the polygynous mating system prevalent in most mammals. This sole explanation is unsatisfying because selection acts on both sexes simultaneously, therefore any explanation of sexual size dimorphism should explain why one sex is relatively large and the other is small. Using mark-recapture techniques and DNA microsatellite loci to assign parentage, we examined sex-specific patterns of annual reproductive success and survival in the yellow-pine chipmunk (Tamias amoenus), a small mammal with female-biased sexual size dimorphism, to test the hypothesis that the dimorphism was related to sex differences in the relationship between body size and fitness. Chipmunks were monitored and body size components measured over three years in the Kananaskis Valley, Alberta, Canada. Male reproductive success was independent of body size perhaps due to trade-offs in body size associated with behavioral components of male mating success: dominance and running speed. Male survival was consistent with stabilizing selection for overall body size and body size components. The relationship between reproductive success and female body size fluctuated. In two of three years the relationship was positive, whereas in one year the relationship was negative. This may have been the result of differences in environmental conditions among years. Large females require more energy to maintain their soma than small females and may be unable to maintain lactation in the face of challenging environmental conditions. Female survival was positively related to body size, with little evidence for stabilizing selection. Sex differences in the relationship between body size and fitness (reproductive success and survival) were the result of different processes, but were ultimately consistent with female-biased sexual size dimorphism evident in this species.     

Body size, age and paternity in common brushtail possums (Trichosurus vulpecula)

Sexual selection should produce sexual size dimorphism in species where larger members of one sex obtain disproportionately more matings. Recent theory suggests that the degree of sexual size dimorphism depends on physical and temporal constraints involving the operational sex ratio, the potential reproductive rate and the trade-off between current reproductive effort and residual reproductive value. As part of a large-scale experiment on dispersal, we investigated the mating system of common brushtail possums inhabiting old-growth Eucalyptus forest in Australia. Paternity was assigned to 20 of 28 pouch-young (maternity known) genotyped at six microsatellite loci. Male mating success was strongly related to body size and age; male body weight and age being highly correlated. Despite disproportionate mating success favouring larger males, sexual size dimorphism was only apparent among older animals. Trapping and telemetry indicated that the operational sex ratio was effectively 1 : 1 and the potential reproductive rate of males was at most four times that of females. Being larger appeared to entail significant survival costs because males 'died-off' at the age at which sexual size dimorphism became apparent (8-9 years). Male and female home ranges were the same size and males appeared to be as sedentary as females. Moreover, longevity appears to be only slightly less important to male reproductive success than it is to females. It is suggested that a sedentary lifestyle and longevity are the key elements constraining selection for greater sexual size dimorphism in this 'model' medium-sized Australian marsupial herbivore.     

Relationship between clutch size,egg volume and hatching success in a Yellow-legged Gull Larus michahellis colony in south-eastern Tunisia

This study aimed to test the hypothesis that clutch size covaries with egg volume and hatching success in the Yellow-legged Gull Larus michahellis. We determined clutch size and egg volume in a sample of 131 nests, and we used the data to check whether egg volume varied among nests according to clutch size, while taking into account the effects of egg laying order. We also estimated hatching success rate and investigated the relationship between hatching success and clutch size. Egg volume varied among clutches according to clutch size, with eggs being larger in three-egg clutches than in two-egg clutches. Moreover, three-egg clutches showed higher daily survival rates, and hence hatching success, than two-egg clutches. Overall, our results suggest that in the Yellow-legged Gull clutch size covaries with egg volume and hatching success, which could possibly reflect an age effect through different mechanisms. Indeed, older females could be hypothesised to exhibit greater breeding performance than younger females because of their higher experience in tapping energy resources for egg formation and defending nests from dangers. Moreover, due to their age, older females are likely to have lower residual reproductive potential and should invest more heavily in current breeding attempts.     

Reproductive Success and Sexual Selection in Wild Eastern Tiger Salamanders (<Emphasis Type="Italic">Ambystoma t. tigrinum</Emphasis>)

Variation in reproductive success is most pronounced in species with strongly biased operational sex ratios, prominent sexual dimorphisms, and where mate competition and choice are likely. We studied sexual selection in eastern tiger salamanders (Ambystoma t. tigrinum) and examined the role of body size on reproductive success. We genotyped 155 adults and 1,341 larvae from 90 egg masses at six microsatellite loci. Parentage analyses revealed both sexes engaged in multiple matings, but was more common among females (64%) than males (27%). However, the standardized variance in mating and reproductive success was higher in males. Bateman gradients were significant and nearly identical in both sexes, suggesting that sexual selection was roughly equal between sexes. Body size was not correlated with mating or reproductive success in either sex. The apparent lack of sexual selection on body size may be a result of sperm storage, sperm competition, alternative mating tactics, and/or random induction of spermatophores.     

Paternal body size affects reproductive success in laboratory-held zebrafish (<Emphasis Type="Italic">Danio rerio</Emphasis>)

Across many fish species, large females tend to exhibit higher individual reproductive success due to elevated fecundity and the provisioning of better conditioned eggs and offspring compared to small females. By contrast, effects of paternal body size on reproductive success are less well understood. We disentangled the maternal- and paternal-size dependent effects on reproductive output and early life history in zebrafish (Danio rerio). In the laboratory, females and males from four size categories (small, medium-sized, large and very large) were allowed to spawn freely in a full factorial design with 10 replicates per size combination. As expected, larger females produced more eggs and better conditioned offspring compared to smaller females. Male body size further contributed to zebrafish reproductive success: offspring sired by large males exhibited higher hatching probability and these offspring also hatched earlier and larger than offspring fertilized by small males. However, the largest males experienced lower mating success and received fewer eggs than males of the smaller size classes. While male body size substantially affected reproductive success in zebrafish, it remained unclear whether and to what degree direct paternal effects (e.g., related to sperm quality) or indirect paternal effects stemming from differential allocation patterns by females were the mechanism behind our findings. Answering this question constitutes an important future research topic.     

Size‐dependent reproductive success of wild zebrafish Danio rerio in the laboratory

Size‐dependent reproductive success of wild zebrafish Danio rerio was studied under controlled conditions in the laboratory to further understand the influence of spawner body size on reproductive output and egg and larval traits. Three different spawner size categories attained by size‐selective harvesting of the F₁‐offspring of wild D. rerio were established and their reproductive performance compared during a 5 day period. As to be expected, large females spawned more frequently and had significantly greater clutch sizes than small females. Contrary to expectations, small females produced larger eggs when measured as egg diameter with similar amounts of yolk compared to eggs spawned by large spawners. Eggs from small fish, however, suffered from higher egg mortality than the eggs of large individuals. Embryos from small‐sized spawners also hatched later than offspring from eggs laid by large females. Larval standard length (L_S)‐at‐hatch did not differ between the size categories, but the offspring of the large fish had significantly larger area‐at‐hatch and greater yolk‐sac volume indicating better condition. Offspring growth rates were generally similar between offspring from all size categories, but they were significantly higher for offspring spawned by small females in terms of L_S between days 60 and 90 post‐fertilization. Despite temporarily higher growth rates among the small fish offspring, the smaller energy reserves at hatching translated into lower condition later in ontogeny. It appeared that the influence of spawner body size on egg and larval traits was relatively pronounced early in development and seemed to remain in terms of condition, but not in growth, after the onset of exogenous feeding. Further studies are needed to explore the mechanisms behind the differences in offspring quality between large‐ and small‐sized spawners by disentangling size‐dependent maternal and paternal effects on reproductive variables in D. rerio.     

Marker-assisted determination of the relationship between body size and reproductive success and consequences for evaluation of adaptive life histories

We tested for differences in the predicted optimal ages at first maturity in brook charr ( Salvelinus fontinalis ) in Freshwater River, Newfoundland, when life-history data were collated based on the marker-assisted estimation of the relationship between body size and reproductive success rather than using fecundity as a surrogate for reproductive success. Jointly with capture–recapture data to estimate the growth and survival costs of reproduction, we found that weak relationships between body size and reproductive success generate selection against delayed maturation. This finding would not have held for females if the relationship between body size and fecundity had been used as a surrogate for the relationship between body size and reproductive success. This shows that predictions of optimal life histories can be qualitatively changed when using molecular markers to directly evaluate age- and/or size-specific effects of body size on reproductive success.     

Directional selection on body size but no apparent survival cost to being large in wild New Zealand giraffe weevils

When an individual's reproductive success relies on winning fights to secure mating opportunities, bearing larger weapons is advantageous. However, sexual selection can be extremely complex, and over an animal's life the opportunity to mate is influenced by numerous factors. We studied a wild population of giraffe weevils (Lasiorhynchus barbicornis) that exhibit enormous intra and intersexual size variation. Males bear an elongated rostrum used as a weapon in fights for mating opportunities. However, small males also employ sneaking behavior as an alternative reproductive tactic. We investigated sexual selection on size by tracking individual males and females daily over two 30‐day periods to measure long‐term mating success. We also assessed how survival and recapture probabilities vary with sex and size to determine whether there might be a survival cost associated with size. We found evidence for directional selection on size through higher mating success, but no apparent survival trade‐off. Instead, larger individuals mate more often and have a higher survival probability, suggesting an accumulation of benefits to bigger individuals. Furthermore, we found evidence of size assortative mating where males appear to selectively mate with bigger females. Larger and more competitive males secure matings with larger females more frequently than smaller males, which may further increase their fitness.     

The effects of dominance rank and group size on female lifetime reproductive success in wild long-tailed macaques,Macaca fascicularis

Demographic changes were recorded throughout a 12-year period for three social groups ofMacaca fascicularis in a natural population at Ketambe (Sumatra, Indonesia). We examined the prediction that females' lifetime reproductive success depended on dominance rank and group size. Average birth rate was 0.53 (184 infants born during 349 female years). For mature females (aged 8–20 yr) birth rate reflected physical condition, being higher in years with high food availability and lower in the year following the production of a surviving infant. High-ranking females were significantly more likely than low-ranking ones to give birth again when they did have a surviving offspring born the year before (0.50 vs 0.26), especially in years with relatively low food availability (0.37 vs 0.10). Controlled comparisons of groups at different sizes indicate a decline in birth rate with rroup size only once a group has exceeded a certain size. The dominance effect on birth rate tended to be strongest in large groups. Survival of infants was rank-dependent, but the survival of juveniles was not. There was a trend for offspring survival to be lower in large groups than in mid-sized or small groups. However, rank and group size interacted, in that rank effects on offspring survival were strongest in large groups. High-ranking females were less likely to die themselves during their top-reproductive years, and thus on average had longer reproductive careers. We estimated female lifetime reproductive success based on calculated age-specific birth rates and survival rates. The effects of rank and group size (contest and scramble) on birth rate, offspring survival, age of first reproduction for daughters, and length of reproductive career, while not each consistently statistically significant, added up to substantial effects on estimated lifetime reproductive success. The group size effects explain why large groups tend to split permanently. Since females are philopatric in this species, and daughters achieve dominance rank positions similar to their mother, a close correlation is suggested between the lifetime reproductive success of mothers and daughters. For sons, too, maternal dominance affected their reproductive success: high-born males were more likely to become top-dominant (in another group). These data support the idea that natural selection has favored the evolution of a nepotistic rank system in this species, even if the annual benefits of dominance are small.     

Sexually antagonistic selection on primate size

Abstract Male intrasexual selection in haplorhine primates has previously been shown to increase male size and to a lesser degree also female size. I address the following questions: (1) why does female size increase when the selection is on males, and (2) why does female size not increase to the same extent as that of males. The potential for correlational selection on females through increased resource competition was analysed with independent contrasts analyses. No such effect was found, nor did matched pairs comparisons reveal females to increase in size because of selection to bear larger male offspring. Instead further matched pairs analyses revealed higher female postpartum investment, as indicated by a longer lactation period, in more sexually selected species, also after correcting for body weight. Concerning the second question, independent contrast analyses showed that large size has had negative effects on female reproductive rate across the primate order. Matched‐pairs analyses on haplorhines revealed that females of species in more polygynous clades have lower reproductive rates than females of species in less polygynous clades. This is also true after the effects of body weight are removed. These results, both when correcting for body weight and when not, suggest that sexual selection has shifted female size from one favouring female lifetime fecundity to one favouring male success in competition. This depicts antagonistic selection pressures on female size and a trade‐off for females between the ecologically optimal size of their foremothers and the larger size that made their forefathers successful.     

An experimental test for body size‐dependent effects of male harassment and an elevated copulation rate on female lifetime fecundity and offspring performance

Many studies investigate the benefits of polyandry, but repeated interactions with males can lower female reproductive success. Interacting with males might even decrease offspring performance if it reduces a female's ability to transfer maternal resources. Male presence can be detrimental for females in two ways: by forcing females to mate at a higher rate and through costs associated with resisting male mating attempts. Teasing apart the relative costs of elevated mating rates from those of greater male harassment is critical to understand the evolution of mating strategies. Furthermore, it is important to test whether a male's phenotype, notably body size, has differential effects on female reproductive success versus the performance of offspring, and whether this is due to male body size affecting the costs of harassment or the actual mating rate. In the eastern mosquitofish Gambusia holbrooki, males vary greatly in body size and continually attempt to inseminate females. We experimentally manipulated male presence (i.e., harassment), male body size and whether males could copulate. Exposure to males had strong detrimental effects on female reproductive output, growth and immune response, independent of male size or whether males could copulate. In contrast, there was a little evidence of a cross‐generational effect of male harassment or mating rate on offspring performance. Our results suggest that females housed with males pay direct costs due to reduced condition and offspring production and that these costs are not a consequence of increased mating rates. Furthermore, exposure to males does not affect offspring reproductive traits.     

Effects of parental body condition and size on reproductive success in a tenebrionid beetle with biparental care

Abstract. 1. The beetle Parastizopus armaticeps (Coleoptera: Tenebrionidae) inhabits the Kalahari desert of southern Africa, constructs breeding burrows after rainfall, and shows extensive biparental care. Previous work has shown that it is predominantly male size, not female size, that determines breeding success; however, in the field these beetles show size assortative mating. This might obscure or override effects of female size on reproduction. Moreover, the inaccessibility of the breeding burrows makes it impossible to test effects of female and male size on offspring development and survival before adulthood. 2. To disentangle the effects of male and female length, body mass, and body condition on reproductive success, males and females were paired randomly in small breeding cages in the laboratory (n = 887 breeding pairs). The construction of the breeding cages allowed a clear view of the brood chamber contents at each stage in offspring development. Larva, pupa, and imago numbers and development were monitored daily, and imago mass at hatching from the pupa (hatchlings), offspring mass, and offspring body length at complete exoskeleton melanisation (juveniles) were determined. 3. There was a weak positive correlation between body condition and body length for females only. Breeding chronology was related to male body condition: males in better condition were fast to start and finish a breeding bout. Males in better condition produced heavier hatchlings and juveniles, and larger‐sized males produced larger‐sized juveniles. In contrast, numbers of larvae and juveniles produced were determined mainly by female length and body condition: larger females in better condition hatched more larvae and produced more offspring. 4. The results suggest that male size and condition will be the most important determinant of reproductive success under relatively dry conditions, when burrow length is critical for reproductive success. Female size might be more important for the pair's reproductive success under wet breeding conditions, when burrow length is less critical for successful reproduction.     

Reproductive success and effective population size in woodrats (Neotoma macrotis)

Discrepancies between the census size and the genetically effective size of populations (N(e)) can be caused by a number of behavioural and demographic factors operating within populations. Specifically, strong skew in male reproductive success, as would be expected in a polygynous mating system, could cause a substantial decrease in N(e) relative to census size. Because the mating system of Neotoma macrotis had previously been described as one nearing harem polygyny, I examined the distribution of reproductive success and genetic variation within a population of this species. Combining genetic data and three years of field observations, I show that variance in reproductive success does not deviate from poisson expectations within either sex and variance in success is similar between the sexes. Furthermore, both males and females had multiple partners across litters in addition to some evidence of multiple paternity within litters. Despite a lack of strong skew in reproductive success, an estimate of N(e) based on a number of demographic parameters suggests that the ratio of N(e)/N in this population is 0.48. Although the ratio of N(e)/N suggests that the population is experiencing higher rates of genetic drift than would be expected based on census size alone, the population maintains high levels of genetic diversity. Estimates of neighbourhood size and patterns of recruitment to the study site suggest that immigration plays an important role in this population and may contribute to the maintenance of high levels of genetic diversity.