Determinants of multiple paternity in a fluctuating population of ground squirrels

Does the potential strength of sexual selection differ between mating systems with and without defensive behaviours? A meta-analysis

The Darwin-Bateman paradigm predicts that females enhance their fitness by being choosy and mating with high-quality males, while males should compete to mate with as many females as possible. In many species, males enhance their fitness by defending females and/or resources used by females. That is, males directly defend access to mating opportunities. However, paternity analyses have repeatedly shown that females in most species mate polyandrously, which contradicts traditional expectations that male defensive behaviours lead to monandry. Here, in an extensive meta-analysis, encompassing 109 species and 1026 effect sizes from across the animal kingdom, we tested if the occurrence of defensive behaviours modulates sexual selection on females and males. If so, we can illuminate the extent to which males really succeed in defending access to mating and fertilisation opportunities. We used four different indices of the opportunity for sexual selection that comprise pre-mating and/or post-mating episodes of selection. We found, for both sexes, that the occurrence of defensive behaviours does not modulate the potential strength of sexual selection. This implies that male defensive behaviours do not predict the true intensity of sexual selection. While the most extreme levels of sexual selection on males are in species with male defensive behaviours, which indicates that males do sometimes succeed in restricting females' re-mating ability (e.g. elephant seals, Mirounga leonina), estimates of the opportunity for sexual selection vary greatly across species, regardless of whether or not defensive behaviours occur. Indeed, widespread polyandry shows that females are usually not restricted by male defensive behaviours. In addition, our results indicate that post-mating episodes of selection, such as cryptic female choice and sperm competition, might be important factors modulating the opportunity for sexual selection. We discuss: (i) why male defensive behaviours fail to lower the opportunity for sexual selection among females or fail to elevate it for males; (ii) how post-mating events might influence sexual selection; and (iii) the role of females as active participants in sexual selection. We also highlight that inadequate data reporting in the literature prevented us from extracting effect sizes from many studies that had presumably collected the relevant data.

Habitat selection in a fluctuating ground squirrel population: Density‐dependence and fitness consequences

Investigating individual‐based habitat settlement decisions is a central theme in ecology, yet studies that quantify density‐dependent habitat selection or tie fitness to resource selection decisions remain rare. We quantified habitat selection in golden‐mantled ground squirrels (Callospermophilus lateralis) across two spatial scales (home‐range placement, and occurrence within the home range) by using 11 consecutive years of data on individual space use, and we used resource selection functions and multilevel modeling to address how habitat preferences may be influenced by density or linked to fitness outcomes. Squirrels preferred dry meadow over other habitat types (wet meadow, aspen, spruce, and willow) at both spatial scales. Squirrels were more likely to use dry meadow that contained shorter vegetation and vision‐enhancing prominences such as rocks (“perches”). The use of dry meadow at each scale was not influenced by changes in density. The use of dry meadow did not lead to increased litter size, pre‐hibernation mass, or survival. However, squirrels that experienced a greater number of perches or lower local densities had higher survival rates. Our results suggest that a lack of visual obstruction, probably facilitating detection of predators, drives habitat selection in this system. Surprisingly, squirrels maintained their preference for dry meadow as density increased, and they experienced reduced survival as a result. This work furthers our understanding about the causes and consequences of changes in habitat use, informing wildlife management and conservation.

Intraspecific variation in male mating strategies in an African ground squirrel (Xerus inauris)

Male mating strategies respond to female availability such that variation in resources that affect spatial distribution can also alter cost–benefit tradeoffs within a population. In arid‐adapted species, rainfall alters reproduction, behavior, morphology, and population density such that populations differing in resource availability may also differ in successful reproductive strategies. Here, we compare two populations of Cape ground squirrels (Xerus inauris), a sub‐Saharan species with year‐round breeding and intense mating competition. Unlike most mammals where males resort to aggressive interactions over females, male X. inauris are tolerant of one another, relying instead on other nonaggressive pre‐ and postcopulatory strategies to determine reproductive success. Our findings suggest that differences in resource availability affect female distribution, which ultimately leads to intraspecific variation in male reproductive tactics and sexual morphology. Sperm competition, assessed by reproductive morphometrics, was more pronounced in our high resource site where females were distributed evenly across the landscape, whereas dominance seemed to be an important determinant of success in our low resource site where females were more aggregated. Both sites had similar mating intensities, and most males did not sire any offspring. However, our low resource site had a higher variance in fertilization success with fewer males siring multiple offspring compared with our high resource site where more individuals were successful. Our results lend support to resource models where variations in female spatial distribution attributed to environmental resources ultimately impact male reproductive behaviors and morphology.

Multiple paternity in garter snakes with evolutionarily divergent life-histories

Many animal species exhibit multiple paternity, defined as multiple males genetically contributing to a single female reproductive event, such as a clutch or litter. Although this phenomenon is well documented across a broad range of taxa, the underlying causes and consequences remain poorly understood. For example, it is unclear how multiple paternity correlates with life-history strategies. Furthermore, males and females may differ in mating strategies and these patterns may shift with ecological context and life-history variation. Here, we take advantage of natural life-history variation in garter snakes (Thamnophis elegans) to address these questions in a robust field setting where populations have diverged along a slow-to-fast life-history continuum. We determine both female (observed) and male (using molecular markers) reproductive success in replicate populations of 2 life-history strategies. We find that despite dramatic differences in annual female reproductive output: 1) females of both life-history ecotypes average 1.5 sires per litter and equivalent proportions of multiply-sired litters, whereas 2) males from the slow-living ecotype experience greater reproductive skew and greater variance in reproductive success relative to males from the fast-living ecotype males despite having equivalent average reproductive success. Together, these results indicate strong intrasexual competition among males, particularly in the fast-paced life-history ecotype. We discuss these results in the context of competing hypotheses for multiple paternity related to population density, resource variability, and life-history strategy.

Factors influencing parentage ratio in canine dual-sired litters

Breeding a bitch with two different sires during a single estrous cycle has the potential to facilitate rapid genetic gain and improve reproductive performance within a canine breeding colony. There is limited data regarding the factors that contribute to the success of dual-sired litters in domestic dogs, and only anecdotal evidence suggesting that these litters rarely produce offspring from more than one sire. The objective of this prospective clinical study was to investigate multiple factors that likely affect the success of dual-sired litters on whelping rate, litter size and parentage ratio. These factors include: timing of artificial insemination (AI), order of sires, number of AI's per cycle, semen type, sperm quality and age of sire and bitch. Data collected over a 10 year period from twenty-nine estrous cycles (28 individual bitches of 10 different breeds) were evaluated after an initial AI with frozen semen from the 'genetically desired' sire and followed up with a second AI with either fresh (n = 9) or frozen (n = 16) semen or natural mating (n = 4) from a different, 'back up', sire. DNA parentage of each pup born was determined by using a primary panel of 288 SNPs. The whelping rate and litter size from previous single sire inseminations per estrous cycle, in the same bitches, (n = 16) over 25 estrous cycles using either fresh (n = 4) or frozen-thawed (n = 21) semen, were analyzed as controls. Of the 29 dual-sired breedings, 26 bitches whelped (89.7%), and 8 litters (30.8%) were of mixed parentage. In the litters of mixed parentage after a dual-sired breeding, a greater proportion of the offspring were from the second sire than the first sire (73.0% and 27.0% respectively; P < 0.05). Interestingly, in litters where all pups were of single sire parentage after a dual-sired breeding, 50.0% of the offspring were by the first sire and 50.0% were by the second sire. For litters of mixed or single paternity produced by dual-sired breeding there was no difference in average litter size. However, on a per estrous cycle basis for each bitch the whelping rate (89.7% v. 76.0%.) and litter size (5.5 ± 2.5 v. 4.0 ± 2.78) of all dual-sired breedings were greater (P < 0.05) compared to previous single-sired breedings (controls) respectively. This study demonstrates that offspring of mixed parentage derived from dual-sired breedings may be achieved. Furthermore, insemination with semen from two different sires may increase the whelping rate and litter size, which is an important consideration when using genetically valuable, or older individuals with potentially reduced fertility.

Multiple mating is linked to social setting and benefits the males in a communally rearing mammal

Individuals in social species may mate with multiple opposite-sex individuals, including members of the same or different social groups. This variation may be linked to genetic benefits, where multiple mating decreases risk of inbreeding. Multiple mating may also be constrained by the sociospatial setting through its effect on availability of mates. Because multiple mating with individuals from same or different groups may determine sex-specific fitness effects, we also examined how multiple mating modulates social benefits of females and males. We used 7 years of data on demography, social organization, and genetics of a natural population of the group-living and colonial rodent, Octodon degus, to determine how kin and sex composition within social groups, and spatial relations between these groups (i.e., colonial habits) influence multiple mating and its fitness consequences. Males (81.3%) and females (64.9%) produced offspring with multiple opposite-sex individuals within groups and with individuals of neighboring groups. Thus, polygynandry was the dominant mating system in the degu population examined. Multiple mating in degus was high when compared with estimates reported in other social mammals. Variation in female and male multiple mating was better explained by social setting through its effect on availability of potential mates rather than by benefits derived from decreasing risk of inbreeding. Finally, our study revealed how multiple mating enhances male, but not female reproductive success.

An Inconvenient Truth: The Unconsidered Benefits of Convenience Polyandry

Polyandry, or multiple mating by females with different males, is commonplace. One explanation is that females engage in convenience polyandry, mating multiple times to reduce the costs of sexual harassment. Although the logic underlying convenience polyandry is clear, and harassment often seems to influence mating outcomes, it has not been subjected to as thorough theoretical or empirical attention as other explanations for polyandry. We re-examine here convenience polyandry in the light of new studies demonstrating previously unconsidered benefits of polyandry. We suggest that true convenience polyandry is likely to be a fleeting phenomenon, even though it can profoundly shape mating-system evolution via potential feedback loops between resistance to males and the costs and benefits of mating.