Mate guarding, male attractiveness, and paternity under social monogamy

Socially monogamous species vary widely in the frequency ofextrapair offspring, but this is usually discussed assumingthat females are free to express mate choice. Using game-theorymodeling, we investigate the evolution of male mate guarding,and the relationship between paternity and mate-guarding intensity.We show that the relationship between evolutionarily stablemate-guarding behavior and the risk of cuckoldry can be complicatedand nonlinear. Because male fitness accumulates both throughpaternity at his own nest and through his paternity elsewhere,males evolve to guard little either if females are very faithfulor if they are very unfaithful. Attractive males are usuallyexpected to guard less than unattractive males, but within-pairpaternity may correlate either positively or negatively withthe number of extrapair offspring fertilized by a male. Negativecorrelations, whereby attractive males are cuckolded more, becomemore likely if the reason behind female extrapair behavior appliesto most females (e.g., fertility insurance) rather than thesubset mated to unattractive males (e.g., when females seek"good genes") and if mate guarding is efficient in controllingfemale behavior. We discuss the current state of empirical knowledgewith respect to these findings.     

Mate fidelity and parentage in Cory's shearwater Calonectris diomedea - field studies and DNA fingerprinting

Field studies on Cory's shearwater Calonectris diomedea , which were carried out in a breeding colony in the Aegean Sea between 1989 and 1993, revealed that almost all breeding mates stay together over many consecutive years. Mates usually changed when one partner disappeared (e.g. through death), whereas 'divorce' occurred at a rate of 2.7%. Since birds are nesting at very close quarters, the potential for extrapair copulation (EPC) and subsequent extrapair fertilization (EPF) seems to be high. Multilocus DNA fingerprints were used to determine the true parentage of 46 offspring (broods contain a single chick only) from 29 pairs (few pairs were studied in two and three successive years). There were no cases of extrapair paternity.     

Extrapair paternity, inclusive fitness, and within-group benefits of helping in western bluebirds

In central coastal California, USA, 3–16% of western bluebird ( Sialia mexicana ) pairs have adult male helpers at the nest. Demographic data on a colour-ringed population over a 13-year period indicate that helpers gain a small indirect fitness benefit through increases in the number of young fledged from nests of close kin. A small proportion of adult helpers (16%) that were able to breed and help simultaneously had higher annual inclusive fitness than males that only bred. These males comprised such a minor proportion of helpers that the mean fitness of helpers was still lower than the mean fitness of independent breeders. We used DNA fingerprinting to determine whether extrapair fertilizations alter within-group benefits enough to tip the balance in favour of helping behaviour. Overall, 19% of 207 offspring were sired by males other than their social father and extrapair fertilizations occurred in 45% of 51 nests. Intraspecific brood parasitism was rare so that mean mother-nestling relatedness approximated the expected value of 0.5. Extrapair paternity reduced putative father-offspring relatedness to 0.38. Mean helper-nestling relatedness was 0.41 for helpers assisting one or both parents and 0.28 for helpers aiding their brothers. Helpers rarely sired offspring in the nests at which they helped. Helping was not conditional on paternity and helpers were not significantly more closely related to offspring in their parents' nests than to offspring in their own nests. Although helpers may derive extracurricular benefits if helping increases their own or their father's opportunities for extrapair fertilizations, within-nest inclusive fitness benefits of helping do not compensate males for failing to breed. Breeding failure and constraints on breeding are the most likely explanations for why most helpers help.     

Breeding synchrony and extrapair fertilizations in two populations of red-winged blackbirds

We tested the relationship between synchrony of breeding andthe frequency of extrapair fertilizations (EPFs) in two populationsof red-winged blackbirds known to differ in female extrapairbehavior. We found no association between the number of simultaneouslyfertilizable females (temporal neighbors) and EPF rate in eitherpopulation, although a significant difference between populationsin the direction of this relationship (positive where femalesinitiated extrapair copulations and negative where males initiatedthem) suggested a modest difference in the influence of synchrony.Males losing offspring to EPFs tended to have more fertilizablefemales at that time than the actual sires in some analysesbut not in others. We also tested several assumptions underlyingtwo competing hypotheses for the effects of synchrony. We foundno evidence that females pursued extrapair copulations moreoften when other females were synchronous. Rather, females weremore likely to gain EFFs with exirapatr males whose social mateswere not yet building their nests. Synchrony also did not consistentlyaffect male pursuit of exirapair copulations or achievementof EPFs. These results suggest that timing of breeding has someeffects on extrapair activity, but that those effects are bothrelatively weak and influenced by other factors that vary betweenyears or populations.     

Extrapair paternity, migration, and breeding synchrony in birds

To understand interspecific patterns in the strength of sexualselection, variation in the costs and benefits of exercisingmate choice needs to be evaluated. One manifestation of sexualselection in birds is the occurrence of greatly variable levelsof extrapair paternity (EPP). A proposed general explanationfor this variation is that the benefits to females in seekingextrapair copulations vary in a predictable manner accordingto the degree of breeding synchrony because females are betterable to assess potential extrapair partners when males are simultaneouslyin breeding condition. This hypothesis predicts a latitudinaltrend in EPP because birds tend to breed more synchronouslyaway from the equator. Expanding on previous geographicallyand taxonomically restricted tropical/temperate comparisons,we used phylogenetically independent standardized linear contraststo show that this positive relationship persists when all birdspecies for which EPP estimates currently exist are considered.However, if a third factor covaries with latitude in the sameway as breeding synchrony and EPP, this relationship need notbe causal. Migration could also account for latitudinal variationin the benefits to females of pursuing EPP, if migration isassociated with (1) hasty or (2) inaccurate mate choice, (3)facilitated assessment of male quality through the condition-dependenceof arrival time, or (4) increased genetic variance in male quality.We show that migration distance is positively related to theproportion of EPP and that migration can statistically explainthe latitudinal trend in EPP, even when confounding factorsare simultaneously controlled. Hence, alternative explanationsfor latitudinal variation in EPP may be feasible, and carefulintraspecific tests are needed to assess their relative importanceand their implications for geographical variation in life-historyevolution.     

Female extrapair mate choice in a cooperative breeder: trading sex for help and increasing offspring heterozygosity

Sexual conflict between males and females over mating is common. Females that copulate with extrapair mates outside the pair-bond may gain (i) direct benefits such as resources or increased paternal care, (ii) indirect genetic benefits for their offspring, or (iii) insurance against infertility in their own social mate. Few studies have been able to demonstrate the different contexts in which females receive varying types of benefits from extrapair mates. Here, I examined sexual conflict, female extrapair mate choice, and patterns of extrapair paternity in the cooperatively breeding superb starling Lamprotornis superbus using microsatellite markers. Although extrapair paternity was lower than many other avian cooperative breeders (14% of offspring and 25% of nests), females exhibited two distinct mating patterns: half of the extrapair fertilizations were with males from inside the group, whereas half were with males from outside the group. Females with few potential helpers copulated with extrapair mates from within their group and thereby gained direct benefits in the form of additional helpers at the nest, whereas females paired to mates that were relatively less heterozygous than themselves copulated with extrapair mates from outside the group and thereby gained indirect genetic benefits in the form of increased offspring heterozygosity. Females did not appear to gain fertility insurance from copulating with extrapair mates. This is the first study to show that individuals from the same population mate with extrapair males and gain both direct and indirect benefits, but that they do so in different contexts.