Multiple paternity in a lek mating system: Females mate multiply when they choose inexperienced sires

Paternity bias and cryptic female choice in chickens

Sperm competition and cryptic female choice (CFC) are 2 significant mechanisms of postcopulatory sexual selection that greatly impact fertilization success in various species. Despite extensive research has conducted on sperm competition and the evolution of sperm traits in internal fertilization, our understanding of the female preferences in selecting sperm is still limited. Here, we aimed to investigate the characteristics of CFC in chickens by utilizing artificial insemination with mixed semen to control for variations in male fertilization success caused by female perception of male quality and mating order. Our results revealed that the offspring from multiple-mated females exhibited mixed paternity. Although the males had an equal number of viable sperm, 1 male consistently exhibited a 15% higher success rate on average, regardless of whether the insemination was performed with fresh or diluted semen. This result suggested that this male demonstrates superior performance in sperm competition, and exhibited a potential advantage in fertilization success. While the dominant male generally made a greater genetic contribution to most offspring, the degree of this advantage varied greatly, ranging from 11.11 to 75%. Furthermore, our study provided evidence of female preferences influenced the precedence of sperm from certain males over others. Interestingly, this bias is not consistently observed among all individuals, as offspring derived from some females were predominantly sired by an overall disadvantaged male while others were predominantly by a different disadvantaged male. Overall, these results underscored the complex processes involved in sperm selection and emphasized the importance of females in sexual selection theory.

Hidden impacts of conservation management on fertility of the critically endangered kākāpō

Background

Animal conservation often requires intensive management actions to improve reproductive output, yet any adverse effects of these may not be immediately apparent, particularly in threatened species with small populations and long lifespans. Hand-rearing is an example of a conservation management strategy which, while boosting populations, can cause long-term demographic and behavioural problems. It is used in the recovery of the critically endangered kākāpō (Strigops habroptilus), a flightless parrot endemic to New Zealand, to improve the slow population growth that is due to infrequent breeding, low fertility and low hatching success.

Methods

We applied Bayesian mixed models to examine whether hand-rearing and other factors were associated with clutch fertility in kākāpō. We used projection predictive variable selection to compare the relative contributions to fertility from the parents' rearing environment, their age and previous copulation experience, the parental kinship, and the number of mates and copulations for each clutch. We also explored how the incidence of repeated copulations and multiple mates varied with kākāpō density.

Results

The rearing status of the clutch father and the number of mates and copulations of the clutch mother were the dominant factors in predicting fertility. Clutches were less likely to be fertile if the father was hand-reared compared to wild-reared, but there was no similar effect for mothers. Clutches produced by females copulating with different males were more likely to be fertile than those from repeated copulations with one male, which in turn had a higher probability of fertility than those from a single copulation. The likelihood of multiple copulations and mates increased with female:male adult sex ratio, perhaps as a result of mate guarding by females. Parental kinship, copulation experience and age all had negligible associations with clutch fertility.

Conclusions

These results provide a rare assessment of factors affecting fertility in a wild threatened bird species, with implications for conservation management. The increased fertility due to multiple mates and copulations, combined with the evidence for mate guarding and previous results of kākāpō sperm morphology, suggests that an evolutionary mechanism exists to optimise fertility through sperm competition in kākāpō. The high frequency of clutches produced from single copulations in the contemporary population may therefore represent an unnatural state, perhaps due to too few females. This suggests that opportunity for sperm competition should be maximised by increasing population densities, optimising sex ratios, and using artificial insemination. The lower fertility of hand-reared males may result from behavioural defects due to lack of exposure to conspecifics at critical development stages, as seen in other taxa. This potential negative impact of hand-rearing must be balanced against the short-term benefits it provides.

Does Size Matter? The Case of the Courtship Pyramids in Red Sea Ghost Crabs (Ocypode saratan)

Display, wherein males attempt to maximize fitness by attracting sexually mature females to mate, is known to drive speciation by Sexual Selection. We researched the Red Sea Ghost Crab (Ocypode saratan; RSGC), in which males build display pyramids to attract females. The study was conducted at the beach in Eilat, Israel. At each session, we measured the height (in cm) of all pyramids and the dimensions (height, breadth; in cm) of the burrow entrance. We assumed that the size of the entrance represented the relative size of the carapace width of the occupant. The mean (± SE) entrance volume was 230.8 ± 11.7 cm, and the height of the pyramid was 11.8 ± 0.49 cm (n = 54). The results of our study did not support our hypothesis because we had expected to find a linear correlation between body size and pyramid height, i.e., the larger the male, the larger the pyramid. However, our results show that the largest males in the population either built small pyramids or not at all, and the cut-off of the larger crab's body size appears to be around 350 cm3. We discovered a step-wise function in the data in that crabs with the smallest body size of ca. 250 cm3 constructed the highest pyramids, with a declining tendency between 250-350 cm3 and extremely low pyramids beyond 350 cm3. However, our findings need to be further studied with a stress on the ambiance and elucidate whether the habitats differ in temperature, humidity, prey-base, etc., before concluding as to why the larger males desist from building pyramids. This study underwrites the importance of studying the mating systems of the macro-fauna of the beaches that are fast disappearing owing to anthropogenic development.

Integrative Studies of Sexual Selection in Manakins, a Clade of Charismatic Tropical Birds

The neotropical manakins (family Pipridae) provide a great opportunity for integrative studies of sexual selection as nearly all of the 51 species are lek-breeding, an extreme form of polygyny, and highly sexually dimorphic both in appearance and behavior. Male courtship displays are often elaborate and include auditory cues, both vocal and mechanical, as well as visual elements. In addition, the displays are often extremely rapid, highly acrobatic, and, in some species, multiple males perform coordinated displays that form the basis of long-term coalitions. Male manakins also exhibit unique neuroendocrine, physiological, and anatomical adaptations to support the performance of these complex displays and the maintenance of their intricate social systems. The Manakin Genomics Research Coordination Network (Manakin RCN, https://www.manakinsrcn.org) has brought together researchers (many in this symposium and this issue) from across disciplines to address the implications of sexual selection on evolution, ecology, behavior, and physiology in manakins. The objective of this paper is to present some of the most pertinent and integrative manakin research as well as introducing the papers presented in this issue. The results discussed at the manakin symposium, part of the 2021 Society for Integrative and Comparative Biology Conference, highlight the remarkable genomic, behavioral, and physiological adaptations as well as the evolutionary causes and consequences of strong sexual selection pressures that are evident in manakins.

Behavioral Sex Differences and Hormonal Control in a Bird with an Elaborate Courtship Display

Gonadal hormones can activate performance of reproductive behavior in adult animals, but also organize sex-specific neural circuits developmentally. Few studies have examined the hormonal basis of sex differences in the performance of elaborate, physically complex and energetic male courtship displays. Here we describe our studies over more than 20 years examining sex difference and hormonal control of courtship in Golden-collared manakins (Manacus vitellinus) of Panamaian rainforests. Our recent studies of birds studied in an artificial "lek" in a rainforest aviary provide many new insights. Wild and captive males and females differ markedly in their performance of male-typical behaviors. Testosterone (T) treatment augments performance of virtually all of these behaviors in juvenile males with low levels of circulating T. By contrast, T-treatment of females (with low circulating T) either failed to activate some behaviors or activated male behaviors weakly or strongly. These results are discussed within a framework of our appreciation for hormonal vs genetic basis for sex differences in behavior with speculation about the neural mechanisms producing these patterns of hormonal activation.

An experimental test to separate the effects of male age and mating history on female mate choice

Should females prefer older males as mates? Male survival to old age might indicate the presence of fitness-enhancing genes that increase offspring fitness. However, many correlational studies show that mating with older males can lower female fecundity and even reduce offspring fitness due to epigenetic or germline mutation effects. One problem in quantifying female choice based on male age is that age is usually confounded with mating history. This begs a question: Do females choose males based on their age or their mating history? The answer requires an experimental approach, but few such studies exist. Here, we test if experimentally induced variation in the mating history of old and young males (12-week difference in postmaturity age) affects female choice in the eastern mosquitofish (Gambusia holbrooki). To vary mating history, adult males were either allowed to freely mate with females for 3 weeks or they only had visual contact with females. Immediately thereafter, we ran four-choice mating trials, using association time, to test the effects of male age and mating history (2 × 2 design) on male attractiveness. Females did not show a clear preference for males based on either characteristic. This was not due to a lack of female choice: females spent significantly more time with larger males. In addition, female choice was significantly repeatable across four trials: twice as a virgin and twice as a nonvirgin. Finally, female mating status (virgin or nonvirgin) did not affect her choice of mate, although virgin females spent significantly more time associating with test males.

Why do females of a lekking species mate with multiple males?

In Focus: Rivers, P. R., & DuVal, E. H. (2020). Multiple paternity in a lek mating system: Females mate multiply when they choose inexperienced sires. Journal of Animal Ecology, 89, 1142-1152. In many socially monogamous species with biparental care, some females engage in extra-pair copulations, despite the apparent risk of losing male help in raising their brood. Why females mate with multiple males remains the focus of intense debate, but may be linked to constraints in social mate choice. If so, multiple mating is not expected in a lekking mating system, where females receive no resources from the male other than sperm and are free to choose the 'best' mate to sire their offspring. Rivers and DuVal (2020) report on the occurrence of multiple paternity in lekking lance-tailed manakins and test several hypotheses to explain it. Their study shows that lower-ranked males and males with less experience were more likely to share paternity. However, the process underlying this pattern remains puzzling. The authors suggest female choice related to male characteristics that change with experience as an explanation, but alternative scenarios cannot be excluded.