The mismeasurement of sexual selection

Sexual selection can explain major micro‐ and macro‐evolutionary patterns. Much of current theory predicts that the strength of sexual selection (i) is driven by the relative abundance of males and females prepared to mate (i.e. the operational sex ratio, OSR) and (ii) can be generally estimated by calculating intra‐sexual variation in mating success (e.g. the opportunity for sexual selection, I_s). Here, we demonstrate the problematic nature of these predictions. The OSR and I_s only accurately predict sexual selection under a limited set of circumstances, and more specifically, only when mate monopolization is extremely strong. If mate monopolization is not strong, using OSR or I_s as proxies or measures of sexual selection is expected to produce spurious results that lead to the false conclusion that sexual selection is strong when it is actually weak. These findings call into question the validity of empirical conclusions based on these measures of sexual selection.     

The Relationship Among Swimming Performance, Courtship Behavior, and Carotenoid Pigmentation of Guppies in Four Rivers of Trinidad

In the laboratory, courtship behavior and carotenoid pigmentation of male guppies are condition-dependent traits, since their expression is affected by physical vigor and environmental factors such as water velocity and diet. Whether these relationships exist in guppies under field conditions is yet to be determined. We compared the swimming performance, courtship behavior, and carotenoid pigmentation of guppies from headwater and downstream localities in four rivers of Trinidad. Swimming performance and courtship behavior of males differed among rivers and between headwater and downstream sites. Guppies from headwater sites swam significantly faster and had higher display rates than those from downstream sites. Mean swimming performance across sites was positively correlated with mean water velocity, but was correlated with the number of orange color spots (carotenoid pigment) in only one river. These results indicate that the courtship behavior of Trinidadian guppies is condition-dependent because the amount of display behavior is positively correlated to swimming performance, a measure of physical endurance. The proximal cause for this condition-dependence may be predator induced variation in microhabitat use by guppies in headwater and downstream locations.     

The opportunity to be misled in studies of sexual selection

It is a challenge to measure sexual selection because both stochastic events (chance) and deterministic factors (selection) generate variation in individuals' reproductive success. Most researchers realize that random events ('noise') make it difficult to detect a relationship between a trait and mating success (i.e. the presence of sexual selection). There is, however, less appreciation of the dangers that arise if stochastic events vary systematically. Systematic variation makes variance-based approaches to measuring the role of selection problematic. This is why measuring the opportunity for sexual selection (I(s) and I(mates)) is so vulnerable to misinterpretation. Although I(s) does not measure actual sexual selection (because it includes stochastic variation in mating/fertilization success) it is often implicitly assumed that it will be correlated with the actual strength of sexual selection. The hidden assumption is that random noise is randomly distributed across populations, species or the sexes. Here we present a simple numerical example showing why this practice is worrisome. Specifically, we show that chance variation in mating success is higher when there are fewer potential mates per individual of the focal sex [i.e. when the operational sex ratio (OSR), is more biased]. This will lead to the OSR covarying with I(s) even when the strength of sexual selection is unaffected by the OSR. This can generate false confidence in identifying factors that determine variation in the strength of sexual selection. We emphasize that in nature, even when sexual selection is strong, chance variation in mating success is still inevitable because the number of mates per individual is a discrete number. We hope that our worked example will clarify a recent debate about how best to measure sexual selection.     

Points of view: Why male cannibalism won't cause a female-biased OSR -- a comment on Smith and Wootton's paper

Reviews in Fish Biology and Fisheries -     

Points of view: Cannibalism and OSR -- a response to Kvarnemo

Reviews in Fish Biology and Fisheries -     

Heritability of male secondary sexual traits in feral guppies in Japan

Secondary sexual traits of male guppies show remarkable geographic variation, and male guppies can flexibly change the conspicuousness of their sexual traits within a few generations when they are introduced into new habitats. We examined the degree of conspicuousness and heritabilities of male secondary sexual traits in a feral guppy population in Okinawa, a subtropical island of Japan. Male guppies in this population showed high variation of their sexual traits such as dorsal and caudal fin lengths and red-orange color spot patterns on their bodies. Offspring–parent regressions revealed significant heritabilities of male body size, dorsal and caudal fin lengths, and the number and relative area of orange spots. Especially, the high heritability of the relative orange spot area of sons compared to that of fathers suggests some Y chromosome-linked contribution of the trait. On the other hand, coloration (hue and saturation) of orange spots did not show significant inheritance, probably because most components of orange spot coloration may be condition-dependent traits. These results compared with previous work in native guppy populations suggest female mate preferences based upon these male secondary sexual traits and low predation pressure in this population. Received: June 19, 2000 / Accepted: September 18, 2000     

Ultraviolet vision and mate choice in the guppy (Poecilia reticulata)

Ultraviolet (UV) vision is well documented for many speciesof vertebrates. UV cues are known to be used in foraging, navigationand in mate choice. We conducted a series of behavioral experimentsto investigate the role of UV perception in mate choice inboth female and male guppies (Poecilia reticulata). In ourexperiments the visual appearance of potential mates was manipulatedusing either UV transmitting (UV+) or UV blocking (UV-) filters.Female guppies significantly preferred UV+ males. Male guppiestended to prefer UV- females, but their preferences were marginallynonsignificant. Further experiments investigating the roleof luminance, indicate that UV wavelengths are probably beingused for color discrimination rather than for detecting differencesin brightness. These experiments raise the possibility thatUV is used in mate assessment in different ways by male andfemale guppies. This may reflect the different strategies thatthe two sexes have in order to maximize reproductive success.To our knowledge, these are the first data showing that UVis used by any fish species in mate selection.     

Genes, copying, and female mate choice: shifting thresholds

Recent experimental work on guppies (Poecilia reticulata) hasexamined the strength of genetic and cultural (copying) factorsin determining female mate choice. Using females from a populationwith a heritable preference for the amount of orange body colorpossessed by males, prior work discovered that a threshold differencein orange color among males existed below which females wouldchoose a less orange male if they observed another female choosethat male, but above which they consistently preferred the moreorange of the males, regardless of whether they viewed anotherfemale prefer the less orange male. I tested whether this thresholdcan be shifted by increasing the amount of mate-copying informationavailable to a female. I demonstrate that when a female hasthe opportunity to see two different model females independentlyprefer the less orange of two males or a single female neara drab male for a longer period of time (twice as long as inprior work), the observer female prefers this drab male evenwhen males dramatically differ in orange coloration.     

Sex differences in the trade-off between feeding and mating in the guppy

Male guppies Poecilia reticulata court almost continuously and often indulge in sneaky mating attempts. Female guppies, on the other hand, devote much of their time to foraging. This paper demonstrates that both sexes reduce their feeding rate in mixed-sex groups relative to single sex ones and suggests that the decrease is less pronounced for females than for males. Thus males make feeding and mating decisions that are dependent on hunger while female foraging behaviour is constrained by sexual harassment. The observed sex differences in behaviour are a consequence of asymmetrical mating costs in males and females.     

Crowding, sex ratio and horn evolution in a South African beetle community

Sexually selected ornaments and weapons are exceptionally variable, even between closely related species. It has long been recognized that some of this diversity can be explained by differences in mating systems between species, but there remains substantial variation between species with similar mating systems. We investigated the roles of sex ratio (measured as operational sex ratio, OSR) and population density (measured as mean male crowding, a measure indicating the average number of conspecific males that an individual male animal will encounter) in determining horn presence in a community of South African dung beetles. Analysis of data from 14 species using a generalized least-squares model incorporating phylogenetic influences found that both OSR and mean crowding were significant predictors of horn presence, with hornless species tending to show female-biased sex ratios and high levels of crowding. The influence of mean crowding on horn diversity between species probably reflects the difficulty of guarding and monopolizing females when many competitors are present, meaning that males who adopt 'scramble' tactics tend to be favoured.