Elisabet Forsgren

Male mate choice selects for female coloration in a fish

Although sexual selection theory has proved successful in explaining a wide array of male ornaments, the function of ornaments occurring in females is largely unknown. Traditionally, female ornaments have been considered nonfunctional, being merely a genetically correlated response to selection for male ornamentation. However, this hypothesis is only relevant to species in which the ornament is basically the same in the two sexes. Alternatively, female ornaments may be influenced by selection acting directly on the females, either through female–female competition or male choice. We tested the latter hypothesis in mate-choice experiments with two-spotted gobies (Gobiusculus flavescens). In this small marine fish, females have bright yellow-orange bellies during the breeding season, a conspicuous trait that is not present in males. We conducted two aquarium experiments to test whether males preferred to mate with more colorful females. In the first experiment, males had a choice between two females that varied in natural coloration (and belly roundness). In the second experiment, we manipulated belly coloration and kept roundness constant. Males spent more time with colorful than with drab females in both experiments and also performed far more courtship displays toward colorful females. Our study provides experimental evidence that males prefer ornamented females in a fish that is not sex-role reversed, supporting the hypothesis that female ornamentation is sexually selected.

Predation Risk Affects Mate Choice in a Gobiid Fish

There has been a growing interest in factors influencing mating dynamics (see, e.g., Bateson 1983; Partridge and Halliday 1984). In particular, the conflicting demands of foraging and predator avoidance have been suggested to affect mating behaviors uch as search strategies, courtship, mate choice, and mating duration (Sih et al. 1990; Crowley et al. 1991; Magnhagen 1991). Facultative changes in mating behavior in response to alterations in predation pressure have been documented in male tungara frogs (Physalaemus pustulosus) (Ryan 1985) and in male guppies (Poecilia reticulata) (Endler 1987). A semiaquatic bug (Microvelia austrina) and a waterstrider (Gerris remigis) reduce their mating activity in the presence of predatory sunfish (Sih 1988; Sih et al. 1990). In a dynamic-game model Crowley et al. (1991) have shown how mate choice and assortative mating might depend on mate density, operational sex ratio, and predation risk over a breeding season. If choosiness is costly in terms of increased mortality caused by predation, the model predicts that females should become less choosy and mate indiscriminately as predation risk increases. Other models (Sutherland 1985; Hubbell and Johnson 1987) indirectly support his prediction as the strength of sexual selection decreases with lower survival and choosiness is affected. Here I present the first experimental study, as far as I know, that examines the direct effects of predation risk on mate choice of individual females. The sand goby (Pomatoschistus minutus) is a marine fish common on the coasts of Europe. It lives 1-2 yr and has only one breeding season, during which both sexes can reproduce several times. Males build nests under mussel shells and attract females by a leading display. After spawning, males care for the eggs until they hatch. There is no sexual dimorphism in size, but males have blue-black pelvic, anal, and tail fins and a bright blue spot on the dorsal fin that is lacking in females. Larger males have an advantage over smaller ones in male-male competition and seem to have greater eproductive success (Lindstrom 1988; Magnhagen and Kvarnemo 1990). The role that females play in determining mating success of males is not clear. The aim of this study was to investigate whether females prefer certain males over others and whether mate choice is affected by predation risk.

Unusually dynamic sex roles in a fish

Sex roles are typically thought of as being fixed for a given species. In most animals males compete for females, whereas the females are more reluctant to mate. Therefore sexual selection usually acts most strongly on males. This is explained by males having a higher potential reproductive rate than females, leading to more males being sexually active (a male-biased operational sex ratio). However, what determines sex roles and the strength of sexual selection is a controversial and much debated question. In this large-scale field study, we show a striking temporal plasticity in the mating competition of a fish (two-spotted goby, Gobiusculus flavescens). Over the short breeding season fierce male–male competition and intensive courtship behaviour in males were replaced by female–female competition and actively courting females. Hence, sex role reversal occurred rapidly. This is the first time that a shift in sex roles has been shown in a vertebrate. The shift might be explained by a large decline in male abundance, strongly skewing the sex ratio towards females. Notably, the sex role reversal did not occur at an equal operational sex ratio, contrary to established sex role theory.

On the function of female ornaments: male bluethroats prefer colourful females

Female ornaments in animals with conventional sex roles have traditionally been considered non–functional, being merely a genetically correlated response to selection for male ornamentation. Alternatively, female ornaments may be influenced by selection acting directly on the females, either through female–female competition or male choice. We tested the latter hypothesis in mate choice experiments with bluethroats (Luscinia s. svecica), a passerine bird in which females vary considerably in coloration of an ornamental throat patch. In outdoor aviaries placed in prime breeding habitat, males were allowed to choose between a colourful and a drab female. We found that males associated more with, and performed more sexual behaviours towards, colourful females. Female coloration was not age–related, but correlated significantly with body mass and tarsus length. Thus, we have demonstrated both a male preference for female ornamentation, and a relationship between ornament expression and female body size, which may be indicative of quality. Our results refute the correlated response hypothesis and support the hypothesis that female ornamentation is sexually selected.

Female sand gobies gain direct benefits by choosing males with eggs in their nests

Abstract In some fish species with paternal care, females prefer to spawn with males whose nests already contain eggs. Several hypotheses have been put forward to explain this behaviour, such as reduced risk of predation or cannibalism (the dilution effect), increased parental investment, and mate copying. This experimental study focuses on female mate choice in the sand goby, Pomatoschistus minutus. Females were found to choose males with eggs in their nests. In addition, hatching success increased with clutch size, mainly because males with larger clutches showed less filial cannibalism. Increased egg survival in large clutches may thus be explained by a combination of the dilution effect and higher parental investment. In another experiment, females did not seem to copy the observed mate choice of other females. In conclusion, female preference for males with eggs in their nests is adaptive, and can be explained by direct benefits, as more surviving offspring are produced.

Mode of sexual selection determined by resource abundance in two sand goby populations

We used field observations and experiments to show that sexual selection in two populations of sand gobies, Pomatoschistus minutus (Pisces, Gobiidae), was affected by differences in resource availability. Male sand gobies rely on empty mussel shells for nest building and spawning. The two populations differed considerably in nest‐site abundance and sexual‐selection regimes. In one population nest sites were scarce, leading to stronger male‐male competition over nests, a higher nest site colonization rate and reduced potential for female choice compared with the other population that had a surplus of nests. In the high‐competition population, males were larger than females, perhaps as a response to selection, whereas the other population was not sexually size dimorphic. The results from the field were confirmed in a pool experiment that demonstrated the effect of nest abundance on nest occupancy and male reproductive success. Larger males were more successful in obtaining nest sites in both high and low nest availability treatments. Larger males were also favored by females as mating partners, but only in the treatment with surplus nest sites. Nest shortage was associated with an increased potential for intrasexual selection (measured as the coefficient of variation), whereas the potential for intersexual selection was increased when nests were common. In conclusion, nest‐site abundance can influence the relative contribution of intrasexual competition and mate choice in a population. Hence, resource availability can contribute to within‐species variation in mating patterns.

Parental behaviour in relation to food availability in the common goby

In the common goby, Pomatoschistus microps (Pisces, Gobiidae), males build nests under mussel shells where they care for the eggs until hatching. To investigate why male common gobies cannibalize their own eggs (filial cannibalism), we conducted a feeding experiment. Males given little food ate from their eggs more often than males given food in excess. However, males given mussel meat in excess did not eat more of their eggs than males fed with both mussel meat in excess and goby eggs. This may suggest that male common gobies cannibalize their eggs to obtain energy rather than essential nutrients lacking in other diets. Moreover, males ate their whole clutch if it was exceptionally small regardless of food treatment, suggesting that males stop investing in their clutch if its reproductive value is less than the cost of guarding it. Thus, whole clutch cannibalism and partial clutch cannibalism seem to be governed by different factors. Furthermore, poorly built nests were associated with starved males, suggesting that nest concealing is costly. There was an association between how well the nest was built and partial clutch filial cannibalism, suggesting that the appearance of the nest may indicate the condition of the male, and thus the risk of filial cannibalism. Copyright 1998 The Association for the Study of Animal Behaviour.

Effects of sex ratio on intra- and inter-sexual behaviour in sand gobies

Reproductive behaviour and reproductive success in a marine fish, the sand goby, Pomatoschistus minutus, were affected by changes in the sex ratio. When the sex ratio was male biased (six males: three females), aggressive male-male interactions were more frequent per individual than in a female-biased situation (three males: six females). Accordingly, in the female-biased treatment females interacted more with each other than in the male-biased treatment. There was no difference between treatments in male interactions towards females, nor in female interactions towards males. Controls, with even sex ratios at two different densities (three or six of each sex), did not differ from each other in intra- or inter-sexual interactions. This shows that the differences in intra-sexual behaviour, found in males as well as females, were caused by the sex ratio and not by density. In the male-biased treatment and in the unbiased controls, nest-building males were larger than non-building males, whereas in the female-biased treatment there was no difference in size between builders and non-builders. Thus, our experiments show that a change in sex ratio changes competition for mates, and that individuals adjust their reproductive behaviour to the current sex ratio.

Seasonal change in female choice for male size in the two-spotted goby

Mate choice may vary according to various factors, such as mate availability and variation in mate quality. We tested seasonal changes in female choice regarding male size in a marine fish, the two-spotted goby, Gobiusculus flavescens. In this species, males occupy nests in which females lay their eggs and males thereafter provide exclusive parental care of the eggs until hatching. Females are predicted to prefer large males if, for example, they are better at competing for and defending nests. In this species, the opportunity for females to be selective should decline over the breeding season as the availability of males decreases. We therefore predicted a reduction in female choosiness with season. We conducted mate choice tests, during the early (May) and later (June) part of the breeding season, in which females were allowed to choose between males differing in body length. Females chose large males early in the season, but became unselective with respect to male size later on. This change in mate choice may be caused by a reduction in overall choosiness as a result of the decline in male availability with season. Alternatively, the change could be caused by a switch in choice cues used by females if factors other than male size become more important for female reproductive success later in the breeding season. Further studies are needed to test between these alternative explanations.

Male preference for colourful females affected by male size in a marine fish

It is becoming increasingly clear that mate preferences are not static, but can vary as a function of ecological conditions and the state of the choosing individual. This applies not only to females, the sex that has usually been the subject in research on mate preferences, but also to males. Under certain conditions, males should be selective in their choice of breeding partner. In the two-spotted goby, Gobiusculus flavescens, a small marine fish, breeding females develop conspicuous yellow-orange bellies, which they actively display to males during courtship. We have recently shown that males prefer more colourful females as mates. In this study, we test if the size of a male affects his preference for colourful females. Using three-compartment mate-choice aquaria, we recorded the interest shown by a male in two females differing in coloration but similar in size. Large and small males were equally eager to court females, but only large males showed a greater interest in the more colourful females. We suggest that small males are unselective because they usually obtain few mating opportunities, as a result of being unsuccessful in mate attraction or male contest competition. This study provides the first demonstration that the size of a male affects his preference for female colour.