Kai Lindström

How cuckoldry can decrease the opportunity for sexual selection: Data and theory from a genetic parentage analysis of the sand goby, Pomatoschistus minutus

Alternative mating strategies are common in nature and are generally thought to increase the intensity of sexual selection. However, cuckoldry can theoretically decrease the opportunity for sexual selection, particularly in highly polygamous species. We address here the influence of sneaking (fertilization thievery) on the opportunity for sexual selection in the sand goby Pomatoschistus minutus, a marine fish species in which males build and defend nests. Our microsatellite-based analysis of the mating system in a natural sand goby population shows high rates of sneaking and multiple mating by males. Sneaker males had fertilized eggs in ≈50% of the assayed nests, and multiple sneakers sometimes fertilized eggs from a single female. Successful males had received eggs from 2 to 6 females per nest (mean = 3.4). We developed a simple mathematical model showing that sneaking in this polygynous sand goby population almost certainly decreases the opportunity for sexual selection, an outcome that contrasts with the usual effects of cuckoldry in socially monogamous animals. These results highlight a more complex and interesting relationship between cuckoldry rates and the intensity of sexual selection than previously assumed in much of the literature on animal mating systems.

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.

Competition Versus Cooperation: Success of Individuals Foraging Alone and in Groups

Using a variant of information-sharing models, we examine the pros and cons of group foraging against the alternative of staying alone. Models of this category-assuming that in groups food finding by one results in food sharing by many-conclude that patch-finding rate improves with group size. In our modification interference among individuals reduces pooled searching efficiency of the group. We introduce a term, s, the probability of an individuals being among the ones sharing the food in a patch found by the group. Not unexpectedly, these fine-tunings prolong food-finding rates that push individuals in the group toward the foraging status of a solitary individual. With phenotype-related differences in s, foraging in groups turns out to be a less profitable option the lower an individual is ranked in the group. The model suggests that, in terms of food finding, individuals have to pay attention to their performance in the foraging group. The option of foraging alone may easily be a better strategy than that of a low-ranking individual foraging in a group. If so, the model also suggests groups assorted by phenotype.

Water turbidity by algal blooms causes mating system breakdown in a shallow-water fish, the sand goby Pomatoschistus minutus.

Eutrophication as a result of human activity has resulted in increased algal blooms and turbidity in aquatic environments. We investigated experimentally the effect of algal turbidity on the mating system and sexual selection in the sand goby, Pomatoschistus minutus (Pallas), a marine fish with a resource–defence mating system and paternal care. Owing to male–male competition and female choice, large males can monopolize multiple mates, while some males do not achieve mating at all. We show that the number of eggs laid was the same in both turbid and clear tanks but that mating success was more evenly distributed among males in turbid than in clear water. The opportunity for sexual selection was lower in turbid conditions. In turbid conditions mating success was less skewed towards large males. Our results suggest that increased turbidity can change mating systems and decrease the opportunity for sexual selection as well as selection intensity.

Environmental deterioration compromises socially enforced signals of male quality in three-spined sticklebacks.

Social costs are often important in promoting the honesty of sexually selected traits. What happens, then, when social costs are relaxed? In species that breed in shallow coastal waters, increases in the frequency and severity of phytoplankton blooms may undermine the value of visual signals by reducing visibility and, in so doing, lead to dishonest signaling by relaxing the social consequences of high signaling effort for poor‐quality individuals. Here, we experimentally test the effects of algally induced water turbidity on the role of male‐male competition in facilitating reliable sexual displays in three‐spined sticklebacks. We found that males in poor condition reduced their courtship effort in the presence of competition in turbid water. This reduction, however, was to a much lesser extent than that observed in clear water. Thus, courtship under conditions of algal turbidity did not reflect male condition as honestly as courtship in clear water. Algal turbidity also influenced breeding coloration, with males in poor condition reducing their area of red nuptial coloration in turbid conditions. Our findings suggest that anthropogenic disturbance to the signaling environment can potentially reduce the evolutionary potential of sexual selection by diminishing the efficacy of visual displays and weakening socially enforced signals of male quality.

The effect of resource holding potential, nest size and information about resource quality on the outcome of intruder-owner conflicts in the sand goby

SummarySand goby, Pomatoschistus minutus Pallas, males need a nest site for successful breeding. The value of the nest increases with size, as larger nests can hold more eggs. In the field using artificial nest sites, takeovers occurred in 29.0% of the cases. The winning intruders were on average 16.3% larger than the owners. These, in turn, were significantly smaller than the nonreplaced males. Replacement probability increased with nest size. Prior ownership was of no importance in experimental situations, but might have affected takeover probabilities in the field. Males assessed the size of a potential nest site on the basis of its external appearance. When intruders and owners were given opposite information about nest size the experiments resulted in takeover rates of around 50% in situations where takeover rates were expected to be extreme. This suggested that intruders might update their information about a sites true value during the fight. The continuous assessment, in turn, might affect their motivation to continue fighting. On average, larger males were more active in initiating displays. However, smaller opponents challenged their larger opponents more actively when they were defending a large nest than when they were defending a small nest. The greater willingness to defend might provide the intruder with more information about the real value of the nest.

Habitat-specific clutch size and cost of incubation in common eiders, Somateria mollissima

Abstract Common eiders, Somateria mollissima, breed on two types of island in the Northern Baltic: open, sparsely vegetated islands and wooded islands with dense mixed forests. On open islands 79.8% of the nests were on open cliffs, exposed to wind and rain whereas on wooded islands 91.7% of the nests were sheltered inside dense spruce and pine thickets. We found that clutch size on open islands was significantly smaller than on wooded islands. Females started breeding simultaneously in the two habitats and they were similar in body size as measured by the length of the radio-ulna. During incubation females on open islands lost weight at a faster rate than females on wooded islands (34 g/day and 19 g/day, respectively). Heat loss is faster on open than wooded islands and therefore we suggest that the faster weight loss of females on open islands result from thermodynamically adverse incubation conditions. Because the eider is an extreme capital breeder, energy used for egg production cannot be used for incubation. To sustain a higher incubation cost on open islands, the optimal clutch size is therefore lower than on wooded islands.

Sexual selection for male parental care in the sand goby, Pomatoschistus minutus

Male parental care is typically thought to come at a cost to mate attraction and future mating success. However, it has also been hypothesized that paternal care may be under sexual, as well as natural, selection, such that good fathers actually attract more mates. Here we show experimentally that in the sand goby, Pomatoschistus minutus, females prefer to mate with males that provide higher levels of parental care. We manipulated male behavior using (1) different nest sizes and (2) an application of low-O2 water in the nests, and found that females consistently preferred males with elevated levels of care in dichotomous mate choice tests. This complements our earlier study in which we showed that males increase the amount and quality of care they provide in the presence of females. Our results demonstrate that male care may have evolved as a result of sexual selection rather than natural selection alone, and furthermore, that male care may not necessarily be in conflict with mate attraction.

Female spawning patterns and male mating success in the sand goby Pomatoschistus minutus

In June 1989 in a study conducted near Träminne Zoological Station, Finland (60° N 23° E) I investigated whether or not male mating success could be explained by female choice for male size in sand gobies (Pomatoschistus minutus). Male mating success was constrained by nest size and increased markedly with increasing nest size. I also found a negative correlation between the length of spawning females and the fullness of the nest. As large females lay more eggs, they also need to find a nest with more space available for the eggs. The size of males without eggs was the same across nest size, whereas the size of males with eggs significantly increased with increasing nest size. This is interpreted as female discrimination against males as mates in nests that are often contested. There was no correlation between a males size and his mating success when males with no eggs in their nests were excluded. A male removal experiment, however, showed that, in a specific nest, when male size increases so does mating success, whereas, if male size decreases, mating success also decreases. It is concluded that in the sand goby females prefer to mate with larger males, especially when the male possesses a high-quality nest that he most probably will have to defend against other males.

Effects of costs and benefits of brood care on filial cannibalism in the sand goby

Abstract This study tested experimentally whether clutch size and the cost of care affect filial cannibalism in the sand goby, Pomatoschistus minutus. Evolutionary models of filial cannibalism suggest that egg eating has evolved as a way for the male parent to prolong his breeding season. These models assume that eggs function as an alternative energy source for the constrained parent. I manipulated clutch size by allowing males to mate with either one or two females, representing a small and a large clutch, respectively. The addition of a small male shore crab, a common egg predator, increased the cost of care. I quantified fat reserves as a measure of the condition of guarding males. Males who did not build nests had lower fat reserves than males who built nests, suggesting that males with low energy reserves do not start breeding. Males with small clutches lost their nest to the crab more often than males with large clutches. Neither filial cannibalism nor the amount of eggs eaten were affected by the treatments. Males who consumed eggs had a higher fat percentage than males who did not eat eggs. The result that males with small clutches lost their nests to the crabs supports the idea that eggs are defended only if the benefit from continued care will outweigh the cost and that males therefore are sensitive to the trade-off between present and future reproductive success.