Malte Andersson

EVOLUTION OF CONDITION-DEPENDENT SEX ORNAMENTS AND MATING PREFERENCES: SEXUAL SELECTION BASED ON VIABILITY DIFFERENCES

The possibility that the evolution of mating preferences and secondary sex traits can be based on heritable differences in viability is examined with a three‐locus model. Earlier genetic models suggested that viability‐based processes alone cannot explain the evolution of mate choice and sex ornaments that reduce survival; a Fisherian mating advantage seemed necessary. The present model is based on a monogamous mating system that precludes such a mating advantage. A key assumption is that ornament development depends on the phenotypic condition and overall genotype of the possessor; there is evidence that secondary sex traits often mirror nutritional status and health, sometimes through hormonal mediation. Ornament and preference can then hitchhike slowly to high frequency with alleles that confer a slight survival advantage, provided that such alleles become available often enough. The evolution of mating preferences and secondary sex traits that reflect overall genotypic constitution therefore can be based solely on viability differences, no Fisherian mating advantage being required. In practice, these and several other mechanisms of sexual selection may occur together.

Parental defence of offspring: A model and an example

Abstract Defence of offspring against predators is an important form of parental investment in many species. We derive a model for the optimal level of parental defence during a predator attack. A higher level of defence increases offspring security, but it also exposes the parent to a higher risk. Other conditions being equal, the model predicts that the optimal level of defence increases with offspring age. This is because the relative difference between parent and offspring in expected future survival decreases with increasing offspring age. Compared with the parent itself, the relative importance of the offspring for parental inclusive fitness therefore increases. The risk that the parent should take in defending offspring therefore increases with its age. The model is applied to fieldfare (Turdus pilaris) nest defence. As predicted, parent fieldfares increase their defence throughout the nest period. The model also predicts the observed decrease in parental defence after the hatching and scattering of a precocial brood of young.

Multiple receivers, multiple ornaments, and a trade-off between agonistic and epigamic signaling in a widowbird

Sexual displays often involve several different ornamental traits. Yet most indicator models of sexual selection based on a single receiver (usually a choosy female) find that multiple handicap signals should be unstable. Here we study reasons for this contradiction, analyzing signal function, signal content, and trade‐offs between signals in the polygynous red‐collared widowbird Euplectes ardens. Males have both a long, graduated tail and a red carotenoid collar badge. Territory‐holding “residents” have slightly shorter tails than the nonbreeding “floaters,” but their carotenoid collars are 40% larger, and they have (on the basis of reflectance spectrometry and objective colorimetry) a 23‐nm more long‐wave (“redder”) hue than floaters. This corroborates experimental evidence that the red collar is selected by male contest competition, whereas female choice is based almost exclusively on male tail length. Tail length is negatively correlated with the carotenoid signal, which together with body size and condition explains 55% of the variation in tail length. The trade‐off in tail length and carotenoid investment is steeper among residents, suggesting an interaction with costs of territory defense. We propose that the “multiple receiver hypothesis” can explain the coexistence of multiple handicap signals. Furthermore, the trade‐off between signal expressions might contribute to the inverse relation between nuptial tail elongation and coloration in the genus Euplectes (bishops and widowbirds).

Genetic similarity between mates and extra-pair parentage in three species of shorebirds

Matings between close relatives often reduce the fitness of offspring, probably because homozygosity leads to the expression of recessive deleterious alleles. Studies of several animals have shown that reproductive success is lower when genetic similarity between parents is high, and that survival and other measures of fitness increase with individual levels of genetic diversity. These studies indicate that natural selection may favour the avoidance of matings with genetically similar individuals. But constraints on social mate choice, such as a lack of alternatives, can lead to pairing with genetically similar mates. In such cases, it has been suggested that females may seek extra-pair copulations with less related males, but the evidence is weak or lacking. Here we report a strong positive relationship between the genetic similarity of social pair members and the occurrence of extra-pair paternity and maternity (‘quasi-parasitism’) in three species of shorebirds. We propose that extra-pair parentage may represent adaptive behavioural strategies to avoid the negative effects of pairing with a genetically similar mate.

On the evolution of hoarding behaviour

Abstract We discuss some simple models of food storing in which we aim to identify the factors influencing the evolution of hoarding behaviour. When an individual stores food in a defended territory, the net benefit derived from hoarding depends on the cost of storing, the probability of recovering a stored item, its food value, and the multiplication rate of non-hoarded items. When hoarders live in a group, an additional cost may arise from ‘cheaters’, who do not store food themselves but parasitize the caches made by other individuals. Hoarding in this context can be adaptive if a hoarder is more likely than any other group member to recover its own items. This could come about either because individuals remember their own caching sites, or if individuals differ in their preferred foraging sites and cache food in their own kind of place. We also discuss conditions for the existence of a stable mixture of hoarders and cheaters in a group.

Nest Parasitism in Goldeneyes Bucephala clangula: Some Evolutionary Aspects

Intraspecific nest parasitism, in which parasitic females lay eggs in the nests of host females of the same species, occurs in many birds and certain insects. Most known cases concern waterfowl (Anatidae). We examined host responses to experimental parasitism, adding eggs to nests of laying goldeneye ducks. Three different hypotheses on host responses yield mutually exclusive predictions. (1) The host is a determinate layer; her own clutch is unaffected by the number of parasitic eggs. (2) The host is an indeterminate layer and responds to parasitic eggs as to her own. She reduces her clutch by the number of eggs added by the parasite. (3) The host is an indeterminate layer and estimates the number of parasitic eggs. She responds in an optimal fashion and reduces her own clutch with approximately half the number of parasitic eggs, hence maximizing the production of host offspring. With 2 + 2 eggs added on adjacent days early during the hosts laying period, prediction (2) is substantiated. Goldeneyes then respond to parasitic eggs as to their own, and tend to adjust their final clutch size to a given total number of eggs. However, the host did not lay fewer eggs if experimental eggs were added late during her laying period. Experimental parasitism with one or 2 + 2 eggs during the laying period did not raise the frequency of nest desertion. When seven eggs were added on the same day, no clutch was incubated, indicating that the host female detected the parasitism and abandoned the nest. This might be adaptive, since a large proportion of the clutch would not be her own.

Optimal foraging area: Size and allocation of search effort

Abstract A model is derived for the optimal spatial allocation of foraging effort for an animal returning with food to a central place in a uniform habitat. The forager is assumed to maximize its yield of food during a given period. Foraging effort is expended on search for food, and on transportation to the central place. It is shown that the allocation of search has been optimal if and only if the “marginal cost” of additional food is equal throughout the foraging area when the period has elapsed. The model is used to predict the optimal area radius and allocation of search time. With realistic parameter values, the optimal time per unit area roughly decreases linearly with the distance from the central place. The influence of food density and forager characteristics is examined.

Clumping versus spacing out: Experiments on nest predation in fieldfares (Turdus pilaris)

Abstract Fieldfares ( Turdus pilaris ), which nest solitarily as well as in colonies, offer an opportunity to assess within one species the relative advantages of clumping versus spacing out. An experiment with artificial, egg-baited nests showed that predation in the absence of fieldfares was higher on clumped than on scattered nests. In another experiment, we tested whether artificial nests run a higher risk of predation near solitary fieldfare pairs than near fieldfare colonies. Nest predation was higher near solitary fieldfare pairs than outside and inside fieldfare colonies. The risk probably was higher outside than inside colonies. Communal antipredator attacks is the likely cause of reduced predation near colonies. Even solitary fieldfare pairs confer some degree of protection, because predation was lower at artificial nests near solitary fieldfare pairs than at nests without neighbouring fieldfares.

PLANT PHENOLS AND NUTRIENTS IN RELATION TO VARIATIONS IN CLIMATE AND RODENT GRAZING

Secondary plant metabolites such as phenols and tannins sometimes have a repellent effect on herbivores and may reduce consumer pressure on plants. Grazer-induced changes in the concentrations of secondary metabolites have been proposed as an important cause of population cycles among small rodents. To test this hypothesis, we measured several secondary metabolites and nutrients in five species of plants during two rodent cycles on alpine heath in northern Sweden. We also tested whether climatic effects on plant chemistry could generate microtine cycles by altering food quality. Phenol concentrations (Folin-Denis, proanthocyanidin, and vanillin phenols) from 1974 to 1981 correlated strongly with each other, but correlated negatively with concentrations of nitrogen, phosphorus, and potassium. Astringent tannins, calcium, and magnesium showed few significant correlations with the other components. There was no evidence that rodent grazing markedly affected the chemical composition of plants during the same or the subsequent year. Secondary metabolites and nutrients also failed to show any clear relationship to the 4-yr cycles in rodent density, which are probably caused by other factors. Instead, the levels of phenols and nutrients seem to vary according to climatic fluctuations. In warm summers, nutrient concentrations decreased and phenols increased in four of the five plant species examined. A possible explanation is that the increased photosynthate in warm summers was partly allocated to synthesis of phenols and other carbohydrates, causing a simultaneous dilution of nutrients.

Why are there so many threat displays

Abstract Many animals have more than one kind of threat signal in their display repertoire. This condition is suggested to arise from a process of frequency-dependent signal efficiency in deterring opponents. Originally, a threat display is efficient because it is a reliable predictor of attack, but later it gains increasing use as “bluff”, without attack necessarily following. As individuals adapt their responses to the reduced reliability of the display, its efficiency decreases to the extent that a new pattern, which predicts attack more reliably than the old one, can gain use as a display. This new signal then will also become increasingly used as bluff. As a consequence, there will be competition between displays, and a state of balance in their use may arise due to frequency-dependent selection, where each display becomes less efficient for chasing away opponents as it becomes increasingly used as bluff. Proliferation of the display repertoire may be limited by restricted availability of behaviour patterns suitable for evolution into threat displays.