Görgen Göransson

CAN VERTEBRATE PREDATORS REGULATE THEIR PREY

Whether vertebrate predators can regulate their prey or not has long been a controversial question. At the one extreme it has been claimed that predators have no impact on prey numbers but consume only a doomed surplus (Errington 1946), and at the other that predators strongly interact with their prey causing either stable equilibria or cycles (Tanner 1975; Keith et al. 1977). However,.there are no field studies showing a regulatory effect of predation among vertebrates. Here we report on such a study.

MHC genotype and male ornamentation: Genetic evidence for the Hamilton-Zuk model

The major histocompatibility complex (MHC) is an immunologically important cluster of highly variable genes that is known to affect fitness in domesticated mammals and birds. Spur length of male pheasants in southern Sweden correlates with male viability, female mate choice, and offspring survival rate. Here we show by genetic analyses that the MHC genotype is associated with variation in both male spur length and male viability. These are the first data that directly support a ‘good genes’ hypothesis by Hamilton and Zuk predicting that females discriminate among males on the basis of secondary sexual characters in order to pass on genes for disease resistance that improve fitness in their offspring.

Male characteristics, viability and harem size in the pheasant, Phasianus colchicus

A population of pheasants was studied for 4 years in southern Sweden to determine how sexual selection operates among males. Morphological characters, viability, dominance, territory quality, date of territorial establishment, harem size and reproductive success of males were measured; 81 males and 101 females were radio-tracked. The spur length of males was the most important predictor of harem size. Phenotypic condition and viability were significantly related to spur length, the best single predictor of the reproductive success of males. These are the first data to show that a sexually selected male character correlates significantly with male viability. The results support models suggesting that viability-based processes can contribute to the evolution of mate choice and secondary sexual characters.

Artificial selection for increased comb size and its effects on other sexual characters and viability in Gallus domesticus (the domestic chicken)

The expression of secondary ornaments is often positively correlated with the bearers condition. So far no study has reported on consequences of directional selection on male sexual ornaments in birds, i.e. it is unknown to what extent male sexual traits are genetically correlated and whether selection for secondary ornaments affects viability. In this study we estimate phenotypic and genotypic parameters of three male sexual traits, namely comb size, spur length and testes mass, by using two different lines of domestic chickens (Gallus domesticus). One of the two lines had been artificially selected for large comb size during nine generations whereas the other line was a randomly mated control line. As a result of the selection, male comb size was almost doubled. Data from the selection line indicate that comb size was negatively genetically correlated with testes mass and spur length. The cockerels from the selection line had accordingly lighter testes and shorter spurs than cockerels from the control line. A simple mate choice experiment with females and males from both lines indicated that preferred males had a larger joint expression of comb size and spur length than nonpreferred males. Data from three different selection lines, all with enlarged comb size, clearly reveal a negative effect of comb size on male viability.

Bird fauna of cultivated energy shrub forests at different heights

A farmland area in southern Sweden used for cereal production was turned into energy production by growing Salix. The breeding birds were censused in these fields after energy foresting had started and the first harvest was taken. The introduction of Salix virninalis in the fields showed to be favourable for species well adapted to bushy habitats like the marsh warbler. The habitat succession from the planted or harvested forest to the four, five years later mature forest is important too, as bird species prefer different growth stages of the brush forest. The whitethroat and the whinchat preferred the sprouts of the recently harvested areas, but the willow warbler and the garden warbler preferred the fully grown bushes preceeding the harvest. Introduction of Salix fields in an open farmland landscape will increase the number of bird species and for some species also the abundance. The biodiversity is improved if the harvest is asynchronous in different subareas. In terms of environment and fauna protection or improvement a limited increase of scattered energy producing Salix fields will probably be favourable, causing an increase in overall biological diversity.

Nest attendance during egg laying in pheasants.

As precocial bird species hatch synchronously, incubation during the egg-laying stage should be disadvantageous because it makes the embryos develop asynchronously. We established the patterns of nest attendance during egg laying and the start of incubation in ring-necked pheasants, Phasianus colchicus, and tested three hypotheses regarding the advantage of early incubation. To determine nest attendance, we measured egg temperatures in real pheasant nests. Females spent more time on the nest as laying progressed, with an average of 6.4 h at a clutch size of 10. At the start of incubation, nest attendance increased to over 20 h/day. On the day before full incubation, time spent on the nest was positively correlated with the females condition and negatively with the number of breeding attempts she had already made that season. The hypothesis that an early start of incubation improves egg viability was rejected, as the predicted relationship between the number of eggs laid after the start of incubation and the number laid before the start of incubation was not significant. We also rejected the possibility that early incubation reduces the risk of nest parasitism, as it was negatively related to the number of females radiotracked around the nest. Our data supported the hypothesis that early incubation reduces the risk of nest predation by shortening the period of exposure, as the number of eggs laid after incubation started was positively related to the number of breeding attempts made by the female, and thus to the perceived predation risk, but was negatively related to the time of season. Copyright 1999 The Association for the Study of Animal Behaviour.