Showing papers by "Tim R. Birkhead published in 1995"

Male phenotype and ejaculate quality in the zebra finch Taeniopygia guttata

Abstract: We tested the idea that female preference for relatively attractive extra-pair males arises because the morphological and behavioural features that females find attractive covary with ejaculate features: Sheldon's (Proc. R. Soc. Lond. B 257 25-30 (1994)) phenotype-linked fertility insurance hypothesis. Two phenotypic traits that female zebra finches find attractive in males are song rate and symmetry of chest band plumage, but we found neither of these to be significantly related to any of the following ejaculate features: number of sperm, percentage of live sperm, absolute number of sperm, sperm length or sperm swimming velocity. Furthermore, and surprisingly, we did not find the predicted negative relationship between male song rate and fluctuating asymmetry of chest band plumage. Because most ejaculate features (except sperm numbers in rested males) show low levels of repeatability, it is unlikely that female zebra finches could reliably obtain a better quality ejaculate by choosing to copulate with a more attractive male. There was thus no evidence for the phenotype-linked fertility insurance hypothesis. Nor did we obtain evidence for the more general fertility insurance hypothesis: we found that female zebra finches paired to a vasectomized male, and hence receiving no sperm, were no more likely to seek an extrapair copulation than females paired to an intact male.

Ejaculate quality and the success of extra-pair copulations in the zebra finch

Abstract: IN many passerine birds, sperm competition1,2 is intense and extra-pair paternity frequent3. The outcome of sperm competition is often determined by relative sperm numbers4,5, and theory predicts that males should maximize the number of sperm they ejaculate during extra-pair copulations6,7. Differences in sperm quality between males also affect the outcome of sperm competition4. Here we report that the swimming velocity of sperm of the zebra finch, Taeniopygia guttata, varies predictably within males, and is determined, together with sperm numbers, by the time since last ejaculation. By performing extra-pair copulations outside their own-pair copulation period, males maximize both the quality and number of sperm in ejaculates. These effects are a consequence of the way sperm are stored and mature in the male reproductive tract. The disportionate success of extra-pair copulations8, also seen in other birds9, may therefore be explained in terms of the independent effects of sperm numbers and velocity.

Sperm precedence in zebra finches does not require special mechanisms of sperm competition

Abstract: Competition between the spermatozoa of different males to fertilize the eggs of a single female acts as a selection pressure on the behaviour of males and females. However, quantitative predictions about behaviour can only be made if the paternity consequences of different patterns of copulation are known. Because exhaustive empirical measurement of these consequences may be impractical, interest has centred on determining the mechanisms by which sperm competition occurs, knowledge of which may allow consequences to be calculated. One method of elucidating mechanisms of sperm competition is to use mathematical models to determine which mechanisms are necessary or sufficient to account for empirical observations. We use this approach for zebra finches Taeniopygia guttata and show that empirically measured rates of disappearance of sperm from the reproductive tract, and differences in the number of sperm in the first and subsequent ejaculates of each male, are sufficient to account for observed levels of sperm precedence. Special mechanisms of sperm competition, such as displacement or stratification of sperm, are therefore unnecessary to explain sperm precedence in this species.

Sperm competition: evolutionary causes and consequences

Abstract: The interaction between functional and mechanistic approaches to sperm competition and between male and female perspectives are described and illustrated by a study of the zebra finch, Taeniopygia guttata. Sperm competition experiments in the laboratory show that last male sperm precedence occurs (as it does in many other taxa) although the mechanism is unknown (as in most other taxa). Empirically-derived values were used to construct a mathematical model of sperm competition in the zebra finch. The model indicates that precedence occurs as a consequence of: (i) the temporal pattern of pair copulations; (ii) the rate at which sperm are lost from the female tract; and (iii) more sperm being transferred during extra-pair copulations than during pair copulations. The latter effect is a consequence of males seeking extra-pair copulations after their own pair copulation period has ended. The effect of sperm numbers on the pattern of sperm precedence may be further increased by: (i) extra-pair males increasing ejaculate size (sperm numbers) (for which there is no evidence); (ii) extra-pair males being of a better quality and transferring more sperm or better quality sperm (for which there is some evidence); and (iii) cryptic female choice. Females eject over 99% of sperm following insemination; if they eject fewer sperm from males chosen as extra-pair copulation partners, the potential for cryptic female choice is considerable. However, this is still being investigated. The model also predicts the optimal time for an extra-pair copulation to occur (from either a male or female perspective). A comparison between the predicted and observed pattern suggests that the optimal timing of extra-pair copulations is constrained in both sexes.

Sperm precedence in the domestic fowl

Abstract: The aim of this study was to examine last-male sperm precedence in the domestic fowl. We used sperm from two different genotypes to assign paternity, and in seven experiments females were artificially inseminated with either equal or unequal numbers of sperm at intervals of 4 or 24 h. We were unable to replicate the results of a previous study by Compton et al . (1978) in which a strong last-male precedence effect had been recorded when two equal sized inseminations were made 4 h apart. We observed no marked last-male sperm precedence and our results did not differ significantly from that predicted by the passive sperm loss model, in which a last-male effect is determined by the rate at which sperm are lost from the female tract and the interval between successive inseminations. The most likely explanation for the disparity between our result and Compton et al .9s is a difference in the timing of inseminations. The implications of this for studies of sperm competition in birds is discussed.

Sperm competition and unhatched eggs in the House Sparrow

Abstract: There is considerable debate over the benefits that female birds obtain from engaging in extra-pair copulations (Westneat et al. 1990, Birkhead and M0ller 1992). Two main classes of benefit have been identified: direct benefits, which includes fertility insurance (e.g. Sheldon 1994), and indirect or genetic benefits (see Birkhead and Moller 1992, Moller 1994). In the House Sparrow Passer domesticus, Wetton and Parkin (1991) found that the occurrence of unhatched eggs was associated with extra-pair paternity in the same clutch and suggested that this might occur because females guard against low fertility of their mate by engaging in a relatively small number of extra-pair copulations. Both Birkhead and M0ller (1992: 15) and Lifjeld (1994) offered alternative explanations for Wetton and Parkin's (1991) result. Lifjeld (1994) suggested that unhatched eggs may have been the consequence of social interactions. That is, female House Sparrows paired to poor quality males would be more likely to seek extra-pair copulations, and because extra-pair copulations in this species are often forced and occur during 'communal displays' (M0ller 1987), these females would be subject to an unusual amount of harassment. As a consequence, not only would these females lay a greater proportion of infertile eggs, they would also produce some eggs fathered by extra-pair males.

Ejaculate features and sperm utilization in peafowl Pavo cristatus

Abstract: We estimated the number and quality of sperm that male peafowl (Pavo cristatus) transferred during copulation to examine differences in these features between males. Ejaculate size was estimated in two ways: directly from sexually rested males copulating with a stuffed female (mean number of sperm per ejaculate 124 $\times $ 10$^{6}$ $\pm $ 39.73 $\times $ 10$^{6}$ s.d.); and indirectly as the number of sperm on the outer perivitelline layers of laid eggs (1386 $\pm $ 116 s.d.). Neither method revealed any significant difference between males. The mean percentage of live sperm in ejaculates was 84% $\pm $ 12 s.d. (n = 6 males) and the number of live sperm per ejaculate was significantly and positively correlated with the number of sperm found on the outer perivitelline layers of eggs laid by females inseminated by those males. There was no significant difference between males in the proportion of live sperm per ejaculate. The instantaneous rate at which sperm were lost from the female tract was 0.0067 sperm h$^{-1}\pm $ 0.0008 s.e. and the mean duration of sperm storage was 26 days $\pm $ 8.6 s.e. (n = 9 females), both these values are intermediate between those found in other galliformes. Beacause neither of the two methods for estimating of the numbers of sperm per ejaculate size differed significantly between males, it seems unlikely that female peafowl could obtain reliable direct fertility benefits from choosing to copulate with particular males.