Udo M. Savalli

Selected Abstracts


Savalli, U.M., Czesak, M.E., & Fox, C.W. 2000. Paternal investment in the seed beetle, Callosobruchus maculatus (Coleoptera: Bruchidae): Variation among populations. Annals of the Entomological Society of America 93: 1173-1178.
Ejaculate size in seed beetles (Coleoptera: Bruchidae) is subject to both sexual and fecundity selection. We examined interpopulation variation and inheritance of ejaculate size in the seed beetle Callosobruchus maculatus. There was significant variation among three populations in both body mass and the proportion of a male's body mass that was transferred to females during mating. The seed upon which beetles were raised had a small effect on male body size but not the size of their ejaculates. To investigate the inheritance of ejaculate size, we established inter- and intra-population crosses with two of these populations. The progeny of interpopulation crosses were intermediate between the intrapopulation (parental) crosses, suggesting additive genetic autosomal inheritance. This differs from an earlier study of a different population that suggested that ejaculate size was maternally inherited.

Savalli, U.M., & Fox, C.W. 1999. The effect of male mating history on paternal investment, fecundity, and female remating in the seed beetle Callosobruchus maculatus. Functional Ecology 13: 169-177.
In many organisms, males provide nutrients to females via ejaculates that can influence female fecundity, longevity and mating behaviour. We determined the effect of male mating history on male ejaculate size, female fecundity, female longevity and female remating behaviour in the seed beetle Callosobruchus maculatus. The quantity of ejaculate passed to females declined dramatically with successive matings. Despite the decline, a male's ability to fully fertilize a female did not appear to decline substantially until his fourth mating. When females multiply mated with males of a particular mated status, the pattern of egg production was cyclic, with egg production increasing after mating. Females multiply mated to virgins had higher fecundity than females mated to non-virgins, and females mated to twice-mated males had disproportionately increased egg production late in their life. Females that mated to multiple virgins, and consequently laid more eggs, experienced greater mortality than females mated only once or mated to non-virgins, suggesting that egg production is costly, and rather than ameliorating these costs, male ejaculates may increase them by allowing or stimulating females to lay more eggs. Females mating with non-virgin males remated more readily than did females mated to virgins. Females given food supplements were less likely to remate than females that were nutritionally stressed, suggesting that females remate in part to obtain additional nutrients.

Savalli, U.M., & Fox, C.W. 1999. The effect of male size, age, and mating behavior on sexual selection in the seed beetle Callosobruchus maculatus. Ethology, Ecology and Evolution 11: 49-60.
We use laboratory mating experiments to examine the effect of male size, age, and mating behavior on fecundity selection and sexual selection in the seed beetle, Callosobruchus maculatus (Coleoptera: Bruchidae), a species in which females are larger than males. Female C. maculatus gain a fitness advantage, in the form of increased lifetime fecundity, from mating with large males (which contribute larger ejaculates), but the partial correlation between male size and fecundity is weaker than the partial correlation between female size and her fecundity. Large males had a mating advantage relative to small males, both when a single male was presented to a female and when two males were present. However, this did not appear to be due to females rejecting male courtship attempts, but instead may be due to male-male competition. When females were mated to two males sequentially, neither the size of the first male nor the size of the second male influenced whether or how quickly a female remated. None of the other potential bases for sexual selection—male age, male mating experience, and male courtship persistence—appeared to influence male mating success. We discuss how patterns of sexual selection on body size and sexual size dimorphism in C. maculatus differ from patterns of sexual selection and dimorphism in another seed beetle, S. limbatus.

Savalli, U.M. & Fox, C.W. 1998. Sexual selection and the fitness consequences of male body size in the seed beetle Stator limbatus. Animal Behaviour 55: 473-483.
We examined sexual selection on male body size in a laboratory population of the seed beetle, Stator limbatus , and the fitness consequences to females of mating with larger males. Large males produced larger ejaculates than small males. Both males and females lost body weight as a consequence of breeding, and large males lost more weight than small males. The amount of weight lost by males correlated as highly with female fecundity as did the amount of weight lost by females. Similarly, male and female body weight correlated equally highly with female fecundity. These results indicate that males make substantial contributions to female fecundity, most likely through nutrients transferred in their ejaculate. As a consequence, fecundity selection should favour large body size in both males and females.
We found no preference for large males when virgin females were presented with only one male but when presented with two males simultaneously, females were more likely to mate with the larger male. This result is consistent with relative female choice or male-male competition, although no overt indications of male-male competition were observed. Females that were mated to small males re-mated sooner than females that were first mated to large males. Females that were first mated to a non-virgin male were also more likely to r-emate than females that were first mated to a virgin male, suggesting that females re-mate to obtain additional sperm or nutrients and not just as a form of mate choice. In addition to the possible benefits from mate choice and male-male competition, large males gain a mating advantage through reduced sperm competition. This large male advantage, combined with fecundity selection on males as well as females, may account for males being larger than females in this species.

Savalli, U.M. & Fox, C.W. 1998. Genetic Variation in Paternal Investment in a Seed Beetle. Animal Behaviour 56: 953-961.
Males of many species invest resources in their offspring. For paternal investment to evolve, it must exhibit heritable variation. Using a standard half-sib quantitative genetic design, we investigated whether genetic variation in male ejaculate size, a trait that affects female fecundity, and copulation duration are present in the seed beetle Callosobruchus maculatus. Ejaculate size was estimated as the amount of weight lost by males during mating. Dams, but not sires, had significant effects on their sons' absolute ejaculate size (both replicates) and relative ejaculate size (proportion of body weight; one replicate only), explaining 21-25% of the variance in absolute ejaculate size and 8-16 % of the variance in relative ejaculate size. These results suggest either a large maternal effect on ejaculate size or sex linkage of loci that affect the variation in ejaculate size. The proportion of phenotypic variance explained by sex linkage (assuming no maternal effects) was 42 and 49% (ejaculate size) and 17 and 31% (relative ejaculate size) in the two replicates. These results indicate that male paternal investment can respond to selection, and that it may be able to do so especially rapidly because sex-linked traits have the potential to evolve much more quickly than autosomal traits. There were only weak negative correlations between ejaculate size and mating duration, contrary to what we predicted. There was additive genetic variation in female copulation duration, but not in male copulation duration, suggesting that copulation duration is under female control.

Savalli, U.M. 1997. The role of male territories in the mating system of the Yellow-shouldered Widowbird Euplectes macrourus. Ibis 139: 374-378.
The territorial system and breeding biology of the Yellow-shouldered Widowbird, Euplectes macrourus (Ploceidae) was investigated in western Kenya. Yellow-shouldered Widowbirds have a resource-defence polygynous mating system: males defend large (mean = 0.95 ha) territories and build the coarse framing for the nests in tall grass. Males had up to five females nesting per territory. Females provide nearly all parental care except for a territorial male seen feeding a fledgling: the first observation of paternal care in the wild for this genus. There was considerable variation in territory size but the cause of this variation remains unknown: territory size was not related to potential indicators of territory quality such as grass height and abundance, did not relate to male morphology (mass, size, and ornament size) or territorial behaviour (boundary displays and singing), and did not affect female preferences. Although resources (territories and nests) are defended by the males, observations that males frequently feed outside their territory and form communal roosts during the breeding season, suggest that this species represents a transitional stage between typical resource-defence polygyny and lek-breeding.

Savalli, U.M. 1995. The evolution of tail-length in widowbirds (Ploceidae): tests of alternatives to sexual selection. Ibis 137: 389-395.
The long tails of male widowbirds (Euplectes, Ploceidae) have been used to test sexual selection theory, but alternatives to sexual selection have not been investigated. This study tests three alternative hypotheses for the evolution of tail-length in widowbirds: aposematism; the unprofitable prey hypothesis; and species recognition. Using museum specimens, geographic patterns of tail-length were examined for evidence of character convergence (as predicted by the aposematism and unprofitable prey hypotheses) or character divergence (as predicted by the species recognition hypothesis) in areas where two pairs of species were sympatric. There was no consistent trend: one species showed evidence of character convergence, and the other some evidence of divergence. Experimental manipulation of tail-length in the Yellow-shouldered Widowbird, E. macrourus , also failed to support the species recognition hypothesis: there was no preference for males with species-typical tail-lengths, but instead a slight, non-significant trend to favour short-tailed males. There was also no evidence that mistaken identity led to territory loss following these experimental manipulations. Tail-length of six Euplectes species did not correlate with unpalatability scores, as predicted by the aposematism hypothesis. These hypotheses do not appear to explain the evolution of long tails in widowbirds, suggesting that sexual selection is the sole factor favouring long tails. Interspecific variation in tail-length remains unexplained.

Savalli, U.M. 1995. Does rainfall constrain the evolution of tail length in widowbirds? Ethology Ecology & Evolution 7: 379-385.
Widowbirds (Euplectes , Ploceidae) are sexually dimorphic weaverbirds in which the males have elongated tails that exhibit a great deal of interspecific variation. One possible cause of such variation is geographic variation in cost due to the possibility that rainfall makes it more difficult to fly. This hypothesis was tested by comparing tail length to the modal and maximum rainfall levels in which the populations occur. Overall, there was a significant negative relationship between maximum rainfall and tail length, and a similar but non-significant trend with modal rainfall. Phylogenetic effects were partially controlled for by testing for the same relationships within two species groups and across species groups: in all cases, the trends were as in the overall test, suggesting that shared descent alone cannot explain the relationship. These results suggest that geographic variation in costs could explain inter-population variation in sexually selected ornaments. Other reasons for the relationship are also possible.

Savalli, U.M. 1994. Tail length affects territory ownership in the yellow-shouldered widowbird. Animal Behaviour 48: 105-111.
Bright and conspicuous colours and ornaments of male birds are often attributed to female choice, but alternative hypotheses, such as male-male competition for females or resources, are rarely considered. Yellow-shouldered widowbirds, Euplectes macrourus, are sexually dimorphic, polygynous birds in which males have long tails. Males with experimentally shortened tails were less likely to acquire or retain territories (4 of 11) than controls (7 of 9) or males with lengthened tails (9 of 10). This was not due to indirect effects of the experimental treatments on male behaviour, since the displays used in territorial encounters did not differ in their frequency among the treatment groups. Tail length did not appear to affect female choice, as there was no correlation between natural tail length and male attractiveness (measured as the number of females nesting in his territory) and no difference between treatments in the number of females nesting in a male's territory. Thus, male-male competition for territories can also lead to the evolution of ornaments and may be an important factor in the evolution of conspicuousness.

Savalli, U.M. 1994. Mate choice in the yellow-shouldered widowbird: correlates of male attractiveness. Behavioral Ecology and Sociobiology 35: 227-234.
Recent investigations of male ornaments in sexual selection have used experimental manipulation of tail length in three widowbirds species, but only for one of these have correlates of male reproductive success been reported. I examined correlates of male attractiveness to nesting females over two breeding seasons for the polygynous yellow-shouldered widowbird, Euplectes macrourus , in order to discover which cues females may be using to select mates. The black, long-tailed (10 cm) males defend large territories and build nest frames, or cock's nests, which females then line and use for nesting. I examined various aspects of male morphology, five behavioral displays, territory characteristics, and the number of cock's nests that males built. Few correlates of mating success were found. The best predictor was the number of cock's nests that a male builds, though one courtship display also correlated with male mating success in one year, as did average grass height. Tail length did not correlate with male mating success. A partial correlation analysis confirmed that cock's nests and, in one year, grass height, were the primary contributers to male success. Females may choose where to nest primarily on the availability of suitable nesting sites. Long tails may be used by females seeking extra-pair copulations or in male-male competition for territories.

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