Sexual selection on alleles that determine body size in the swordtail Xiphophorus nigrensis

Summary In a natural population, we document changes in the frequencies of alleles influencing body size and size-correlated alternative male reproductive tactics, and we examine the possible role of sexual selection in producing these changes. Male swordtails (Xiphophorus nigrensis, Rio Choy) exhibit three body size classes (small, intermediate, and large) that primarily derive from allelic variation (s, I, L) at the Y-linked pituitary (P) locus. Some XX individuals are male. They are small and can be fathered either by XX or XY males. We compared the frequencies of Y-linked P alleles across two generations in a natural population. There was a significant decrease in the Y-s genotype relative to Y-I and Y-L genotypes. Laboratory experiments suggest that the disadvantage of Y-s resulted, at least in part, from female preference for larger courting males relative to small chasing males. All larger courting males are of the Y-I or Y-L genotypes. Although the frequency of Y-linked P alleles changed across generations, there was no change in the distribution of male body size classes. The stability of the phenotypic distribution across generations, despite changes in P allele frequencies, was maintained by the production of small XX sons by Y-I and Y-L males. Offprint requests to: M.J. Ryan     

Do Midas cichlids win through prowess or daring? It depends

Summary Individual Midas cichlids (Cichlasoma citrinellum) show persistent differences in aggressive behavior toward dummy fish. The starting question was whether the level of such behavior can be used to predict the winner of a fight. In the first (long-term) experiment two fish were matched for size, color, and sex. After 24 h the opaque barrier separating the 2 fish was removed; they immediately behaved aggressively. Scores for aggression toward dummies did not predict the winners, nor did taking the initiative in escalation. But weight did foretell the winners, who averaged only 2% heavier than their opponents. The conventional display phase of such fights was brief (20% of total duration), and escalation was rapid. Both winners and losers sustained damage, but losers accumulated damage faster than winners. In the subsequent (short-term) experiment the fish were separated only 1–2 h. Now aggression scores predicted winners, and winners were the fish who escalated. Weight of fish had no effect. The conventional phase was relatively much longer, about half the length of the fight. Losers accumulated damage at the same rate as the losers in the long-term experiment, but the fights were shorter; winners suffered little damage. The fish had difficulty assessing one another. Fighting prowess was remarkably uniform when weight was factored out. Daring to escalate, in contrast, varied among individuals and correlated with aggression scores. Prowess (=weight) determined the outcome in the long-term experiment, which may find its parallel in nature in intraterritorial disputes. Daring to escalate determined the winner in the short-term experiment; this may be comparable to establishing a territory in nature.Prowess probably results from strong directional selection because it has low costs and high benefits. In contrast, daring is subject to bi-directional selection because both costs and benefits are high. Resource holding potential is conventionally viewed as deriving from prowess of self and opponent and value of resource; to that one must add the individual's inherent aggressiveness.     

Species-specific differences in pure tonal whistle vocalizations of five western North Atlantic dolphin species

Summary Pure tonal whistle vocalizations from five species of dolphins found in the western North Atlantic had consistent, species-specific characteristics. The degree of differences between species, as based on the results of multivariate discriminant analysis (Fig. 2), correlated with the taxonomic and zoogeographic relations of the five dolphin species. Congeneric species had more similar vocalizations than species of different genera. Differences between sympatric species were greater than differences between allopatric species. Of the six whistle parameters measured, maximum frequency had the lowest coefficient of variation for all five species, and duration and number of inflection points had the highest coefficients of variation for all five species.     

Differential attraction of females to dominant males: Olfactory discrimination and mating preference in the brown lemming (Lemmus trimucronatus)

Summary This study investigated differential attraction of estrous brown lemmings (Lemmus trimucronatus) to conspecific males recently exposed to each other for a 10-min agonistic encounter. In tests conducted 5 min, 1 h and 24 h after agonistic encounters, females preferred the oder of dominant males to that of defeated males when both odors were presented simultaneously in a Y-maze olfactometer. Defeat in an agonistic encounter did not reduce the propensity of male lemmings to initiate sexual behavior. In one-male, one-female tests conducted 5 min after agonistic encounters, dominant males achieved higher mount and thrust scores while defeated males obtained higher scores for attempted mounts. The sexual behavior of dominant and defeated males did not differ significantly in similar tests conducted 1 h and 24 h later. In contrast, females readily mated with dominant males and tended to avoid defeated males in two-male tethering tests conducted 5 min after agonistic encounters. In these tests, females still showed a preference for dominant males 1 h and 24 h after male agonistic encounters.     

Epidemic Disease and the Catastrophic Decline of Australian Rain Forest Frogs

In the montane rain forests of eastern Australia at least 14 species of endemic, stream-dwelling frogs have disappeared or declined sharply (by more than 90%) during the past 15 years. We review available information on these declines and present eight lines of evidence that collectively suggest that a rapidly spreading, epidemic disease is the most likely responsible agent. The extreme virulence of the putative frog patbogen suggests it is likely exotic to Australian rain forests. We propose that exotic pathogens may be responsible for some recent declines of amphibian populations on other continents and that the intercontinental spread of such pathogens is greatly facilitated by human activities such as the thriving international trade in aquarium fish. Our hypothesis may help explain why some amphibian populations in seemingly pristine environments have mysteriously declined.     

Antennal responses of the western pine beetle, <Emphasis Type="Italic">Dendroctonus brevicomis</Emphasis> (Coleoptera: Curculionidae), to stem volatiles of its primary host, <Emphasis Type="Italic">Pinus ponderosa</Emphasis>, and nine sympatric nonhost angiosperms and conifers

Summary. Stem volatile extracts from ten trees that are sympatric with the western pine beetle, Dendroctonus brevicomis LeConte (Coleoptera: Curculionidae) were assayed by gas chromatographic-electroantennographic detection analysis (GC-EAD). The extracts were from the primary host, ponderosa pine, Pinus ponderosa Dougl. ex Laws. (Pinaceae); two nonhost angiosperms, California black oak, Quercus kelloggii Newb. (Fagaceae), and quaking aspen, Populus tremuloides Michx. (Salicaceae); and seven nonhost conifers, white fir, Abies concolor (Gord. & Glend.) Lindl. ex Hildebr. (Pinaceae), incense cedar, Calocedrus decurrens (Torr.) Florin (Cupressaceae), Sierra lodgepole pine, P. contorta murrayana Grev. & Balf. (Pinaceae), Jeffrey pine, P. jeffreyi Grev. & Balf. (Pinaceae), sugar pine, P. lambertiana Dougl. (Pinaceae), Douglas-fir, Pseudotsuga menziesii (Mirb.) Franco (Pinaceae), and mountain hemlock, Tsuga mertensiana (Bong.) Carr. (Pinaceae). Sixty-four compounds were identified from the ten trees, 42 of which elicited antennal responses in D. brevicomis, usually in both sexes. In addition, several synthetic compounds, including a number of the antennally-active compounds from the extracted trees and some bark beetle pheromone components, elicited antennal responses in a manner similar to that observed with the extracts. Of the antennally-active compounds known to be present in trees sympatric with D. brevicomis, only geraniol was unique to its host. Four antennally-active compounds were found in the host and in other conifers; five compounds were found only in nonhost conifers; eight compounds were found in either or both of the nonhost angiosperms; eight compounds were found in either or both of the angiosperms and in nonhost conifers, but not in the host; and 19 were found in both the host and in angiosperms and/or nonhost conifers. Several bark beetle pheromone components were found in the stem volatile extracts. Conophthorin was identified from both nonhost angiosperms; exo-brevicomin was identified in A. concolor; verbenone was identified from a number of nonhost conifers; and chalcogran was identified from P. tremuloides. The number of nonhost volatile chemicals that D. brevicomis encounters and is capable of detecting, and the diversity of sources from which they emanate, highlight the complexity of the olfactory environment in which D. brevicomis forages. This provides a basis for further work related to chemically-mediated aspects of foraging in this insect and perhaps other coniferophagous bark beetles, and highlights the need to consider foraging context in the design and implementation of semiochemical-based management tactics for tree protection.     

Position preferences within groups: do whirligigs select positions which balance feeding opportunities with predator avoidance?

The benefits and costs of group living are likely to be asymmetric within a group. Animals at the edge of a group are more at risk from predators, according to the selfish herd hypothesis, but are also more likely to obtain scattered food resources. Does an animal's choice between these two conflicting positions depend on its body reserves? The hunger level of marked whirligig beetles (Coleoptera: Gyrinidae) was manipulated and the positions of individuals relative to the rest of the group on the surface of the water were determined with image analysis software. In 12 out of 13 groups, of approximately 18 beetles each, hungry beetles were closer to the edge of a group and had a higher distance to their nearest neighbor than well-fed beetles. Hungry beetles at the edge obtained nearly all of the food particles dropped onto the surface of the water. These results show that position preferences within groups may involve a dynamic feedback between foraging, predator avoidance, and shortterm hunger levels.