Acoustic and postural displays in a miniature and transparent teleost fish, Danionella dracula.

Acoustic behavior is widespread across vertebrates, including fishes. We report robust acoustic displays during aggressive interactions for a laboratory colony of Danionella dracula, a miniature and transparent species of teleost fish closely related to zebrafish (Danio rerio), which are hypothesized to be sonic based on the presence of a hypertrophied muscle associated with the male swim bladder. Males produce bursts of pulsatile sounds and a distinct postural display - extension of a hypertrophied lower jaw, a morphological trait not present in other Danionella species - during aggressive but not courtship interactions. Females show no evidence of sound production or jaw extension in such contexts. Novel pairs of size-matched or -mismatched males were combined in resident-intruder assays where sound production and jaw extension could be linked to individuals. In both dyad contexts, resident males produced significantly more sound pulses than intruders. During heightened sonic activity, the majority of the highest sound producers also showed increased jaw extension. Residents extended their jaw more than intruders in size-matched but not -mismatched contexts. Larger males in size-mismatched dyads produced more sounds and jaw extensions compared with their smaller counterparts, and sounds and jaw extensions increased with increasing absolute body size. These studies establish D. dracula as a sonic species that modulates putatively acoustic and postural displays during aggressive interactions based on residency and body size, providing a foundation for further investigating the role of multimodal displays in a new model clade for neurogenomic and neuroimaging studies of aggression, courtship and other social interactions.

Sexual dimorphism in body size and the origin of sex-determination systems.

Diverse sex-determining systems occur in vertebrates, including environmental sex determination (ESD), genetic sex determination (GSD) of type XX/XY (heterogametic males), and GSD of type ZZ/ZW (heterogametic females). The origins of the two genetic types are poorly understood. We use protected invasion theory to derive a model that generates testable predictions about the origins of the two genetic types from ESD. Protected invasion theory predicts biases in the evolutionary origins of new traits by focusing on the probability that a selectively favored trait will avoid loss by genetic drift when rare. We show that the theory makes predictions about the conditions under which XY or ZW systems are more likely to arise from an ancestral state of ESD. In particular, assuming that there is an average trend toward increasing body size in lineages, the origins of XY systems are predicted to be accompanied by increases in male∶female body size ratio. In contrast, ZW systems are predicted to be accompanied by decreases in male∶female body size ratio. We find support for these predictions in the form of a marked association among vertebrates between sex-determining system and body size dimorphism in paired comparisons independent of shared phylogeny.

Z linkage of female promiscuity genes in the moth Utetheisa ornatrix: support for the sexy-sperm hypothesis?

Female preference genes for large males in the highly promiscuous moth Utetheisa ornatrix (Lepidoptera: Arctiidae) have previously been shown to be mostly Z-linked, in accordance with the hypothesis that ZZ-ZW sex chromosome systems should facilitate Fisherian sexual selection. We determined the heritability of both female and male promiscuity in the highly promiscuous moth U. ornatrix (Lepidoptera: Arctiidae) through parent-offspring and grandparent-offspring regression analyses. Our data show that male promiscuity is not sex-limited and either autosomal or sex-linked whereas female promiscuity is primarily determined by sex-limited, Z-linked genes. These data are consistent with the "sexy-sperm hypothesis," which posits that multiple-mating and sperm competitiveness coevolve through a Fisherian-like process in which female promiscuity is a kind of mate choice in which sperm-competitiveness is the trait favored in males. Such a Fisherian process should also be more potent when female preferences are Z-linked and sex-limited than when autosomal or not limited.

Female control of the distribution of paternity in cooperative breeders.

Models of reproductive skew have shed light on why animal societies vary in the partitioning of reproduction among group members. However, their application to cooperative vertebrate societies remains controversial. A particular problem is that previous models assume that skew in paternity is determined by interactions among males and males only. This conflicts with observations from many species that indicate that females exert control over the distribution of paternity. Here we address this shortfall in the current theory by developing two models to explore the expected patterns of skew in three member groups in which a female controls the allocation of paternity among two males. The first "staying incentive" model extends previous "transactional" (or "concession") models to examine the conditions where females will be willing to share reproduction among a dominant and a subordinate male to retain the subordinate in the group. The second "work incentive" model explores patterns of skew where females allocate paternity in order to maximize the amount of care their offspring receive. The models make contrasting predictions about the nature of male-female conflict over reproduction and also about the relationships between skew and relatedness, ecological constraints, the relative quality of the subordinate male, and the relative cost of care for the two males. These divergent predictions provide a schema by which the evolutionary causes of variation in skew among males can be evaluated.

A Transactional Theory of Within-Group Conflict.

Transactional models of social evolution emphasize that dominant members of the society can be favored to donate parcels of reproduction to subordinate members in return for cooperation. I construct a formal theory of intragroup conflict within the framework of transactional models by determining the maximum extent to which colony members can be selfish without destabilizing the group. The difference between the maximum value of the subordinate's fraction of group reproduction that the dominant can tolerate before ejecting the subordinate and the minimum value required by the subordinate to stay and cooperate peacefully in the group defines the "window of selfishness," which in turn predicts the frequency of within-group conflict. The window of selfishness tends to increase with increasing group reproductive output, increasingly harsh ecological constraints on solitary breeding, and, counterintuitively, increasing relatedness between subordinate and dominant. Increasing fighting ability of the subordinate can either widen or narrow the window of selfishness, the latter being most likely when ecological constraints on group living are strong. Although increasing relatedness is predicted to increase the rate of within-group aggression, the mean intensity of an aggressive act should decline, as predicted by the general theory of honest signaling between relatives and the tug-of-war models of within-group selfishness. In the bidding game, in which multiple dominants bid for the services of a subordinate, the window of selfishness is predicted to have zero width. A zero-width window of selfishness and low conflict also are predicted for saturated N-person groups, that is, groups whose total output is a concave function of group size and in which the dominant is not favored to admit additional subordinates. The model's predictions are compared to empirical evidence and to predictions of alternative models of intragroup aggression, including the value-aggression model and the pure tug-of-war model.

GENETIC VARIABILITY, QUEEN NUMBER, AND POLYANDRY IN SOCIAL HYMENOPTERA.

Several hypotheses have been put forward to explain the adaptive significance of interspecific variation in mating frequencies by eusocial hymenopteran queens. Four of these hypotheses assert that polyandry is advantageous to queens because of the resultant increase in genetic variability within colonies (referred to as the "GV" hypotheses). Here we compare the frequency of polyandry between monogynous (single-queen) and polygynous (multiple-queen) ant species to test the hypotheses that (1) multiple mating functions primarily to increase intracolonial genetic variability, and (2) mating has costs (such as increased energetic losses or risk of predation or venereal disease). If one of the GV hypotheses is true and mating is costly, the frequency of polyandry should be lower in polygynous species (in which the presence of multiple queens results in low relatedness among workers) than in monogynous species. As predicted by the GV hypotheses, polyandry is less common among polygynous than among monogynous species. Furthermore, it seems that the causal relationship underlying this association is that the number of matings by queens depends on the number of queens present in the colony (rather than the number of queens being influenced by the number of matings), which also supports the GV hypotheses together with the assumption that mating has costs.