Abstract Colonial organisms host a large diversity of symbionts (collectively, parasites, mutualists, and commensals) that use vertical transmission (from parent colony to offspring colony) and/or horizontal transmission to disperse between host colonies. The early life of some colonies, characterized by the dispersal and establishment of solitary individuals, may constrain vertical transmission and favor horizontal transmission between large established colonies. We explore this possibility with the miniature cockroach Attaphila fungicola, a symbiont of leaf‐cutter ants and the mutualist fungal gardens they cultivate. The early life of a leaf‐cutter colony is characterized by the dispersal of a female alate (winged “queen”) carrying a fungal pellet, and the subsequent establishment of a foundress (workerless “queen”) raising her incipient fungal garden and colony. Roaches hitchhike on female alates during leaf‐cutter nuptial flights, which strongly suggests that roaches are vertically transmitted to foundresses and their incipient colonies; however, weak compatibility between roaches and incipient gardens may constrain roach vertical transmission. Reciprocally, opportunities for horizontal transmission between large established colonies with abundant fungal gardens may weaken selection against roach‐induced harm (virulence) of incipient gardens. We use a laboratory experiment, behavioral observations, field surveys, and a transmission model to estimate the effect roaches have on the survivorship of incipient gardens and the frequency of roach vertical transmission. Contrary to traditional assumptions, our results indicate that roaches harm incipient gardens and predominantly use horizontal transmission between established leaf‐cutter colonies. Ultimately, “costs of generalism” associated with infecting disparate stages of a host's lifecycle (e.g., incipient vs. established colonies) may constrain the vertical transmission of roaches and a broad range of symbionts.

Choice of social group can affect the likelihood of survivorship and reproduction for social species. By joining larger social groups—shoals—small freshwater fish like the mosquitofish Gambusia affinis can reduce predation risk and forage more efficiently. We tested shoal choice in mosquitofish to determine whether such choices are economically rational, i.e. consistent and optimal. Although many studies of decision-making assume rational choice, irrational decision-making is common and occurs across contexts. We tested rationality of shoaling decisions by testing the constant ratio rule, which states that the relative preference for two options should not change in the presence of a third option. Female mosquitofish upheld this rule when tested for shoal preference based on group size. Our results contrast with other studies showing violations of the constant ratio rule in foraging and mate choice decision-making contexts. These results suggest that decisions that immediately influence survivorship or decision-making along a single dimension may favour rational decision-making.

Preferences for the larger shoal for the fish in this study.

Biologists have long been interested in intransitive preferences: circular preferences in which options cannot be ranked and no single option dominates, similar to a game of rock-paper-scissors. Intransitive preferences violate rational decision-making, an assumption made by models of evolution by mate choice. Despite its potential importance in the study of sexual selection, few studies have tested for intransitive preferences. Even fewer have asked whether females differ in whether they choose mates transitively or intransitively and what factors might predict (in)transitive choice. Though intransitive choice is thought to be more common as options become more complex, this prediction is untested in animals. To fill this gap, we tested whether female Xiphophorus nigrensis swordtails can rank digitally animated males differing in size, courtship intensity, or both size and courtship intensity, and whether female responses were predicted by a female's age. Females choosing among males that varied only in size showed higher than expected levels of intransitivity, whereas females choosing among males that varied in their courtship or both properties did not. Older females were more likely to be irrational than younger females when evaluating male size, suggesting that experience modifies transitive decision-making processes. These results show that mate choice irrationality may vary by a female's experience and the signal characteristics during decision-making.

Female mating preferences can be both context dependent and based on the assessment of multicomponent male signals. Here, we assess the social context dependence of female mating preferences for two components of a male's multicomponent signal. We dissected the visual signal of male Xiphophorus nigrensis swordtails, a species in which males vary by both size and degree of courtship, to test (1) how the identity of males in a given choice influences female mating preferences and (2) how females perceptually integrate a male's multicomponent signal. We used validated male animations that generate repeatable female responses to test mating preferences for size and courtship vigour, separately and together, using dichotomous choice tests. When keeping courtship vigour constant, females discriminated between males only when there was a large size difference between them. When keeping size constant, the identity of males in a choice reversed a preference for a vigorously courting male. We found no evidence that females perceptually bind the separate components of a male's signal additively. However, females were faster to approach males when the males varied in both size and courtship than when the males only varied in size, perhaps favouring the evolution of multicomponent signals in males.

Why are asexual vertebrates so rare? One seldom explored avenue to understanding the evolutionary persistence of extant asexual species is their sensory ecology—how they perceive and respond to the environment. Asexual species formed by hybridization have been hypothesized to have an expanded sensory repertoire because they carry 1 allele from each of their parental species, including alleles that impact sensory function. The ability to detect odorants in the environment is a likely candidate for this expansion but has never been explored in this context. Here, we explore the olfactory abilities of the asexual Amazon molly, a gynogenetic fish formed by hybridization 100000 years ago. We test whether Amazon mollies can use only olfactory cues to detect conspecifics, detect heterospecific males, and discriminate between males infected with a common parasite. We further explore whether a female’s size, a proxy for age, explains any variation in her behavior. We find strong evidence that Amazon mollies use olfactory cues to detect conspecifics but surprisingly may avoid heterospecific males based on olfactory cues alone. We find no evidence that females use chemical cues to discriminate between infected and noninfected males. We also find that smaller Amazon mollies are more likely to use chemical cues. This study highlights the potential importance of sensory systems in asexual vertebrates.

Multiple mating by females is difficult to explain in primarily socially monogamous taxa such as birds because mating outside the pair bond often provides no obvious benefit to females. Although indirect selection is often invoked to explain the evolution of polyandry, current evidence suggests that selection on indirect benefits of mating is weak. Here, I consider a direct benefit of remating in birds: increased fertilization success. I test whether increased hatching success of a female's eggs is related to rates of extra-pair paternity (EPP), a proxy of polyandry, across 113 bird species. I use two statistical approaches, control for phylogenetic uncertainty, and assess the fit of competing evolutionary models. Results show there is indeed a positive relationship between rates of EPP and hatching success in birds. I propose that by mating with many males, females may increase their fertility. I end by discussing the biological rationale for this explanation, alternative interpretations of the results, and how this study furthers our understanding of polyandry and mating system evolution.