Risk trading in mating behavior: forgoing anti-predator responses reduces the likelihood of missing terminal mating opportunities

Age, but not an immune challenge, triggers terminal investment in the Pacific field cricket, Teleogryllus oceanicus

The terminal investment hypothesis proposes that, when individuals are faced with a threat to survival, they will increase investment in current reproduction. The level of the threat necessary to elicit terminal investment (the dynamic terminal investment threshold) may vary based on other factors that also influence future reproduction. Here, we tested whether there is an interactive effect of age and an immune challenge on the dynamic terminal investment threshold in the Pacific field cricket, Teleogryllus oceanicus. We measured the courtship call, mating attractiveness, ejaculate size, and offspring production of T. oceanicus males. We found only limited support for the dynamic terminal investment threshold: there was no consistent evidence of a positive interaction between male age and immune challenge intensity. However, we found evidence for age-related terminal investment: older males produced a larger spermatophore than younger males. Older males also had a slower calling rate compared to younger males, suggesting a potential trade-off between these two pre- and post-copulatory traits. As some, but not all, reproductive traits responded plastically to cues for terminal investment, our research highlights the importance of considering a broad range of pre-and post-copulatory traits when exploring the potential for terminal investment to occur.

Consumption of endemic arbovirus mosquito vectors by bats in the southeastern United States

Mosquitoes affect human health and well-being globally through their roles as disease-causing pathogen vectors. Utilizing genetic techniques, we conducted a large-scale dietary study of three bat species common to the southeastern U.S.A., Lasiurus seminolus (Seminole bat), Nycticeius humeralis (evening bat), and Myotis austroriparius (southeastern myotis). Through next-generation sequencing of a 180 bp portion of cytochrome oxidase subunit I (COI) of mitochondrial DNA from 180 bat guano samples, we documented consumption of 17 species of mosquitoes by bats, including six endemic arbovirus vectors. Culex quinquefasciatus, Culex coronator, Culiseta melanura, Culex salinarius, Culex erraticus, and Coquillettidia perturbans were consumed by 51.3%, 43.7%, 27.2%, 22.8%, 18.0%, and 12.7% of bats sampled, respectively. Consumption of two of these mosquito species was explained by spatial variables reflecting the prevalence of mosquito larval habitat, five were explained by bat traits (bat mass, bat species), and two were explained by these factors plus temporal variables (maximum daily temperature, time since sunset, date), making it challenging to offer specific guidance on how best to promote bats as a means of reducing arbovirus vector species. Our results show that common bat species of the southeastern U.S.A. consume endemic, but not exotic, arbovirus mosquito vectors. Future studies are needed to understand the impact of bat consumption on mosquito numbers and public health.

Meta-analysis shows the evidence for context-dependent mating behaviour is inconsistent or weak across animals

Animals often need to invest significantly in mating behaviour in order to successfully mate. However, the expression of mating behaviour can be costly, especially in unfavourable environments, so animals are expected to adjust their behaviour in a context-dependent way to mitigate these costs. I systematically searched the literature for studies measuring animal mating behaviour (sexual signalling, response to sexual signals or the strength of mate choice) in more than one environment, and used a phylogenetically controlled meta-analysis to identify environmental factors influencing these behaviours. Across 222 studies, the strength of mate choice was significantly context-dependent, and most strongly influenced by population density, population sex ratio and predation risk. However, the average effect sizes were typically small. The amount of sexual signalling and the strength of response to sexual signals were not significantly related to the environment. Overall, this suggests that the evidence for context-dependent mating behaviour across animals is surprisingly weak.

Predation risks of signalling and searching: bats prefer moving katydids

Males signalling their attractiveness to females are at risk from predators that exploit mating signals to detect and locate prey. Signalling, however, is not the only risky activity in sexual interactions: mate searching can incur risk as well. Male Neotropical pseudophylline katydids produce both acoustic and vibrational signals (tremulations). Females reply to male signals with tremulations of their own, and both sexes walk to find one another. We asked if movement increases predation risk, and whether tremulation or walking was more attractive to predators. We offered the Neotropical gleaning bat Micronycteris microtis a series of two-choice tests, presenting the bats with katydid models that were motionless or moved in a way to mimic either tremulating or walking. We found that prey movements do put prey at risk. Although M. microtis can detect motionless prey on leaves, they preferred moving prey. Our study shows that movement can put searching or signalling prey in danger, potentially explaining why silent female katydids are frequently consumed by gleaning bats.

The influence of predation risk on mate signaling and mate choice in the lesser waxmoth Achroia grisella

Mating signals are often conspicuous and can be eavesdropped on by predators. Therefore, it is expected that predation risk will shape the evolution of sexual communication in both senders and receivers. Males of the lesser waxmoth Achroia grisella produce ultrasonic signals to attract females, and females have preferences for certain signal traits. When they detect predator cues such as bat echolocation calls, males typically stop signaling and females freeze. Here we examine different ways in which predation risk affects male signaling and female choice. We exposed calling males to predator cues under three different treatments: in isolation, within a simulated lek composed of attractive males, or within a simulated lek of unattractive males. We also tested if female choice depends on predation risk by submitting females to a two-choice test between an attractive and an unattractive signal, in the presence and the absence of predator cues. We found that risk-taking behavior is not only impacted by the presence of competitors, but by the competitors’ attractiveness as well. Additionally, more attractive males take more risks when calling in isolation, but not when lekking. In females, we found that preference for the more attractive call disappeared when the attractive call was associated with higher predation risk. These results reinforce the trade-off between traits that increase survival and mate attraction, and bring new insight into the evolution of leks in this species.

A dynamic threshold model for terminal investment

Although reproductive strategies can be influenced by a variety of intrinsic and extrinsic factors, life history theory provides a rigorous framework for explaining variation in reproductive effort. The terminal investment hypothesis proposes that a decreased expectation of future reproduction (as might arise from a mortality threat) should precipitate increased investment in current reproduction. Terminal investment has been widely studied, and a variety of intrinsic and extrinsic cues that elicit such a response have been identified across an array of taxa. Although terminal investment is often treated as a static strategy, the level at which a cue of decreased future reproduction is sufficient to trigger increased current reproductive effort (i.e., the terminal investment threshold) may depend on context, including the internal state of the organism or its current external environment, independent of the cue that triggers a shift in reproductive investment. Here, we review empirical studies that address the terminal investment hypothesis, exploring both the intrinsic and extrinsic factors that mediate its expression. Based on these studies, we propose a novel framework within which to view the strategy of terminal investment, incorporating factors that influence an individual's residual reproductive value beyond a terminal investment trigger - the dynamic terminal investment threshold.

Evolution of directional hearing in moths via conversion of bat detection devices to asymmetric pressure gradient receivers

Small animals typically localize sound sources by means of complex internal connections and baffles that effectively increase time or intensity differences between the two ears. However, some miniature acoustic species achieve directional hearing without such devices, indicating that other mechanisms have evolved. Using 3D laser vibrometry to measure tympanum deflection, we show that female lesser waxmoths (Achroia grisella) can orient toward the 100-kHz male song, because each ear functions independently as an asymmetric pressure gradient receiver that responds sharply to high-frequency sound arriving from an azimuth angle 30° contralateral to the animal's midline. We found that females presented with a song stimulus while running on a locomotion compensation sphere follow a trajectory 20°-40° to the left or right of the stimulus heading but not directly toward it, movement consistent with the tympanum deflections and suggestive of a monaural mechanism of auditory tracking. Moreover, females losing their track typically regain it by auditory scanning-sudden, wide deviations in their heading-and females initially facing away from the stimulus quickly change their general heading toward it, orientation indicating superior ability to resolve the front-rear ambiguity in source location. X-ray computer-aided tomography (CT) scans of the moths did not reveal any internal coupling between the two ears, confirming that an acoustic insect can localize a sound source based solely on the distinct features of each ear.

Along came a spider who sat down beside her: Perceived predation risk, but not female age, affects female mate choosiness

Organisms often exhibit behavioral plasticity in response to changes in factors, such as predation risk, mate density, and age. Particularly, female mate choosiness (the strength of female's attraction to male traits as they deviate from preferred trait values) has repeatedly been shown to be plastic. This is due to the costs associated with searching for preferred males fluctuating with changes in such factors. Because these factors can interact naturally, it is important to understand how female mate choosiness responds to these interactions. We studied the interaction between perceived predation risk and female age on the variable field cricket, Gryllus lineaticeps. Females were either exposed or not exposed to predation cues from a sympatric, cursorial, wolf spider predator, Hogna sp. We then tested the females at one of three adult ages and measured their choosiness by recording their responsiveness to a low quality male song. We found female choosiness plasticity was affected by neither age nor the interaction between age and perceived predation risk. Perceived predation risk was the only factor to significantly affect the plasticity of female mate choosiness: females were less choosy when they perceived predation risk and were more choosy when they did not. Predation may be such a strong source of selection that, regardless of differences in other factors, most individuals respond similarly.

A two-resource model of terminal investment

The most widely known theoretical basis for the hypothesis of terminal investment is the classic model by George C. Williams (1966). Although this model predicts that reproductive effort (i.e. the proportion of available resources devoted to reproduction) increases with decreasing reproductive value, it implies that reproductive allocation in absolute terms should remain stable. This contrasts with the empirical evidence on terminal investment reported to date: the vast majority of positive case studies report an increase in some aspect of reproductive allocation in absolute terms. Also, a substantial number of studies have failed to record terminal investment, despite expectations. Here, I present a simple conceptual model which explains such results. I argue that to explain terminal investment, an organism's reproductive capacity must not be considered as a common pool of resources (often described by the term 'reproductive value'), but as a set of different resources which are not easily convertible to each other, and should be exhausted in balance. Thus, if one resource accidentally decreases, in response, the others must be expended at higher rate. To test this model, each reproductive allocation should be measured in a more specific currency (or currencies) than traditional 'reproductive effort'. The model is consistent with both the positive and the negative case reports on terminal investment.