Evolutionary events have potentially dramatic implications for organisms. I took advantage of an evolutionary event, the loss of a sexual signal in the field cricket Teleogryllus oceanicus, to elucidate the potential consequences of such a change. In some Hawaiian populations, males possess a wing mutation, flatwing, that precludes song production, rendering them obligately silent. Signal loss has the effect of altering the acoustic landscape experienced by all crickets, representing a change in the social environment. Previous studies indicate that the lack of acoustic input alters several phenotypic traits, perhaps by changing perceived population density. I investigated the role of the acoustic environment in mediating male reproductive investment and aggressive behavior. Males reared in song-dense environments, representing populations with many rivals, invested more in the ejaculate but were less aggressive than males from silent environments.

In addition, because flatwing males lack the ability to produce song, they face difficulties in attracting mates, since females locate males by responding to calling song. In some other species with multimodal communication, individuals may compensate for the temporary loss of a signal modality by switching to or increasing the use of an alternative, pre-existing modality. Communication in my study species involves acoustic and chemical cues, so I predicted that males lacking the ability to produce song would compensate by producing more attractive chemical cues. A female-response assay showed that females responded to chemical cues from both normal-winged and flatwing males, but did not exhibit a preference for either, indicating that flatwing males do not compensate for a lack of song with chemical cues.

Further, on the Hawaiian islands, calling males are subject to lethal parasitism from an acoustically-orienting North American parasitoid fly, Ormia ochracea, and males in parasitized populations do not live as long as males from unparasitized populations. Given that flatwing males are unable to produce song and are thus protected from parasitism from O. ochracea, I predicted that flatwing males would have a greater average age than their normal-winged counterparts in the same population. Contrary to my predictions, flatwing males were not older than normal-winged males.

Abstract

Two models have been suggested to predict the presence of behavioral syndromes (correlations between different behaviors or the same behavior in different contexts). According to "genetic constraint" models, such correlations, where present, should persist regardless of environmental changes, while "adaptive divergence" models posit that syndromes will exist only where beneficial, and one such case is a high predation environment. We examined the plasticity and potential correlation between two behaviors - female responsiveness to male calls (responsiveness) and latency to emerge from shelter after a simulated predator threat (boldness). The subjects were female Pacific field crickets (Teleogryllus oceanicus) from populations with different predation pressures, and as an additional, experimental proxy for predation pressure, they were reared in an environment simulating either high or low population density. As expected, females reared in incubators where low population density was simulated, were more responsive to male calls, since such environments signal less mating opportunities. However, regardless of rearing conditions, females from the population where parasitoid predation pressure was strongest, were more responsive to male calls and also less bold, which suggests that there are genetic differences between the populations as well. As predicted by divergence models, the only population in which a behavioral syndrome (bold/more responsive) was detected, suffered the highest rate of parasitoid larvae infestation. If predation has driven the formation of bold/more responsive and shy/less responsive coping styles in females in this population, such strategies could be adaptive due to a tradeoff mechanism. A boldness/responsiveness syndrome was also present in both the high and low population density environments, which could be explained by both genetic constraint and adaptive divergence models - different experiments, and in some instances larger sample sizes, are needed to clarify this result.

The Hamilton-Zuk hypothesis proposes that females choose elaborately ornamented males as mates because such ornaments indicate parasite resistance. Females benefit from selecting resistant males by passing resistance genes on to the offspring. Many studies have tested this hypothesis using immunity as a proxy for parasite resistance; however, the relationship between the strength of the immune response and the quality of the sexual signal remains controversial, and such studies rarely consider whether immunity provides a good measure of parasite resistance. Here I examined the relationship between immune response, parasite load, and sexual signaling, as well as what factors affect mating and reproductive success in western bluebirds (Sialia mexicana). Coloration appeared to act as a signal in social mate choice, and social pairs mate assortatively by UV-blue coloration. Males with brighter UV-blue coloration also sired more of the offspring within their own nests. However, coloration played no role in extra-pair mate choice. Likewise, immune response did not predict coloration (sexual signal quality) or male reproductive success. Immune response did relate to parasite loads, however, and individuals that had a stronger immune response tended to have lower levels of infection with avian malaria. Parasite infection and age both predicted male success at siring extra-pair offspring; extra-pair males tended to be older and uninfected. This suggests that females select extra-pair males based on their ability to resist parasites, consistent with the Hamilton-Zuk hypothesis. However, uninfected males did not have higher reproductive success than infected males, despite the advantage of siring extra-pair offspring. Double brooding, another method by which birds can increase their reproductive success in a season, occurred rarely in our population, with only 5% of pairs producing second broods. Pairs that produced second broods tended to breed earlier and the females in these pairs tended to be heavier. Environment also played a role in double brooding, with more pairs producing second broods during years in which the breeding season was long, warm, and dry. This highlights the importance of parasites and environmental factors in determining mate choice and reproductive success.

Cooperative breeding is an evolutionarily curious behavior, because helpers appeared to altruistically forgo opportunity for their own reproduction. In a population of Kalij Pheasants Lophura leucomelanos introduced to Hawai'i, we recorded cooperative breeding behaviors including caring for chicks, defending against conspecific intruders, and vigilance against predators. While cooperative breeding mostly occurs in altricial species, in which offspring need substantial parental care to survive, such behavior in the precocial Kalij Pheasants with relatively independent offspring provides an excellent opportunity to examine cooperative breeding and its influencing factors without the constraint of intense offspring needs. In our study population, Kalij pheasants formed stable social groups that usually contained one female and one to six males, and larger groups maintained larger year-round home ranges in general. One male was dominant among others within a social group, and age was the only factor found to determine dominance, indicating helpers can possibly stay in social groups and queue for dominance. Because high density was observed, we hypothesized that breeding habitat may be saturated, and that subordinate males cannot establish independent breeding habitat and subsequently remain in social groups. The removal of about one third of social groups resulted in a significant decrease in number and proportion of multiple male groups, and suggested that habitat saturation contributed to cooperative breeding in this population. To examine if helpers gained genetic benefits, we used 12 autosomal microsatellites and applied relatedness and parentage analyses to 88 adult and 82 offspring samples. On one hand, we found 34.4% subordinate males were related to the dominant male of the social group, indicating some helpers can gain inclusive fitness by helping kin. On the other hand, subordinate males fathered 16.5% of offspring sampled, suggesting that helpers can gain direct fitness be participate in reproduction. These results helped to understand the maintenance of cooperative breeding in this population; however, the causes for cooperative breeding can be complex in reality. In addition to examined factors, group-living benefits and life history traits may also have played a role in cooperative breeding in Kalij Pheasants.