Communication via color signals is common in natural systems. Ultraviolet (UV)-blue patches located on the outer-ventral scales of some Lacertid lizards are thought to be involved in male-male competition. However, the mechanisms that maintain their honesty remain unknown. Here, we use the common wall lizard Podarcis muralis to test whether the lateral UV-blue spots are conventional signals, the honesty of which is guaranteed by receiver-dependent costs, and discuss their potential role as an amplifier of body size. We first described the morphology and reflectance properties of lateral UV-blue spots in common wall lizards and investigated how they influence malemale competition. Spot size and number, UV chroma, and conspicuousness (calculated using vision models) were significantly greater in adult males relative to adult females and adult males relative to juveniles. Total spot area (and not spot number) of adult males was positively correlated with body size. We conducted staged competition encounters between focal males and smaller or larger rivals with control or manipulated spots. Spots were enlarged in small rivals and reduced in large rivals to disrupt the phenotypic correlation between spot area and body size. Aggressiveness and dominance were positively influenced by body size in control encounters. Spot manipulations resulted in greater submission and less aggressiveness in focal males. These results contradict the predictions associated with conventional signals and amplifiers, but suggest that spots contributed to opponent evaluation during shortdistance encounters between competing males.

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Vertebrates confronted with challenging environments often experience an increase in circulating glucocorticoids, which result in morphological, physiological, and behavioral changes that promote survival. However, chronically elevated glucocorticoids can suppress immunity, which may increase susceptibility to disease. Since the introduction of avian malaria to Hawaii a century ago, low elevation populations of Hawaii Amakihi (Chlorodrepanis virens) have undergone strong selection by avian malaria and evolved increased resilience (the ability to recover from infection), while populations at high elevation with few vectors have not undergone selection and remain susceptible. We investigated how experimentally elevated corticosterone affects the ability of high and low elevation male Amakihi to cope with avian malaria by measuring innate immunity, hematocrit, and malaria parasitemia. Corticosterone implants resulted in a decrease in hematocrit in high and low elevation birds but no changes to circulating natural antibodies or leukocytes. Overall, leukocyte count was higher in low than high elevation birds. Malaria infections were detected in a subset of low elevation birds. Infected individuals with corticosterone implants experienced a significant increase in circulating malaria parasites while untreated infected birds did not. Our results suggest that Amakihi innate immunity measured by natural antibodies and leukocytes is not sensitive to changes in corticosterone, and that high circulating corticosterone may reduce the ability of Amakihi to cope with infection via its effects on hematocrit and malaria parasite load. Understanding how glucocorticoids influence a host's ability to cope with introduced diseases provides new insight into the conservation of animals threatened by novel pathogens.

Emerging infectious diseases are spreading at unprecedented rates and affecting wildlife worldwide, with particularly strong effects on islands. Since the introduction of avian malaria to Hawaii a century ago, the disease has contributed to the decline and extinction of several endemic Hawaiian honeycreeper species. At low elevation, where avian malaria is prevalent, Hawaii Amakihi Chlorodrepanis virens honeycreeper populations have experienced strong selection by the disease and have evolved increased malaria resilience, the ability to recover from infection. The goals of this study were to describe malaria infection rates across Hawaii Island, to examine the role of innate immunity in malaria resilience, and to determine the effects of resilience and chronic infection on Amakihi health. We measured malaria infection and metrics of innate immunity and health in low elevation Amakihi populations, which are more resilient to malaria, and high elevation populations, which have experienced weak malaria selection and are less resilient to malaria. Avian malaria infection was higher at low than high elevation. Measures of innate immunity were higher in low than high elevation Amakihi, but also depended on whether the birds were from the leeward side of the island (low rainfall) or windward side of the island (high rainfall) and whether they were freeliving or captive. Regarding measures of health, body condition was better in malaria-infected than uninfected Amakihi, while hematocrit did not vary by malaria infection but was higher in high than low elevation birds. These are among the first results to describe variation in immunity and health according to avian malaria selection and infection in Hawaiian honeycreepers. Understanding the impacts of introduced diseases on island species, which are particularly susceptible to novel pathogens, will provide critical insight into how introduced disease may affect endangered species.

Reproductive territoriality can be influenced by external and internal variables. Trade-offs between reproductive behaviors and other costly biological processes, such as immunity, exist across taxa, but the effects of novel diseases on these trade-offs remain poorly understood. Since the introduction of avian malaria to Hawaii in the early 1900s, low elevation Hawaii Amakihi (Chlorodrepanis virens) populations, which have undergone strong selection by the disease, have evolved increased malaria resilience. However, the effects of malaria selection on trade-offs between immunity and reproduction in Amakihi remain largely unknown. To begin exploring this relationship, we conducted simulated territorial intrusions on Amakihi at low elevation (where malaria selection has been stronger) and high elevation (where selection has been weaker) on Hawaii Island during the breeding season. We hypothesized that selection by avian malaria has favored greater investment in avian malaria resilience at the cost of reproductive behaviors. We predicted that low elevation Amakihi would be less territorial compared with high elevation Amakihi, while recognizing that variables other than disease pressures that may differ across elevation (e.g., competition, predation) could explain behavioral variation. Territoriality was reduced in low compared with high elevation Amakihi as measured by proximity and chases in response to the intrusion. Low elevation Amakihi generally flew less than high elevation individuals, although this relationship varied across the breeding season. Our correlational results demonstrate that territoriality is greater in high compared with low elevation Amakihi. Further investigations would help determine which factors differing across elevation underlie this variation in territoriality.