Summary1. Reproduction and immune function are critical processes, but organisms can rarely optimize both traits. Resultant reproduction-immunity trade-offs may be 'facultative', occurring only when resources are scarce, or they may be 'obligate', occurring regardless of resource availability.2. Previous research has tested for the 'facultative' or 'obligate' nature of reproduction-immunity trade-offs by measuring resource allocation (e.g. follicle size). However, measuring resource allocation alone may be insufficient when gauging the fitness consequences of reproduction-immunity trade-offs because the number and quality of eggs or offspring trade off with one another. 3. We used the Texas field cricket (Gryllus texensis) to provide the most direct test to date of whether a fitness trade-off between these two traits is 'facultative' or 'obligate'. We used a factorial design to manipulate food availability and immune status throughout adulthood. We then estimated lifetime fecundity, hatching success and their product (reproductive success), and we also measured several aspects of offspring quality (e.g. egg size and protein content, and hatchling size and energy stores). 4. A reproduction-immunity trade-off was 'obligate' in this species because immune challenge reduced reproductive success estimates regardless of food availability. Females with unlimited food were more fecund and produced more and larger hatchlings, but neither food availability nor immune status affected egg size, egg phenoloxidase activity, incubation duration, hatching success or hatchling energy stores. We observed a trade-off between offspring size and number -females favouring offspring size over fecundity produced fewer hatchlings, but their hatchlings were of higher quality (larger and more robust). 5. By demonstrating that not all eggs are created equal, we provide key insight into the role of reproductive allocation in the fitness trade-off between reproduction and immunity.

Animal body temperature (Tbody) varies over daily and annual cycles, affecting multiple aspects of biological performance in both endothermic and ectothermic animals. Yet a comprehensive comparison of thermal performance among animals varying in Tbody (mean and variance) and heat production is lacking. Thus, we examined the thermal sensitivity of immune function (a crucial fitness determinant) in Vertebrata, a group encompassing species of varying thermal biology. Specifically, we investigated temperature-related variation in two innate immune performance metrics, hemagglutination and hemolysis, for 13 species across all seven major vertebrate clades. Agglutination and lysis were temperature dependent and were more strongly related to the thermal biology of species (e.g., mean Tbody) than to the phylogenetic relatedness of species, although these relationships were complex and frequently surprising (e.g., heterotherms did not exhibit broader thermal performance curves than homeotherms). Agglutination and lysis performance were positively correlated within species, except in taxa that produce squalamine, a steroidal antibiotic that does not lyse red blood cells. Interestingly, we found the antithesis of a generalist-specialist trade-off: species with broader temperature ranges of immune performance also had higher peak performance levels. In sum, we have uncovered thermal sensitivity of immune performance in both endotherms and ectotherms, highlighting the role that temperature and life history play in immune function across Vertebrata.

Reproductive investment and output are integral fitness components, often incorporated into life-history trade-off models and important to population dynamics. The trade-offs associated with reproduction can be dramatic in species such as snakes that make especially large investments into reproduction. Unfortunately, traditional methods used to determine reproductive investment and output are effective in many (but not all) situations. Thus, we used portable ultrasonography to serially estimate reproductive investment and reproductive output in three python species that exhibit significant variation in phylogeny, geographic range, body size, egg size, and clutch size: ball pythons (Python regius), Children's pythons (Antaresia childreni), and water pythons (Liasis fuscus). At each time point of measurement (range: 1-49 days pre-oviposition), ultrasound estimates of viable clutch size were highly accurate in all three species. However, ultrasound estimates of mean viable egg mass, and thus viable clutch mass, significantly differed from the actual values (range: 23-73% error). Interestingly, this error was considerably smaller as females approached oviposition, suggesting that female pythons transfer a significant amount of water into their eggs during the week before oviposition. Thus, water balance during late-stage egg development may be an integral part of reproductive success. The results obtained in the present study form the foundation for future assessments of reproductive investment, and also provide insight into the use of ultrasound technology to assist such efforts.

Fever can reduce mortality in infected animals. Yet, despite its fitness-enhancing qualities, fever often varies among animals. We used several approaches to examine this variation in insects. Texas field crickets (Gryllus texensis) exhibited a modest fever (1 °C increase in preferred body temperature, T pref) after injection of prostaglandin, which putatively mediates fever in both vertebrates and invertebrates, but they did not exhibit fever during chronic exposure to heat-killed bacteria. Further, chronic food limitation and mating status did not affect T pref or the expression of behavioural fever, suggesting limited context dependency of fever in G. texensis. Our meta-analysis of behavioural fever studies indicated that behavioural fever occurs in many insects, but it is not ubiquitous. Thus, both empirical and meta-analytical results suggest that the fever response in insects 'is widespread, although certainly not inevitable' (Moore 2002). We highlight the need for future work focusing on standardizing an experimental protocol to measure behavioural fever, understanding the specific mechanism(s) underlying fever in insects, and examining whether ecological or physiological costs often outweigh the benefits of fever and can explain the sporadic nature of fever in insects.

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