The effect of condition on mate searching speed and copulation frequency in the Cook Strait giant weta

Passive plasticity and a sex difference in the predictability of mobility

Behavioural predictability describes the behavioural variability of an individual. Unpredictability can arise from many sources including non-adaptive passive plasticity in which an environmental factor acts directly on the individual to create non-adaptive phenotypic variation. In this study, I use radiotelemetry to field test the hypothesis that Cook Strait giant weta Deinacrida rugosa (Orthoptera: Anostostomatidae) exhibit a sex difference in the predictability of their nightly travel distance due to passive behavioural plasticity. As predicted, I found that male mobility (i.e. nightly travel distance) was less predictable than female mobility. Females travel short and predictable distances each night for food and refuges that are close by and readily available. In contrast, male travel is less predictable because they search for female mates that are stochastically dispersed across the landscape. Therefore, their travel distance can vary considerably across nights.

Dietary macronutrient balance and fungal infection as drivers of spermatophore quality in the mealworm beetle

Males of many insects deliver ejaculates with nutritious substances to females in the form of a spermatophore. Different factors can affect spermatophore quality. We manipulated males' diet and health to determine the balance of macronutrients deposited in the spermatophores of Tenebrio molitor beetles. For diet, we varied the concentration of proteins and carbohydrates, while for health status we used a fungal infection. Males with different condition copulated with unmanipulated females, and spermatophores were extracted to measure the amount of proteins, lipids and carbohydrates. Diet and infection had an effect on the quality of the spermatophore. Diets with high protein and low carbohydrate contents produced spermatophores with higher protein, carbohydrate, and lipid contents. In contrast, diets with little protein and high in carbohydrates led to low quality spermatophores. Infected males produced spermatophores with the highest amount of all three macronutrients. In general, spermatophore content was carbohydrates>proteins>=lipids. The fact that sick males produced richer spermatophores can be explained as a terminal investment strategy. The large investment of carbohydrates may be related to the preparation of spermatozoa in males, and eggs in females.

Effects of nutrient limitation on sperm and seminal fluid: a systematic review and meta-analysis

Theory predicts that costly sexual traits should be reduced when individuals are in poor condition (i.e. traits should exhibit condition-dependent expression). It is therefore widely expected that male ejaculate traits, such as sperm and seminal fluid, will exhibit reduced quantity and quality when dietary nutrients are limited. However, reported patterns of ejaculate condition dependence are highly variable, and there has been no comprehensive synthesis of underlying sources of such variation in condition-dependent responses. In particular, it remains unclear whether all ejaculate traits are equally sensitive to nutrient intake, and whether such traits are particularly sensitive to certain dietary nutrients, respond more strongly to nutrients during specific life stages, or respond more strongly in some taxonomic groups. We systematically reviewed these potential sources of variation through a meta-analysis across 50 species of arthropods and vertebrates (from 71 papers and 348 effect sizes). We found that overall, ejaculate traits are moderately reduced when dietary nutrients are limited, but we also detected substantial variation in responses. Seminal fluid quantity was strongly and consistently condition dependent, while sperm quantity was moderately condition dependent. By contrast, aspects of sperm quality (particularly sperm viability and morphology) were less consistently reduced under nutrient limitation. Ejaculate traits tended to respond in a condition-dependent manner to a wide range of dietary manipulations, especially to caloric and protein restriction. Finally, while all major taxa for which sufficient data exist (i.e. arthropods, mammals, fish) showed condition dependence of ejaculate traits, we detected some taxonomic differences in the life stage that is most sensitive to nutrient limitation, and in the degree of condition dependence of specific ejaculate traits. Together, these biologically relevant factors accounted for nearly 20% of the total variance in ejaculate responses to nutrient limitation. Interestingly, body size showed considerably stronger condition-dependent responses compared to ejaculate traits, suggesting that ejaculate trait expression may be strongly canalised to protect important reproductive functions, or that the cost of producing an ejaculate is relatively low. Taken together, our findings show that condition-dependence of ejaculate traits is taxonomically widespread, but there are also many interesting, biologically relevant sources of variation that require further investigation. In particular, further research is needed to understand the differences in selective pressures that result in differential patterns of ejaculate condition dependence across taxa and ejaculate traits.

Effect of nutritional stress and sex on melanotic encapsulation rate in the sexually size dimorphic Cook Strait giant weta (Deinacrida rugosa)

Nutritional condition and sex are known to influence efficacy and investment in immune function. A poor diet is costly to immune function because it limits the resources (e.g., protein) available to effector systems (e.g., melanotic encapsulation), whereas males and females are expected to differ in how they allocate resources to fitness-related traits. Males are expected to invest less in immunity, and more in mating, than females, but this pattern could be reversed if fitness is more condition-dependent in males than in females. I tested the effects of nutritional condition and sex on melanotic encapsulation rate in the Cook Strait giant weta (Deinacrida rugosa Buller, 1871), an orthopteran insect exhibiting strong female-biased sexual size dimorphism that is, at least in part, the result of strong sexual selection for small male size. I found that male D. rugosa have a stronger encapsulation response than females, while nutritional condition has only a small positive effect on this particular effector system in both sexes. Whether the observed sex difference in encapsulation ability is due to a physiological constraint in females or whether males allocate more resources to this effector system because their fitness is more condition-dependent than female’s remains to be determined.