The postabsorptive and postprandial metabolic rates of praying mantises: Comparisons across species, body masses, and meal sizes

Post-feeding thermophily in a scorpion is associated with rapid digestion and recovery of maximal nocturnal activity

A postprandial increase in metabolic rate is typical in all studied animal groups. The phenomenon, termed specific dynamic action (SDA), is understudied in terrestrial arthropods, and arachnids in particular. To the best of our knowledge, this is the first report of SDA properties in scorpions, which are temperature-dependent as in other poikilotherms. Metabolic rates of scorpions are low compared with similarly-sized arthropods, and as they often feed on relatively large prey the cost of digestion is expected to be notable. This prompted us to study the extent of SDA and its characteristics in scorpions at two different ecologically-relevant temperatures. We also hypothesized that post-feeding behavioral thermoregulation would reflect benefits to the scorpion energy balance. On average, fed adult Hottentotta judaicus (Buthidae) expressed a 3 °C increase in preferred surface temperature, although we did not find evidence for lower costs of digestion at higher temperatures. However, SDA duration was significantly shorter at 30 compared with 25 °C. Fast processing of their meal at 30 °C was correlated with recovery of elevated nocturnal metabolic rates, which are not digestion-related. This suggests that post-feeding choice of higher temperatures accelerates digestion and recovery of other elevated metabolic states such as locomotion and lower sensory threshold, which may enhance foraging success.

Metabolic responses to starvation and feeding contribute to the invasiveness of an emerging pest insect

Metabolic rate, and the flexibility thereof, is a complex trait involving several inter-linked variables that can influence animal energetics, behavior, and ultimately, fitness. Metabolic traits respond readily to ambient temperature variation, in some cases increasing relative or absolute energetic costs, while in other cases, depending on the organism's metabolic and behavioral responses to changing conditions, resulting in substantial energy savings. To gain insight into the rapid recent emergence of the indigenous South African longhorn beetle Cacosceles newmannii as a crop pest in sugarcane, a better understanding of its metabolic rate, feeding response, digestion times, and aerobic scope is required, in conjunction with any behavioral responses to food availability or limitation thereof. Here, we therefore experimentally determined metabolic rate, estimated indirectly as CO₂ production using flow-through respirometry, in starved, fasted, and fed C. newmannii larvae, at 20 °C and 30 °C. We estimated multiple parameters of metabolic rate (starved, standard, active, and maximum metabolic rates) as well as aerobic scope (AS), specific dynamic action (SDA), and the percentage time active during respirometry trials. Additionally, in individuals that showed cyclic or discontinuous gas exchange patterns, we compared rate, volume, and duration of cycles, and how these were influenced by temperature. Standard and active metabolic rate, and AS and SDA were significantly higher in the larvae measured at 30 °C than those measured at 20 °C. By contrast, starved and maximum metabolic rates and percentage time active were unaffected by temperature. At rest and after digestion was complete, 35% of larvae showed cyclic gas exchange at both temperatures; 5% and 15% showed continuous gas exchange at 20 °C and 30 °C respectively, and 10% and 0% showed discontinuous gas exchange at 20 °C and 30 °C respectively. We propose that the ability of C. newmannii larvae to survive extended periods of resource limitation, combined with a rapid ability to process food upon securing resources, even at cooler conditions that would normally suppress digestion in tropical insects, may have contributed to their ability to feed on diverse low energy resources typical of their host plants, and become pests of, and thrive on, a high energy host plant like sugarcane.

Progressive behavioural, physiological and transcriptomic shifts over the course of prolonged starvation in ticks

Ticks are obligatorily hematophagous but spend the majority of their lives off host in an unfed state where they must resist starvation between bouts of blood feeding. Survival during these extended off-host periods is critical to the success of these arthropods as vectors of disease; however, little is known about the underlying physiological and molecular mechanisms of starvation tolerance in ticks. We examined the bioenergetic, transcriptomic and behavioural changes of female American dog ticks, Dermacentor variabilis, throughout starvation (up to nine months post-bloodmeal). As starvation progressed, ticks utilized glycogen and lipid, and later protein as energy reserves with proteolysis and autophagy facilitating the mobilization of endogenous nutrients. The metabolic rate of the ticks was expectedly low, but showed a slight increase as starvation progressed possibly reflecting the upregulation of several energetically costly processes such as transcription/translation and/or increases in host-seeking behaviours. Starved ticks had higher activity levels, increased questing behaviour and augmented expression of genes related to chemosensing, immunity and salivary gland proteins. The shifts in gene expression and associated behavioural and physiological processes are critical to allowing these parasites to exploit their ecological niche as extreme sit-and-wait parasites. The overall responses of ticks to starvation were similar to other blood-feeding arthropods, but we identified unique responses that could have epidemiological and ecological significance for ticks as ectoparasites that must be tolerant of sporadic feeding.

The hindgut microbiota of praying mantids is highly variable and includes both prey-associated and host-specific microbes

Praying mantids are predators that consume a wide variety of insects. While the gut microbiome of carnivorous mammals is distinct from that of omnivores and herbivores, the role of the gut microbiome among predatory insects is relatively understudied. Praying mantids are the closest known relatives to termites and cockroaches, which are known for their diverse gut microbiota. However, little is known about the mantid gut microbiota or their importance to host health. In this work, we report the results of a 16S rRNA gene-based study of gut microbiome composition in adults and late-instar larvae of three mantid species. We found that the praying mantis gut microbiome exhibits substantial variation in bacterial diversity and community composition. The hindgut of praying mantids were often dominated by microbes that are present in low abundance or not found in the guts of their insect prey. Future studies will explore the role of these microbes in the digestion of the dietary substrates and/or the degradation of toxins produced by their insect prey.

Food consumption in ground beetles is limited under hypoxic conditions in response to ad libitum feeding, but not restricted feeding

Habitats on land with low oxygen availability provide unique niches inhabited by numerous species. The occupation of such hypoxic niches by animals is hypothesized to come at a cost linked to the limitations of aerobic metabolism and thus energy budget but may also provide benefits through physical protection from predators and parasitoids or reduced competition for food. We investigated the effects of hypoxic conditions on standard metabolic rate (SMR) and specific dynamic action (SDA) in male Carabus nemoralis. SMR and SDA were determined under three manipulated oxygen availabilities: 7, 14 and 21% O₂ and two feeding regimes: limited or ad libitum food consumption. In both hypoxic conditions, C. nemoralis was able to maintain SMR at levels similar to those in normoxia. When the meal size was limited, SDA duration did not differ among the oxygen availability conditions, but SDA was smaller under hypoxic conditions than at normoxic levels. The relative cost of digestion was significantly higher in normoxia than in hypoxia, but it did not affect net energy intake. In contrast, when offered a large meal to simulate ad libitum food conditions, beetles reduced their food consumption and net energy gain by 30% under hypoxia. Oxygen availability may influence the consumed prey size: the hypoxic condition did not limit net energy gain when the beetles fed on a small meal but did when they fed on a large meal. The results indicate that meal size is an important variable in determining differences in physiological costs and whole animal energy budgets at different concentrations of environmental oxygen levels.