Inactivity Is Nycthemeral, Endogenously Generated, Homeostatically Regulated, and Melatonin Modulated in a Free-Living Platyhelminth Flatworm.

Introduction: Sleep either appeared once early in the evolution of animals, or at multiple instances over evolutionary time. Understanding whether sleep is a diagnostic trait for members of the kingdom Animalia has important implications for our understanding of the evolution of sleep and sleep functions. Unfortunately, knowledge on the phylogenetic breadth of sleep is restricted to vertebrates, a few arthropods and molluscs, and one species of nematode. There is a dearth of information on the other 30 or so animal phyla. Aims and Methods: Here, we provide original data on a previously unstudied group of animals with respect to sleep: platyhelminth flatworms. These free-living animals are relatively simple, with a rudimentary central nervous system and absence of many other specialized physiological systems. Results: Despite this simplicity, inactive flatworms appeared to be sleeping. Specifically, quiescence was organized in a circadian manner, occurring largely during the daytime. This basic rhythm persisted under constant darkness, suggesting that it was endogenously generated. Active flatworms responded more readily to stimulation, and flatworms recovered lost sleep by sleeping longer after a 3-hour period of inactivity deprivation. We were also able to increase inactivity in a dose-dependent manner with exposure to melatonin, a hormone that increases sleep in diurnal animals. Conclusions: Taken together, these data expand our understanding of the phylogenetic extent of sleep and reinforce the idea that sleep evolved early in the evolutionary history of animals. However, additional studies on other types of animals are required for a comprehensive understanding of the origin(s) and evolution of sleep.

Fall field crickets did not acclimate to simulated seasonal changes in temperature.

In nature, many organisms alter their developmental trajectory in response to environmental variation. However, studies of thermal acclimation have historically involved stable, unrealistic thermal treatments. In our study, we incorporated ecologically relevant treatments to examine the effects of environmental stochasticity on the thermal acclimation of the fall field cricket (Gryllus pennsylvanicus). We raised crickets for 5 weeks at either a constant temperature (25°C) or at one of three thermal regimes mimicking a seasonal decline in temperature (from 25 to 12°C). The latter three treatments differed in their level of thermal stochasticity: crickets experienced either no diel cycle, a predictable diel cycle, or an unpredictable diel cycle. Following these treatments, we measured several traits considered relevant to survival or reproduction, including growth rate, jumping velocity, feeding rate, metabolic rate, and cold tolerance. Contrary to our predictions, the acclimatory responses of crickets were unrelated to the magnitude or type of thermal variation. Furthermore, acclimation of performance was not ubiquitous among traits. We recommend additional studies of acclimation in fluctuating environments to assess the generality of these findings.

Isopods failed to acclimate their thermal sensitivity of locomotor performance during predictable or stochastic cooling.

Most organisms experience environments that vary continuously over time, yet researchers generally study phenotypic responses to abrupt and sustained changes in environmental conditions. Gradual environmental changes, whether predictable or stochastic, might affect organisms differently than do abrupt changes. To explore this possibility, we exposed terrestrial isopods (Porcellio scaber) collected from a highly seasonal environment to four thermal treatments: (1) a constant 20°C; (2) a constant 10°C; (3) a steady decline from 20° to 10°C; and (4) a stochastic decline from 20° to 10°C that mimicked natural conditions during autumn. After 45 days, we measured thermal sensitivities of running speed and thermal tolerances (critical thermal maximum and chill-coma recovery time). Contrary to our expectation, thermal treatments did not affect the thermal sensitivity of locomotion; isopods from all treatments ran fastest at 33° to 34°C and achieved more than 80% of their maximal speed over a range of 10° to 11°C. Isopods exposed to a stochastic decline in temperature tolerated cold the best, and isopods exposed to a constant temperature of 20°C tolerated cold the worst. No significant variation in heat tolerance was observed among groups. Therefore, thermal sensitivity and heat tolerance failed to acclimate to any type of thermal change, whereas cold tolerance acclimated more during stochastic change than it did during abrupt change.

Mothers forewarn offspring about predators: a transgenerational maternal effect on behavior.

Predation risk has long been known to exert a strong influence on behavior, but no study to date has determined whether predators influence offspring antipredator behavior via maternal effects. Here, we provide a unique example of a transgenerational maternal effect in antipredator behavior that takes the form of a "warning" about predators that female fall field crickets Gryllus pennsylvanicus transmit to their offspring. Specifically, the offspring of gravid crickets exposed to a wolf spider Hogna helluo exhibit greater antipredator immobility in response to Hogna chemical cues than do offspring of nonexposed females. These "forewarned" crickets exhibit greater survival in the presence of Hogna than do those not forewarned. Accordingly, gravid crickets from areas with significant Hogna populations produce offspring that are more responsive to Hogna cues than do those from nearby Hogna-free areas. Such transgenerational maternal effects may be more common than currently realized.

Energy availability influences microclimate selection of hibernating bats.

Many species hibernate to conserve energy during periods of low food and water availability. It has long been assumed that the optimal hibernation strategy involves long, deep bouts of torpor that minimize energy expenditure. However, hibernation has ecological (e.g. decreased predator avoidance) and physiological (e.g. sleep deprivation) costs that must be balanced with energy savings; therefore, individuals possessing sufficient energy reserves may reduce their use of deep torpor. We tested the hypothesis that energy (fat) availability influences temperature selection of two fat-storing bat species during hibernation. We predicted that individuals with small energy reserves would select colder temperatures for hibernation in order to minimize energy expenditure, while individuals with larger energy reserves would choose warmer temperatures to minimize the costs of hibernation. Results from our field experiment indicate that little brown myotis (Myotis lucifugus) hibernating in warm microclimates were significantly heavier than individuals hibernating in cooler microclimates. To determine if energy availability was mediating this relationship, we limited fatty acid availability with mercaptoacetate (MA) and quantified its effect on torpid metabolic rate (TMR) and thermal preference of big brown bats (Eptesicus fuscus). Administration of MA caused a 43% drop in TMR at 10 degrees C and caused bats to choose significantly colder temperatures for hibernation. Our results suggest that fat-storing bats minimize torpor expression using both physiological and behavioral mechanisms.

The perils of picky eating: dietary breadth is related to extinction risk in insectivorous bats.

Several recent papers evaluate the relationship between ecological characteristics and extinction risk in bats. These studies report that extinction risk is negatively related to geographic range size and positively related to habitat specialization. Here, we evaluate the hypothesis that extinction risk is also related to dietary specialization in insectivorous vespertilionid bats using both traditional and phylogenetically-controlled analysis of variance. We collected dietary data and The World Conservation Union (IUCN) rankings for 44 Australian, European, and North American bat species. Our results indicate that species of conservation concern (IUCN ranking near threatened or above) are more likely to have a specialized diet than are species of least concern. Additional analyses show that dietary breadth is not correlated to geographic range size or wing morphology, characteristics previously found to correlate with extinction risk. Therefore, there is likely a direct relationship between dietary specialization and extinction risk; however, the large variation in dietary breadth within species of least concern suggests that diet alone cannot explain extinction risk. Our results may have important implications for the development of predictive models of extinction risk and for the assignment of extinction risk to insectivorous bat species. Similar analyses should be conducted on additional bat families to assess the generality of this relationship between niche breadth and extinction risk.