Cognitive skills of common shrews (Sorex araneus) vary with seasonal changes in skull size and brain mass.

In a rare phenomenon, shrews and a few other species cope with seasonal environments by reducing and regrowing brain size, potentially at the cost of changes in cognitive abilities. Here, we confirm an extensive seasonal shrinkage (21.4%) and regrowth (17.0%) of brain mass in winter and spring, respectively, in the common shrew (Sorex araneus L.) in Southern Germany. In a spatial learning task experiment, individuals with reduced winter brain size covered larger distances to find food, compared with the relatively large-brained summer juveniles and regrown spring adults. By reducing their brain mass, these shrews may reduce their energetic demands, but at the cost of cognitive performance, implying a complex trade-off for coping with seasonally fluctuating resources. These results are relevant for our understanding of evolution and the dynamics of mammalian nervous systems in response to environmental changes.

Associative memory or algorithmic search: a comparative study on learning strategies of bats and shrews.

Two common strategies for successful foraging are learning to associate specific sensory cues with patches of prey ("associative learning") and using set decision-making rules to systematically scan for prey ("algorithmic search"). We investigated whether an animal's life history affects which of these two foraging strategies it is likely to use. Natterer's bats (Myotis nattereri) have slow life-history traits and we predicted they would be more likely to use associative learning. Common shrews (Sorex araneus) have fast life-history traits and we predicted that they would rely more heavily on routine-based search. Apart from their marked differences in life-history traits, these two mammals are similar in body size, brain weight, habitat, and diet. We assessed foraging strategy, associative learning ability, and retention time with a four-arm maze; one arm contained a food reward and was marked with four sensory stimuli. Bats and shrews differed significantly in their foraging strategies. Most bats learned to associate the sensory stimuli with the reward and remembered this association over time. Most shrews searched the maze using consistent decision-making rules, but did not learn or remember the association. We discuss these results in terms of life-history traits and other key differences between these species. Our results suggest a link between an animal's life-history strategy and its use of associative learning.

Field vole (Microtus agrestis) seasonal spacing behavior: the effect of predation risk by mustelids.

There are numerous studies showing that predation risk may change different aspects of the behavior of prey, such as habitat use, activity pattern, and foraging. Prey should exhibit the strongest antipredatory response against their most deadly predator. Small mustelids are considered the most important mammalian predators of voles. Nevertheless, there is no general agreement as to whether strong antipredatory reactions exist in natural free-living populations of voles. Here, we studied the field vole Microtus agrestis spatial reaction to high predation risk from small mustelids in the breeding (August) and nonbreeding (October) seasons under natural conditions. Voles were exposed to a caged weasel (Mustela nivalis) and a stoat (Mustela erminea), as well as to the odors of these predators. The reactions of 30 field voles were monitored with radiotelemetry. The field voles were found to display antipredator reactions that varied with season. In the breeding period, in response to predation risk, voles reduced locomotory activity and daily-range size, whereas in the nonbreeding period they did not. Changes in home range position were similar for control and treatment voles, in both the breeding and nonbreeding periods. The results indicate that mustelid predators modify the spatial behavior of small rodents in natural conditions depending on season. This might be a reflection of differences in state-dependent responses to predation from sexually active or inactive individuals. This suggests that the basic antipredatory reaction of voles under high predation risk from small mustelids limits their locomotory activity.

Neonatal DHT but not E2 speeds induction of sexual receptivity in the musk shrew.

Neural aromatization of testosterone (T) to estrogen during development is thought to be important for sexual differentiation of many altricial mammals. We evaluated the effects of neonatal injections of the non-aromatizable androgen dihydrotestosterone propionate (DHTP) and estradiol (E2) on the copulatory behavior of the female musk shrew, an altricial insectivore. Following adult ovariectomy and replacement T, animals were paired with a stimulus female for two 60-minute copulatory behavior tests. The latency to induce sexual receptivity (in the form of tail-wagging by the female), mount latency and total number of mounts were recorded in experimental females and in a group of untreated control males. While neither hormone treatment significantly affected mounting behavior, DHTP-treated animals induced receptivity faster and with latencies not significantly different from intact males, suggesting that early non-aromatizable androgens can have masculinizing actions by either increasing sexual motivation or making the treated animal more attractive to the stimulus female. Reliance on androgenic rather than estrogenic metabolites for the differentiation of courtship behaviors conforms to the pattern seen more typically in primates than rodents.

Mating behavior is controlled by acute changes in metabolic fuels.

Mild food restriction for 48 h inhibits mating behavior in female musk shrews (Suncus murinus). However, mating behavior is restored after a 90-min feeding bout. In this series of experiments, we examined the role of metabolic fuels in this behavioral restoration. First, drugs reported to block glycolysis or fatty acid oxidation were given 2 h before mating. Both treatments inhibited mating in food-restricted females that were refed after treatment. Blood glucose levels were assessed in females that were fed ad libitum, food restricted, or food restricted and refed for 90 min. Food restriction significantly lowered blood glucose compared with ad libitum feeding or food restriction in combination with 90 min of refeeding. However, neither glucose nor fat alone could substitute for food and promote mating behavior in food-restricted females. In addition, analysis of ketone bodies and body composition in females demonstrated low or undetectable levels of these energy substrates. Our data suggest that musk shrews have relatively little stored energy. Therefore, female musk shrews rely on continuous food intake and monitor multiple cues acutely, including glucose availability and fatty acid oxidation. This ensures that mating does not occur when adequate energy is unavailable.

Temporal aspects of female musk shrew (Suncus murinus) sexual behavior.

The effects of testing condition on sexual behavior were examined in female musk shrews (Suncus murinus). Females were tested in 2 conditions, a pair test and a paced test. The pair test was similar to traditional sex test conditions in which the female and male are placed into the same chamber together until mating occurs. The paced condition allowed the female to leave the male's chamber and revisit him at will, thus "pacing" the interaction. Females displayed receptivity continuously for 14 days in both conditions. In the paced condition, females were less likely to become receptive within 30 min and mate to ejaculation. However, few additional differences were found between test conditions. Because this is the 1st experiment to use a pacing test paradigm in a species with induced ovulation, the authors speculate that the absence of pacing behavior during mating may be shown by other species that have induced ovulation.

Behavioral interactions have rapid effects on immunoreactivity of prohormone and gonadotropin-releasing hormone peptide.

The nervous system responds to both internal and external cues, integrating these signals to coordinate behavior and physiology. Mating interactions can promote dramatic changes in neuroendocrine cells which trigger successful copulation, ovulation, fertilization, and pregnancy. The neurons that transduce behavioral cues into neuroendocrine signals are distributed in a loose continuum along the medial ventral forebrain where they produce and secrete gonadotropin-releasing hormone (GnRH). In the past we have reported changes in GnRH-immunoreactive (GnRH-ir) cell numbers in brains of female musk shrews sacrificed during, and after, brief mating interactions. The purpose of the current study was twofold: first to determine which aspect of intracellular GnRH production is stimulated by behavioral interactions; second, to characterize the specific aspects of the social exchange that trigger GnRH production. We report that 1 h after copulation the production of proGnRH protein is stimulated. Non-copulatory behavioral interactions resulted in a rapid decrease in the numbers of neurons containing GnRH-ir peptide. This change was accompanied by an increase in the GnRH-ir fibers in the median eminence, but no surge in luteinizing hormone. These data suggest that behavioral interactions stimulate release of mature GnRH peptide from cell bodies followed by accumulation of available GnRH in cell terminals. Copulation triggers increased production of proGnRH in cell bodies. The data highlight the usefulness of behavioral paradigms for the examination of the dynamics of neuropeptide production.

[Diurnal rhythm of drinking activity in the house musk shrew].

A diurnal rhythm of drinking activity in 7 male and 6 female house musk shrews (Jic: SUN) aged about one year was observed over a period of 10 days under a schedule of 12 hr light and 12 hr darkness (light on at 07:00). In general, the pattern of drinking activity was similar among both sexes, with around 24-hr diurnal rhythm. A few typical drinking patterns of these animals were represented as follows: 1) Drinking interval was very close in the dark phase, while it was a little too sparse in the light phase (n = 4). 2) Its interval remains stationary through a whole day (n = 5). 3) Drinking was performed between the latter half of light and the first half of dark phases (n = 4).

Inheritance and breeding of the waltzing mutant in the musk shrew (Suncus murinus, Insectivora) characterized by the circling and head-shaking behaviors.

A behavioral mutant was found in the laboratory-bred musk shrew (Suncus murinus). The affected shrews were characterized by the behaviors of tight circling in both directions and frequent head shaking in horizontal. They could definitely be identified by at least day 10 after birth. These abnormal behaviors were steady and permanent through life. Mating experiments demonstrated that the mutant character is expressed by a single autosomal recessive gene in homozygote with complete penetrance. The pedigree analysis indicated that the gene was derived from one heterozygous male captured in Ginowan city, Okinawa. The name, waltzing, was proposed for this mutant character with the gene symbol wz. An abnormality of the balance organ was predicted for a cause of the abnormal behaviors, since, besides the circling and head shaking behaviors, the affected shrews could not keep the body stretching but twisted it frequently when they were held up by the tail and further they could not keep the head on the water surface at all. Nevertheless, the affected shrews were almost normal in gestation period, litter size, weaning ratio and body weight in comparison with the phenotypically normal ones. The mutant shrews have been maintained as a closed colony, the WZ line involved more than 30 individuals at every one generation.

Circadian rhythms in the short-tailed shrew, Blarina brevicauda.

Circadian rhythms of wheel running and feeding were measured in the short-tailed shrew. Shrews were strongly nocturnal, and their activity rhythms entrained to both long-day (LD 16:8) and short-day (LD 6:18) photocycles. Under conditions of continuous light (LL) or darkness (DD), the activity rhythms free-ran with average periodicities of 25.1 hours and 24.1 hours, respectively. In LL the level of activity was depressed, and in some cases wheel running was completely inhibited. No significant sex differences were observed in the period or amplitude of the monitored circadian rhythms. All shrews fed throughout the day and night; however, unlike in previous reports, ultradian periods of feeding behavior were not found. The results are related to Aschoff's four observations for the effect of light on activity rhythms in nocturnal rodents.