Average daily metabolic rate of gerbils of two species:Gerbillus pyramidumandGerbillus allenbyi

Energy requirements, length of digestive tract compartments and body mass in six gerbilline rodents of the Negev Desert

Energy requirements of an animal are size dependent and, in this study, the average daily metabolic rate (ADMR) of six Negev Desert gerbilline rodents, ranging in body mass (m_b) from 10g to over 200g, scaled to m_b^0.57. Although gerbilline rodents are considered 'granivores', these rodents consume substantial amounts of green vegetation and the largest gerbil is a strict herbivore. We predicted that the lengths of the compartments of the digestive tract would scale allometrically to mb^0.33 and that ADMR would scale allometrically to the lengths of the compartments to the exponent of 1.73. Using log-transformed data, the length of the colon scaled to m_b^0.50 (r²_adj = 0.74; p= 0.02), of the caecum to m_b^0.45 (r²_adj=0.80; p= 0.01) and of the small intestine to m_b^0.30 (r²_adj=0.59; p < 0.05). Therefore, the exponents for the colon and caecum were higher than predicted and were close to the exponent for ADMR generated for the rodents. The absolute lengths of the colon (r²_adj=0.68; p= 0.03; slope = 0.99) and of the caecum (r²_adj=0.79; p= 0.01; slope = 1.19) were related significantly to ADMR, but of the small intestine was not (r²_adj=0.04; p=0.33; slope = 0.85). The exponents implied that the relationships were isometric and not allometric as predicted and that the rates of increase of the lengths of the intestine compartments were at the same rate as the increase in ADMR. The lengths of the colon and caecum were highly correlated between each other (r²_adj=0.98; p< 0.001; slope = 1.12) and explained most of the variation in ADMR. Green vegetation could be a nutritional bottleneck for rodents as it is bulky and, consequently, limits the dietary intake, and fermentation occurs in the caecum and colon, whereas seeds, which are compact and are digested in the small intestine, would limit intake to a much lesser degree. However, when the effect of body mass was eliminated by using residuals of the variables on body mass, only the length of the small intestine was significant (r²_adj=0.86; p< 0.005; slope = -1.33) and was related negatively to ADMR. Therefore, when effects of body size were removed, most of the variation in ADMR was explained by the length of the small intestine and implied that the length of the small intestine increased with a decrease in ADMR. A higher energy expenditure was related to a shorter small intestine and, therefore, by implication, a higher concentration of metabolizable energy yield of the diet. We also questioned whether there are differences in the morphology of the digestive tract due to differences in dietary consumption. The digestive tracts of the gerbils were not diverse and could be characterized as structurally homogenous. All the gerbils had a uni-locular, hemi-glandular stomach and the differences in the digestive tract among species did not seem to be of functional importance, but rather were related to the taxon. However, some important morpho-functional characteristics of the digestive tract emerged that apply to the whole group.

From endosymbionts to host communities: factors determining the reproductive success of arthropod vectors

Elucidating the factors determining reproductive success has challenged scientists since Darwin, but the exact pathways that shape the evolution of life history traits by connecting extrinsic (e.g., landscape structure) and intrinsic (e.g., female's age and endosymbionts) factors and reproductive success have rarely been studied. Here we collected female fleas from wild rodents in plots differing in their densities and proportions of the most dominant rodent species. We then combined path analysis and model selection approaches to explore the network of effects, ranging from micro to macroscales, determining the reproductive success of these fleas. Our results suggest that female reproductive success is directly and positively associated with their infection by Mycoplasma bacteria and their own body mass, and with the rodent species size and total density. In addition, we found evidence for indirect effects of rodent sex and rodent community diversity on female reproductive success. These results highlight the importance of exploring interrelated factors across organization scales while studying the reproductive success of wild organisms, and they have implications for the control of vector-borne diseases.

Age, intensity of infestation by flea parasites and body mass loss in a rodent host

Parasitism by the flea Synosternus cleopatrae does not affect the body mass of its principal rodent host, Gerbillus andersoni under natural infestation levels. We hypothesized that the lack of negative effects of flea parasitism on rodent body mass could be related either to the low level of natural infestation or to the differential susceptibility of rodent age cohorts to flea parasitism. We tested these hypotheses by measuring body mass change under flea parasitism in (a) adult rodents infested with fleas above the natural infestation level (the first hypothesis) and (b) juvenile rodents infested with fleas at natural infestation levels (the second hypothesis). Adult individuals parasitized by a number of fleas higher than in nature lost body mass at higher rates than non-parasitized control individuals. Parasitism significantly affected daily body mass change of juvenile gerbils. Juvenile rodents parasitized by fleas at the natural level of infestation lost body mass faster and gained body mass slower than control animals. We suggest that some regulating mechanisms may limit natural flea densities at a point at which the negative effect on hosts is below the accuracy of our measurements. However, natural flea densities are sufficiently high to harm the more susceptible, juvenile cohort.

Ectoparasites and age-dependent survival in a desert rodent

Host age is one of the key factors in host-parasite relationships as it possibly affects infestation levels, parasite-induced mortality of a host, and parasite distribution among host individuals. We tested two alternative hypotheses about infestation pattern and survival under parasitism in relation to host age. The first hypothesis assumes that parasites are recruited faster than they die and, thus, suggests that adult hosts will show higher infestation levels than juveniles because the former have more time to accumulate parasites. The second hypothesis assumes that parasites die faster than they are recruited and, thus, suggests that adults will show lower infestation levels because of acquired immune response and/or the mortality of heavily infested juveniles and, thus, selection for less infested adults. As the negative effects of parasites on host are often intensity-dependent, we expected that the age-related differences in infestation may be translated to lower or higher survival under parasitism of adults, in the cases of the first and the second hypotheses, respectively. We manipulated ectoparasite numbers using insecticide and assessed the infestation pattern in adult and juvenile gerbils (Gerbillus andersoni) in the Negev Desert. We found only a partial support for age-dependent parasitism. No age-related differences in infestation and distribution among host individuals were found after adjusting the ectoparasite numbers to the host's surface area. However, age-related differences in survival under parasitism were revealed. The survival probability of parasitized juveniles decreased in about 48% compared to unparasitized hosts while the survival probability of adults was not affected by ectoparasites. Our results suggest that the effect of host age on host-parasite dynamics may not explicitly be determined by age-dependent differences in ectoparasite recruitment or mortality processes but may also be affected by other host-related and parasite-related traits.

Simultaneously model-unbiased, design-unbiased estimation

This paper proposes a class of inferential procedures (incorporating both design and estimation elements) that yield estimates of means that are simultaneously model unbiased and design unbiased. Classical regression procedures yield conditionally unbiased estimators for the mean (conditioning on the model and choice of observation points). In contrast, design-based methods yield estimators that are unconditionally unbiased on matter what the form of the underlying model. Variance properties of the proposed class are examined, and applications to bioavailability, water quality from mine run-off, and finite population regression estimation are considered. The proposed procedures perform well, especially in the typical case where a model is only approximately correct.

Energy expenditure and efficiency of energy use in rodents: desert and non-desert species

Feeding and the search for food are the predominant activities of most rodents. Since desert rodents inhabit regions of generally low food availability, it has been theorized that their energetic requirements would be lower and/or their efficiency to use energy for maintenance and growth would be higher than for rodents from more temperate regions. Indeed, the basal metabolic rate, average daily metabolic rate and field metabolic rate of desert rodents have been found to be lower than those of non-desert rodents. However, the ability to use energy intake for maintenance by desert rodents was calculated to be lower than that of non-desert rodents. In addition, granivores were not different from omnivores and herbivores in energy use efficiency, but herbivores were more efficient than omnivores.