Feast or famine: evidence for mixed capital–income breeding strategies in Weddell seals

Evolved patterns of resource expenditure for reproduction have resulted in a life history continuum across species. A strictly capital-breeding strategy relies extensively on stored energy for reproduction, whereas income breeding uses energy acquired throughout the reproductive period. However, facultative income breeding has been shown in some classically capital-breeding animals, and was originally thought to provide a nutritional refuge for smaller females incapable of securing sufficient reserves during pre-partum foraging. We examined milk composition and milk output for the Weddell seal to determine to what degree lactation was aided by food intake, and what factors contributed to its manifestation. Milk composition was independent of maternal post-partum mass and condition, but did change over lactation. Changes were most likely in response to energetic and nutritional demands of the pup at different stages of development. During early lactation, females fasted and devoted 54.9% of total energy loss to milk production. Later in lactation 30.5% more energy was devoted to milk production and evidence suggested that larger females fed more during lactation than smaller females. It appears that Weddell seals may exhibit a flexible strategy to adjust reproductive investment to local resource levels by taking advantage of periods when prey are occasionally abundant, although it is restricted to larger females possessing the physiological capacity to dive for longer and exploit different resources during lactation. This supports the assumption that although body mass and phylogenetic history explain most of the variation in lactation patterns (20–69%), the remaining variation has likely resulted from physiological adaptations to local environmental conditions. Our study confirms that Weddell seals use a mixed capital–income breeding strategy, and that considerable intraspecific variation exists. Questions remain as to the amount of energy gain derived from the income strategy, and the consequences for pup condition and survival. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

A validated model of passive skeletal muscle to predict force and intramuscular pressure

The passive properties of skeletal muscle are often overlooked in muscle studies, yet they play a key role in tissue function in vivo. Studies analyzing and modeling muscle passive properties, while not uncommon, have never investigated the role of fluid content within the tissue. Additionally, intramuscular pressure (IMP) has been shown to correlate with muscle force in vivo and could be used to predict muscle force in the clinic. In this study, a novel model of skeletal muscle was developed and validated to predict both muscle stress and IMP under passive conditions for the New Zealand White Rabbit tibialis anterior. This model is the first to include fluid content within the tissue and uses whole muscle geometry. A nonlinear optimization scheme was highly effective at fitting model stress output to experimental stress data (normalized mean square error or NMSE fit value of 0.993) and validation showed very good agreement to experimental data (NMSE fit values of 0.955 and 0.860 for IMP and stress, respectively). While future work to include muscle activation would broaden the physiological application of this model, the passive implementation could be used to guide surgeries where passive muscle is stretched.     

Synergistic effects of p-fluorophenylalanine and supra-optimal temperatures on cell division in Tetrahymena pyriformis.

The combined effects of sub-optimal (20 degrees) or supra-optimal (31 degrees C and 34 degrees C) temperatures and the amino acid analogue p-fluorophenylalanine on preparations for cell division in Tetrahymena pyriformis were analysed. A supra-optimal temperature of 31 degrees produced a marked exaggeration of the division-delaying capacity of the analogue. The analogue also sensitises cells when combined with the normal heat shock temperature of 34 degrees, which indicates that division proteins continue to be synthesized at this temperature but cannot be effectively utilized by the cell. These results support the model that preparations involve interactions between and formation of assemblies of proteins on a strict time schedule shortly before division.     

Analysis of ear sebum of the Syrian hamster (Mesocricetus auratus) reveals pronounced sexual dimorphism

External ear of male and female Syrian hamsters (Mesocricetus auratus) were extracted with hexane and separated by class on thin-layer chromatography (TLC). Separated lipid classes were eluted and saponified, and non-saponifiable lipids further characterized by TLC, gas-liquid chromatography (GLC), UV and IR spectroscopy and functional group analyses. Many sex differences were observed, most notably the presence of sex-specific sterols of males and females. Mature animals were found to have greater quantities of ear sebum, but the characteristic qualitative lipid profiles of each sex were apparent in immature animals.