The effect of thermoregulatory substitution on optimal energy reserves of small birds in winter

Previous models have predicted the body mass of small birds in winter on the basis of a trade-off between starvation and predation. Many of these models have assumed that energy expenditure while active increases with body mass. The implications of the fact that the metabolic cost of activity can substitute for internal heat production and help keep the bird warm have not been investigated. In this paper we show that if thermoregulatory substitution occurs then there is a critical level of energy reserves above which an active bird is thermoneutral. This critical level increases as temperature decreases. Below this level, substitution of energy results in higher optimal levels of reserves than would be predicted in the absence of substitution. Our model thus predicts that at low temperatures body mass will be higher when thermoregulatory substitution occurs.     

The scaling of diving time budgets: insights from an optimality approach

Simple scaling arguments suggest that, among air-breathing divers, dive duration should scale approximately with mass to the one-third power. Recent phylogenetic analyses appear to confirm this. The same analyses showed that duration of time spent at the surface between dives has scaling very similar to that of dive duration, with the result that the ratio of dive duration to surface pause duration is approximately mass invariant. This finding runs counter to other arguments found in the diving literature that suggest that surface pause duration should scale more positively with mass, leading to a negative scaling of the dive-pause ratio. We use a published model of optimal time allocation in the dive cycle to show that optimal decisions can predict approximate mass invariance in the dive-pause ratio, especially if metabolism scales approximately with mass to the two-thirds power (as indicated by some recent analyses) and oxygen uptake is assumed to have evolved to supply the body tissues at the required rate. However, emergent scaling rules are sensitive to input parameters, especially to the relationship between the scaling of metabolism and oxygen uptake rate at the surface. Our results illustrate the utility of an optimality approach for developing predictions and identifying key areas for empirical research on the allometry of diving behavior.     

Sperm allocation strategies and female resistance: a unifying perspective

The classical viewpoint in sperm competition theory, which holds that males evolve sperm allocation strategies in response to a given degree of sperm competition, has recently been challenged by an alternative viewpoint, which holds that the degree of sperm competition is itself a consequence of these same strategies. Here, we present a game theory model that unites these alternative views as the endpoints of a continuum. Based on the recognition that female control over mating may limit the extent to which male strategies affect the degree of sperm competition, we investigate sperm allocation strategies in a setting where females can resist excessive matings more or less successfully. We discuss how conflicting predictions made by previous theory relate to implicit assumptions about female resistance behavior. Moreover, we show that female resistance, while being highly relevant to the predicted relationship between ejaculate size and the degree of sperm competition, has little effect on the predicted positive correlation between relative testis size and the degree of sperm competition. This result strengthens one of the central predictions of sperm competition theory and is in accordance with empirical findings from a wide range of taxa.     

The antihyperglycemic effect of estrone sulfate in genetically obese-diabetic (ob/ob) mice is associated with reduced hepatic glucose-6-phosphatase.

Excessive glucose production by the liver contributes significantly to diabetic hyperglycemia. The enzyme system glucose-6-phosphatase plays a key role in regulating hepatic glucose production and therefore its inhibition is a potential therapeutic target for the correction of hyperglycemia. It has previously been shown that sulfated steroids, such as estrone sulfate and dehydroepiandrosterone sulfate, inhibit the glucose-6-phosphatase system in vitro, principally through inhibition of endoplasmic reticulum glucose-6-phosphate transport. We report here that in the obese/diabetic ob/ob mouse model, orally administered estrone sulfate reduces the abnormally elevated hepatic glucose-6-phosphatase enzyme activity and enzyme protein levels that are characteristic in the ob/ob mouse, and that this reduction is associated with normalization of blood glucose levels. Other sulfated and non-sulfated steroids also reduced, to a lesser extent, glucose-6-phosphatase enzyme activity - with the exception of dehydroepiandrosterone sulfate, which had no apparent effect on this system in ob/ob mice. Estrone sulfate is therefore an effective antihyperglycemic agent in ob/ob mice, and the glucose-6-phosphatase system can be successfully targeted for the therapeutic management of hyperglycemia in this animal model of non-insulin-dependent diabetes mellitus.     

Impaired flight ability--a cost of reproduction in female blue tits

When prey are attacked by predators, escape ability has an obvious influence on the probability of survival. Laboratory studieshave suggested that flight performance of female birds mightbe affected by egg production. This is the first study of changesin take-off ability, and thus potentially in predation risk,during reproduction in wild birds. We trapped individual maleand female blue tits repeatedly during the breeding season.Females were 14% heavier and flew 20% slower (probably as aconsequence of a lower ratio of flight muscle to body mass)during the egg-laying period than after the eggs had hatched.However, flight muscle size did not change to compensate for changes in body mass over this period. In contrast, males showedno changes in either body mass, muscle size, or flight abilityover the same period. Furthermore, the impairment of flightin females increased with the proportion of the clutch thathad been laid, an effect that was independent of body mass and muscle size. This indicates that egg production causes additional physiological changes in the female body that produce impairedlocomotor performance. We suggest that courtship feeding offemale blue tits by their mates might reduce predation riskduring the period when female take-off ability is impairedby reducing the time females have to spend foraging and thusreducing the time they are exposed to increased predation.     

Specialized Subregions of the Bifunctional hisB Gene of Salmonella typhimurium

Forty-three hisB mutants of Salmonella typhimurium have been screened to determine the molecular size of the resulting histidinol phosphate phosphatase activity, one of the activities of a bifunctional enzyme produced by this gene which also controls imidazole glycerol phosphate dehydrase activity. Mutation in hisB can lead to the loss of both phosphatase and dehydrase activities, or only of dehydrase activity. Through the use of nonsense mutants lacking dehydrase activity, a distinct point of transition was detected near the middle of hisB at which a dramatic change occurs in the size of the phosphatase enzyme that is synthesized. A missense mutant with a lesion in this region has a high-molecular-weight enzyme which is eluted in the void volume of a Sephadex G-200 column. The enzyme from nonsense mutants near the transition point have molecular weights near 40,000. Even though the buffer conditions are designed to favor the stabilization of the high-molecular-weight form, some mutants have both high- and low-molecular-weight forms. The polypeptide chain specified by the operator proximal part of hisB is sufficient to allow the expression of phosphatase activity. The synthesis of substantially less than the complete product of hisB resulted in association into a form similar to the native enzyme which was found in the void volume of a Sephadex G-200 column.     

Writing in New World Civilizations

Mesoamerican Writing Systems: Propaganda, Myth, and History in Four Ancient Civilizations . Joyce Marcus