Does maternal condition or predation risk influence small mammal population dynamics?

There is strong debate over whether the intrinsic traits of individuals or the extrinsic environment exert the greater influence on small mammal population dynamics. We test the roles of maternal effects (an intrinsic factor) and predation risk (an extrinsic factor) in the population dynamics of wild strain house mice using a 2-factor enclosure experiment. Pre-release supplemental feeding with a high-fat diet created female treatment founders that were 6–10% heavier than controls, a condition that we predicted would be passed on as a maternal effect. Predation risk was enhanced using regular application of predator (red fox Vulpes vulpes ) scats. Founder populations of six females and six males released into eight, 15×15 m enclosures showed near exponential population growth over 17 weeks (maximum 3 generations). But there were no responses to either treatment in terms of survival, inherited body weights, fecundity or population size. We suggest that elevated maternal condition may have only minor and transient intergenerational effects with little long-term consequence. We also suggest that the general significance of predator scats as a cue to predation risk to alter prey behaviour may have been overestimated. Hence our results question the role of either factor in causing long-term responses that influence condition to affect population processes.     

Catecholaminergic contributions to the neuronal machinery of the olfactory bulb: aftoradiographic, immunohistochemical and evoked feild potential studies

aftographic exeperiments on the localization of radiolabelednoradrenaline, dopamine and dopa, as well as immunohistochemicalstudies on hydroxylase-like activity, are summarized and comparedin both rat and turtle olfactory bulbs. Evoked field potentialstudies on effects of dopamine are also discussed. The histochemicalstudies suggest that dopaminergic periglomerular neurons arethe most significant cellular component of the catecholaminergicsystem in the olfactory bulb of both species. Scattered fluorescentcell group was also present in the internal plexiform layerand superficial granule cell layer of the turtle olfactory bulb.Other fibres, not related to intrinsic bulbar neuronal cellbodies, were also labeled, mostly in the granule cell layerbut also in the external plexiform layer. These might belongto a centrifugal catecholaminergic system from brain stem neurons.In the in vitro turtle olfactory bulb, dopamine and apomorphinedepressed the amplitude of field potentials evoked by a singlevolley in the olfactory nerve or lateral olfactory tract, andreduced the depression and latency of reponses when paired volleywere delivered. It is suggested that catecholaminergic systemsplay a key role in modulating mitral cell activity through actionsin both superficial (glomerular) and deep (granule) layers.This may involve direct actions, or other, non-catecholaminergicinterneurons.     

Ecosystem function and species loss – a microcosm study

There are too many kinds of organisms to be able to study and manage each, yet the loss of a single species can sometimes unravel an ecosystem. Such `fusewire species'– critical in the same sense that an electrical fuse can cut out a whole circuit – would be a rewarding focus for research and management effort. However, this approach can only be effective if these `fusewires' represent but a small proportion of the number of species in the system.  

Aim

To demonstrate methods for measuring what proportion of the species in a system are critical to ecosystem function.  

Methods

The prevalence of fusewire species was measured in manipulative experiments on an aquatic microcosm.  

Results

No single genus deletion caused changes in key characteristics of the system.  

Main conclusions

Comparison of these results with other published studies shows that the proportion of critical fusewire species varies amongst different ecosystems. The oxidation pond microcosms were shown to contain no single species indispensable to system function. They appear to be ill-suited to a management strategy which focuses on priority eukaryote species. However, a single study provides no evidence that this result is general or even typical of other kinds of ecosystems; it is presented here as an empirical model. Other methods of investigation are available; they are less experimentally rigorous but more practical. These could provide important guidance in planning an approach to management in a particular ecosystem.     

Ntau-methylhistidine excretion is a valid index of myofibrillar protein breakdown in the guineapig (Cavia porcellus).

The recovery of radioactivity in the urine of guineapigs following a bolus intravenous dose of chromatographically pure 14C-Ntau-methylhistidine was measured in order to test whether the excretion of Ntau-methylhistidine (Ntau-MH) is a valid index of myofibrillar protein breakdown in these animals. Four male and four female guineapigs were dosed and after 7 days, 91.65+/-2.82% and 3.58+/-0.91% of injected radioactivity was recovered in the excreta and tissues, respectively. The average total recovery of 95.2+/-3.0% was not significantly different from 100%. Male guineapigs excreted the radioactivity more slowly than females (70% of the dose excreted within 74 h vs 39 h, respectively) but cumulative excretion at 7 days was the same for each sex. Chromatographic analysis of the urine showed almost all of the radioactivity to be associated with a single peak corresponding to Ntau-MH, indicating a lack of significant metabolism. These data show that although the clearance of 14C-Ntau-MH is slower than in rats or humans the urinary excretion of Ntau-MH is a valid index for myofibrillar protein degradation in the guineapig.     

Increased Concentrations of 3-Hydroxykynurenine in Vitamin B6 Deficient Neonatal Rat Brain

Increased concentrations of the endogenous tryptophan metabolite 3-hydroxykynurenine (3-HK) were measured in the brains of vitamin B6 deficient neonatal rats. Mean concentrations of 3-HK in B6 deficient cerebellum, corpus striatum, frontal cortex, and pons/medulla ranged from 9.7 to 18.6 and 102 to 142 nmol/g of wet tissue at 14 and 18 days of age, respectively. 3-HK was not significantly increased in control neonatal or adult rat brain, vitamin B6 deficient rat brain at 7 days of age, or in brains from adult rats deprived of vitamin B6 for 58 days. The administration of daily intraperitoneal injections of vitamin B6 from the 14th to the 18th day of age decreased the concentration of 3-HK to control levels. 3-HK has been shown by other investigators to produce seizures when injected into the cerebral ventricles of adult rodents. Thus, our studies show the accumulation in brain of a putative endogenous convulsant as the result of a nutritional deficiency.     

Patterns of junctional communication in skin: Studies on cultured keratinocytes

Junctional communication has long been suggested to play a role in coordinating the development of multicellular tissues. A better understanding of the patterns of communication between cells in such tissues is important for the identification of areas where this process may have a role. We have investigated the patterns of communication in cultures of human epidermal keratinocytes by iontophoretic injection of Lucifer Yellow CH, using involucrin expression as a marker of cells undergoing terminal differentiation. Cells that lack involucrin (i.e., the basal, proliferating cells) transfer dye preferentially to other involucrin-negative cells, whereas involucrin-positive cells either are not coupled or transfer dye with similar frequency to involucrin-positive and involucrin-negative neighbors. This decrease in communication associated with terminal differentiation was observed in both the presence and the absence of assembled desmosomes. Our observations lead us to speculate that loss of junctional communication may influence the commitment of basal keratinocytes to terminal differentiation.