Seasonal patterns of nitrogen utilization by pocket gophers, Thomomys bottae

Adult pocket gophers, fed diets of natural vegetation in the laboratory during summer and winter, remained in positive N balance in all feeding trials. Nitrogen budgets for free-ranging pocket gophers were calculated and revealed that total N influx exceeded 2400 mg N . kg-1 day-1 in summer and 2300 mg N . kg-1 day-1 in winter. Apparent N assimilation was 50 and 53% in summer and winter, respectively, and truly digestible N requirements were congruent to 680 mg N . kg-1 day-1. These budgets indicate that, in spite of seasonally-changing quality of available forage, pocket gophers remain in positive N balance, due largely to shifts in forage preference with season.     

Karyotypic variation following an elevational gradient in the pocket gopher,Thomomys bottae grahamensis Goldman

A system of non-Robertsonian chromosomal polymorphism is described for populations of the pocket gopher, Thomomys bottae grahamensis Goldman, from the Graham Mountains of southeastern Arizona. The polymorphism occurs in three of the four populations sampled, but the range of chromosomal variation within each population is less than for the whole sample. The diversity varies clinally with elevation and is strongly correlated with both habitat and moisture gradients. Each population is considered to be at an adaptive norm with respect to the occupied habitat, and the karyotypic changes are presumed to have played a role in this adaptability. Maintenance of the polymorphic conditions in each population is considered to be due to karyotypic stability (either frequency equilibrium or population monotypy) coupled with gene flow from adjacent populations.     

The relationship between genetic variability and growth rate among populations of the pocket gopher, Thomomys bottae

Perhaps the oldest unresolved debate inconservation genetics is whether geneticvariability matters – in other words, whetherrelatively low average genetic variationcontributes to deficits in individual andpopulation level vigor and fitness. Using astatistically powerful paired sampling designin which each of three pairs of populationsconsisted of one high genetic variability andone low genetic variability population from aparticular subspecies of the pocket gopher,Thomomys bottae, we tested the hypothesisthat individuals from populations with lowergenetic variability have lower growth rates (acommonly used surrogate for fitness) than thosefrom populations with higher variability. Wemeasured genetic variability using averageallozyme heterozygosity and two measures of DNAfingerprint band sharing (Jeffreys 33.15 andMS1 probes). The population rankings of thelevels of genetic variability among the threemeasures were concordant. The least squaresmean growth rate (controlling for sex,subspecies and initial mass) of gophers fromlow variability populations (0.41 ± 0.06g/day, n = 48) was less than half that ofgophers from high variability populations (1.04± 0.07 g/day, n = 45). This result lendscredence to the premise that differences inpopulation level genetic variability havesignificant fitness consequences andunderscores the importance of maintaininggenetic variability in managed populations.     

Responses of pocket gophers (Thomomys bottae) to changes in diet quality

Summary We examined digestibility of dry matter, nutrients, and fiber, and food intake, metabolic fecal losses, weight change, and gut size of pocket gophers (Thomomys bottae) in relation to diet quality in the laboratory. Pocket gophers were maintained for 15–20 days on one of seven diets which contained from 18% to 56% neutral detergent fiber (NDF). NDF content of the diet was an excellent predictor of diet quality. Digestibility of dry matter, NDF, and nitrogen all decreased with increasing NDF content of the diet. In general, pocket gophers compensated for low diet quality by increasing dry matter intake, but those given high quality forage before the lowest quality diet reduced their intake. Thus, the response of pocket gophers to low quality diets may depend on their body condition. Because increased food intake resulted in increased total metabolic fecal losses and metabolic fecal nitrogen losses, decreasing food intake on low-quality diets may be advantageous. A further response of pocket gophers to decreased food quality was an increase in size of cecum and large intestine, suggesting that fermentation of cell walls became increasingly important as diet quality decreased.     

Burrow limitations and group living in the communally rearing rodent, Octodon degus

Group living is thought to evolve whenever individuals attain a net fitness advantage due to reduced predation risk or enhanced foraging efficiency, but also when individuals are forced to remain in groups, which often occurs during high-density conditions due to limitations of critical resources for independent breeding. The influence of ecological limitations on sociality has been studied little in species in which reproduction is more evenly shared among group members. Previous studies in the caviomorph rodent Octodon degus (a New World hystricognath) revealed no evidence that group living confers an advantage and suggest that burrow limitations influence formation of social groups. Our objective was to examine the relevance of ecological limitations on sociality in these rodents. Our 4-year study revealed no association between degu density and use of burrow systems. The frequency with which burrow systems were used by degus was not related to the quality of these structures; only in 1 of the 4 years did the frequency of burrow use decrease with decreasing abundance of food. Neither the number of females per group nor total group size (related measures of degu sociality) changed with yearly density of degus. Although the number of males within social groups was lower in 2008, this variation was not related clearly to varying density. The percentage of females in social groups that bred was close to 99% and did not change across years of varying density. Our results suggest that sociality in degus is not the consequence of burrow limitations during breeding. Whether habitat limitations contribute to variation in vertebrate social systems is discussed.     

Fluctuating asymmetry and allozymic heterozygosity among natural populations of pocket gophers (Thomomys bottae)

Fluctuating asymmetry, or random deviations from bilateral symmetry, has been widely used as a measure of developmental stability. The relationship between fluctuating asymmetry (FA) and allozymic heterozygosity was evaluated using 18 natural populations of pocket gophers ( Thomomys bottae ). Heterozygosity in local populations of pocket gophers ranges over more than an order of magnitude (1.5—18.4%), making this burrowing rodent particularly apt for such studies. Two measures of FA in mensural skull characters were examined: absolute deviations between left and right sides and the variance of signed differences. After log transformations, levels of FA among individuals and populations were not related to size. Repeated-measures analyses of variance showed that FA was significant relative to measurement error, both across populations and within them. Asymmetries of different characters were uncorrelated, despite positive significant correlations among the characters themselves. FA levels varied only slightly among populations of gophers, and this variation was not significant for most characters. FA levels of populations were not correlated with allozymic heterozygosity, and analyses of variance in FA employing heterozygosity were not significant. Heterozygosity levels in these rodents appear more strongly related to aspects of population history (especially effective size and gene flow) than to developmental stability. Because so many genomic and environmental factors can affect morphological variation, caution is needed in interpreting correlations between genetic and phenetic variation.     

Prevalence of anomalies in the appendicular skeleton of a fossorial rodent population

The prevalence of macroscopic bone anomalies in the appendicular skeleton of wild rodents and, particularly, fossorial species is not well known. We examined 8,257 bones corresponding to 564 collection specimens (249 males and 315 females) of a fossorial form of water vole (Arvicola terrestris monticola). Animals were obtained monthly from July 1983 to December 1984 in the Aran Valley (Pyrenees). Most macroscopic anomalies were healed fractures or exostoses. The prevalence of anomalies was not significantly different between males and females but was clearly higher in adults than in juveniles and subadults. The frequency of alterations in the thoracic limb long bones was significantly higher than that in the pelvic counterparts. Aggressive intraspecific interactions and biomechanical factors related to burrowing may be associated with these differences. In females, remodeling of the innominate shape because of pregnancy and parturition could enhance fractures and exostoses in this structure.     

Burrow fractal dimension and foraging success in subterranean rodents: a simulation

For animals that forage underground, the success with whichfood items are located may be closely related to burrow architecture.Fractal dimension, which describes how a burrow explores thesurrounding area in a way that is independent of burrow length,is an obvious choice for a single metric describing burrow shape.Although it is often assumed that burrows of high fractal dimensionwill be associated with greater foraging success, this has notpreviously been demonstrated. In this study, we use computersimulations to study the success with which burrows of differentfractal dimensions locate randomly distributed food items. Inaddition, we examine the effect of different patterns of fooddistribution (in particular the patchiness with which food itemsare distributed) and consider how using different criteria forlocating food items affects the relationship between fractaldimension and foraging success. We conclude that, under a widerange of plausible assumptions about the ways in which subterraneanrodents forage, burrows of high fractal dimension are more successfulat locating food items than burrows of lower fractal dimension.     

Thermal biology of the fossorial rodent Ctenomys fulvus from the Atacama desert, northern Chile

The Andean tuco-tuco, Ctenomys fulvus (Rodentia: Ctenomyidae) inhabits one of the most arid regions of the world, the Salar de Atacama, Northeast of Antofagasta, Chile (23°17′06″S, 68°05′43″W; 2.240 m.a.s.l). We found that a stable microclimate in burrows, a low evaporative water loss (EWL), and a diet of roots (59% water content) are the main factors that permit the survival of this fossorial species in harsh desert conditions. Large circadian variation in T_a was observed above ground. Daily ΔT_a (T_a max − T_a min) = 37.9±0.2°C in summer and in winter. In contrast, circadian variation of T_a inside the burrows was only 5.8±0.5°C in the same seasons. Relative humidity (RH) was 1.9–3.1% during the day, increasing to maximum values of 27% at night and early morning. Inside the burrows RH was higher and quite stable, ranging between 53.1 and 65%, independent of the time of day and season. EWL, measured between 10 and 25°C, was low (1.26 mg/g h), and a moderate increase of 13–20% was observed at higher temperatures. The low EWL may prevent dehydration. However, because of the low heat loss capability, animals became hyperthermic (0.8–1.6°C) in dry air at T_a=30–35°C. As T_a during afternoon normally exceeded 35°C, the microclimate of burrows provided the only way to avoid the lethal effects of hyperthermia.     

Breeding biology of the fossorial rodent Ctenomys talarum (Rodentia: Octodontidae)

The paper describes the breeding biology of the South American herbivorous fossorial rodent Ctenomys talarum (tuco-tuco), covering the following aspects: length of breeding season, prenatal mortality, mean litter size and its correlates, sex ratio and its density-dependence, and late sexual maturity of both sexes. These parameters are compared and contrasted with those of other conspecifics and other subterranean rodents.     

Locomotory activity of captive Cryptomys hottentotus, (Mammalia: Bathyergidae), a fossorial rodent

Locomotory activity of six mole-rats kept singly and in various sized groups was monitored for a total of 1728 hours by use of photocells, event recorder, and a 21 metre long, transparent burrow system. Movement in tunnelways was recorded for all hours of a 24 hour period. Locomotory activity for individuals and a two member group average14–24% of the total time, while a four member group averaged 57% of the total time. Approximately five rest periods per day averaged 50 minutes duration each. The effects of social isolation, burrow length, diet, and individual variation on locomotory activity of fossorial mammals are discussed.     

Of hummingbirds and helicopters: hovering costs, competitive ability, and foraging strategies

Wing morphology and flight kinematics profoundly influence foraging costs and the overall behavioral ecology of hummingbirds. By analogy with helicopters, previous energetic studies have applied the momentum theory of aircraft propellers to estimate hovering costs from wing disc loading (WDL), a parameter incorporating wingspan (or length) and body mass. Variation in WDL has been used to elucidate differences either among hummingbird species in nectar-foraging strategies (e.g., territoriality, traplining) and dominance relations or among gender-age categories within species. We first demonstrate that WDL, as typically calculated, is an unreliable predictor of hovering (induced power) costs; predictive power is increased when calculations use wing length instead of wingspan and when actual wing stroke amplitudes are incorporated. We next evaluate the hypotheses that foraging strategy and competitive ability are functions of WDL, using our data in combination with those of published sources. Variation in hummingbird behavior cannot be easily classified using WDL and instead is correlated with a diversity of morphological and physiological traits. Evaluating selection pressures on hummingbird wings will require moving beyond wing and body mass measurements to include the assessment of the aerodynamic forces, power requirements, and power reserves of hovering, forward flight, and maneuvering. However, the WDL-helicopter dynamics model has been instrumental in calling attention to the importance of comparative wing morphology and related aerodynamics for understanding the behavioral ecology of hummingbirds.     

Acoustic communication and burrow acoustics are reflected in the ear morphology of the coruro (Spalacopus cyanus, Octodontidae), a social fossorial rodent

We studied the middle and inner ears of seven adult coruros (Spalacopus cyanus), subterranean and social rodents from central Chile, using free-hand dissection and routine staining techniques. Middle ear parameters that were focused on here (enlarged bullae and eardrums, ossicles of the "freely mobile type") are believed to enhance hearing sensitivity at lower frequencies. The organ of Corti was of a common mammalian type and revealed three peaks of higher inner hair cell densities. Based on a position frequency map, frequencies were assigned to the respective peaks along the basilar membrane. The first peak at around 300-400 Hz is discussed with respect to the burrow acoustics, while the peak around 10-20 kHz is probably a plesiomorphic feature. The most pronounced peak at around 2 kHz reflects the frequency at which the main energy of vocal communication occurs. The morphology of the ear of the coruro corresponds to the typical pattern seen in subterranean rodents (low frequency and low-sensitivity hearers), yet, at the same time, it also deviates from it in several functionally relevant features.