Modeling Natural Photic Entrainment in a Subterranean Rodent (Ctenomys aff. knighti), the Tuco-Tuco

Subterranean rodents spend most of the day inside underground tunnels, where there is little daily change in environmental variables. Our observations of tuco-tucos (Ctenomys aff. knighti) in a field enclosure indicated that these animals perceive the aboveground light-dark cycle by several bouts of light-exposure at irregular times during the light hours of the day. To assess whether such light-dark pattern acts as an entraining agent of the circadian clock, we first constructed in laboratory the Phase Response Curve for 1 h light-pulses (1000lux). Its shape is qualitatively similar to other curves reported in the literature and to our knowledge it is the first Phase Response Curve of a subterranean rodent. Computer simulations were performed with a non-linear limit-cycle oscillator subjected to a simple model of the light regimen experienced by tuco-tucos. Results showed that synchronization is achieved even by a simple regimen of a single daily light pulse scattered uniformly along the light hours of the day. Natural entrainment studies benefit from integrated laboratory, field and computational approaches.     

Rhythmic 24 h Variation of Core Body Temperature and Locomotor Activity in a Subterranean Rodent (Ctenomys aff. knighti), the Tuco-Tuco

The tuco-tuco Ctenomys aff. knighti is a subterranean rodent which inhabits a semi-arid area in Northwestern Argentina. Although they live in underground burrows where environmental cycles are attenuated, they display robust, 24 h locomotor activity rhythms that are synchronized by light/dark cycles, both in laboratory and field conditions. The underground environment also poses energetic challenges (e.g. high-energy demands of digging, hypoxia, high humidity, low food availability) that have motivated thermoregulation studies in several subterranean rodent species. By using chronobiological protocols, the present work aims to contribute towards these studies by exploring day-night variations of thermoregulatory functions in tuco-tucos, starting with body temperature and its temporal relationship to locomotor activity. Animals showed daily, 24 h body temperature rhythms that persisted even in constant darkness and temperature, synchronizing to a daily light/dark cycle, with highest values occurring during darkness hours. The range of oscillation of body temperature was slightly lower than those reported for similar-sized and dark-active rodents. Most rhythmic parameters, such as period and phase, did not change upon removal of the running wheel. Body temperature and locomotor activity rhythms were robustly associated in time. The former persisted even after removal of the acute effects of intense activity on body temperature by a statistical method. Finally, regression gradients between body temperature and activity were higher in the beginning of the night, suggesting day-night variation in thermal conductance and heat production. Consideration of these day-night variations in thermoregulatory processes is beneficial for further studies on thermoregulation and energetics of subterranean rodents.     

Energetics and thermoregulation during digging in the rodent tuco-tuco (Ctenomys talarum)

For subterranean rodents, searching for food by extension of the tunnel system and maintenance of body temperature are two of the most important factors affecting their life underground. In this study we assess the effect of ambient temperature on energetics and thermoregulation during digging in Ctenomys talarum. We measured VO2 during digging and resting at ambient temperature (Ta) below, within, and above thermoneutrality. Digging metabolic rate was lowest at Ta within the thermoneutral zone and increased at both lower and higher temperatures, but body temperature (Tb) remained constant at all Tas. Below thermoneutrality, the cost of digging and thermoregulation are additive. Heat production for thermoregulation would be compensated by heat produced as a by-product of muscular activity during digging. Above thermoneutrality, conduction would be an important mechanism to maintain a constant Tb during digging.     

Bioenergetics of reproduction and pup development in a subterranean rodent (Ctenomys talarum)

The objective of this study was to evaluate the maternal costs of reproduction and pup development in the subterranean rodent Ctenomys talarum (Thomas 1898). Statistical differences were detected in whole-animal metabolic rates between nonreproductive and pregnant or lactating females. Whole-animal metabolic rates during pregnancy and lactation were 128% and 151% of the resting metabolic rate (RMR) observed in nonreproductive females. The total additional energy cost of reproduction (above the nonreproductive level) was similar for both the gestation and lactation periods. Mass-specific RMR revealed an upregulation of cell or tissue metabolism during lactation but not during gestation. The mass-specific metabolic rate of pups was 237% of the adults' metabolic rates. No differences were observed in body temperature among nonreproductive, pregnant, or lactating females. No differences were detected in body mass at birth among pups from litters with different numbers of nestlings. Pups increased their body temperature, reaching adult temperature at 30 d of age, when they were near weaning. Milk constituted the exclusive food for pups until they started eating solid food at 10 d old. Suckling time decreased with age of pups, and at the same time, mother chases directed toward their pups increased. These reproductive characteristics may contribute to successful existence in a subterranean habitat.     

Population structure and landscape genetics in the endangered subterranean rodent Ctenomys porteousi

In order to devise adequate conservation and management strategies for endangered species, it is important to incorporate a reliable understanding of its spatial population structure, detecting the existence of demographic partitions throughout its geographical range and characterizing the distribution of its genetic diversity. Moreover, in species that occupy fragmented habitats it is essential to know how landscape characteristics may affect the genetic connectivity among populations. In this study we use eight microsatellite markers to analyze population structure and gene flow patterns in the complete geographic range of the endangered rodent Ctenomys porteousi. Also, we use landscape genetics approaches to evaluate the effects of landscape configuration on the genetic connectivity among populations. In spite of geographical proximity of the sampling sites (8–27 km between the nearest sites) and the absence of marked barriers to individual movement, strong population structure and low values of gene flow were observed. Genetic differentiation among sampling sites was consistent with a simple model of isolation by distance, where peripheral areas showed higher population differentiation than those sites located in the central area of the species’ distribution. Landscape genetics analysis suggested that habitat fragmentation at regional level has affected the distribution of genetic variation among populations. The distance of sampling sites to areas of the landscape having higher habitat connectivity was the environmental factor most strongly related to population genetic structure. In general, our results indicate strong genetic structure in C. porteousi, even at a small spatial scale, and suggest that habitat fragmentation could increase the population differentiation.     

Nutritional responses to different diet quality in the subterranean rodent Ctenomys talarum (tuco-tucos)

Nutritional response to different diet quality was examined in the subterranean rodent Ctenomys talarum (tuco-tuco). Animals maintained in captive conditions were fed with three plant species that differed in their fibre content. Tuco-tucos showed the ability to perform adjusts in short time lapse in response to diet quality; food ingestion, egestion and feces ingestion changed in animals under different plant species diets. Time budget, mainly time devoted to feeding and activity accompanied such changes. Coprophagy was practiced along the day and night following the arrhythmic activity pattern found for this species. Feces reingestion was not associated to resting. Furthermore, it was observed during fresh food ingestion, being pellets chewed. Soft and hard feces differed in morphological and nutritional characteristics.     

Maximal thermogenic capacity and non-shivering thermogenesis in the South American subterranean rodent Ctenomys talarum

Subterranean rodents inhabit closed tunnel systems that are hypoxic and hypercapnic and buffer aboveground ambient temperature. In contrast to other strictly subterranean rodents, Ctenomys talarum exhibits activity on the surface during foraging and dispersion and hence, is exposed also to the aboveground environment. In this context, this species is a valuable model to explore how the interplay between underground and aboveground use affects the relationship among basal metabolic rate (BMR), cold-induced maximum metabolic rate (MMR), shivering (ST), and non-shivering thermogenesis (NST). In this work, we provide the first evidence of the presence of NST, including the expression of uncoupling proteins in brown adipose tissue (BAT), and shivering thermogenesis in Ctenomys talarum, a species belonging to the most numerous subterranean genus, endemic to South America. Our results show no differences in BMR, cold-induced MMR, and NST between cold- (15?°C) and warm- (25?°C) acclimated individuals. Furthermore, thermal acclimation had no effect on the expression of mitochondrial uncoupling protein 1 (UCP1) in BAT. Only cytochrome c oxidase (COX) content and activity increased during cold acclimation. When interscapular BAT was removed, NST decreased more than 30?%, whereas cold-induced MMR remained unchanged. All together, these data suggest that cold-induced MMR reaches a maximum in warm-acclimated individuals and so a probable ceiling in NST and UCP1 expression in BAT. Possible thermogenic mechanisms explaining the increase in the oxidative capacity, mediated by COX in BAT of cold-acclimated individuals and the role of ST in subterranean life habits are proposed.     

Phylogeographical structure in the subterranean tuco-tuco Ctenomys talarum (Rodentia: Ctenomyidae): contrasting the demographic consequences of regional and habitat-specific histories

In this work we examined the phylogeography of the South American subterranean herbivorous rodent Ctenomys talarum (Talas tuco-tuco) using mitochondrial DNA (mtDNA) control region (D-loop) sequences, and we assessed the geographical genetic structure of this species in comparison with that of subterranean Ctenomys australis, which we have shown previously to be parapatric to C. talarum and to also live in a coastal sand dune habitat. A significant apportionment of the genetic variance among regional groups indicated that putative geographical barriers, such as rivers, substantially affected the pattern of genetic structure in C. talarum. Furthermore, genetic differentiation is consistent with a simple model of isolation by distance, possibly evidencing equilibrium between gene flow and local genetic drift. In contrast, C. australis showed limited hierarchical partitioning of genetic variation and departed from an isolation-by-distance pattern. Mismatch distributions and tests of neutrality suggest contrasting histories of these two species: C. talarum appears to be characterized by demographic stability and no significant departures from neutrality, whereas C. australis has undergone a recent demographic expansion and/or departures from strict neutrality in its mtDNA.     

Relationship between helminth parasites and demographic attributes of a population of the subterranean rodent Ctenomys talarum (Rodentla: Octodontidae)

The helminth parasite fauna of a natural population of the octodontid, Ctenomys talarum, was studied. Parasites that were found included the nematodes Heligmostrongylus sp. and Trichuris sp. Total prevalence of parasitism was 92.3%, mean intensity of infection was 22.7 worms, and mean abundance was 21 worms. Prevalence and mean abundance of infection with Heligmostrongylus sp. were higher in C. talarum males relative to females. Ecological and physiological causes, as well as the mating system of the host species, influence the likelihood of sex differences in parasite infection. The low parasite burden and diversity of C. talarum are associated with restrictions imposed by the subterranean habitat and with life-history traits of these rodents. Whether these findings apply to other Ctenomys spp. is unknown.     

Acoustic signals of a solitary subterranean rodent Ctenomys talarum (Rodentia: Ctenomyidae): physical characteristics and behavioural correlates

The purpose of this study was to analyse and describe vocalizations of a solitary subterranean rodent, Ctenomys talarum. In adult C. talarum five different sounds (four true vocalizations and one mechanical sound) were recorded during different behavioural contexts. Using data from the laboratory and literature, we classified these vocalizations as territorial, distress, and mating calls. We found that the vocalization range in C. talarum is shifted towards low frequencies, which transmit better in underground burrows and match well the hearing range described for other species of subterranean rodents. These low-frequency vocalizations, also found in other non-related subterranean rodents, may reflect an adaptation to the acoustic conditions of the habitat. Electronic Publication     

Role of maternal odors on foraging behavior during postnatal development in a solitary subterranean rodent Ctenomys talarum

We investigated the role of maternal odors emanating from excretory deposits and/or glandular secretions during the postnatal development of foraging and related behaviors in young Los Talas’ tuco-tucos (Ctenomys talarum), a solitary rodent, under seminatural conditions. During cafeteria tests, we offered them two of the grasses most preferred by adults of this species (bromegrass, Bromus unioloides and panicgrass, Panicum racemosum). We scent marked one dietary option in a treatment group, and the other option in a second treatment group, keeping a third group with no odors as control. Pups did not show to prefer a special grass in particular, and maternal odors showed no effect on the behavioral variables evaluated in this study. When they reached their weaning age, they showed an ontogenetic change in food preferences, where they preferred bromegrass, and maternal odors influenced negatively on the consumption of panicgrass. During dispersal, preference for bromegrass was expressed independently of the presence of maternal odors. Overall, their effect was less pronounced than age’s effect, and it was evident in only a subset of the conducted experiments. This study shows that the influence of maternal odors is not essential for the establishment of feeding or site preferences in this species in seminatural conditions, contrarily to what was observed in other studies performed with social mammal species (i.e., rats and rabbits). It is suggested that other factors (e.g., individual learning) might influence the observed preferences.     

Metabolic rate and thermoregulation in two species of tuco-tuco, Ctenomys talarum and Ctenomys australis (Caviomorpha, Octodontidae)

1. Resting metabolic rate and body temperature in function of ambient temperature were determined for two species of Ctenomys. 2. Oxygen consumption was lowest between 25 and 30 degrees C and was 0.946 +/- 0.030 and 0.968 +/- 0.022 in Ctenomys talarum (from Mar de Cobo and Necochea, respectively). Resting metabolic rate was 0.343 +/- 0.053 at 30 C in C. australis. 3. Mean rectal temperature at thermoneutrality was 36.1 +/- 0.13 and 37.3 +/- 0.17 in C. talarum and C. australis, respectively. 4. Limited thermoregulation occurred in C. talarum down to 20 degrees C but C. australis maintained body temperature down to 10 degrees C. 5. Both species of tuco-tucos became hyperthermic at ambient temperatures above thermoneutrality.     

Use of vegetation chemical signals for digging orientation in the subterranean rodent Ctenomys talarum (Rodentia: Ctenomyidae)

Sensory modalities involved in the localization and selection of food represent critical information for understanding the foraging behavior of subterranean rodents. The objective of this study was to determine if a subterranean rodent that forages on aboveground plant parts, Ctenomys talarum, is able to detect the presence of odorous chemicals released by the plants into the soil to guide its excavation, as has been shown for subterranean species that forage on roots and bulbs. Individuals were introduced into an artificial Y‐maze, whose arms were filled with sandy soil in which plants either had or had not been growing (control). Digging activity exhibited in each soil type was recorded. The following plant species, present in the natural habitat of C. talarum, were used: Panicum racemosum (three different concentrations), Cortadeira sp., Margiricarpus pignatus, Ambrosia sp. and Raphanus sativus. The individuals allocated more time to excavation in the soil in which vegetation had been growing in comparison with the control for the five analysed plant species, except in the case of P. racemosum at its lowest concentration. The number of individuals that completed excavation in the tube‐containing soil in which plants had been growing was significantly higher than in the tube‐containing soil in which no plants had been growing, with the exception of P. racemosum at its lowest concentration and R. sativus. The results of the present study suggest that C. talarum have the ability to use olfaction to orient their digging while foraging even though members of this species consume mainly aboveground plant parts.     

Olfactory Discrimination of Individual Scents in the Subterranean Rodent Ctenomys talarum (tuco-tuco)

Biological odours that convey cues regarding individual identity can provide valuable information mediating many aspects of mammalian social relationships such us dominance hierarchies, group memberships, territorial and mating activities. The ability of the subterranean rodent, tuco‐tuco (Ctenomys talarum), to discriminate between soiled shavings, urine and faeces from different individuals was investigated using the habituation–discrimination paradigm. Discrimination by both males and females was tested using scents obtained from same‐ and opposite‐sex individuals. Each test subject was habituated for three consecutive days to odour samples from the same individual; on the fourth day a scent from a novel individual was provided. For all odour sources, animals spent significantly less time investigating the habituation scent over successive trials, indicating that animals perceived the stimulus as familiar. For all stimuli, test subjects spent more time investigating the novel odour, rather than the familiar one. Animals spent more time investigating soiled shavings than urine or faeces. Both males and females discriminated novel from familiar odours in shavings and urine regardless of the gender of the odour donors. In contrast, test animals discriminated between familiar and novel odours in feces only when the fecal donors were of different sex from subjects. Possible territorial and reproductive functions of individual scent discrimination are discussed.     

Seasonal changes in body composition of Ctenomys talarum (Rodentia: Octodontidae): an herbivore subterranean rodent

Ctenomys talarum is a subterranean herbivorous rodent whose burrow systems exhibit particular characteristics, distinct from other subterranean environments. We studied seasonal variation in body composition of C. talarum in relation to energetic requirements. Body lipid content seasonally changed in C. talarum, related to reproductive cycle and thermorregulatory mechanisms. A decrease in protein body content was found only in spring. Ash content of females was lowest when most of them are in post partum estro. Observed variations in water body content could be associated with plant water content and/or metabolic regulation. Our results show the occurrence of seasonal variations in body composition in C. talarum, which could be related to the high cost of reproduction and the subterranean life style of this species.     

Crystal structures of N-acetylmannosamine kinase provide insights into enzyme activity and inhibition

Sialic acids are essential components of membrane glycoconjugates. They are responsible for the interaction, structure, and functionality of all deuterostome cells and have major functions in cellular processes in health and diseases. The key enzyme of the biosynthesis of sialic acid is the bifunctional UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase that transforms UDP-N-acetylglucosamine to N-acetylmannosamine (ManNAc) followed by its phosphorylation to ManNAc 6-phosphate and has a direct impact on the sialylation of cell surface components. Here, we present the crystal structures of the human N-acetylmannosamine kinase (MNK) domain of UDP-N-acetylglucosamine-2-epimerase/N-acetylmannosamine kinase in complexes with ManNAc at 1.64 Å resolution, MNK·ManNAc·ADP (1.82 Å) and MNK·ManNAc 6-phosphate·ADP (2.10 Å). Our findings offer detailed insights in the active center of MNK and serve as a structural basis to design inhibitors. We synthesized a novel inhibitor, 6-O-acetyl-ManNAc, which is more potent than those previously tested. Specific inhibitors of sialic acid biosynthesis may serve to further study biological functions of sialic acid.     

Graphidioides subterraneus n. sp. (Nematoda: Trichostrongyloidea) from the South American subterranean rodent Ctenomys talarum Thomas, 1898 (Rodentia: Octodontidae)

A new nematode species, Graphidioides subterraneus n. sp., found in the stomach of C. talarum from Argentina is described. The new species more closely resembles G. mazzai Lent & Freitas, 1935, parasite of Galea leucoblephara from Argentina, and G. kravetzi Sutton & Durette-Desset, 1995, parasite of Holochilus brasiliensis from Uruguay. It can be distinguished by shorter spicules, by the shape of the gubernaculum, by shorter uterine branches, and by a different number of ridges of the synlophe all along the body.     

Energetics and thermoregulation during digging in the rodent tuco-tuco (Ctenomys talarum)

For subterranean rodents, searching for food by extension of the tunnel system and maintenance of body temperature are two of the most important factors affecting their life underground. In this study we assess the effect of ambient temperature on energetics and thermoregulation during digging in Ctenomys talarum. We measured V˙o₂ during digging and resting at ambient temperature (T_a) below, within, and above thermoneutrality. Digging metabolic rate was lowest at T_a within the thermoneutral zone and increased at both lower and higher temperatures, but body temperature (T_b) remained constant at all T_as. Below thermoneutrality, the cost of digging and thermoregulation are additive. Heat production for thermoregulation would be compensated by heat produced as a by-product of muscular activity during digging. Above thermoneutrality, conduction would be an important mechanism to maintain a constant T_b during digging.     

Structural studies of neuropilin/antibody complexes provide insights into semaphorin and VEGF binding

Neuropilins (Nrps) are co-receptors for class 3 semaphorins and vascular endothelial growth factors and important for the development of the nervous system and the vasculature. The extracellular portion of Nrp is composed of two domains that are essential for semaphorin binding (a1a2), two domains necessary for VEGF binding (b1b2), and one domain critical for receptor dimerization (c). We report several crystal structures of Nrp1 and Nrp2 fragments alone and in complex with antibodies that selectively block either semaphorin or vascular endothelial growth factor (VEGF) binding. In these structures, Nrps adopt an unexpected domain arrangement in which the a2, b1, and b2 domains form a tightly packed core that is only loosely connected to the a1 domain. The locations of the antibody epitopes together with in vitro experiments indicate that VEGF and semaphorin do not directly compete for Nrp binding. Based upon our structural and functional data, we propose possible models for ligand binding to neuropilins.