Energy and distribution in subterranean rodents: Sympatry between two species of the genus Ctenomys

The low basal metabolic rate (BMR) observed in subterranean rodents, compared to that of surface-dwelling species of comparable size, has been proposed to be an adaptation to underground life. Two main hypotheses have been proposed to explain this finding, the cost of burrowing and the thermal stress. The former states that the low BMR is due to the high cost of extending the tunnel system whereas the other relates it to the possibility of overheating in burrows where evaporative and convective heat exchange are restricted. Additionally, both hypotheses related the energetics of subterranean rodent with spatial distribution. The genus Ctenomys is an excellent model to evaluate the cost of burrowing or thermal stress, since they are widely distributed, with members differing markedly in body mass. The aim of this study was to assess digging and basal energetics in two Ctenomys species that live in sympatry in a coastal grassland, but differ in their microspatial distribution by soil preference. We used the obtained energetic data to test both energy-distribution hypotheses. We measured BMR and digging metabolic rate (DMR) through open flow respirometry in two species exposed to soft and hard soils. In brief, DMR in Ctenomys talarum (100-170 g), as in Ctenomys australis (250-600 g), was unaffected by soil hardness. Within thermoneutral zone of each species, DMR/RMR quotient was lower in the smaller species. Our data did not support the thermal stress hypothesis, but the cost of burrowing hypothesis was not rejected. Other alternative hypotheses are proposed to explain the distribution of C. talarum and C. australis.

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.

Effect of tunnel inclination on digging energetics in the tuco-tuco, Ctenomys talarum (Rodentia: Ctenomyidae)

Burrows play an important role for many species, providing them with shelter and access to food resources. For subterranean rodents, living underground imposes constraints on morphology and physiology. The convergence in burrow architecture among subterranean rodents has been related to the energy demands imposed by the cost of constructing an entire system. The low frequency of tunnels with downward angles steeper than 40 degrees appears to be a common feature in burrow design. In the subterranean habitat, movements through the soil are expensive and gravity can exert important restrictions on digging energetics when individuals push out the soil removed in steeper digging angles. The aim of this study was to determine the effect of digging angle on digging energetics in Ctenomys talarum. The mass of the removed soil and burrowing speed were similar while digging metabolic rate and net cost of transport were higher in individuals digging in tunnels with angles >40 degrees than in those digging tunnels with angles <40 degrees . The cost of constructing a burrow in the horizontal plane differed by 20% from others in which the natural representation of tunnels >40 degrees was considered. Even given that tunnels >40 degrees represented only 6% of the total burrow length, burrow architecture appears to be constrained by the high energetic cost of constructing in steeper angles.

Cost of foraging in the subterranean rodent Ctenomys talarum: effect of soil hardness

Subterranean burrows provide inhabitants with shelter, a relatively stable thermal environment, and potentially access to food resources. However, one cost of living in such burrows is the energetically expensive mode of locomotion. Soil hardness and the physiological capabilities of animals are likely important factors that affect the cost of burrow construction, and hence, distribution of burrows. We assessed the effect of soil hardness on the cost of digging by captive individual Ctenomys talarum Thomas, 1898 in soft soils. Digging metabolic rate (DMR) was higher in harder soil than in softer soil (408.30 ± 51.35 mL O2·h–1 vs. 267.59 ± 20.97 mL O2·h–1, respectively). In C. talarum, a higher soil hardness augments DMR by increasing, in terms of the cost of burrowing model, the costs of shearing and of pushing the removed soil. Additionally, these costs differ between C. talarum and other subterranean species (e.g., Thomomys bottae (Eydoux and Gervais, 1836)), depending on soil hardness and digging mode. Thus, the relationship between digging cost and soil hardness appears to be one of the most important factors that affect burrowing efficiency in subterranean rodents.

Thermoregulatory development in pups of the subterranean rodent Ctenomys talarum

The effect of the mother's contact and huddling with nest mates on the mass-specific metabolic rate (RMR) and body temperature (T(b)) of pups of Ctenomys talarum from 2 to 45 days of age was evaluated at ambient temperatures (T(a)) within and below the adult thermoneutrality range (25 and 19 degrees C, respectively, the latter corresponding to the one recorded in burrows during the spring, when pups are born). Under these conditions, we recorded the percentage of time that pups spent huddled with nest mates, with their mother and suckling. At 19 degrees C, huddling and contact with the mother significantly reduced pups' body heat loss until they were 15 days old but did not affect their RMR. Fifteen-day-old pups showed an increase in their RMR, associated with the onset of independent thermoregulation. Pups older than 15 days showed a less variable T(b) and their RMR decreased. 2- to 30-day-old pups spent 80% of the time in contact with their mother and, when she was absent, they spent 70% of the time huddled with their nest mates. However, these results did not differ between the two T(a) evaluated. Forty-five-day-old pups reached adult T(b) and spent significantly less time in contact with their mother and nest mates. Huddling did not have a significant effect on energy expenditure of young tuco-tucos, being this related to the stable thermal conditions found in natural burrows and pups' mode of development.

Assessing the effect of litter size on growth pattern and homeothermy acquisition in the Pampas mice Akodon azarae (Rodentia, Muridae)

We evaluated in pups of Akodon azarae both the growth pattern from birth to 48 days of age and the effect of litter size on growth pattern and homeothermy acquisition from birth to weaning age. Individual pups gained weight as expected by a Gompertz growth pattern. Until weaning, litter size affected both the slope of the relationship between body temperature and age and the rate of growth of pups. Pups from small litters increased both body temperature and weight until body temperature and body weight of adults at higher rates than those from large litters.

Digging energetics in the South American rodent Ctenomys talarum (Rodentia, Ctenomyidae)

Ctenomys is the most speciose among subterranean rodents. There are few studies on energetics of Ctenomys, and none of them have focused on the energetics of digging. The present study aims to quantify the energetic cost of burrowing in Ctenomys talarum in natural soil conditions and to compare the energetics data with those reported for other subterranean rodents. Digging metabolic rate (DMR) in gravelly sand for C. talarum was 337.4 ± 65.9 mL O2·h–1 (mean ± SD). No differences in DMR were detected between sexes. Moreover, DMR was 295.9% of resting metabolic rate. In terms of a cost of burrowing model, the mass of soil removed per distance burrowed (Msoil) in gravelly sand was 44.5 ± 6.7 g·cm–1. Coefficients of the equation that related the energy cost of constructing a burrow segment of length S and Msoil(Eseg/Msoil) were Ks = 0.33 ± 0.32 J·g–1, which is the energy cost of shearing 1 g of soil, and Kp = 0.0055 ± 0.0042 J·g–1·cm–1, which is the energy cost of pushing 1 g of soil 100 cm. Regarding the cost of burrowing model, our data showed that C. talarum has the lowest DMR in gravelly sand among unrelated subterranean rodents analyzed. Moreover, despite C. talarum feeding aboveground, the foraging economics was similar that of to other rodents.

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.

Reproductive energetics and thermoregulatory status of nestlings in pampas mice Akodon azarae (rodentia: sigmodontinae)

We evaluated metabolic rates during reproduction and the thermoregulatory status of preweaning pups of Akodon azarae (Fisher 1829). Metabolic rates during late pregnancy and lactation were 159% and 200%, respectively, of the basal metabolic rate. Metabolic rates of 10-d-old pups were 447% of the adult's metabolic rates. No difference in metabolic rates of pups was detected among different ambient temperatures. Differences were detected in body temperatures between pups without mothers before and after exposure to different ambient temperatures below the thermoneutral zone. Differences were not detected in body temperatures among solitary or grouped pups.