Table 2.
A summary of common phenotypic characteristics of subterranean rodents
| Phenotype | Naked mole-rat | Damaraland mole-rat | Blind mole-rat | Mouse |
|---|---|---|---|---|
| Eye size (diameter mm) | ~0.75 | ~1.5 | Completely degenerate | ~3 |
| Retina | Regressive and disorganized | Irregularly structured | Vestigial and reorganiz ed | Strict organization |
| Detection of light images | Yes/no | Yes/no | Yes/no | Yes/can detect detailed image |
| External ear pinnae | No | No | No | Yes |
| Highly developed tactile vibrissae all over body | Yes | Yes | Yes | On face only |
| Internal testes in males | Yes | Yes | Yes | No |
| Mode of digging | Chisel tooth | Chisel tooth | Head–lift and chisel tooth | Feet |
| Strict Herbivores | Yes | Yes | Yes | No |
| Body temperature (°C) | 33 | 35 | 35 | 37 |
| Mass specific metabolic rate (% predicted by mass) | 50 | 57 | 84 | 130 |
| Thermal conductance (% predicted by mass) | 254 | 134 | 150 | 100 |
| Precision of thermoregulation | Predominantly thermally labile | Predominantly homeothermic | Predominantly homeothermic | Strictly homeothermic |
| NST capacity (fold increase relative to BMR) | 4 | 2 | 3 | 2 |
| Heart rate (beats per min) | 200–370 | 150–300 | 110–200 | 350–850 |
| Heart rate (% expected) | 45 | 45 | 43 | >120 |
| Hematocrit (%) | 46 | 48 | 45 | 38 |
| Hemoglobin (g/dL) | 14 | 14 | 15 | 12 |
| Tolerance of hypoxia/hypercapnia | High | High | High | Low |
Subterranean rodents share a number of features considered adaptations for life underground. The three mole-rat species (the naked, Damaraland, and blind mole-rat) highlighted here represent the Heterocephalidae, Bathyergidae, and Spalacidae families. All are morphologically streamlined (lack of ear pinnae and cryptorchidism), visually impaired with greater reliance on the somatosensory system. Linked to life in a sealed burrow system where gas exchange is restricted to diffusion through soil, all show reduced metabolic rates, heart rates, and oxygen consumption with concomitant changes in blood oxygen carrying capacity. Not surprisingly, mole-rats are resistant to hypoxia and hypercapnia; neither convective cooling nor evaporative water loss is particularly effective in humid sealed burrows, rather loss of metabolic heat is primarily facilitated by high rates of thermal conductance. Low metabolic rates coupled with high rates of thermal conductance give rise to lower resting body temperatures and less strict regulation of body temperature than observed in species that live above ground. Data obtained from (Bennett and Faulkes 2000; Lacey 2000; Cernuda-Cernuda et al. 2002; Begall et al. 2007)