Compared to Terran males, the average Homo-Reptilicus Sirius male is slightly taller and heavier. Conversely, the average Sirian female is shorter and smaller than her Terran counterpart.
| Average Height | Average Weight | Average lifespan | |
| Sirian Male | 5 Foot 10 inches | 190 pounds | 81 Earth Years |
| Sirian Female | 5 Foot 3 inches | 139 pounds | 73 Earth Years |
Note for females: Gestation period is 2.5 months; all Sirian young are carried in an oviparous state and birthed in a soft-shell egg after which hatching occurs immediately.
The reptilian Sirians are similar in chemical makeup and physiological nature to Terrestrial reptiles. However, Sirians are the dominant lifeform on their planet, comprising over 80% of total lifeforms on Sirius IV.
Like Terran Homo sapiens, Sirians walk upright and erect. The Sirian skeletal structure is similar to that of a human, although slightly lower in calcium. The bones of the spinal vertebrae are smaller and terminate in a short tail, a vestigial remnant of the past. The skull is ossified and bony, as in Terrestrial reptiles, and is fixed except for a distendable jawbone to facilitate consumption of large prey. The bony plates that protrude above the eyes and across the head are vestigial; once used in defense they now help in the body's cooling.
Sirian internal organs are closely similar to those of a Terrestrial crocodile. The heart is divided into five chambers and beats 52 times per minute. The circulatory system is intricate, branching out into fibrous bunches toward the extremities. Due to the complexity of this circulatory system, Sirians are warm-blooded and need no outside temperature augmentation to remain comfortable. Sirian blood is copper-based, hence its green color, and rich in antibodies and sparse in corpuscles. Those Sirians in the Terran fleet have been extensively vaccinated against most Terran bacteria.
The digestive and ingestive system of Sirians is similar to that of Terrestrial snakes, including a highly developed stomach. Sirians prefer live prey or freshly-killed meat. Sometimes by choice but often due to lack of meat, Sirians consume raw, mineral-rich vegetables as a dietary supplement. Fried or prepared foods cause gastric upset. The excretory system makes provision for solid and liquid waste through a single large kidney organ.
Sirian lungs are both functional and operate best in low-altitude topography and damp climates. Sirian skin is covered by dermal plates, or scales, and is extremely osmotic. To keep their dermal from drying and cracking, Sirians bathe regularly in oil-rich water. For Sirians in the Terran fleet, a silicon-based lubricant coats the inner layer of their artificial "human skin." This skin is devoid of feeling, but is thin enough to allow full sensory transmittal and gas permeability.
The optical arrangement of Sirians is purely reptilian, with a single vertical slit instead of a round pupil. The Sirian eye possesses sufficient cones to allow pale color vision, and it is extremely sensitive to light due to the density of rod neurons. Olfactory senses are less highly developed than that of Terrans, but are more functional in filtering out airborne particles. The Sirian auditory system is composed of a vibrating tympanic membrane directed inward by a simple ear arrangement.
Sirians have a built-in defense mechanism in the form of an acidic neurotoxin similar to Terrestrial rattlesnake venom. The toxin is located in a maxillary gland (like the human parotid gland) and can be sprayed by a Sirian voluntarily. The atomized venom is airborne and irritating to humans, causing dizziness, heart fibrillation, shortness of breath, and in cases of extreme alergy, the eyes can go blind, if the venom hits there, and in some cases, even death is possible.