information:highlighting, stylistic:style_transfer, structural:expansion, stylistic:tone
Anatomy

Echidnas and platypuses are the only egg-laying mammals, the monotremes.The average lifespan of an echidna in the wild is estimated at 14–16 years. Fully grown females can weigh about 4.5 kilograms (9.9 lb), the males 33% larger, at about 6 kilograms (13 lb). Though the internal reproductive organs differ, both sexes possess an identical single cloaca opening for urination, defecation, and mating.

Male echidnas have non-venomous spurs on the hind feet, similar to the venomous male platypus.

Due to their low metabolism and accompanying stress resistance, echidnas are long-lived for their size; the longest recorded lifespan for a captive echidna is 50 years, with anecdotal accounts of wild individuals reaching 45 years.

The echidna's brain is half neocortex, compared to 80% of a human brain. Contrary to previous research, the echidna does enter REM sleep, but only in a comfortable temperature around 25 °C (77 °F). At lower or higher temperatures of 15 °C (59 °F) and 28 °C (82 °F), REM sleep is suppressed.