Deer

Antlers, not horns

Deer are unique among modern mammals in having antlers — bony growths on the head that regrow each year. The cycle:

1. Spring — antlers begin growing, covered in soft “velvet” skin with blood vessels.
2. Summer — antlers reach full size, hardening; velvet dries and is rubbed off.
3. Fall — males use antlers in dominance battles during the rut.
4. Winter — antlers are shed.
5. Spring — cycle restarts.

Each year’s antlers are larger and more elaborate than the previous year’s, until the deer reaches peak age. The annual regrowth involves an extraordinary rate of bone formation — antler tissue grows at up to 2.5 cm per day.

Antlers contrast with horns (cattle, sheep, goats), which are permanent and grow continuously throughout life.

A successful suburban species

White-tailed deer in North America have thrived in human-altered landscapes. Their population has grown from ~500,000 in 1900 to over 30 million today — driven by:

• Forest fragmentation — deer prefer edge habitat, which suburbs create.
• Predator removal — wolves and cougars eliminated from most of their range.
• Agricultural food sources — corn, soybeans, garden plants.
• Hunting regulation — managed for sustained populations.

The result is dense suburban deer populations causing major ecological problems: car collisions, Lyme disease vector, severe overgrazing of native plants, garden destruction, and forest regeneration failure.

Many species

Cervidae includes about 50 species worldwide:

• White-tailed deer, mule deer — North American
• Red deer, roe deer, fallow deer — European
• Reindeer / caribou — Arctic and subarctic
• Moose — largest deer, North America and northern Eurasia
• Elk / wapiti — North American and East Asian
• Sika deer — East Asian
• Sambar, axis, muntjac — Asian tropical
• Marsh deer, white-lipped deer — South American

Reindeer are the only deer species where females also have antlers.

Lyme disease vector

Deer are the primary host for the adult black-legged tick (the vector of Lyme disease in North America). The deer themselves don’t get Lyme disease, but they support large tick populations that infect mice, which then infect humans. Suburban deer expansion has correlated closely with Lyme disease spread.