Older or Younger? (HTML Version)
Older Dog's Hips
Younger Dog's Hips
Domestic Dog
(Canis familiaris)

Which x-ray is from an older dog?
Look at bones.

The x-ray on the top is from an older dog. Look at the hip joints. The bones in young dogs have smooth edges. The bones of older dogs are more jagged.

(X-rays courtesy of J. Veronika Kiklevich, DVM.)

Younger worm
Video Clip of Younger Worm (611KB Quick Time video clip. Free QuickTime 6 player required.)
Older worm
Video Clip of Older Worm (240KB Quick Time video clip. Free QuickTime 6 player required.)

(Caenorhabditis elegans)

Which worm is older?
Look at movement.

The worm on the bottom is older. Changes in movement are the first signs of aging. The older worm is close to the end of its two-week life span. Old worms show signs of muscle loss similar to those seen in human aging.

C. elegans is a tiny 1-millimeter (as long as a sesame seed) nematode that lives in the soil. Because of its short life span, small size, transparent body, and genetic simplicity, C. elegans is an excellent model for studying the genetics of aging.

(Video courtesy of Dr. Cynthia Kenyon, University of California, San Francisco.)

Skull of older horse.
Skull of younger horse.
Domestic Horse
(Equus caballus)

Which horse skull is older?
Look at teeth.

The skull on the top is from an older horse - about 17 years old. The younger skull is from a 6 to 8 month-old horse.

Compare the skulls:

  • Size: The older skull is much larger than the younger one.
  • Number of teeth: The older horse has premolars. Premolars are found in the space between the front and back teeth. Premolars appear and migrate forward between 9 months and 5 years of age.
  • Shape of teeth: The younger horse has smaller teeth. The older horse's teeth are longer and more grooved.

(Horse skulls prepared and donated by Division of Veterinary Anatomy, Washington State University, Pullman, Washington.)

Younger dog
Older dog
Domestic Dog
(Canis familiaris)

Which dog is older?
Look at hair.

The dog on the bottom is older. Look at the muzzle. In some breeds, gray hair first appears on the muzzle at around 8 years of age. A very old dog may have gray hair all over its body.

(Photo courtesy of Denise Fleener.)

Younger Gorilla
Older Gorilla
(Gorilla gorilla)

Which gorilla is older?
Look at hair.

The "silverback gorilla" on the bottom is older. Like humans, some animals' hair turns gray with age.

Older Rat
Younger Rat
Domestic Rat
(Rattus norvegicus)

Which rat is older?
Look at fur.

The rat on the top is older. Older rats have rougher fur. The younger rat is sleeker and smoother.