Form and Function of Sharks 51
from the mother tail-first. However, an expanded rostral cartilage would
easily hang up in the mother’s birth canal. To facilitate passage, the head
cartilages of newborn hammerhead sharks are soft and pliable at birth and
fold up as they pass outside. They then stiffen and straighten shortly after.
What kind of brains do sharks have?
Receiving sensory information, interpreting it, and acting on it through
muscular activity and behavior requires brain power. Sharks, in contrast
to their media image as mindless killers, have relatively large and complex
brains.
Unlike a mammal’s brain such as ours, which is folded back on itself,
shark brains are arranged in a line, divided into hindbrain, midbrain, and
forebrain. The spinal cord enters through the skull and connects with the
hindbrain, which is then divided into brainstem and cerebellum. Cranial
nerves associated with hearing, balance, and electric sense enter the brain-
stem. The cerebellum sits on top of the brainstem and coordinates muscu-
lar activity.
The top of the midbrain has two swellings, the optic lobes. These are
relatively small and extend out via the optic nerves and eyeballs and regu-
late vision. The forebrain consists of olfactory organs associated with smell
and are relatively large. The front part of the forebrain is the cerebrum,
which deals with learning and memory. The forebrain also houses the hy-
pothalamus and pituitary gland. The hypothalamus produces chemicals
(hormones) that affect heart rate, thermoregulation, metabolic rate, salt
and water balance, food intake, and digestion. It also houses the epiphysis,
the organ that senses day length and perhaps sun position. The pituitary
secretes a number of hormones, including those that regulate blood pres-
sure and the concentration of various chemicals in the blood associated
with feeding, growth, kidney function, calcium levels, metabolic rate, and
sexual activity.
How large is a shark’s brain?
Researchers describe shark brain size in terms of the ratio of brain
weight to body weight. This is thought to be a better measure for com-
paring animals than absolute size because bigger animals will tend to have
larger brains. Squatinid angel sharks and cetorhinid Basking Sharks have
the smallest brains relative to body weight. Squalid dogfishes and rajid
skates also have relatively small brains. White Sharks have larger brain-to-
body weight ratios than dogfishes or skates, although a 3.7-m (12-ft) White
Shark’s brain weighs only 35 g (1.2 oz), or 0.008% of its total body weight,