Does melatonin secretion occurs when Weddell seals are exposed to darkness?

In many animals, the hormone melatonin is secreted from the pineal gland to regulate physiological changes in relation to length of daylight. Weddell seals are exposed to 24 hour sunlight during the summer months and have been found not to secrete the hormone melatonin during this time of the year.

This study investigated whether the seals have a potentially active pineal gland during summer. Seven non-lactating seals were placed, one at a time, in darkness and blood samples were collected every 30 minutes for 2 hours at which point, they were exposed to sunlight and sampling continued for another 2 hours. Blood samples were analysed to determine whether pineal function was activated by darkness and caused a corresponding elevation of melatonin concentration in the blood.