Journal of biological rhythmsJan 1, 1992

How removing the pineal gland affects daily melatonin levels in house sparrows and European starlings

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Updated Mar 28, 2026

Abstract

Pinealectomy resulted in low melatonin levels at all times in both European starlings and house sparrows.

Intact and sham-pinealectomized birds exhibited a melatonin rhythm with low daytime and high nighttime levels.
The removal of the pineal gland abolished the nighttime peak of melatonin in both species.
Circadian activity rhythms responded differently to pinealectomy, with house sparrows showing arrhythmic locomotor activity in constant darkness, while starlings maintained a rhythm.
In synchronized light-dark cycles, pinealectomized birds of both species displayed a similar activity pattern.
House sparrows exhibited a damped rhythm after transitioning from light-dark to constant darkness, indicating variability in circadian regulation.
These findings suggest that melatonin output from the pineal gland is only one component of a more complex system regulating circadian rhythms, with species-specific differences.

Simplified

We determined 24-hr plasma melatonin profiles in intact, sham-pinealectomized, and pinealectomized European starlings (Sturnus vulgaris) and house sparrows (Passer domesticus) in a light-dark (LD) cycle and in constant darkness (DD). In the intact and sham-pinealectomized birds of both species, a melatonin rhythm was found, with low levels during the day and high levels during the night. Pinealectomy abolished the nighttime peak of melatonin in both species; hence, levels were low at all times sampled. This uniform response of plasma melatonin to pinealectomy contrasts with the differential response of circadian activity rhythms to pinealectomy for these two species. In DD, locomotor activity in pinealectomized house sparrows is usually arrhythmic, whereas in starlings a rhythm usually persists. This suggests that in the latter species free-running circadian rhythms are not necessarily dependent on a rhythm in plasma melatonin. The same is true for the synchronized activity rhythm observed in pinealectomized birds of both species in LD, as well as for the damped rhythm that persists in pinealectomized house sparrows following an LD-to-DD transfer. The results are consistent with the hypothesis that the pineal and its periodic output of melatonin constitute only one component in a system of at least two coupled pacemakers. They also suggest that there are species differences in the relative role played by the pineal and other pacemakers in controlling circadian rhythms in behavior.

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How removing the pineal gland affects daily melatonin levels in house sparrows and European starlings

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