Melatonin release patterns in pigs kept under different light regimens

Melatonin is a hormone controlling the biorhythm of mammals and might be an indicator for harmony between individuals and their environment. The major ‘zeitgeber’ for the release of melatonin by the pineal gland, regulated by the hypothalamus, is environmental light. It synchronizes the melatonin secretion with the 24- hour day/night cycle. Normally, melatonin levels are high in the night and low during the day. There is a lot of discussion about building “mega” pig stables in The Netherlands. In these discussions animal welfare plays a big role. Since pig breeding is more and more intensified, several adaptations to the way of pig stabling are introduced, including adapting the light regimen to feeding regimen. Until now no objective parameter is available to measure animal welfare in mammals. Because melatonin could be a possible parameter to estimate the (non) adaptation to another light regime. We formed two groups of six pigs. One group was kept under a single light phase regimen, for a minimum of eight hours of light per day. The other group was kept under a phased light regimen, three periods of three hours of light spread over 24 hours. Each pig was sampled during a 24 hour period, by a saliva sample taken every hour. In both groups no nocturnal rise in melatonin concentration was found. Both groups show a higher (no significant) mean melatonin concentration during light periods. There was no significant rise or fall in melatonin concentration caused by the change of environmental light from light to darkness and from darkness to light. There was no correlation between rise in melatonin concentration and food intake. We could conclude we did not find a difference in melatonin release pattern between pigs kept under a discontinuous multiple light regimen and pigs kept under a continuous light regimen. If melatonin could be a useful parameter for measuring animal welfare cannot be confirmed by this experiment