Perspectives Online

Night and Day, Research demonstrates that photoperiod can have profound effects on birds. By Dave Caldwell

Dr. Thomas Siopes adjusts the intensity of a light to study turkeys' photophysiology - the way they respond to light intensity, light color and photoperiod (the length of darkness and light in a 24-hour period).
Photo by Becky Kirkland

What a College of Agriculture and Life Sciences researcher is learning about how birds respond to day length could improve turkey reproduction and make the birds healthier. It might even have implications for human health.

Dr. Thomas Siopes, professor of physiology in the Department of Poultry Science, studies photophysiology in turkeys and quail. Photophysiology is the way animals respond to photoperiod, light intensity and light color.

Photoperiod is the length of darkness and light in a 24-hour period, and it plays a major role in the lives of turkeys. As is the case with many animals, including humans, the amount of daylight turkeys experience acts as a trigger that prompts and synchronizes various physiological activities. For example, turkeys will not breed unless the days are sufficiently long.

Where turkeys are concerned, it seems likely that many responses to photoperiod are genetic adaptations that enhance survival in the wild. Poults hatched during the summer, when days are longer, are more likely to survive than poults hatched in the dead of winter, when the days are shorter. Turkeys have adapted to use day length as a signal to breed at a time of year with favorable temperature and abundant food to maximize survival.

This seasonality suits Mother Nature just fine but is a problem for the turkey industry, which must produce birds year round. To get turkeys to breed all year, growers must house the birds in light-controlled barns, where they can expose them to the light equivalent of short, winter-like days, which fools the hens into expecting the immediate arrival of longer spring-like days. When the light/dark ratio is then adjusted to mimic longer days, a reproductive "season" ensues.

But this system isn't completely adequate, and much of Siopes' work is focused on understanding why by learning more about a condition called photorefractoriness. Siopes describes photorefractoriness as "a state of nonresponsiveness to the long days initially used to induce lay." In other words, birds stop responding to photoperiod. That, of course, is not what you want if you're a turkey grower and you're adjusting photoperiod to get your birds to lay eggs.

Working with colleagues and graduate students, Siopes is experimenting with natural and pharmacological means to intervene during photorefractoriness, restoring sensitivity to photoperiod, thus improving the efficiency of turkey reproduction. Much remains to be learned of the mechanism of photorefractoriness, says Siopes, and it remains one of the major unresolved issues in avian reproduction, let alone turkey reproduction.

Because turkeys breed when signaled by day length that the time of year is suitable for survival, growers can use light-controlled barns to get turkeys to breed year round. Siopes (above) is working to halt an impediment to the process - a condition of non-response to photoperiod called photorefractoriness in turkeys.
Photo by Becky Kirkland
Indeed, Siopes says it is likely that a better understanding of photorefractoriness may lead to methods of increasing turkey egg production, which is among the poorest in domestic birds. Turkeys produce only about a third as many eggs as chickens.

While Siopes is interested in the role photoperiod plays in turkey breeding when days are long, he is also interested in how turkeys and quail respond to shorter periods of light, the equivalent of the short days of winter.

Siopes has found that photoperiod plays a pivotal role in regulating quail and turkey immune systems. Exposing birds to 24-hour periods when the dark period is considerably longer than the light period boosts the birds' immune systems.

"We've developed a way to regulate the immune system of an animal in a simple, inexpensive way without pharmacology," says Siopes.

uch of Siopes' work on photoperiod response has been done with Dr. Herb Underwood, professor of zoology in the College of Agriculture and Life Sciences, while he is also working with Dr. Jim Millam at the University of California-Davis; with Dr. John Proudman at the U.S. Department of Agriculture Biotechnology and Germplasm Lab in Beltsville, Md.; and Dr. John Barnes in the College of Veterinary Medicine at N.C. State. A past graduate student, Dr. Chris Moore, now a postdoctoral fellow at the Lineberger Comprehensive Cancer Center at UNC-Chapel Hill, also contributed to the work.

Siopes explains that exposing turkeys to the light equivalent of short days stimulates the pineal gland, which produces melatonin, a hormone that is a powerful immune system enhancer. The more daily darkness, the more melatonin.

The immune response to the light-dark cycle occurs quickly. "We can see effects in one to two weeks," Siopes says. The immune systems of birds kept in short-day conditions are much more robust than birds living under longer light regimes.

Siopes has found that photoperiod plays a pivotal role in regulating the immune systems in quail (above) and turkeys (below). His research suggests new ways for turkey growers to manage the health of their flocks.
Photo by Becky Kirkland
This research suggests new ways for turkey growers to manage the health of their flocks. The daily light-dark cycle might be used to reduce the use of chemicals such as antibiotics and vaccines now used to manage flock health.

Siopes is working to determine how the immune system boost that photoperiod adjustment makes possible interacts with antibiotics and vaccines. He'd also like to know if photoperiod enhancement of immune function translates to what growers call improved performance, the rate at which birds grow and how efficiently they turn feed to meat.

It would seem logical that if birds are healthier, they'll grow faster and reproduce more efficiently, but actual data are scarce. Siopes also wants to know whether it's possible to boost a bird's immune system too much, whether birds suffer any ill effects from a high-powered immune system.

And he's experimenting with other ways to get the same kind of immune system response that the changes in photoperiod cause. Because the amino acid tryptophan is the body's source of melatonin, Siopes is looking at adding tryptophan to turkey diets to enhance melatonin production.

"In effect, this is a dietary means to simulate photoperiod effects," he says.

Why does photoperiod, and short days in particular, have such an effect on a turkey's immune system? Siopes speculates that the explanation may be pretty straightforward. The survival chances of a wild turkey whose immune system functions at a higher level in the shortened days of winter, when food may be scarce and the environment harsh, are increased. Granted, the turkeys raised by farmers bear little resemblance to wild turkeys, but in this case, it seems domesticated and wild turkeys are similar.

Another result of Siopes' research is not so easily explained.

It seems that 3 to 5 percent of turkey hens develop spontaneous ovarian tumors. Siopes first noticed the cancers a number of years ago when doing necropsies on turkeys. When he confirmed the masses he found in some of his birds were cancerous tumors, Siopes decided to see whether photoperiod had any effect on the cancers.

What he found was startling. By adjusting photoperiod for a shorter day length, Siopes was able to eradicate tumors. When he put the turkeys whose tumors had completely disappeared back on longer days, the tumors returned.

"We can literally make these cancers disappear and reappear by manipulating photoperiod," Siopes says. He's also found that melatonin injections will slow tumor development. Moore, the former graduate student who is now a postdoctoral fellow at the Lineberger Cancer Center, contributed to Siopes' cancer studies.

This line of research raises some intriguing questions. For example, do mammals exhibit similar behavior? If so, might photoperiod and/or melatonin adjustment be used as part of a cancer treatment for humans?

Siopes isn't close to answering either question, but he does think that turkeys might be used as a model system, as is already the case with chickens, to study human ovarian cancer.