Ever fought over the last piece of pie? Aggressive competition is common among animals, and hormones play a major role in such behavior. One of these hormones is testosterone, which is produced by the testes and directly interacts with brain regions involved in social interactions. The brain can also convert testosterone into estradiol, another hormone that is a key regulator of male aggression. Hungry animals can be especially aggressive over food, but it is not known which hormones regulate food-related aggression. Testosterone from the testes is not likely to be involved because fasting is stressful and stress suppresses testosterone production by the testes. A testosterone precursor – called dehydroepiandrosterone (DHEA) – could regulate food-related aggression because fasting increases its levels and the brain can convert DHEA to testosterone and estradiol. In Kiran Soma's lab at the University of British Columbia, Canada, postdoctoral researcher H. Bobby Fokidis is using social songbirds to investigate whether DHEA is the molecular link between fighting and food.

Fokidis and his colleagues decided to stage a bird food fight in the lab. First, the team housed adult male zebra finches in groups of two to four birds for 6 h with full access to food (fed), an empty food dish (fasted) or 6 h with an empty food dish followed by 15 min with full access to food (re-fed). Then, they removed all of the food dishes and all of the birds were offered food from a point-source feeder that only allowed one bird at a time to perch and access food. The team then filmed the birds to see how much jostling went on at the feeder, and measured the birds' levels of testosterone, DHEA and estradiol to find out which of these hormones was controlling the food fight.

The team found that the energetic zebra finch does not like to go without food. Compared with the fed group, the fasted group was more aggressive when the point-source feeder was introduced, and frequently chased and displaced other birds in their group. Aggression in the re-fed group was intermediate between that of the fed and fasted groups, showing that food status was directly involved in these behaviors.

Next, the team looked at how hormone levels compared between the three treatment groups. They found that testosterone was lower in blood taken from the brachial veins of birds that were fasted for 6 h, even if they were then re-fed. Blood in the brachial vein has not been to the brain yet, so any hormones found here were made in peripheral organs. Therefore, the team ruled out testosterone made in the testes as the hormone responsible for turning the feathered friends into foes. When the team measured DHEA levels in the brachial vein, they found that it was higher in the fasted group than in either the fed or re-fed groups, and when the team checked the two organs that produce it – the liver and adrenal glands – they found higher DHEA there too. However, the team found no differences in DHEA levels in blood taken from the jugular veins of birds in any of the treatment groups. Because blood in the jugular vein has just left the brain, the team thought that the extra DHEA made by the liver and adrenal glands of the fasted birds must have been converted to another hormone by the brain. In fact, Fokidis and his colleagues found more estradiol, which can be made from DHEA, in several brain regions that mediate social aggression in both the fasted and re-fed birds compared with the fed birds.

So, this study shows that DHEA may mediate food-related aggression after it is converted to estradiol in neural circuits that regulate social interactions, and drives who gets the last crumb in a zebra finch food fight.

H. B.
N. H.
K. K.
Fasting increases aggression and differentially modulates local and systemic steroid levels in male zebra finches