WASHINGTON — Men who voted for Republican John McCain in last year’s US presidential election saw their testosterone levels fall significantly when they learned he had lost to Barack Obama, a study showed Thursday.

Saliva samples collected from 163 men on the evening of the election showed that voters for both McCain and Obama had similar testosterone levels when polling stations closed on the east coast, but the levels in McCain backers fell when Obama was announced as the winner.

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By contrast, testosterone levels among men who voted for Obama remained stable. Taking into account the fact that polls shut and results were announced at night — a time when men’s testosterone levels usually decline — the study said the level was equivalent to a rise.

“Political elections are dominance competitions. When men win a dominance competition, their testosterone levels rise or remain stable to resist a circadian decline; and when they lose, their testosterone levels fall,” said the study, published online by the Public Library of Science (PLOS One).

Supporters of McCain or Libertarian Party candidate Bob Barr — who the study noted didn’t have a hope of getting elected — showed “significantly larger testosterone decreases” than Obama supporters from the time when polls closed to 40 minutes after Obama was declared the winner.