AMPLITUDE MODULATION AS A SOURCE OF LOW FREQUENCY FACILITATING THE PROPAGATION OF MARMOT (MAMMALIA, RODENTIA) VOCAL SIGNALS IN BURROWS Article Nikol’skii A.A., Chi W., Vanisova E.A., Lisitsyna T.Y. Doklady biological sciences : proceedings of the Academy of Sciences of the USSR, Biological sciences sections / translated from Russian. Vol. 463. 2015. P. 193-199