One plus one: Binary alarm calls retain individual signature for longer periods than single-note alarms in the European ground squirrel (Spermophilus citellus)

•Number of alarm call elements differs between and within ground squirrel species.•European ground squirrel’s alarm call contains two structurally different elements.•Both elements have high potential to encode individuality.•Combining both elements does not enhance encoded individuality.•Higher time stability of individuality can be achieved when combining both elements. Ground squirrels emit species-specific alarm calls that, among other characteristics, differ by the number of elements. Unlike some species that produce single-element calls, e.g., the Speckled ground squirrel (Spermophilus suslicus), individual European ground squirrels (S. citellus) frequently emit binary-element calls in addition to single-element calls. We tested the hypothesis that the time stability of individuality encoded in alarm calls might be better retained by complicating their acoustic structure by adding extra elements. In a semi-captive colony of individually marked European ground squirrels, we repeatedly recorded alarm calls that were produced towards a human by 12 adult (2 males and 10 females) live-trapped animals. Repeated recordings occurred within time spans of a few hours, 2days and 1year from the first recording. Our results showed that individual calls were highly similar within recordings, but less similar between recordings separated by time spans. Individual differences were best retained when we used nine acoustic variables from both elements. The differences were worse when we used nine variables from only the first element and worst when we used nine variables from only the second element. These results supported the caller reliability hypothesis for species that produce multiple-note alarms, e.g., the Richardson’s ground squirrel (S. richardsonii).