Sex-biassed site fidelity and orientation behaviour in reproductive natterjack toads (Bufo calamita)

Abstract

Site fidelity, initial orientation and homing performance were studied in the natterjack toad Bufo calamita near Bonn, Western Germany, during three breeding periods. Capture-mark-recapture experiments demonstrated that most males stayed within areas of about 0.5 km² during the whole study period. If rainfall provided ephemeral ponds suitable as spawning sites within these areas, local males advertised the position of the ponds by calling. Homeward orientation and homing success of displaced males showed that males were strictly attached to a pond once chosen for several days. The paths of individuals were registered using a mechanical tracking device. However, displaced reproductive females did not return to the breeding pond, if another pond with calling males was less distant from the release site. The initial orientation of untreated males towards their home pond remained unaffected by the presence or absence of a guiding breeding chorus at the home pond. Not even breeding choruses at other nearby sites biassed the directional choice. In contrast, displaced females always approached the less distant breeding chorus. Males with attached bar magnets were disoriented as a group but every individual followed a relatively straight path. Deprivation of olfactory cues impaired the homeward orientation of males but did not impede it entirely or affect the straightness of the path. Anosmic females showed the same directional choice as untreated ones. Blindfolded males were disoriented as a group and also as individuals moving frequently in irregular or spiral paths. Blindfolding reduced the mean locomotory activity significantly to about 40% of that of controls. The homing success of magnetically disturbed or anosmic males was the same as in untreated males but both treatments reduced the homing speed. In conclusion, males used magnetic, olfactory, and visual but not acoustic cues for initial orientation towards the calling site. Even if deprived of magnetic or olfactory information during the return journey, they were still able to home using alternative cues after a temporal delay. Reproductive females, however, used mainly acoustic information (conspecific breeding choruses) for long distance piloting towards suitable breeding sites.