Androgen changes and flexible rutting behaviour in male giraffes

Male giraffes can switch their mating activity within days or weeks. A bull actively investigating females can soon enter a quieter phase, rather than remaining in the same reproductive state for an entire season. In wild giraffes, these changes also appeared in the hormones excreted in their dung.

Peter Seeber, Patrick Duncan, Hervé Fritz and André Ganswindt observed 73 individually recognised bulls in Zimbabwe's Hwange National Park from March to May 2011. They recorded behaviours such as testing females' urine, guarding potential mates and attempting to mate. Sixty-six dung samples from 39 known males supplied a measure of androgen metabolites, allowing reproductive activity to be compared with hormone levels without repeatedly capturing the animals.

The most mature-looking males showed the greatest sexual activity and generally higher androgen levels. Within that group, sexually active bulls had higher levels than inactive ones; repeated observations of three males linked switches into inactivity with sharp hormone declines. The association does not reveal whether hormones initiated the behavioural changes or followed social circumstances. It does suggest brief, individually timed mating phases rather than one synchronised rut for all males. A bull's age and imposing appearance therefore do not tell the whole story of his reproductive activity on a particular day.