The mane characteristics of lions Panthera leo in the greater Tsavo ecosystem (GTE) were compared with those of lions from the equatorial middle-elevation plains (EMEP). Contrary to popular belief, most full-grown GTE lions are not maneless; 87% had manes, with 49% possessing good manes. The manes of GTE lions, however, were poorer on average, relative to age, than the manes of EMEP lions. For both groups, there was a significant relationship between age and mane type. In EMEP lions, mane development started early and grew to a full mane by age 4–5. In GTE lions, mane development began later and developed more slowly. Delayed onset and a slower rate of development are correlated with the consistently hot Tsavo climate. Poorly maned but fully mature lions mated actively, showing dissociation between mane development and sexual maturity. The correlation between climate and mane development suggests that climatic adaptation results in the inhibition and/or delay in the development of a secondary sexual character without compromising reproductive viability.