We don't see any normal lack of secondary sexual characteristics in the great apes - except in the orang-utan. The undeveloped male in these two species, without the large dewlap-like flange, cannot change back to this morph but usually develops into the mature morph over a variable period of time. Those apes that remain in the less mature state can do so for many years and still reproduce somewhat successfully, although with lower female interest. One of these records has a male developing flange 30 years after siring offspring.
The two species, Pongo pygmaeus, from Borneo and Pongo abelii, from Sumatra, vary in their male development. The flanged males were more common in Tuanan (Borneo), while at Suaq (Sumatra), there were more unflanged males. A lot of the Tuanan males became flanged during each year of study. Unflanged males are also small, near female size, and without the long fur and inflatable throat sac of fully mature males. Lynda P Dunkel and her colleagues published a paper making acute observations on male maturation in the journal, 'Frontiers in Zoology,' working from Universtas Nasional in Jakarta, Universitas Indonesia in Depok and Zurich University.
It's probably best to explain how females react on the two islands to male presence. Sumatra has the girls gathered within earshot of a flanged male, who will make regular calls for his "harem." They consort together for long periods of time. These males don't mind the "unflanged boys" of various ages, partly because their extra weight prevents a successful chase. The unflanged males of Sumatra have quite an active sex life. It is also unsuccessful as far as breeding goes ! The unflanged males probably copulate with more females than non-dominant flanged males, because of this access to females.
In Borneo, flanged males fight each other more. Perhaps related is a shorter time spent with females and more forced mating. The females seem more promiscuous too, and don't remain near (within earshot of) the dominant males, who also wander about much more. There should logically be more flanged males in the Borneo population and they should grow flanges more over a given period. There should also be more transient flanged males on Borneo (and more transient unflanged males on Sumatra.
Results of the observers work over 5 years all agreed that there were many more Sumatran unflanged males than on Borneo; there was a much higher rate of flange development in Borneo.
All in all, the chances of selective mortality due to logging or hunting can be discounted. Logging had been discontinued in Sumatra for decades before the study. In similar fashion other causes such as differences in dispersal are discounted by the paper's authors because the males are the dispersing sex on both islands. The differences between the two species are limited, but the key is assumed to sexual monopoly by the flanged male. This is much more successful in Sumatra, where unflanged males achieve frequent copulation with non-reproducing females. Borneo flanged orang-utan males all have access to fertile females (who are in oestrous) and the unflanged individuals will have low access to females.
A lot more study is needed to understand this unique and now rare ape, including, the authors suggest, comparison with various macaque species, mandrills, and even other vertebrates that have this unfamiliar arrested developmental stage in their reproductive lives.href="https://earthtimes.org/environment/primates/index.html">Primates