Caudal and pedal flags in medium-size duikers of Gabon
All of the 16 or more species of duikers (Cephalophus) hide in dense vegetation, and their colouration is accordingly inconspicuous to predators. However, certain parts of the body are, when in motion, pale enough to be conspicuous in the context of secretive social communication at close range (up to ten metres). Duikers are described as 'solitary', but it is likely that there is often one other individual (either the juvenile progeny in the case of females or a prospective mate in the case of males) in view. The main social messages are 'this is where I am right now', 'I am walking in this direction' and 'I am chewing'.
Duikers are often described as 'red' or even 'orange', but we can ignore these hues when assessing the adaptive colouration of ungulates. This is because the visual systems of both ungulates and Carnivora are insensitive to the colours of the rainbow and instead particularly sensitive to movement. It is any pattern of pale-dark contrast that tends to be noticeable to these mammals; and such patterns become even more noticeable when they are put into motion.
However, in the dim light of forests, dark markings tend to lose their visibility. We can therefore expect the conspicuous features in duikers to be pale ones.
The photo-guide at https://www.zooniverse.org/talk/1971/697125 allows me to use five species in Gabon as initial examples.
Caudal flag in Cephalophus callipygus https://www.africahunting.com/media/peters-duiker-hunt-congo.42204/ and https://www.youtube.com/watch?v=XXxrzYkuokk.
Caudal flag in Cephalophus leucogaster https://www.youtube.com/watch?v=J2SIi5lvoAQ
Caudal flag in Cephalophus nigrifrons https://www.facebook.com/watch/?v=118759616767008
Pedal flag in Cephalophus ogilbyi crusalbum https://emammal.si.edu/rabi-forestgeo and https://www.youtube.com/watch?v=ESpC2yXf2tc
Caudal flag in Cephalophus dorsalis https://www.africahunting.com/media/hunting-congo-bay-duiker.52665/ and https://www.pinterest.com.au/pin/32862272268559020/
All of these species flick the tail as they walk, but only four show enough pale fur to qualify as possessing caudal flags. In the case of Cephalophus ogilbyi crusalbum it is instead the lower legs, particularly the hindlegs, that are pale enough to be conspicuous while in motion.
The paleness under the tail is most marked in Cephalophus dorsalis, possibly because this species is so strictly nocturnal that any social signal needs to be pale enough to show up even in darkness.
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In small duikers such as Cephalophus natalensis, the tail is small and the pale part of the tail minimal. In this case, I would classify the feature as a caudal semet, not a caudal flag (see https://www.youtube.com/watch?v=FW2npjdOa54). The conspicuousness of the tail is mainly owing to the movement of the tail, and only slightly owing to pale accentuation.
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