Predators are dark-spotted, but their ungulate prey are pale-spotted. Why?

@tonyrebelo @paradoxornithidae @beartracker @davidbygott @botswanabugs

Spotting is a form of camouflage for both predators and their prey.

However, what remains to be explained is:
Why is the pattern inverted in the two categories in mammals?

Spots are dark against a relatively pale ground-colour in

By contrast, spots are pale against a relatively dark background in

One way to approach this puzzle is to examine any exceptions.

However, this turns out not to be particularly enlightening.

In the case of Carnivora, I cannot think of any real exceptions.

Broadening the search, an exception can be found in an Australian carnivorous marsupial. The spotted-tailed quoll (Dasyurus maculatus, https://www.inaturalist.org/taxa/40166-Dasyurus-maculatus and https://wildlife.org.au/spotted-tailed-quoll/) certainly has pale spots on a relatively dark background.

In the case of ungulates, an obvious exception is giraffes (Giraffa, see https://www.flickr.com/photos/isaacpacheco/14402047495).

However, these are also by far the largest land-animals with camouflage colouration, complicating any comparisons. Why giraffes are spotted in the first place is a question unto itself.

One reason why the nearly categorical difference between predators and prey is surprising is that all of the spotted Carnivora are themselves vulnerable to their largest local relatives. The lion (Panthera leo, https://www.inaturalist.org/taxa/41964-Panthera-leo), after all, readily kills all other Carnivora, regardless of whether it finds them acceptable as food.

This makes it hard to know whether the spotting of the cheetah (Acinonyx jubatus, https://www.inaturalist.org/taxa/41955-Acinonyx-jubatus), for example, serves more to hide it from its prey, or from its own predators, namely

Even in the case of the spotted hyena, second only to the lion in the African hierarchy, the spotting is puzzling. This is because this species hunts by pursuit, to the apparent exclusion of camouflage-dependent stalking.

Any naturalist who has spent time pondering adaptive colouration will know how enigmatic this field of biology can be. The patterns of colouration in animals seem to defy generalisation and prediction, discouraging further enquiry.

However, in the riddle of the dark-spotted predators versus the pale-spotted prey, we at least have an unusually clear-cut question, encapsulated by the following photos (https://www.reddit.com/r/natureismetal/comments/a9gmwh/leopard_carrying_its_kill_in_front_of_a_herd_of/ and https://www.reddit.com/r/HardcoreNature/comments/e3orpz/leopard_with_a_chital_deer_kill/).

Are we up to the challenge of solving at least this major puzzle?

Posted on April 29, 2021 02:43 AM by milewski milewski

Comments

One obvious difference, is that camouflage serves prey for initial sightings, but once located, it is of only secondary value, whereas for predators, camouflage is most useful in the final stages of stalking. This translates to camouflage being most useful for far sightings for prey, and near sightings for carnivores.
Is there anything suggesting that white disrupts far patterns whereas black better disrupts nearby patterns?

Posted by tonyrebelo over 1 year ago

@tonyrebelo

I like your line of thinking.

Another possibility has to do with the day/night difference.

Dark spotting is obviously less visible than pale spotting at night. This means that the adaptive value of dark spotting is likely to be mainly in bright illumination.

Could it be that deer are spotted mainly to hide at night, whereas the leopard is spotted mainly to hide by day (including when it is inactive, and subject to mobbing/harassment?)

Posted by milewski over 1 year ago

@tonyrebelo

Thinking further along the lines that pale spotting is adaptive mainly in relative darkness:

The impala (Aepyceros melampus, http://eswatininaturereserves.com/mammals/redlist/2016MammalRedListImpala-Aepyceros-melampus-melampus_LC.pdf) and the kob (Kobus kob) are, collectively, so widespread and common in Africa that they should have ecological counterparts in Eurasia.

The closest counterpart for the impala in Asia is the chital (Axis axis). The fallow deer (Dama dama) is also worth considering, despite being extratropical.

What is striking is that the Eurasian forms in this comparison are spotted, whereas the African forms are not.

I note that the impala is specialised for diurnal activity, retiring gregariously to special, open places at night. I do not know whether the kob is similar in this way.

By contrast, the fallow deer has an unremarkable pattern of activity, preferring crepuscular emergence, but able to become nocturnally active where it is persecuted. Is the chital similar?

On this basis I hypothesise that one reason for the spotlessness of the impala (despite this being unusually deer-like among bovids, in several ways) is its diurnal specialisation. The impala would not benefit from spotting, because a) pale spots provide poor camouflage by day, and b) at night it is in the open, where there is no cover anyway, i.e. with no background to blend into.

By contrast, the guib (Tragelaphus scriptus) - which is the most spotted of all bovids - is far more nocturnal than the impala and, probably, the kob (with which the guib is widely sympatric).

By the way, it seems well-known among game-managers that reintroductiins of the impala to game ranches tend to be unsuccessful unless a certain minimum number of individuals (more than a dozen?) is released simultaneously. I wonder if this is partly because of the specialisation on gregarious retirement to open places at night.

Posted by milewski over 1 year ago

Pale spotting in Moschidae (infants and juveniles):

https://www.naturepl.com/stock-photo-nature-image01598589.html

Posted by milewski over 1 year ago

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