Can precociality in the colouration of impalas (Aepyceros) be explained by their confusing nature as 'sedentary plains game'? part 1

@adamwelz @syddddney

INTRODUCTION

The basic nature of impalas (Aepyceros) has been confusing for naturalists.

This is possibly because it has not been fully realised that impalas are 'plains game turned sedentary'.

In this Post, I explain the adaptive colouration of infants in this context.

IMPALAS AS DECEPTIVE 'PLAINS GAME'

The rationale goes as follows.

Impalas are associated with woody cover, rather than treeless grassland.

However, they conform with large ungulates of open, short vegetation in

gregariousness (https://www.gettyimages.com.au/detail/photo/impala-herd-royalty-free-image/1278026487?phrase=african+impala&adppopup=true),
visual reliance, with large eyeballs placed laterally on the face for horizon-wide vigilance,
ambling gait and cursoriality (https://www.gettyimages.com.au/detail/photo/cheetah-acinonyx-jubatus-is-hunting-impalas-masai-royalty-free-image/1173639957?phrase=african+impala&adppopup=true),
reluctance to jump fences,
limited sexual dimorphism,
seasonality of reproduction despite being tropical,
dependence on green grass as food for lactating mothers, and
long-legged precociality of infants (https://www.gettyimages.com.au/detail/photo/the-hunt-royalty-free-image/1216744415?phrase=african+impala&adppopup=true and https://www.gettyimages.com.au/detail/photo/impala-and-fawn-royalty-free-image/1219697783?phrase=african+impala&adppopup=true).

At the same time, impalas differ from most other 'plains game' in a basic way, viz. in not being migratory/nomadic.

Instead, throughout their range, they are thoroughly sedentary (https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2028.1982.tb00301.x and https://www.researchgate.net/publication/230185512_Home_range_dispersal_and_the_clan_system_of_impala).

Impalas achieve this sedentariness by resorting in the dry season to foods other than grass, particularly

foliage of non-deciduous Vachellia (https://en.wikipedia.org/wiki/Vachellia), and
fleshy fruits in the form of the carob-like pods of Vachellia, Faidherbia, and Dichrostachys (https://www.jstor.org/stable/2560055 and https://www.cambridge.org/core/journals/journal-of-tropical-ecology/article/abs/dispersal-of-acacia-seeds-by-ungulates-and-ostriches-in-an-african-savanna/4941148A8BCBEB27B465A6B238EAD158 and https://www.cabidigitallibrary.org/doi/full/10.1079/cabicompendium.2342).

What this means is that the well-known association of impalas with trees and shrubs may be misleading. Their 'cover-dependence' is trophic (https://www.vocabulary.com/dictionary/trophic), rather than anti-predatory.

Unlike reduncins (https://en.wikipedia.org/wiki/Reduncinae) and tragelaphins (https://en.wikipedia.org/wiki/Tragelaphini) of similar body size, impalas are not 'cover-dependent' w.r.t. predators.

Instead, impalas are typical of 'plains game' in their

reliance on gregarious vigilance,
diurnal rather than nocturnal foraging, drinking, and parturition,
extreme gaits in reaction to predators, and
abandonment of concealment - even as infants.

Essentially, impalas have made a tradeoff, paying the costs (downsides) of certain benefits (upsides).

The main upside is the food provided by woody plants, obviating the need to shift location.

However, the main downsides are that

these plants provide cover not for the impalas, but for their predators, and
sedentariness exacerbates parasitism by ticks, mites, and flies.

The result of the unusual ecological strategy of impalas is a combination of biological traits that has confused naturalists in the past.

In this Post, I focus on only part of this confusion: that regarding adaptive colouration, and how it changes - or, more precisely, hardly changes - from youth to maturity.

ADAPTIVE COLOURATION RELATIVE TO TYPICAL 'PLAINS GAME'

The above characterisation provides a basis for assessing the adaptive colouration of infants in impalas.

In particular, it offers insight into their surprising emancipation from cover-dependence.

Unlike most 'plains game', adults of impalas do not have adaptively conspicuous colouration. I refer to dark/pale features so bold that they advertise the figures at distance.

Instead, their colouration is configured differently from both

'plains game', and
cover-dependent ruminants such as Redunca arundinum (https://www.inaturalist.org/taxa/42305-Redunca-arundinum) and Nyala angasi (https://www.inaturalist.org/taxa/42342-Tragelaphus-angasii), which have either plain (cryptic) or disruptive (camouflage) colouration.

RELEVANT FEATURES OF COLOURATION IN INFANTS, as per https://www.inaturalist.org/journal/milewski/89191-bleezes-flags-and-semets-in-the-adaptive-colouration-of-impalas-aepyceros-part-1#

Throughout this Post, I refer to Aepyceros melampus and Aepyceros petersi jointly, except where specified.

Infants of impalas have colouration similar to that of their mothers and fathers (https://www.gettyimages.com.au/detail/photo/impala-and-fawn-royalty-free-image/1219697789?phrase=african+impala&adppopup=true).

Impalas are unusual among the relatively large-bodied (body mass 50 kg or more) ungulates of the world - including most spp. of 'plains game' - in a certain way.

This is that the colouration of mature males is approximately complete already at birth, at least w.r.t. pigmentation, as opposed to sheen.

In other words, the pattern of pigmentation of the pelage in impalas is remarkably precocial.

However, not all features of colouration are equally well-developed in neonates.

Thus, a subsidiary aim of this Post is to document and illustrate these minor chronological (= ontogenetic) variations.

LATERAL FLAG:

The lateral flag hypothetically facilitates the gregariousness of impalas, dependent on certain conditions of illumination.

This flag is undeveloped (i.e. present only in incipient form) in infants (https://www.alamy.com/stock-photo-three-impala-walking-along-in-warm-lighting-18765797.html?imageid=4BAAA4C2-8374-4CA7-BCE5-788C0800662B&p=34420&pn=2&searchId=97e69abec044f22cfa320b375090cc9f&searchtype=0 and https://www.shutterstock.com/image-photo/female-impala-her-calf-kruger-national-1453377443 and https://www.alamy.com/impala-ewe-with-calf-aepyceros-melampus-okonjima-nature-reserve-namibia-africa-image230532633.html?imageid=75A1E27D-EC24-4482-98B2-1A80F2C9DB47&p=71799&pn=1&searchId=f389d6fe91e8459064d1ae8d87ceca96&searchtype=0 and https://wildadventuresblog.files.wordpress.com/2023/11/rixaka-and-baby-impala-1.jpg and https://www.dreamstime.com/impala-ewe-aepyceros-melampus-lamb-image242030063).

This is in contrast to the pedal flag (see below).

In particular, the sheen effect is absent in infancy from the relatively pale panel on the flanks (https://www.gettyimages.com.au/detail/photo/impalas-mother-and-young-aepyceros-melampus-royalty-free-image/128112952?phrase=african+impala&adppopup=true).

The following shows that, when the sheen effect is 'switched off', adults resemble infants in that the pattern on the flanks is subdued (https://www.gettyimages.com.au/detail/photo/impala-in-rain-storm-lake-nakuru-kenya-royalty-free-image/126377157?phrase=african+impala&adppopup=true).

The 'punctuation' of the lateral flag, by a patch of dark, bare skin at the stifle-fold, is particularly lacking in infants (https://www.shutterstock.com/image-photo/impala-mother-baby-on-green-grass-2125521815 and https://www.shutterstock.com/image-photo/mother-child-impala-standing-close-together-2129759222).

On balance, the lateral flag is among the least precocial of the patterns described in this Post. However, it appears after only about one month of age (https://www.alamy.com/stock-photo-africa-kenya-masai-mara-national-reserve-young-impalas-walking-aepyceros-42109588.html?imageid=143E541A-8495-4DA2-A237-1CBED6420840&p=147756&pn=6&searchId=0171166ff0fdcc816dc79c8195263b39&searchtype=0).

https://www.mindenpictures.com/stock-photo-impala-aepyceros-melampus-mother-running-with-her-calf-following-naturephotography-image00630132.html

https://www.dreamstime.com/impala-lamb-masai-mara-national-reserve-young-aepyceros-melampus-kenya-image243636698

https://www.offset.com/photos/an-impala-and-young-calf-aepyceros-melampus-on-the-edge-of-woodland-963170

https://www.superstock.com/asset/impala-calf-baby-impala-antelope-wilderness-africa/6001-16208619

https://www.inaturalist.org/observations/4425491 and https://www.africanflyingadventures.com/life-and-death-in-the-bush-impala/

https://www.gettyimages.com.au/detail/photo/portrait-of-deer-standing-on-field-kruger-national-royalty-free-image/1425287723?adppopup=true)

https://www.gettyimages.com.au/detail/photo/impalas-female-with-young-animals-in-the-steppe-royalty-free-image/927944266?phrase=african+impala&adppopup=true

It is possible that the difference between infants and adults, in the expression of the lateral flag, is owing largely to the proportional 'filling out' of the torso, as the animal progresses from milk to a bulky diet of grass.

ANTERIOR AURICULAR FLAG:

The anterior auricular flag is hypothetically an anti-predator adaptation. It emphasises communication to the predator that it has been spotted, and has lost the advantage of surprise.

The anterior auricular flag of impalas is complete in infants (https://www.alamy.com/young-animal-botswana-chobe-national-park-impala-aepyceros-melampus-savute-bush-daytime-wildlife-tourism-nature-no-people-outdoors-safari-young-animals-cute-animals-in-the-wild-big-5-animal-calf-africa-image595891120.html?imageid=1355A1C6-6044-4D46-BD19-D855A978C069&p=2325997&pn=1&searchId=2c70548f127dc53b817942155023dd85&searchtype=0 and https://www.shutterstock.com/image-photo/impala-family-nakuru-national-park-368215919). This precociality exceeds that of the dark stripes on the buttocks.

https://www.alamy.com/stock-photo-two-baby-impala-antelopes-in-the-african-bush-29057562.html

https://www.dreamstime.com/royalty-free-stock-photo-impala-antelope-lambs-two-small-aepyceros-melampus-lake-nakuru-national-park-kenya-image40154625

https://www.gettyimages.com.au/detail/photo/impala-with-her-young-aepycaros-melampus-masai-mara-royalty-free-image/57255400?phrase=african+impala&adppopup=true

https://www.gettyimages.com.au/detail/photo/aepyceros-melampus-female-with-young-ndutu-tanza-royalty-free-image/90062064?phrase=african+impala&adppopup=true

POSTERIOR CORONAL FLAG:

The posterior coronal flag hypothetically facilitates gregarious vigilance in impalas.

It is located on the highest part of the figure, and emphasise the orientation of attention/alarm, particularly as individuals raise their heads from foraging.

This is illustrated in the opening footage in https://www.google.com.au/search?q=Impala+jumping+fence+on+game+ranch&sca_esv=85ac6a756d1f0a52&sxsrf=ACQVn0-8qDhHDaQQj4ijdrqeQi9DmroxJw%3A1708062330222&ei=evbOZc2YDfLvseMPytGE2A8&ved=0ahUKEwjNvcmDlK-EAxXyd2wGHcooAfsQ4dUDCBA&uact=5&oq=Impala+jumping+fence+on+game+ranch&gs_lp=Egxnd3Mtd2l6LXNlcnAiIkltcGFsYSBqdW1waW5nIGZlbmNlIG9uIGdhbWUgcmFuY2gyCBAAGIAEGKIEMggQABiABBiiBEijD1C0DFi0DHACeACQAQCYAd8BoAHfAaoBAzItMbgBA8gBAPgBAcICDhAAGIAEGIoFGIYDGLADiAYBkAYC&sclient=gws-wiz-serp#fpstate=ive&vld=cid:f7bf2184,vid:ueMZh53DSBY,st:0.

Infants tend not to show the posterior coronal flag, even when the associated adults do show it (https://www.alamy.com/baby-impala-with-mom-cute-impala-calf-image334465986.html?imageid=39ACF71F-D5CD-499A-B7CD-32669CF8CF74&p=1760945&pn=13&searchId=d273b17bcb1f0d1c84c7995c6c50d374&searchtype=0 and https://www.alamy.com/baby-impala-with-mom-cute-impala-calf-image334466064.html?imageid=14A68E2E-F5DA-4EB0-9841-8D17885FEF3A&p=1760945&pn=13&searchId=d273b17bcb1f0d1c84c7995c6c50d374&searchtype=0).

However, the posterior coronal flag may be more precocial than the lateral flag.

In the following, sheen has appeared on the posterior surface of the crown of the head, at an age too young for it to appear on the flanks of the torso.

https://www.dreamstime.com/royalty-free-stock-photos-antelope-image7622658

The following shows that a hair-whorl (https://en.wikipedia.org/wiki/Hair_whorl) - which may perhaps help to explain the sheen effect - occurs at the location of the posterior coronal flag.

https://www.gettyimages.ie/detail/photo/the-small-horns-of-a-young-male-impala-royalty-free-image/617783819?phrase=impala+the+animal&adppopup=true

The following show the posterior coronal flag in adults. A main point to note is that, although there is a definite pattern on the buttocks, it is the pallor (owing to sheen) on the posterior surface of the crown that renders the figures conspicuous.

https://www.shutterstock.com/image-photo/herd-female-impala-serengeti-national-park-1895436763

https://www.shutterstock.com/image-photo/herd-impala-aepyceros-melampus-timbavati-game-2207433769

https://www.dreamstime.com/impala-aepyceros-melampus-kruger-national-park-mpumalanga-south-africa-image135031275

PEDAL FLAG:

The pedal flag hypothetically facilitates gregariousness, particularly at night, when impalas tend to congregate on lawns.

This flag is peculiarly precocial (https://www.dreamstime.com/stock-photo-young-impala-baby-stands-watching-other-antelopes-game-reserve-south-africa-image70429327).

It is fully developed in infants, despite