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

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

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

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

  • the lack of sheen elsewhere on the pelage of infants, and
  • the expectation that the metatarsal glands are relatively poorly-developed at birth.

https://www.dreamstime.com/common-impala-calf-walks-stony-track-image260101943 and https://www.dreamstime.com/common-impala-calf-stands-stony-track-image260102489 and https://www.dreamstime.com/baby-common-impala-walks-along-dirt-track-image257794099

https://www.inaturalist.org/observations/165329533

https://www.inaturalist.org/observations/142346786

https://www.inaturalist.org/observations/106255260

https://www.inaturalist.org/observations/9839721

https://www.inaturalist.org/observations/9356668

to be continued in https://www.inaturalist.org/journal/milewski/89824-can-precociality-in-the-colouration-of-impalas-aepyceros-be-explained-by-their-confusing-nature-as-sedentary-plains-game-part-2#...

Publicado el febrero 11, 2024 01:11 MAÑANA por milewski milewski

Comentarios

The following, of adult male Aepyceros melampus, shows the sheen effect on the anterior of the pale-pigmented flank-band, just posterior to the scapula. The pedal flag is also apparent.

https://www.gettyimages.com.au/detail/photo/impala-ram-royalty-free-image/148240374?phrase=african+impala&adppopup=true

Publicado por milewski hace 5 meses

The following shows the caudal flag in adult male Panthera pardus, and the gular semet in adult male Aepyceros melampus:

https://www.gettyimages.com.au/detail/photo/side-view-of-cheetah-walking-on-field-narok-county-royalty-free-image/1280535780?phrase=african+impala&adppopup=true

Publicado por milewski hace 5 meses

The following shows the precociality of the dark pattern on the face of Aepyceros petersi:

https://www.alamy.com/stock-photo-black-faced-impala-calf-15782959.html

Publicado por milewski hace 5 meses

FEATURES OF COLOURATION ON THE HEAD OF AEPYCEROS

All facial vibrissae are short and fine, not long and coarse (e.g. compare Antilope)

Narrow dark rhinarium (but far larger than in Antilope)

Superciliary pale

Somewhat pale ventral to eye (Litocranius is paler than Aepyceros in this case)

Somewhat dark on rostrum, variable specifically and individually, the dark pelage falling well-short of the rhinarium

Dark marking on crown, the 'wings' of which run towards the eye while remaining disjunct from the dark spot above the eye

Dark spot above eye (= upper part of vestigial/incipient malar stripe)

Pale upper lips (with dark vibrissal 'peppering'), oddly unapparent in profile

Pale lower lips and chin, bordered by dark fawn in a characterisitic configuration that is extremely consistent individually, by sex, by age, and by species

White interramal patch (on ventral surface of head), disjunct from chin, extending to crook-of-throat

Medium-tone, S-shaped, blurry 'stripe' (= malar stripe in incipient/residual form), anterior to eye, ending on edge of upper lip

Whitish long hairs on anterior surface of ear pinna, replaced by dark short hairs at apex (individually consistent, and possibly largest in petersi) and near ventral edge (relatively faint and individually variable); narrow dark rim mid-dorsally on mid-pinna

Flesh-coloured (inconspicuous) bare skin (when visible owing to relaxation of ling white hairs) on anterior surface of ear pinna

Medium-tone short baits on posterior surface of ear pinna, except for apical third (dark, more extensive than its counterpart on anterior surface of ear pinna), and tendency to pale near ventral margin

Fairly pale pelage on posterior surface of crown and adjacent nape, looking paler with sheen effect

Ambivalent colouration in lateral base of ear, where hairs pale but dark skin shows through

No anterior or posterior auricular semets (compare various spp. of Cervidae)

No 'fang-mark' (compare various spp. of deer)

Publicado por milewski hace 5 meses

DIFFERENCE IN PRECOCIALITY OF PEDAL FLAG BETWEEN AEPYCEROS AND ANTILOPE

Adults of both genera possess pedal flags.

However, the infants differ.

Impalas are born with the pedal flag complete. The same is not true of the blackbuck.

The blackbuck differs more in colouration between infants and adult females than is the case in impalas.

In impalas, infants do lack a few small features seen in juveniles and adults (e.g. a dark spot on the point of each hock). However, the pale (based mainly on a peculiar kind of sheen) on the pasterns is fully precocial.

This is complicated by the fact that the pedal flag in the blackbuck is sexually dimorphic.

Publicado por milewski hace 5 meses

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