The unique sexual dimorphism of the northern oribi (Ourebia montana), an ungulate combining polygamy with male diminution

In iNaturalist, Ourebia (https://www.inaturalist.org/taxa/42380-Ourebia-ourebi) seems to be photographed in pairs (https://www.alamy.com/stock-photo-oribi-ourebia-ourebi-kafue-national-park-zambia-11846018.html?imageid=857A4ABE-7DBB-44E5-A844-B03268BAA80C&p=74587&pn=3&searchId=b18d9220987fab88ffb57be4cfa504ab&searchtype=0 and https://www.inaturalist.org/observations/110886912) more frequently than is any other ruminant.

Several small-bodied bovids are monogamous. In these spp., males tend to be smaller-bodied than females (please see comment below for details in the southern African fauna).

So, at first sight, it would seem that Ourebia is yet another example of this syndrome.

However, on closer scrutiny, Ourebia is revealed as odd, in the following ways.

Firstly, it is not monogamous. Territorial males consort with up to four individual females (https://www.inaturalist.org/observations/135350368). Therefore, it is anomalous that males are smaller-bodied than females, in Ourebia montana.

(Please see https://www.inaturalist.org/journal/milewski/70050-the-three-main-types-of-oribi-ourebia-at-a-glance# for the differences between Ourebia montana, the common oribi, and Ourebia ourebi, the southern oribi.)

Indeed, it seems possible that O. montana is the only polygamous ungulate, on Earth, in which males are smaller-bodied than females.

Secondly, adult males of O. montana not only weigh less than adult females, they also have a disproportionately shorter body and neck.

What seems to be unique, in Ourebia, is the way in which adult females differ in body shapes from adult males.

Exaggerating somewhat, in order to give readers a search-image:

Females in O. montana have body shapes similar to reduncins (https://en.wikipedia.org/wiki/Reduncinae).

By contrast, males in Ourebia montana have body shapes similar to those of 'neotragin dwarf antelopes' (https://en.wikipedia.org/wiki/Neotragini), i.e. the smallest-bodied antilopins.

So, when one views a sexual pair of O. montana, it can look like a 'marriage' of two different tribes of bovids, somehow combined within a single species (https://www.inaturalist.org/observations/7215985).

Females of O. montana resemble a diminutive version of reedbucks (Reduncini: Redunca), while males resemble the largest-bodied of the neotragin 'dwarf antelopes'.

To be more precise about the sexual difference in body shapes:

Adult females have a long torso and neck (https://www.ourbreathingplanet.com/oribi/ and https://www.shutterstock.com/image-photo/oribi-ourebia-ourebi-1243168594 and https://www.shutterstock.com/image-photo/portrait-oribi-ourebia-ourebi-murchison-falls-2188507957).

This makes females of O. montana look somewhat like a scaled-down version of adult females of the common reedbuck (Redunca arundinum, https://en.wikipedia.org/wiki/Southern_reedbuck), which actually weigh threefold more than females of O. montana.

Meanwhile, adult males have the unremarkable proportions of a generalised 'dwarf antelope' (https://www.alamy.com/oribi-ourebia-ourebi-masai-mara-national-park-kenia-image60113750.html?imageid=AD7C8970-246D-4C7D-B8E4-36915DA3494F&p=191343&pn=3&searchId=b18d9220987fab88ffb57be4cfa504ab&searchtype=0).

The body mass of O. montana is about 15 kg, with males (about 14-15 kg) at least 2 kg lighter than females (about 16-17 kg). (This is different from Ourebia ourebi ourebi, in which both sexes average 14 kg, and females are, if anything, slightly smaller-bodied than males, https://www.inaturalist.org/observations/8778172.)

However, when one views the sexes apart from each other,

• males (https://www.alamy.com/oribi-ourebia-ourebi-kidepo-valley-national-park-uganda-image239682510.html?imageid=709434DD-DBFE-4793-864B-83FAC66C2086&p=22059&pn=1&searchId=d592d238c126347ec4d3725c2a7bbfc9&searchtype=0 and https://www.istockphoto.com/photo/male-oribi-gm93469926-3976026?phrase=oribi%20ourebia%20ourebi%20wild%20in%20uganda) look about the size of the steenbok (Raphicerus campestris), which in reality weighs about 10 kg, while
• females look about the size of an imaginary reduncin weighing more than 20 kg (https://www.istockphoto.com/photo/a-female-oribi-in-murchison-falls-gm1303121908-394667565?phrase=oribi%20ourebia%20ourebi%20wild%20in%20uganda).

I have chosen the following to show the sexes separately, but in similar postures and perspectives.

Female https://www.inaturalist.org/observations/66999204
Male https://www.inaturalist.org/observations/66999205

Female https://www.inaturalist.org/observations/91161425
Male https://www.inaturalist.org/observations/8490581

Female https://www.inaturalist.org/observations/8490569
Male https://www.inaturalist.org/observations/8490590

Female https://www.inaturalist.org/observations/6577553
Male https://www.inaturalist.org/observations/6441964

Female https://www.inaturalist.org/observations/49107418
Male https://www.inaturalist.org/observations/8490545

Female https://www.inaturalist.org/observations/7103895
Male https://www.inaturalist.org/observations/77578819

Female https://www.inaturalist.org/observations/67005615
Male https://www.inaturalist.org/observations/6183578

Female https://www.inaturalist.org/observations/8490553
Male https://www.inaturalist.org/observations/67005621

Female https://www.inaturalist.org/observations/8490547
Malehttps://www.inaturalist.org/observations/8490577

Female https://www.inaturalist.org/observations/8490546
Male https://www.inaturalist.org/observations/8490548

Female https://www.inaturalist.org/observations/34293509
Male https://www.inaturalist.org/observations/6560837

Female https://www.inaturalist.org/observations/44892361
Male https://www.inaturalist.org/observations/8490568

Female https://www.inaturalist.org/observations/16097725
Male https://www.inaturalist.org/observations/38669966

It is unlikely that this dimorphism in O. montana is directly related to differences between the sexes in the height of foraging.

This is because Ourebia

• depends on low, herbaceous plants throughout the year, being the smallest specialised grazer among all the ungulates of the world, and
• forages only quadrupedally, and does not rise to bipedal postures to seek food above its normal reach.

However, there are potential advantages of the absolutely and relatively longer neck of females of O. montana, in vigilance.

This would be understandable, given that

• typical habitats of Ourebia contain patches of relatively tall grass (about 1 m high), in which the animals tend to rest around midday (https://www.alamy.com/africa-kenya-northern-serengeti-plains-maasai-mara-oribi-wild-ourebia-ourebi-hiding-in-grass-habitat-the-sole-member-of-its-genus-image387309325.html?imageid=F62EBFE4-0DD6-4164-AB70-35E1BDDB505D&p=88978&pn=1&searchId=b51320e4aa8f23d8475c9a13e60a54fb&searchtype=0), and
• O. montana may differ from other small-bodied ruminants in that, when lying down to hide, it is said to keep the neck upright (although Estes, 1991, states on page 62 that it puts the head down; possibly this refers only to O. ourebi).

A possible explanation for the absolutely longer torso in females than in males is as follows.

Grass-eating ruminants tend to have capacious stomachs, compared to those eating mainly the foliage of shrubs, which tends to be more nutritious than grass.

Ourebia montana is at the lower limit of body mass at which specialisation on grazing can be sustained. Therefore, there is likely to be particular pressure on the gestating and lactating sex, to accommodate the necessary 'bulk and roughage' in the gut.

The additional bulk could be accommodated by either an increase in the girth of the torso, or an increase in its length. The latter may possibly be more compatible with the extreme locomotion in this gracile, lightly-built antelope.

The following illustrations reinforce the impression that, in O. montana, the neck is not only absolutely, but also relatively, longer, in females than in males.

https://www.inaturalist.org/observations/7994502

scroll in https://www.zoochat.com/community/threads/ungulate-taxonomy-revisited-the-evidence-for-the-splits-of-g-g.467230/page-2#post-1046851

https://www.shutterstock.com/image-photo/sexual-dimorphism-oribi-antelope-only-bucks-2040799799
https://www.shutterstock.com/image-photo/sexual-dimorphism-oribi-antelope-only-bucks-2040800168

https://www.alamy.com/stock-photo-oribi-ourebia-ourebi-maze-national-park-ethiopia-85989749.html?imageid=6645CFFF-4910-460B-86C0-CC35B462628D&p=22059&pn=3&searchId=b18d9220987fab88ffb57be4cfa504ab&searchtype=0

https://www.alamy.com/stock-photo-oribi-ourebia-ourebi-murchison-falls-national-park-uganda-92320381.html?imageid=0589FEED-640D-432F-BA8E-3EF832819C00&p=22059&pn=3&searchId=b18d9220987fab88ffb57be4cfa504ab&searchtype=0

https://www.inaturalist.org/observations/131699606

In the past, it was assumed that Ourebia belonged to Antilopini. This was partly because it seemed to provide a link, in body size and habits, between 'neotragin' antilopins and gazelle-like antilopins.

However, there has been a growing realisation that Ourebia deserves its own tribe (Ourebini), among Bovidae.

My findings, in this Post, seem to support the distinctiveness of Ourebia.

This is partly because one of its spp., namely O. montana, seems to emerge as unique in two, related ways, viz.

• it shows masculine diminution, despite retaining the polygamy typical of specialised grazers, and
• this diminution is in not only body mass, but also the lengths of torso and neck relative to body mass.

Furthermore, I have noticed another odd combination, in Ourebia, corroborating its phylogenetic distinctiveness.

This is the combination of stotting behaviour by adults and extreme hiding by juveniles.

Ourebia is small-bodied among ungulates that stot. However, Madoqua kirkii damarensis is half as massive (body mass 5 kg) and stots, as follows: "In reaction to sudden fright they may...bound away stiff-legged, stotting, their legs tucked up under their bodies, at each contact with the ground giving a short explosive whistle" (Smithers and Chimimba 2005). What is noteworthy is that, in M. kirkii, infants hide for only three weeks.

Most bovids that show stotting behaviour tend to have infants that hide for only a few weeks. Furthermore, in these spp., it is infants and juveniles that are most likely to stot. (Eudorcas and Antidorcas are typical examples.)

In Ourebia, the hiding period is extended beyond infancy into the juvenile phase, until up to three months old. I assume that juveniles forage at night. However, their hiding by day remains noteworthy, given that adults forage mainly diurnally.

So, does this make Ourebia the only grazing ungulate, on Earth, that hides like an infant until it is nearly half-grown?

Furthermore, it seems possible that, in Ourebia, juveniles are less likely to stot - even in play - than adults are. Does this make Ourebia the only ungulate, on Earth, in which adults stot more frequently than juveniles do?

In summary, here are the anomalous aspects of Ourebia, particularly O. montana, starting with those most supported by the evidence, so far:

• smallest-bodied ungulate on Earth that is specialised for grazing,
• only ungulate with male diminution despite polygamy,
• only ungulate with infantile hiding extended to the juvenile phase,
• only ungulate with male diminution in lengths of torso and neck, relative to body mass, and
• only ungulate in which adults stot more frequently than juveniles do.

Posted on September 15, 2022 12:37 AM by milewski