Is the common hippopotamus (Hippopotamus amphibius) analogous with orangutans (Pongo spp.) in sexual bimaturism? part 1

@markdeeble

(Dear Naturalists, before you read this Post, please realise that - contrary to the impression given in the literature - the following is the 'real' sexual dimorphism:
adult female https://www.inaturalist.org/observations/130503280
fully mature male https://www.inaturalist.org/observations/131393008
together https://www.gettyimages.com.au/detail/news-photo/two-los-angeles-zoo-hippopotamuses-were-transported-tuesday-news-photo/563551061?adppopup=true and https://www.inaturalist.org/observations/182685912 and https://www.gettyimages.com.au/detail/photo/hippopotamus-pair-eating-grass-in-national-safari-royalty-free-image/521352284?phrase=hippo+zoo&adppopup=true and https://www.alamy.com/stock-photo-hippopotamus-at-haller-park-in-mombasa-kenya-22825689.html?imageid=EF0770FF-1288-4768-BB13-927BA0955796&p=2963&pn=1&searchId=5b66b8a6ffa55eec2d24512dc0c4aaa7&searchtype=0 and at minutes 4:00 and 5:00 in https://www.google.com.au/search?sca_esv=ea2cb3181ae7dcdc&sxsrf=ADLYWIKrH74FcLWCSbPkFH_MN-SOiTgFYA:1724375529805&q=Hippo+zoo+berlin&tbm=vid&source=lnms&fbs=AEQNm0BKxFXqFZETuC92mLOmXO9xJMdcEc6vsS8xotR_o6JIE4V6fjYfCiBijvGcXvcw0A1foGVwwGEV12VfBpqJrQOcY1lJEcwoWHBLO3iBa7iZ5QeKaux7sBp-0kIsG2kUeAyUyzSAO-CZcgzBYoFkfzufWRMKGShC9yzL_fiVgQjXh-cDB0Dw_GCPhaIOVi8Sfx8gJtP8O5qwqhDugIYu7nYGZD8J6w&sa=X&ved=2ahUKEwi_hKe394mIAxVNSGcHHbYHEDIQ0pQJegQIDhAB&biw=1004&bih=537&dpr=2.7#fpstate=ive&vld=cid:afd86476,vid:mj62wbabXhQ,st:0).

INTRODUCTION

The common hippopotamus (Hippopotamus amphibius) presents a sociosexual paradox, particularly w.r.t. sexual dimorphism (https://en.wikipedia.org/wiki/Sexual_dimorphism).

This is because it combines extreme polygyny (https://www.merriam-webster.com/dictionary/polygyny) with what seem to be only small differences in body size and shape between females and males.

In general among large mammals, the more polygynous the species, the more sexually dimorphic it tends to be (https://www.perplexity.ai/search/among-large-mammals-which-are-P0S8kU_wSxq3fN_jn2MA9A and https://onlinelibrary.wiley.com/doi/pdf/10.1002/ece3.6246 and https://onlinelibrary.wiley.com/doi/pdfdirect/10.1111/j.0014-3820.2002.tb01438.x and https://www.evolutionary-ecology.com/abstracts/v01/1019.html and https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7297761/).

However, sexual dimorphism in the common hippopotamus seems so limited that it is often difficult to tell the sexes apart (https://core.ac.uk/reader/511307320 and https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8492169/ and https://research.bangor.ac.uk/portal/en/theses/sexual-dimorphism-in-the-common-hippopotamus(755f4355-916f-4340-be69-2a0283ea0037).html and https://royalsocietypublishing.org/doi/10.1098/rsbl.2021.0368 and https://core.ac.uk/reader/511307320 and https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8492169/
and https://www.proquest.com/openview/bf7fe20014d547603c96a18e52506e58/1?pq-origsite=gscholar&cbl=44156 and https://www.youtube.com/watch?v=ct910mx2c8w and https://research.bangor.ac.uk/portal/files/34590613/Sandler_Mres_2020Sandler_MRes_2020_1.pdf and https://www.inaturalist.org/posts/96927-is-the-common-hippopotamus-hippopotamus-amphibius-analogous-with-orangutans-pongo-spp-in-sexual-bimaturism#activity_comment_6175c422-b596-4675-816f-ae658f17f257).

Furthermore, the incisor and canine teeth, although extremely odd among ungulates, have a similar appearance in the two sexes (https://www.inaturalist.org/posts/96927-is-the-common-hippopotamus-hippopotamus-amphibius-analogous-with-orangutans-pongo-spp-in-sexual-bimaturism#activity_comment_71bbd753-64ad-49d7-aa91-abbf419d0618 and https://www.perplexity.ai/search/describe-the-sexual-dimorphism-l9kejg52TsmP5RjO38mZrA).

Given that the common hippopotamus is polygynous (https://ielc.libguides.com/sdzg/factsheets/hippopotamus/reproduction and https://animaldiversity.org/accounts/Hippopotamus_amphibius/), how can this puzzle be resolved?

SEXUAL DIMORPHISM RELATIVE TO TERRITORIALITY

In the habitat of the common hippopotamus, masculine rivalry in the various large-bodied artiodactyls - all of them polygynous - is based either on

non-territoriality, in which mating success of males depends mainly on status conferred by brawn and bone, or
territoriality, in which mating success of males depends mainly on status conferred by temporary tenure of foraging grounds attractive to females.

Conforming to the category of non-territoriality are the following examples, both of which show extreme sexual dimorphism:

the African bush elephant (Loxodonta africana, https://www.facebook.com/watch/?v=125347847483926 and https://www.alamy.com/stock-photo-elephants-mating-loxodonta-africana-showing-the-bull-mounting-a-cow-15405326.html), and
the common eland (Taurotragus oryx, https://www.alamy.com/stock-photo-common-eland-tragelaphus-oryx-male-and-female-19011598.html and https://www.inaturalist.org/observations/177150984 and https://www.researchgate.net/figure/a-An-adult-eland-bull-left-and-female-in-a-tending-bond-Note-the-black-facemask_fig4_272891284 and https://africageographic.com/stories/eland/).

Conforming to the category of territoriality are alcelaphin bovids (https://en.wikipedia.org/wiki/Alcelaphinae), which show minimal sexual dimorphism (https://www.sciencedirect.com/science/article/abs/pii/016815919190264X and https://zslpublications.onlinelibrary.wiley.com/doi/full/10.1111/jzo.13134 and https://zslpublications.onlinelibrary.wiley.com/doi/full/10.1111/jzo.13134).

The common hippopotamus is territorial in a certain sense, and non-territorial in another.

This is because it differs from alcelaphins and other artiodactyls in that its territoriality is confined to the watery refuges of the species, where there is negligible food. The territories completely exclude the extensive lawns on which the species forages, which are free-for-all.

Therefore:
Whichever way the comparison is made, it is anomalous that the common hippopotamus shows minor sexual dimorphism despite its major polygyny. Past studies of the species have failed to resolve this puzzle.

A NEW PERSPECTIVE ON THE PUZZLE

Other authors may have missed a crucial point, which means that the limited sexual dimorphism in of the common hippopotamus is misleading.

The point is that this species may show a kind of 'arrested/suppressed development' w.r.t. the full potential of secondary sexual features of masculinity.

The common hippopotamus is remarkably long-lived for an artiodactyl (https://www.inaturalist.org/posts/97769-lifespan-relative-to-fecundity-in-suids-suidae-compared-with-bovids-bovidae#activity_comment_fbdd32c5-28a4-4f37-8a58-3ae3f00b2ac4 and https://www.gbif.org/species/113274921#:~:text=The%20genitals%20of%20the%20female,function%20of%20these%20is%20unknown).

Maximum lifespan has been recorded as about 65 years in females, and about 62 years in males (https://www.zoochat.com/community/threads/oldest-common-hippopotamuses-in-captive-care.484137/).

However, most adult males in any population remain sexually subordinate, and this may persist for the whole lifetime.

In the sociosexual system of the common hippopotamus,

territory-holders have priority in mating with females, and are presumably the males most attractive to females,
a remarkably small percentage of adult males acquire territories, and
the territorial tenure of a given male individual can be remarkably protracted, up to several decades, so that many/most other males never get a turn to reproduce.

It is possible that, in the common hippopotamus, success in claiming a territory leads to a change in the hormonal status of the individual male.

This then produces additional growth in the body, particularly the head, and even more particularly the lower jaw and its canine and incisor teeth. These teeth remain similar to those of females in shape, but enlarge, both absolutely and relative to the size of the body.

This additional mass enables the territory-holder to prevail continually over the many would-be rivals.

This system would, metaphorically-speaking, be a case of 'nothing succeeds like success', corresponding approximately to 'sexual bimaturism' (https://en.wikipedia.org/wiki/Sexual_bimaturism).

Renewed post-adult growth would, over several years, produce males obviously different in appearance from most adult males in the population. These individuals, although unlikely to be reflected in the many males sampled in culling programmes such as that studied by Laws in Uganda (https://www.researchgate.net/publication/229967272_Dentition_and_ageing_of_the_hippo), would represent the true degree of sexual dimorphism intrinsic to the common hippopotamus.

Most adult males in any population of the common hippopotamus have body mass only modestly exceeding that of adult females. Once the 'arrestment/suppression' of full masculine development is alleviated by territorial success, body mass may increase to double the average for adult females.

POSSIBLE ANALOGY WITH ORANGUTANS

'Sexual bimaturism' has a precedent in orangutans (Pongo spp., https://www.perplexity.ai/search/orangutans-show-bimaturism-whi-8dKevPaeTGKXmx8Crw90Mw and https://www.cambridge.org/core/books/abs/sexual-selection-in-primates/alternative-male-reproductive-strategies-male-bimaturism-in-orangutans/3BF0810A3FFD8D8930E3670B2AD453A3 and https://www.researchgate.net/publication/284553386_Alternative_male_reproductive_strategies_Male_bimaturism_in_orangutans and https://academic.oup.com/beheco/article/13/5/643/327741?login=false and https://d1wqtxts1xzle7.cloudfront.net/54459928/Marty_et_al-2015-Orangutan_bimaturism-libre.pdf?1505715811=&response-content-disposition=inline%3B+filename%3DEndocrinological_correlates_of_male_bima.pdf&Expires=1722723757&Signature=Bjbdyc1fG37eNCtWdr5o6qy-IgklUJ-TY6DOfsscWfI5Qx65EtR6W41yk3T9khxzoK-y~od1gXLszM7Uy70TYpPSxIcsc5ce0EHNHWZf-KPd9OPa4m9NEHwPXSjtG8yY~iVwvD6daWOczjhOU5fVA0Jc0SX2FVy90vG1KWRruzJyTFKfpuwHWiMZO3HB-6sZsSC3NlwEFusa~Ww0gTJKX3XsxdRveOv0qiIuTOFkHzfSRAIm9kxRm6m-bZVw3SbevS2MzNMbPqOMGR431xscWLU01pNQ5aSCRH8jGoUejkJCPuFyiZxfaHBKUQUkukqWRFhuKB-Z35uC~L3Jb-ujLA__&Key-Pair-Id=APKAJLOHF5GGSLRBV4ZA and https://rucore.libraries.rutgers.edu/rutgers-lib/61912/#:~:text=DescriptionBimaturism%20in%20orangutans%20is,physiological%20status%20regulate%20flange%20development. and https://www.researchgate.net/figure/Sexual-bimaturism-in-orangutans-Flanged-A-and-unflanged-B-male-orangutans-from_fig3_232281829 and https://www.eva.mpg.de/documents/Springer/Banes_Male_BehEcoSoc_2015_2191410.pdf and https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10857694/ and https://scholarscompass.vcu.edu/uresposters/241/ and https://www.sciencedirect.com/science/article/abs/pii/S0047248406001795 and https://onlinelibrary.wiley.com/doi/full/10.1002/ajp.23535).

In this phenomenon, full masculine maturity is 'condition-dependent'. However, condition itself depends on success in rivalry, leading to a kind of positive feedback, mediated hormonally (https://pubmed.ncbi.nlm.nih.gov/26235914/).

Furthermore, there is an analogy between the common hippopotamus and orangutans in that the additional developments in masculine anatomy occur on the head, jowls, and neck.

It is noteworthy that both the common hippopotamus and orangutans are remarkably long-lived.

However, the term 'sexual bimaturism' has been used in an unsatisfactory way in the past, for the following two reasons:

Firstly, there has been conflation between the pattern seen in orangutans and that in mammals (possibly including the African bush elephant and gorillas, Gorilla spp.) which simply take a long time to reach full masculine maturity via indeterminate growth. One way to think of the distinction is between 'differentially retarded masculine maturity' and 'delayed masculine maturity'.
Secondly, even if strictly defined, bimaturism has been an awkward category, in the sense that it has contained only one genus of mammals, namely Pongo.

With the search-image created in this Post, it may be worth re-examining the sociosexual system of the common eland, which possibly also conforms with bimaturism rather than simply indeterminate growth and delayed masculine development.

ADULT FEMALES

The following show the proportionately small head (particularly the lower jaw) of females throughout life:

https://www.inaturalist.org/observations/185587511

https://www.inaturalist.org/observations/171876695

https://animals.howstuffworks.com/animal-facts/is-hippo-milk-pink.htm

Second photo in https://www.inaturalist.org/observations/197828198

https://www.inaturalist.org/observations/32076998

https://www.inaturalist.org/observations/190127894

https://www.inaturalist.org/observations/218461874

https://www.inaturalist.org/observations/3820827

https://www.inaturalist.org/observations/57170046

https://www.dreamstime.com/generated-image-image316220977

https://en.wikipedia.org/wiki/Hippopotamus#/media/File:Portrait_Hippopotamus_in_the_water.jpg

https://www.inaturalist.org/observations/232962788

https://www.inaturalist.org/observations/227720326

https://www.inaturalist.org/observations/205936385

https://www.inaturalist.org/observations/201224495

https://www.inaturalist.org/observations/197162402

https://www.inaturalist.org/observations/194695310

https://www.inaturalist.org/observations/190764350

https://www.inaturalist.org/observations/190054337

https://www.inaturalist.org/observations/186526305

https://www.inaturalist.org/observations/183622070

https://www.inaturalist.org/observations/183310834

https://www.inaturalist.org/observations/182837999

https://www.istockphoto.com/photo/animal-hippopotamus-mother-baby-wildlife-africa-care-nature-safari-river-gm925368832-253942777?searchscope=image%2Cfilm

https://www.inaturalist.org/observations/172773350

https://www.inaturalist.org/observations/168698847

https://www.inaturalist.org/observations/167197548

https://www.inaturalist.org/observations/161474315

https://www.inaturalist.org/observations/149918549

https://www.inaturalist.org/observations/125411308

https://www.inaturalist.org/observations/91239857

https://www.inaturalist.org/observations/88626882

https://www.inaturalist.org/observations/86568312

https://www.inaturalist.org/observations/69880035

https://www.inaturalist.org/observations/58818629

https://www.inaturalist.org/observations/57079975

https://www.inaturalist.org/observations/51625751

https://www.inaturalist.org/observations/9944882

https://www.inaturalist.org/observations/9400962

https://www.inaturalist.org/observations/8926835

https://www.inaturalist.org/observations/29037

https://www.inaturalist.org/observations/224186883

https://www.inaturalist.org/observations/196988622

https://www.inaturalist.org/observations/117092368

MALES, ADULT BUT WITH HYPOTHETICALLY 'ARRESTED/SUPPRESSED' DEVELOPMENT OF SECONDARY SEXUAL FEATURES

The following show that many of the adult males, although they possess an enlarged head (particularly the lower jaw), are so similar to adult females that they can be confused with adult females:

https://www.istockphoto.com/photo/isolated-hippopotamus-on-white-background-gm528295822-92931099?searchscope=image%2Cfilm

https://www.inaturalist.org/observations/199820452

https://www.inaturalist.org/observations/200792328

https://www.inaturalist.org/observations/61636965

https://www.inaturalist.org/observations/99321313

https://www.inaturalist.org/observations/104349123

https://www.inaturalist.org/observations/9398378

https://depositphotos.com/photo/hippopotamus-hippopotamus-amphibius-grazing-bull-evening-light-bank-chobe-river-381793214.html

https://upload.wikimedia.org/wikipedia/commons/8/84/Hippopotamus_in_Chobe_National_Park_02.jpg

https://www.inaturalist.org/observations/104195832

https://www.gettyimages.com.au/detail/photo/single-hippopotamus-and-zebras-on-the-savannah-royalty-free-image/948895248?phrase=grazing+hippopotamus&adppopup=true

https://www.facebook.com/alexwalkersserian/photos/a.853569811352767/2877290685647326/?type=3

https://www.gettyimages.com.au/detail/photo/hippo-at-wild-walking-on-dirt-road-royalty-free-image/1157831488?phrase=grazing+hippopotamus&adppopup=true

https://www.dreamstime.com/stock-photo-wounded-hippopotamus-roaring-square-photograph-old-bull-large-round-wound-gaping-mouth-glaring-eye-one-image84144073

https://www.istockphoto.com/photo/hippo-displaying-territorial-behavior-mpumalanga-south-africa-gm166553772-23575755

https://upload.wikimedia.org/wikipedia/commons/thumb/b/b3/Hipop%C3%B3tamo_(Hippopotamus_amphibius),parque_nacional_de_Chobe,_Botsuana,_2018-07-28,_DD_82.jpg/640px-Hipop%C3%B3tamo(Hippopotamus_amphibius),_parque_nacional_de_Chobe,_Botsuana,_2018-07-28,_DD_82.jpg

https://www.inaturalist.org/observations/103993366

https://www.inaturalist.org/observations/103148015

https://www.inaturalist.org/observations/44380745

https://www.inaturalist.org/observations/16316895

https://www.inaturalist.org/observations/217026021

https://www.inaturalist.org/observations/227460909

https://www.inaturalist.org/observations/222033303

https://www.inaturalist.org/observations/217475755

https://www.inaturalist.org/observations/216525950

https://www.inaturalist.org/observations/207187846

https://www.inaturalist.org/observations/205639372

https://www.inaturalist.org/observations/204704697

https://www.inaturalist.org/observations/199328894

https://www.inaturalist.org/observations/198598844

https://www.inaturalist.org/observations/196988605

https://www.inaturalist.org/observations/196813314

https://www.inaturalist.org/observations/196137731

https://www.inaturalist.org/observations/195619422

https://www.inaturalist.org/observations/194629109

https://www.inaturalist.org/observations/190190748

https://www.inaturalist.org/observations/134265024

https://www.inaturalist.org/observations/127495774

https://www.inaturalist.org/observations/118180804

https://www.inaturalist.org/observations/186577453

https://www.inaturalist.org/observations/185479171

https://www.inaturalist.org/observations/182083121

https://www.inaturalist.org/observations/179594302

https://www.istockphoto.com/photo/hippopotamus-gm485563607-38094122?searchscope=image%2Cfilm

https://www.inaturalist.org/observations/170846673

https://www.inaturalist.org/observations/169650595

https://www.inaturalist.org/observations/187129711

https://www.inaturalist.org/observations/190190748

FULLY MATURE (PRESUMABLY TERRITORY-HOLDING) MALES

The following show that some males, in the same populations, have the head (particularly the lower jaw) and neck so enlarged that their masculinity is immediately noticeable (I suspect that their body mass is also greatly increased, to about three tonnes in contrast to the approximately 1.5 tonnes of most adults in the population):

https://www.inaturalist.org/observations/198786138

https://www.inaturalist.org/observations/110526650

https://www.inaturalist.org/observations/13772994

https://www.inaturalist.org/observations/110522141

https://www.inaturalist.org/observations/45468532

https://www.inaturalist.org/observations/91794820

https://www.inaturalist.org/observations/85734913

https://www.inaturalist.org/observations/77513963

https://www.inaturalist.org/observations/46601791

https://www.inaturalist.org/observations/70166024

https://www.inaturalist.org/observations/68351235

https://www.inaturalist.org/observations/38631403