Is the common hippopotamus (Hippopotamus amphibius) analogous with orangutans (Pongo spp.) in sexual bimaturism? part 2

...continued from https://www.inaturalist.org/journal/milewski/96927-is-the-common-hippopotamus-hippopotamus-amphibius-analogous-with-orangutans-pongo-spp-in-sexual-bimaturism-part-1#

Sexual dimorphism in the common hippopotamus is complicated and subtle. This is because of the following combination:

• the lower canine teeth are absolutely much larger in males (even when still 'arrested/suppressed' in development) than in females,
• this is an insufficient clue to sex in the field, because the mandible (lower jaw) also differs in size between the sexes, which means that the relative sizes of the lower canines remain similar (https://www.inaturalist.org/observations/102547968), and
• the absolute size of the lower canines is probably greater again in territory-holding (physically fully-developed) males (https://www.gettyimages.in/detail/photo/close-up-of-hippopotamus-eating-royalty-free-image/948309904?adppopup=true), but this has yet to be measured and is a 'blind spot' in the literature.

The aim of this, part 2, is to describe precisely the difference in the size of the lower canines and mandibles in females vs males (excluding the few males that have succeeded in becoming territorial).

The following is a good reference for the ontogeny (https://en.wikipedia.org/wiki/Ontogeny) of sexual dimorphism in the lower canines of the common hippopotamus.

Pienaar et al. (1966, https://koedoe.co.za/index.php/koedoe/article/view/778/0 and https://www.researchgate.net/publication/47296377_An_experimental_cropping_scheme_of_Hippopotami_in_the_Letaba_river_of_the_Kruger_National_Park and https://knowledge.caribencana.id/detail/eyJpdiI6Img1NEJpTTFvUVZNck1JeDJtTlo5MlE9PSIsInZhbHVlIjoiZkptaXpvNVUrQW1HTTlVVjNXR0VGQT09IiwibWFjIjoiMjdiOTU0MTlkOTVmNWM2Y2Q1ODc2NmZiMDhiZjlhM2MzODAwYWNmNWM5NTk4ZDcyYzc0NWQ4NjE3ZTFiNDU1OSJ9) state:

"The lower canines are cut at about four months and attain a length of about 2.5-3.8 cm during the following twelve months. At two years these canines measure about 6.35 cm in females and from [sic] 10.2-11.4 cm in males. At three years they are about 10.2-11.4 cm in females and 12.7-14 cm in males."

Page 29:

Length of lower canines in adult males (n=32) ranged from 12.7 cm to 31 cm. Within this sample (which seems not to include any territorial individuals), asymptotic length (23-31 cm) of the lower canines is reached when male body mass reaches 1400 kg.

Page 30:

Length of lower canines in adult females (n=36) ranged from 7.6 cm to 20.3 cm, and most values are 12.7-16.5 cm. Within this sample, asymptotic length (about 15 cm) of the lower canines is reached when body mass reaches 1130 kg.

My commentary:

Even in those male individuals (by far the majority of the male population) that have not expressed the full masculine growth, the lower canines are absolutely far longer (about 27 cm) than those of females (about 15 cm).

However, this hardly helps with the sexing of the animals in the field. This is because the lower jaw of males is also larger in males than in females.

I scrutinised the data in order to compare the length of the lower canines in the largest individuals, among female adults, which happen to that have similar body mass to relatively small individuals among adult males.

I found the values to be about 15 cm in females and about 24 cm in males. This is certainly sexually dimorphic. However, it is not dimorphic enough, overall, to make the sex of most adults distinguishable in the field.

The following is another relevant reference.

Laws (1968, https://www.researchgate.net/publication/229967272_Dentition_and_ageing_of_the_hippo and https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1365-2028.1968.tb00899.x and https://www.semanticscholar.org/paper/DENTITION-AND-AGEING-OF-THE-HIPPOPOTAMUS-Laws/fb59f56b17cd2d5afbb472621de5b643617e7309) studied the teeth and mandibles of the common hippopotamus in great detail, relative to age from birth to 45 years old.

Laws (1968) found the following.

Mass of lower canine teeth (sample sizes: females n=82, males n=77):

Page 38:
"In the male growth is rectilinear up to about 25 years (corresponding to a mean weight of 2.0-2.1 kg), but subsequently declines, probably due to breakage and wear, so that the mean weight falls slightly. In the female the growth rate appears to be similar to the male growth rate up to about three years, but is thereafter slower. It has an approximately rectilinear pattern up to an age of about 20-25 years (corresponding to a mean weight of 1-1.2 kg), later levelling off between 1.1 and 1.2 kg. Thus, if age is taken into account, the female canines at full maturity are slightly less than half the weight of the male canines."

The difference between females and males in the size of the canines and incisors does not appear until about 8 years old.

Mass of the mandible (sample sizes: females n=87, males n=85):

Pages 36-37:
"There is a very obvious sex difference in jaw growth and size. Up to...seven years...growth is similar in the two sexes, but the female growth rate declines while the male mandible continues to increase at a steady rate up to about 25 years. In both sexes there is a decline in mandible weight after 25 years. In the male this results in a decrease in weight of some 5 kg from 25 to 40 years, whereas in the female the corresponding decrease is much smaller - about 0.8 kg. The male weight change represents a decrease of 33 per cent., the female only 8.5 per cent. This decrease in the male is not due solely to breakage and wear of teeth - the combined effect of this would not exceed 2.5 kg...and it is necessary to assume that there is a considerable resorption of bone at these higher ages, at least in the male."

Figure 10, on page 36, shows that, at about 30 years old, the mass of the mandible in (non-territorial) males is nearly double that in females.

Growth in length of the mandible (sample sizes: females n=87, males n=83):

Page 37:
"Although the male jaw lengths at age are slightly higher than the female values at ages above...11 years...there is a considerable amount of overlap."

By 40 years old, the mandible has begun to shrink in length, and "jaw feels very light for its size owing to resorption of bone, particularly in the male'. By 43 years old, 'roots of [molar] teeth reduced by resorption to small stumps."

My commentary:

The data in Laws (1968) suggest to me that

• males can live their whole, long lives without fully expressing the secondary sexual potential of the mandible - including the lower canines - in mass and length, and
• this 'suppressed/arrested' state applies to most male individuals in the population.

It seems that Laws (1968), like most other authors, lacked any search image for 'sexual bimaturism' in the common hippopotamus. His otherwise thorough study thus missed any true representation of a crucial component of the population, viz. territory-holding males.

The following illustrate the sizes of the lower canine teeth in the common hippopotamus:

Adult female
https://www.gettyimages.in/detail/photo/close-up-of-crocodile-on-field-royalty-free-image/1584114188?adppopup=true
https://www.gettyimages.in/detail/photo/hippopotamus-yawning-royalty-free-image/84304415?adppopup=true
https://www.gettyimages.in/detail/photo/hippo-with-open-mouth-royalty-free-image/522198598?adppopup=true
https://www.gettyimages.in/detail/news-photo/regular-visitor-to-london-zoo-captain-pfeiffer-placing-his-news-photo/2635009?adppopup=true
https://www.gettyimages.in/detail/photo/wild-bull-hippo-breaching-in-south-africa-royalty-free-image/1464135240?adppopup=true
https://www.alamy.com/hippo-showing-its-teeth-hippopotamus-amphibius-image7445976.html
Adolescent female
https://www.gettyimages.in/detail/news-photo/hippopotamus-tetsuo-opens-his-mouth-to-let-his-teeth-news-photo/475393128?adppopup=true

Adult male
https://www.gettyimages.in/detail/news-photo/hippopotamus-or-hippo-hippopotamidae-news-photo/182129887?adppopup=trueAdolescent male
https://www.dailymail.co.uk/news/article-1246797/Open-wiiiide--Orion-hippopotamus-born-Pablo-Escobars-ranch-visit-dentist.html
https://www.gettyimages.com.au/detail/news-photo/zoo-worker-jorge-aguirre-prepares-an-hippo-named-orion-for-news-photo/96209895
Scroll to third photo in https://www.washingtonpost.com/climate-environment/2021/01/11/invasive-hippos-escobar-colombia-castrate/
https://www.gettyimages.in/detail/news-photo/zoo-worker-jorge-aguirre-makes-a-dental-procedure-to-an-news-photo/96292892?adppopup=true
https://www.gettyimages.co.nz/detail/news-photo/zoo-worker-jorge-aguirre-prepares-an-hippo-named-orion-for-news-photo/96209870
https://www.gettyimages.com.au/detail/news-photo/zoo-worker-jorge-aguirre-jumps-a-fence-during-a-dental-news-photo/96210087

Also see
https://www.gettyimages.in/detail/news-photo/happy-the-hippo-has-his-tusks-filed-by-keeper-anthony-news-photo/143313307?adppopup=true
https://www.gettyimages.in/detail/news-photo/happy-the-hippo-has-his-tusks-filed-by-keeper-anthony-news-photo/143313019?adppopup=true

Publicado el agosto 19, 2024 07:21 TARDE por milewski