Okay, this article is not really about Canadian rewilding efforts, or really Eurasian ones either. This will focus more on the possibilities of using rhinos in warmer places such as the United States, Mexico, and areas of Central and South America. Although rhinos and other large thick-skinned herbivores have been historically present in the colder areas of the world, it is my impression that it would be impractical and quite possibly inhumane to use any extant rhino species in such areas, due to their lack of thick fur or significant levels of subcutaneous fat. That being said, if some compromise can be created in which they are only used in the warmer times of the year, with other accommodations being provided in the colder times, their potential range may be greatly expanded. This would certainly still be preferable to an entirely captive situation for these animals.
To provide some background information on the history of rhinos, they are perissodactyls, odd-toed herbivores of the same order as horses and tapirs, a very old lineage originally from North America. Rhinos evolved here, but have not been present since about four and a half million years ago, long after becoming successful in Eurasia and Africa. Today, there are five living species, of four different genera. These include the African white and black rhinos (Ceratotherium simum and Diceros bicornis), the Indian and Javan rhinos (Rhinoceros unicornis and Rhinoceros sondaicus), and the Sumatran rhino (Dicerorhinus sumatrensis). The white rhinoceros is considered "near-threatened", and the Indian rhinoceros is considered "vulnerable". The other three species are all considered "critically endangered", with very small populations, especially for the Javan and Sumatran species.
Rhinos are of interest for conservation in the Americas because they have similar ecological roles to several large-bodied herbivores. These include toxodonts, glyptodonts, and various ground sloths. Toxodonts were large, rhino-like members of a now extinct order called the notoungulates, South American relatives of the perissodactyls, one member of which (Mixotoxodon larensis) made it as far north as the southern United States. Glyptodonts were giant armadillos with great, turtle-like shells. One species of glyptodon (Glyptotherium floridanum) also made it as far as the southern United States, along with a smaller (but still quite large) armadillo species called the American giant armadillo (Holmesina septentrionalis). The ground sloths were the most successful of the South American megaherbivores to migrate to North America, with four species settling there. These were the Shasta ground sloth (Nothrotheriops shastensis), American giant ground sloth (Eremotherium laurillardi), and Harlan's ground sloth (Paramylodon harlani) of the southern states and Mexico, and Jefferson's ground sloth (Megalonyx jeffersoni) which ranged from Mexico to as far north as Alaska.
Now, when finding replacements for extinct or extirpated species, I find there are four important factors to consider, diet, size, habitat, and behaviour. The most important is diet, whether or not these creatures eat the same things and have similar digestive systems. Rhinos, like horses, tapirs, and elephants, are hind-gut fermenters. This means that they can eat greater amounts of food in a day than fore-gut fermenters like bison and elk, but do so less efficiently. This makes them great plant dispersers because many seeds go through their systems unharmed and are planted upon ejection. White, Indian, and Javan rhinos are considered grazers, meaning they eat grasses and forbes. Black and Sumatran rhinos are primarily browsers, eating more abrasive plants such as leaves and twigs. Toxodons are known to have been browsers and most likely had a similar digestive system as close relatives of the perissodactyls (though it is difficult to be sure), but the digestive systems of glyptodonts and ground sloths are not well understood. However, parallels can be made with their extant relatives. For example, tree sloths have complex stomachs that allow them to compartmentalize and digest food, which is similar to hind-gut fermenters which store food in a cecum and also do not regurgitate their food during digestion, this takes a long time and offers little energy. This is probably the reason why there are no small perissodactyls as they would be like tree sloths in their need to have slow movement and metabolism, which would be a weakness for a creature that has no claws to climb trees. Larger animals can use their size as protection and need less nutrition in comparison to their size, which is probably why large ground sloths were more easily able to protect themselves and travel long distances than their arboreal relatives. Ground sloths did however, have a similar diet to tree sloths, eating leaves as the primary staple of their diet. Modern armadillos have very simple digestive systems with no cecum, as they do not typically eat foods that are hard to digest. Glyptotherium has been theorized to be a grazer due to it's teeth morphology, but the exact nature of it's digestive system remains unknown. The jury is still out as to the diet of the American giant armadillo Holmesina, but it seems to be that it was also a grazer.
As to size the following weights are of importance: Mixotoxodon larensis weighed from 2,000 to 3,000 kilograms, Glyptotherium floridanum weighed 1,000 to 2,000 kilograms, Holmesina septentrionalis weighed from 200 to 300 kilograms, Megalonyx jeffersoni weighed from 500 to 1,000 kilograms, Nothrotheriops shastensis weighed from 200 to 300 kilograms, Eremotherium laurillardi weighed from 2,500 to 3,500 kilograms, and Paramylodon harlani weighed from 1,000 to 1,500 kilograms. For comparison: Ceratotherium simum weighs from 1,500 to 3,500 kilograms, Diceros bicornis weighs from 1,000 to 3,000 kilograms, Rhinoceros unicornis weighs from 1,500 to 3,500 kilograms, Rhinoceros sondaicus wighs from 1,000 to 2,500 kilograms, and Dicerorhinus sumatrensis weighs from 500 to 1,000 kilograms.
Rhinos inhabit several kinds of habitats. White, black, and Indian rhinos inhabit grassland and savannah habitats, while the Javan and Sumatran rhinoceroses live in dense forest habitats. Sloths were known to inhabit open and forested places depending on the species, with the Shasta ground sloths found in desert environments, Harlan's ground sloth preferring grasslands, Jefferson's ground sloth preferrring forests, and the giant ground sloth being partial to savannah environments. Mixotoxodonts were mainly grassland creatures, as were glyptodonts and giant armadillos.
As to behaviour, these animals all have something in common, they use their size and durability to avoid predation. They don't often run or hide, and it's really only undefended young that are ever really in danger. They would also all have slow travel times and large home ranges. They are also as far as we know all solitary, not too intelligent creatures without strong social structures.
As to which species should be used as substitutes for what, I propose the following proxies for use in northern Mexico and the southern United States:
Ceratotherium simum: Glyptotherium floridanum
- Much larger but with similar diet
- Similar habitat and behaviour
- Major distributor of grasses
Diceros bicornis: Mixotodon larensis
- Slightly smaller but with similar diet
- Similar habitat and behaviour
- Major distributor of brush plants
As for now these are the only species/proxies that I feel are particularly solid for use in these areas. As sympatric species they are evolved to exist in the same habitat. I feel that a large browser/grazer dichotomy has been not present from the southern states and Mexico since the Pleistocene extinctions, and that a return of such species could trigger a positive ecological change through the proliferation of plant species and the creation of mixed grasslands in more arid regions. Rhinos are also wallowers, which makes them great creators of pools and sandpits which are ideal macroinvertebrate habitats.
I don't feel that any rhino species quite fills the role of any ground sloth due to their inability to reach high foliage, and thus other proxies may be found. They are also much to large to accurately fill the role of the American giant armadillo. I believe (provisionally) that the other three species of rhino not mentioned above might be implemented in South America as proxies for various glyptodonts and toxodonts there.
Possible proxies for various ground sloths that I may investigate include members of the genera Giraffa, Okapia, Litocranius, and Budorcas. Possible proxies to be investigated for the American giant armadillo must be medium sized grazers, but I do not yet have any idea what to use though giant tortoises briefly came to mind.
It is important to note that there was an eighth genera of South American megafauna to become established in North America, and that was Neochoerus, or the American capybara, which might quite conceivably be replaced with modern greater capybaras of which there may already be a feral population breeding in Florida.
I'd like to stress that this isn't about finding perfect replacements for all extinct megafauna. It's about giving damaged environments the raw material necessary to heal ecological wounds and create new ecological relationships, and using ecological history to tell us what has worked in the past.
Picture Credits in order of appearance:
White Rhino: https://www.flickr.com/photos/dkeats/4979320206/in/photolist-8A1jqo-8A1jGN-5G52J-5G13y-p58HXy-6eSHtw-9FrWS7-4aZRZx-a6Cd43-6qhy6J-5pCeyq-5FZJS-qjPR9U-7RDcHu-pcatWj-oARFjb-gvribf-sm6gfR-oQb7dp-cQdq87-55Sg6t-qxvvdq-9CQCAZ-rPZbtM-bu3FHR-rH1rL6-82idXc-rmUa9M-nsigAG-9M5nyU-7RyxsH-5G4WK-8zXcB8-rE1nvR-sDFFpt-pTCxgv-jbXWDu-8os7hc-dRshjZ-bNsovR-cFnJcb-9SAXwN-3NXMKq-nyRh6T-5q8P17-g1N6yt-8SeZs-m2vo8Y-bu3FKX-qyf7k1
Black Rhino: https://www.flickr.com/photos/ixion/3962066901/in/photolist-737Csi-8yfJYR-c9aCKq-pP3Zmy-737Evv-649VM6-o9Wb3m-coZ9U7-opoR7q-coZff9-73bCVL-88ZzMb-coZ3Wy-opoRSd-o9W84N-otbvma-73bAQf-rMX3ho-ordFLs-737E6M-o9W4Cj-coZnmL-coZ8Kf-or9xFe-6PEb8D-coZ6qQ-coZcCu-coZbeC-tY4Fv1-opoZ2U-coZgDY-ordxZq-o9Xfmi-o9W7zj-o9X7GR-dGU3dE-coZ7yo-dQRrYd-21UXit-q8US24-4yLwNw-kXMen4-21V1GR-dK6n6F-7XmNRM-dK6pm8-rram3-dKbfkE-eNp8mx-ietfmR
Indian Rhino: https://www.flickr.com/photos/spencer77/6928399112/in/photolist-byeRF7-5eePqN-5dWDCJ-4CKE9y-5dWDp3-xEURQW-8QuF2b-5dWDdu-5dWDtW-5eePdq-5dWDhy-5dWD5w-5dSjfF-5eePcC-69wSHK-eNp8vx-6PJiLE-ouFuG7-ouG4QQ-7SdAMy-4CFoDM-4HW2kC-oM9H3m-4HRMsg-oM9t1o-5TqzYT-oMbaGK-9hxQeu-5dWDbh-oK9DFC-X7RhW-oLUAsV-61HhLs-5dWDz7-5dWDn3-5dWDvN-5dSjLv-5dWDv3-5eePob-5dSjMH-5dWCYW-5dWE97-5dWDgG-5dWDVW-5dWE5S-5earF6-5earQr-5earup-5dWDSu-5dSjmi
Javan Rhino: no recent pictures available
Sumatran Rhino: https://www.flickr.com/photos/rhinosirf/7435080178/in/photolist-5WJV9K-bAheCn-bX7jfC-c4kEuS-bAheCt-5wWwNW-ck1XuL-ck1HRd-obmwTi-ck24Kh-mkTmFD-ck1KfA-ck1Wi5-ck26oC-ck1HjJ-5WK3r6-ck1LZ5-ck1Lwh-ck1Jiy-ck1YtU-6PG1py
