Biologists have theorized this divergence has happened over the past 100 years, the rodent population on Nevis was introduced unintentionally to St. Kitt’s when people from Nevis began visiting the island by ship in frequency during the 1900’s. With recent environmental conservation laws these trips are now very infrequent and must meet quarantine standards.

This divergence has caused the allopatric speciation of the Nevis Squirrel (Sciurus Nevis) into the St. Kitt’s Tree-climbers (Sciurus Kittus). The lack of frequent voyage between islands and inability of the rodents to swim the distance between them has caused habitat isolation, with each habitat being slightly different the two populations have diverged. The island of St. Kitts is far more mountainous and difficult to traverse, this selection pressure resulted in longer legs being preferable for continuation of the species. St. Kitt’s Tree-climber is able to hop higher and further due to this. This allotropic differentiation is the cause of the inability to bolster the St. Kitt’s population with those from Nevis, they are too distinct genetically to interbreed.

Another reason for the speciation of these two species could be due to behavioral isolation specifically seen in the difference in average time in courtship display. The St. Kitt’s Tree-climber spends 12.6 seconds in courtship whereas Nevi’s Squirrel spends 21.3 seconds. The courtship ritual doesn’t proceed to completion as a result of this. 

Thupm 12C

 

The St Kitts Island rodent (Kittis Sciuridae) has become under threat from human population development on their island. They are generally faster, shorter, higher jumpers from their larger hind legs and have shorter gestation times than their neighbour species on the island of Nevis (Nevius Sciuridae). 

This difference in gestation period and the courtship displays is likely to contribute to the likely Prezygotic reproductive isolation of these two rodent species. This is as a result of habitat isolation. 

 

Additionally, the fur colour of each species is slightly altered. The St Kitts Island rodent has a dark brown coat while the Nevis Island rodents generally have a lighters coloured coat. 

 

It can be inferred from this data and the observed activities of these different species that the altered stature and appearance of the Nevius Sciuridae is because of it mainly choosing to dwell on the ground. The Kittis Sciuridae hold its initial habitat in the treetops on its island and this habitat split is a great differential factor in these two species mutations. 

 

Temporal isolation is also relevant to this study as when places together in St Kitts Island in a effort to boost population never there, these two species were found to not breed at the same time of year resulting in little to no encounters during key mating season. This is a form of Prezygotic reproductive isolation. 

 

In conclusion, clear speciation is visible between these two now very district types of rodent, from their appearance, to their mating and their habitat choice, the geographical spit of these two populations has produced separate species adapted to their similar but slightly different environments and as such, the Nevis island variety would not assist in saving the reduced population on St Kitts Island. 

A hypothesis for this speciation follows below.

During the Pleistocene ice age, a small population of Sciurus igniventris, the northern Amazon red squirrel, migrated across the then-extant land bridge to the area of St Kitts and Nevis, which themselves were likely connected by a land bridge. As the climate grew warmer, the land bridge connecting the island to the mainland began to dissolve, leaving the population of Sciurus igniventris isolated, interbreeding and forming a new ancestral species to form. Towards the end of the last ice age, the land bridge connecting St Kitts and Nevis dissolved, leaving the populations of this ancestral species on their respective islands isolated from one another.

Those squirrels on St Kitts were likely subject to a selective pressure (such as small species size) which encouraged fast movement and long leaping -as shown by their long forelimbs of 4.2 cm as compared to the Nevis red squirrel (3.9cm) – perhaps a more arboreal habitat, or the presence of a predator. Genetic drift changed the duration of their mating displays, as well as possible natural selection in the presence of a hypothetical predator requiring mating to be quicker. The reduced gestation period (29.3 days for St. Kitts 42.7 days for Nevis) compared to the Nevis red squirrel can be explained by the presence of predators, as during gestation the female squirrel tends to be more sluggish and vulnerable to predation.

Those squirrels on Nevis were likely subject to less arboreal habitat and predation, and perhaps increased foods scarcity as compared to the St Kitts red squirrel; the reduced mobility and speed of the Nevis red squirrel reduces its energy requirements, which may be beneficial in an environment where food is scarce. A lack of predators accounts for the longer mating displays, as sexual selection may drive longer time for mating to ensure successful fertilisation, and the longer gestation periods, as a more-developed squirrel being born increases the chance of offspring survival in the food-scarce environment, since it would sooner be able to forage for itself.

In conclusion, the St. Kitts species diverged in morphology and behavior into a more efficient species in terms of physical speed, gestation time and possibly lower body weight. Due to genetic drift, natural selection and geographic isolation leading to allopatric speciation the two species are now unable to breed. 

 

There once was mighty squirrel from the caribbean,

But thousands of years ago

its island split by seas 

Allopatric separation, one species become two

 

Sciuridae Kittus and Sciuridae Nevis 

Looking identical, 

though Kittus was hunted, quick and nimble

Nevis was safe, grown fat and slow

 

When humans started building over their homes

Kittus’ speed was for naught

Today there are few 

To help the humans sought

 

Reproductive isolation, behavioural differences don’t let them copulate

But this the humans did not know

Thinking they looked identical

They missed Nevis was Slow

 

Isolation drove them apart

And natural selection made them change

Each process played its part

The humans attempt to help was thwarted

 

 

 

 

 

 

 

While trying to repopulate the St Kitts island with squirrels by using Nevis island squirrels as breeding stock, I found that the 2 populations perhaps had some fundamental differences under the skin and perhaps started from one population but split into two through ways I will describe. It seems the initial population started out on nevis as Spermophilus Fattus , then perhaps by a land bridge from the past or by humans manually migrating a select population thousands of years ago, a new species occurred on St Kitts, Spermophilus Fittus. The Fattus squirrel has shorter limbs, can’t jump as high, and moves slower than the Fittus squirrels. This difference could be attributed to Nevis having flatter terrain so food is on the ground and easy to reach, while St Kitts is rugged and mountainous meaning the squirrels there needed to move faster and be able to scale terrain to get food. The initial population evolved into this fitter squirrel by way of natural selection, where those who couldn’t reach food died, and the ones who could passed on this ability to offspring. The 2 now different species wouldn’t have interbred after the initial; migration due to habitat isolation where they couldn’t physically reach each other due to the body of water between the island. Now they are in contact again through my re-population effort, they won’t breed and produce offspring. This could be attributed to Behavioral isolation as the Fattus squirrels have a different length mating dance to the Fittus, meaning they won’t be attracted to each others dance and won’t mate. 

The St Kitts (Sciurus speedus) rodent and Nevis rodent (Sciuris rodentus) live on separate islands on the West Indies due to separation of the islands, this resulted in allopatric speciation. 

 

Due to the island’s vast differences in landscape, it caused these rodents to go down different evolution paths, for instance the St Kitts (Sciurus speedus) lives on the St Kitts island which has much more rough and mountainous terrain to it meaning longer hind limbs (7.8cm compared to 4.2cm. on the Nevis rodent (Scirius rodentus) and a higher leap (1.4m to 0.4m). 

 

The behavioural courtship ritual also could impact the struggling reproduction rate of the St Kitts (Scirius speedus), as it has a much shorter courtship than the Nevis rodent (Scirius rodentus), by 8.7 seconds. 

 

Due to reduced hybrid fertility and gene flow between the two species this has resulted in the St Kitts (Sciurus speedus) being unable to reproduce with the Nevis rodent (Sciurus rodentus) because of reduced hybrid fertility, ultimately resulting in the St Kitt’s (Sciurus speedus) declining population. 

It is possible that due to differences in gestation time as well as behavioural differences, such as clear variance in mating rituals, that these two species will not be able to effectively breed and create fertile offspring. Therefore, even if the two species were reunited, there is no guarantee that gene flow could be reestablished.

After the conclusive evidence that they will not reproduce together I decided to find out why. 

It did not take long for more reasons to come to light, the more I looked the more I saw how different the two Rodents were. Even just on a physical view point there were differences, the St. Kitts rodent was built with longer hind limbs, averaging 7.8 cm while the Nevis Rodent was only averaging 4.2 cm. This led to the St. Kitts Rodent being able to run faster, leap higher than the Nevis Rodent. Further I knew that the rodents had a different reproductive seasons with the gestational time for St. Kitts rodents being 29.3 days on average with a courtship display of 12.6 seconds while the Nevis Rodent had a gestation period of 42.7 days and a courtship of almost double the time at 21.3 seconds.

It became apparent that the two Rodent groups were incompatible.

As an expert in mammalian reproductive strategies this is embarrassing as I should have considered allopatric speciation and temporal isolation before enacting my plan.The microevolution has caused barriers causing the rodents never be able to mix gene pools or have successful reproduction. Meaning gene flow would never have worked.

The following case study is in reference two separate populations of rodent residing on the islands of St Kitts and the much smaller Nevis. The larger rodent residing on the island of St Kitts has been identified as Hydrochoerus magna-rat meaning its related to the Humble Capybara or Hydrochoerus hydrochaeris (this is a hill I’m willing to die on, I don't like squirrels ) Whilst the rodent on the island of Nevis is also apart of the Hydrochoerus family, the species has been identified as Hydrochoerus  luminaire-rat. Due to dwindling population in the Hydrochoerus magna-rat population attempts to increase reproduction via the introduction of Hydrochoerus  luminaire-rat have seemly failed as not one case of reproduction between Hydrochoerus magna-rat and Hydrochoerus  luminaire-rat have been recorded.  This has left a bigger question why can’t these two very similar rodent breed and have offspring. 

Section 1 Habitat isolation: Due to the geography of St Kitts and Nevis it was possible that only species of rodent however due natural forces and changing of the geographical landscape the population of said species could have been separated. Thus, meaning over time the two populations evolved to be similar but also much different at the same time, this habitat isolation limited the gene flow / gene pool to two distinctive islands meaning those species may have very different genes to one another.

Section 2 Gene Flow: As already explained in the section above the gene flow was severely limited due to the geographical isolation experienced between these two species. It is possible during the early stages when there was a possible land bridge or way across the gap between those two islands the gene flow was steady and contained a lot of variation, However a lot of time has passed since then and the gene flows have been limited to different islands meaning the variation in the gene flow has significantly dropped this means the possibility of both species sharing a gene has dropped as time goes on meaning that the probability of a successful mate between these two species goes lower as time goes on.

Section 3 Mutation: Both Species have had different mutations that suit their own habitats, however these two habitats are different two each other a mutation that helps Hydrochoerus  luminaire-rat may be detrimental to Hydrochoerus magna-rat thus meaning Magna rats that have similar mutations die out, due to natural selection. This can be seen in physical measurements recorded on both species. Hydrochoerus magna-rat has recorded a much faster top speed than that of Hydrochoerus  luminaire-rat meaning the Magna-Rat has developed a mutation that allows it move much faster. Evidence can support that Magna-rat has mutated powerful lower body capabilities compared to luminaire-rat. As seen from the recorded leap height. On average Magma-Rat was able to jump at least 1.4 meters into the air whilst luminaire-rat on average was only able to jump 0.4 M, this suggests that the magna-rat due environmental factors favoured a mutation that gave Magna-rat a more robust and powerful lower body. Measurements of hind limbs can support this claim Magna-rat has a hind limb length of 7.8cm whilst luminaire-rat only has a hind limb length of 4.2 cm suggesting Magna-rat has experienced a different mutation to that of luminaire-rat, this mutation allows Magna-rat to adapt to its ever-changing environment.

Conclusion:

These two species are very similar however genetically are very different which prevents them from breeding and capybaras are cool.

Through observation of behavioural and physiological characteristics of both species Sciurus. Humuspabulatus (Common Name: Caribbean Ground Rodent) and Sciurus. proceritascandere (Common name: Caribbean Climbing rodent) it can be inferred how these two similar rodents formed two separate species. 

The island of St.Kitts contains larger vegetation such as taller tree species which is more suitable for tree dwelling species  as opposed to the lower vegetation of the island of Nevis which favours ground feeding. This can be observed in of  S. proceritascandere behavioural traits spending more time above the ground level. This is also observed in physiological data pertaining to the species, the size of both the hind limb (avg. 7.8cm) and forelimb (avg. 4.2cm) of S. proceritascandere used for higher leaping heights for climbing (Up to 1.4m). This contrasts the on average smaller limbs of S. Humuspabulatus (Hindlimg 4.2cm, Forelimb 3.9cm) which results in a lower leap height (Up to 0.4m). This behavioural adaption has resulted in physiological adaptation which has most likely lead to prezygotic isolation of the two species via behaviour.

Furthermore through observation it has been shown that each species has shown a distinct gestation and reproductive period that temporally isolate the two species. S. Humuspabulatus has a much longer gestation period (Avg. 42.7 days) thus it can be inferred that this species breeds in separate intervals to one another; S.Proceritascandere (Avg. 29.3 days) gestation. These temporal isolations indicating how the species would have separated.

 

You wouldn’t believe me but locals on Nevis used to throw these rodents for sport over the gap between the islands,  believing they were common rats. Moreover, once there were enough on the southern part of the isle, their numbers exploded. An old fisherman recalled to me that he never fed had to feed his cats when he was younger as they fed on the rodents. This St Kitts population of rodents it would seem, underwent a natural selection process and after a few generations, resulted in sympatric speciation. 

 

From the capture and release programs, I can tell you that the natural selection required this new subspecies to have longer hind limbs, (4.2cm on Nevis Subspecies and 7.8cm on St Kitts), thus resulting in a top speed that is doubled on average within the new species. With this in mind I named this new species the sprinter chinchilla (Chinchilla sprintus).

 

As you would expect this results in rezygotic reproductive isolation which hindered the success of the repopulation program on St Kitts Island, yet a colleague has recently been doing woodworking and suggested using a ramp. If all else fails we might have to give that a go. 

 

It has been a while since we have discussed our scientific expeditions but I have come across something that I think you will find rather intriguing. I have recently been working on St. Kitts Island and Nevis Island to try and save a local species Sciurus roccalias. The population on St. Kitts is far smaller and so I attempted to breed the two populations to improve the genetic diversity and numbers of the smaller population, but alas to no avail! I have not once observed the two groups mate. I got some of my crew to investigate the two groups and I have discovered something most peculiar! On St. Kitts island, the rodents have far longer hind legs- 7.8cm compared to the 4.2 of the Nevis population. This is carried through to their jump height which averages to 1.4m compared to the Nevis 0.4. Looking at the environments of the two islands, I have found St. Kitts island to be far more developed, meaning ledges, stairways and other terrain less gradual than the natural island. I believe I may be the new Darwin and these are my finches. 

My current hypothesis is that after the islands separated, the development began. Some of the St Kitts species must have randomly gotten a mutation for longer hind legs which benefitted them in this new environment, allowing them to remain far from predators and natural selection thus benefited this trait. Due to the habitat isolation, these new alleles were unable to be passed to the species on Nevis Island and voila! They are now too different to breed. Perhaps the difference in hind leg length inhibits intercourse.

I have taken to calling the St. Kitts group Sciurus bubbalias after you my dearest friend. You simply must come and see the species to help me decide if this truly is my breakthrough.

Your dearest friend,

Dr Jonah

Scientist from Florida have been struggling to revive the dwindling population of isolated Rodent on the island of St. Kitts.

The Rodent is believed to be in the Squirrel family but looks closer to a Chinchilla.

In an effort to revive the populations on St. Kitts, the group of scientists have taken a population of the rodents from the neighbouring island Nevis in order to boost the population on the mainland to encourage breeding.

It sounds like a great plan in theory but there has been a slight miss calculation in their efforts. The rodents don’t seem to be breeding with one another.

Upon investigation, it appears that even though they look remarkably similar, they are in fact different species.

Upon fist glance you can see there is a mechanical issue when it comes to breeding in terms of the size of their legs making it difficult for the males and females to copulate. There is also a couple of prezygotic isolation issues - one being behaviour and one being habitat. The St. Kitts set of rodents seem to do most of their breeding interactions at dusk where as the Nevis’ breeding interactions occur early in the morning. On top of this, the St. Kitts rodents are terrestrial and the Nevis rodents are arboreal creating yet another issue which the scientists call temporal.

With all of these details beginning to surface it has made the ground team much more aware of what you see and study at first glance isn’t always what you expect long term.

So, how do you think we can solve the issue? Is it a matter of letting the St. Kitts species go extinct? Or should they go on another more detailed search of a more comparable rodent? What will be the impact of putting the Nevis species on St. Kitts?

Follow this story line to continue reading about the scientists journey.