Manifestation of “Island Syndrome” in a Population of Arctic Foxes
The Mednyi Island fox (Vulpes lagopus semenovi*) is a little-known subspecies of Arctic fox found only on the Commander Islands in the Bering Sea. In fact, it is one of the only species of terrestrial mammal on the islands. The foxes’ main source of food are the thriving colonies of seabirds that nest there.
"Island syndrome" refers to a suite of behavioral changes commonly found in populations of vertebrates isolated in a small area** from the rest of the species. Largely studied in rodents on islands that have few predators, these changes include:
- Gigantism (if originally a small species) or dwarfism (if originally a large species)
- Higher population density
- Greater longevity
- Delayed sexual maturity
- Lower fertility
- Lower aggression
- Greater territorial overlap
- Lower tendency of dispersal
This process is also sometimes referred to as “self-domestication” because it confers similar behavioral and physiological effects as domestication.
The Mednyi island fox is a particularly interesting specimen because it comes from a carnivorous species that is widely dispersed over large territories on the mainland; some foxes disperse 100 miles or more from their natal territory. Obviously, that level of dispersal is impossible on the 32-mile long island.
Other factors that differ between the mainland and island populations is that the island population has a large, constantly available, and easily accessible food source- seabird colonies- while the mainland population’s resources are much more scattered; that the island arctic foxes do not face the drastic temperature fluctuations that their mainland relatives do; and that the island foxes have no competition or predators in the form of other terrestrial mammals.
The authors of the study I’m about to discuss took advantage of a unique situation: an outbreak of mange had just drastically reduced the Mednyi island fox population, which was slowly recovering. This provided them with the opportunity to discern whether behavioral and physical changes between the mainland and island populations were a short-term response to overpopulation (and would therefore not be present in the reduced population) or were a product of long-term evolution.
Here is a summary of their results:
- All but one of the island syndrome traits were present in Mednyi foxes. The exception: while Mednyi foxes had much smaller territories than the foxes on the mainland, they were extremely territorial and aggressive with their neighbors.
- Unlike their monogamous mainland cousins, Mednyi foxes showed a high tendency towards polygyny and alloparenting.
- Nonbreeding helpers (likely offspring from previous years, usually female) sometimes stayed with the group.
As stated, the Mednyi foxes followed most tenants of the island syndrome traits outlined above. The reduced fertility and longer time to reach fertility in island species can be explained by a few factors: first, due to a lack of predators and a reliable food source, infant mortality gets much lower, meaning that litters can become smaller, with larger, more robust offspring instead. Second, organisms with larger body sizes also tend to take longer to mature.
The enlarged body size of the Mednyi foxes is the opposite of what the island syndrome theory would predict, given that species larger than rodents tend to get smaller. However, the authors point out that carnivores often break this rule because they have selection pressures that herbivores do not: namely, the size and availability of their prey***. The Mednyi foxes had larger and more freely available prey than the mainland foxes did, hence they tended to get larger.
While polygyny is rare in canid species, it is more common in species with food available in large patches (such as the bat-eared fox). Thus, the high and stable availability of food led the foxes to be able to support more than one litter at a time per male parent.
The sex-biased dispersal patterns of Mednyi foxes also reflect this, as well as the restricted population. Having one sex (males) disperse farther from the home range meant that there was a reduced likelihood of inbreeding, important in a confined population.
Since all these traits were still present after the population was reduced for over ten generations, the authors were able to confirm that the species was not merely reacting to overpopulation, but had seriously evolved.
However, for me, the most intruiging question is why the Mednyi foxes were more territorial and aggressive than mainland foxes, the opposite of what island syndrome theory would predict? Normally, higher population densities mean that a species has to become more tolerant of its own kind out of pure necessity. The authors don’t offer much of an explanation as to why the Mednyi foxes don’t follow the pattern, so I’ll provide my own suggestion: as with body size, different factors affect the territoriality of carnivores than herbivores. Herbivore territoriality often has to do with sexual competition, while the territoriality of terrestrial carnivores more often has to do with prey availability. In this sense, the patchy nature of island prey availability would mean that specific parts of the island were highly valuable and specific parts were next to worthless. Thus, fierce territorial defense developed.
Goltsman, M., Kruchenkova, E. P., Sergeev, S., Volodin, I., & Macdonald, D. W. (2005). ‘Island syndrome’ in a population of Arctic foxes (Alopex lagopus) from Mednyi Island. Journal of Zoology, 267(04), 405-418.
Photo source: UNDP
*The paper lists the genus as “Alopex,” but arctic foxes have since been moved to the genus Vulpes.
**Island syndrome also occurs in species on the mainland that are isolated by factors such as severe habitat disruption (i.e., deforestation). This makes it particularly important to study in terms of conservation.
***The authors compared the Mednyi foxes to another island species, the island fox, which got smaller than its mainland cousin the grey fox. This is probably due to the fact that it still scavenges for small prey items like its progenitor.