Do bats follow the ‘island rule’?

The search for a yardstick to gauge geographic variation in a taxonomic context yielded answers to broader biogeographical questions

Above: Geographic variants in the Allen’s common moustached (Pteronotus fuscus). Left, cranium and mandible of a specimen from Paraguaná Peninsula (CVULA 8197). Center, cranium and mandible of a specimen from Venezuela south of the Orinoco River (CVULA 8155). Right above, wing of CVULA 8155. Right below, wing of another specimen from Paraguaná Peninsula (CVULA 8150).

Islands come in many sizes, ages, and kinds: from small to large; old to recent; isolated or part of archipelagos; continental or oceanic; and combinations thereof. On continents, there are also the so-called ecological islands—e.g. unconnected habitat patches, caves, and lakes—including ‘sky’ islands (mountainous areas surrounded by drastically different lowland environments) that also vary in size, age, and degree of isolation. Marine organisms, particularly those inhabiting isolated benthonic patches, have been postulated to be insular. The barriers that separate islands hamper gene flow thus are a major cause of speciation worldwide.

Since Darwin’s time, botanists and zoologist have been busy describing and cataloguing insular biodiversity, and islands have been fundamental as natural laboratories to study evolution. In the 1960’s, these efforts flourished in the form of the MacArthur-Wilson Equilibrium Theory of Island Biogeography, which postulated that the number of species on an island is related to its area, its distance from the mainland, and its balance between immigration and extinction. In the 1970’s, the ‘island rule’ was formulated, postulating that after colonizing islands animals become smaller if they were large and larger if they were small on the mainland; that is, it is predicted that they will converge to ‘optimal’ intermediate sizes thanks to the release from mainland predators and competitors failing to colonize the same islands.

I am a zoologist from Venezuela, a megadiverse country in northern South America. My interests include the taxonomy of Neotropical mammals, especially bats. In this and other animal groups, the degree of continuity and magnitude of geographic variation are of paramount importance to decide how many species and subspecies need to be recognized, or be included in conservation plans. One of the greatest complexities of taxonomic work involves deciding consistently how much geographic variation is sufficient to be formally reflected in scientific nomenclature. Thus taxonomists can be characterized as individuals who are perpetually searching for, refining, and applying morphological yardsticks to gauge geographic variation in their study organisms. I became interested in the island rule as part of this search.

Geographic variants in the Allen’s common moustached (Pteronotus fuscus). Above, typical specimen from the Venezuelan mainland. Below, specimen from Paraguaná Peninsula, in northwestern Venezuela.

Island rule studies caught my attention not only because they deal with geographic variation, but also because their fundamental metric, namely the size ratio between the members of the allopatric populations being compared, could be the yardstick that I needed. As I familiarized myself with the theme, I met a number of problems. First and foremost, despite the availability of information, no comprehensive study of the island rule existed for bats. Second, most island rule research was devoted to the comparison of island organisms with their mainland relatives, thus largely ignored within-mainland and inter-island size variation, which are relevant not only to taxonomy, but also as a frame of reference for the island rule itself. Third, bat taxonomists do not generally use body mass as a character to differentiate species; instead they use cranial and wing measurements because they are more constant. Body mass—in order to increase sample size, often inferred from diverse linear measurements—is the dependent variable generally used in island rule research. Thus most information found in island rule literature was inapplicable to taxonomy. To fill these gaps, I initiated the study on bats that has just been published in the Journal of Biogeography.

Editors’ choice / Cover article: (Free to read online for two years.)
Molinari, J. (2023). A global assessment of the ‘island rule’ in bats based on functionally distinct measures of body size. Journal of Biogeography, 50. 

The search for a morphological yardstick was successful. This is exemplified by the Allen’s common moustached bat (Pteronotus fuscus). Although allopatric populations of this species—or species complex—were known to differ substantially in cranial and wing dimensions (see figures above), now we can affirm that such morphometric variation is unusual, overall the greatest of the 251 bat species included in the study.

The results of the study transcended the initial goal of finding a yardstick to gauge geographic variation, and were amenable to address broader biogeographic questions. Thus I tested:

1) Whether bats follow the island rule, which has previously been concluded to be pervasive in mammals and other vertebrates. I found this not to be the case. The most likely explanation for this exception is that bats do not follow this rule owing to limitations imposed by flight and echolocation.

2) Whether on islands bat body sizes converge to intermediate supraspecific optima, as predicted by theoretical studies. I concluded that this is not the case and that instead sizes converge to species-specific optima, as the general pattern of geographic variation in bats—which appears to be dependent on ecological niches rather than on adaptive zones—would suggest.

3) Whether bats would be ranked in a similar order by skull size, by wing dimensions, and by body mass. Again, I also found this not to be the case. The most likely reason is that the three size measures are functionally distinct—being respectively most relevant to the feeding, movement, and physiological ecology of bats)—thus are subjected to different selective forces.

4) Whether a bias has existed to give formal taxonomic recognition with greater frequency to bats distributed across mainland-to-island ranges than to those distributed across island-to-island or within-mainland ranges. I concluded that this is the case. The explanation is that, owing to the long-standing fascination exerted by islands on evolutionary biologists, there has been a high level of interest in describing morphological differences between island species and their mainland counterparts.

Where do we go from here? In 2006, Mark Lomolino, a pioneer of island rule studies, and his collaborators, proposed a research agenda calling for the use of a comparative approach expanded to include a greater diversity of species, to test the island rule and other ecogeographic patterns and their exceptions. This agenda remains fully valid today. More ecogeographic studies of all kinds of organisms are needed that address ordinal and familial level variation across different kinds of geographic range.

Written by:
Jesús Molinari
Zoologist and ecologist at the Universidad de Los Andes, Venezuela

Additional information:

Published by jbiogeography

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