According to the ecomorphological paradigm, species with similar morphology should exhibit similarities in behaviour and ecology. This prediction, however, raises the possibility of competition between such species when they occur in sympatry. Inter-specific competition takes place when two (or more) species with similar ecological requirements consume resources that are limited in supply. Nevertheless, the stable coexistence of competitors will be possible if their respective niches differ sufficiently.
Niche differentiation is easy to conceptualise as a consequence of inter-specific competition. However, concrete evidence in support of it is difficult to acquire, because the demonstration of niche differentiation per se does not necessarily indicate anything about the contribution of competition. Removal or demographic response experiments, if adequately designed, may demonstrate a cause-effect relationship between niche differentiation and inter-specific competition[4, 5]. However, these experiments are inappropriate for rare, elusive, K-selected, or endangered species, and currently, non-disruptive, more inductive approaches are the only practical alternatives. These alternative approaches usually compare morphology, behaviour, and ecology of two (or more) species which occur under allopatric and sympatric conditions and assume that any niche displacement is directly associated with competition.
Morphologically similar species are numerous among bats, and the number is increasing as molecular tools uncover cryptic species-complexes comprising genetically isolated taxa[8, 9]. An excellent illustration is the discovery that the most abundant and best-known European bat species, the pipistrelle, is in fact a cryptic complex of two species: the common pipistrelle (Pipistrellus pipistrellus) and the soprano pipistrelle (P. pygmaeus)[10, 11]. Despite being morphologically almost indistinguishable, ecological data show a marked divergence in ecological requirements[12, 13], contradicting ecomorphological predictions.
Bats depend on flight as their principal means of locomotion, so wing morphology greatly influences foraging behaviour. Additionally, most bats use echolocation to obtain information about their environment, the precision of which depends on the structure of the echolocation signal. Thus, characterisation of bats’ wing morphology and echolocation signals, facilitate inferences about ecological significance. For example, bats with narrow, pointed wings usually emit low-frequency calls and tend to forage in open spaces, whereas bats with broad, rounded wings usually emit high-frequency signals and tend to forage in cluttered space (i.e. forests)[16, 17]. Some species in the families Vespertilionidae and Molossidae, exhibit flexibility in the structure of search-phase echolocation signals[18, 19], enabling them to access a wide variety of habitats. Other species, for example in the families Mormoopidae and Rhinolophidae, emit unique, stereotyped signal structures with little quantitative variation; these bats are more rigid in the use of foraging habitats[21, 22].
Rhinolophids have a highly specialised auditory system that discriminates the modulated echoes of the beating wings of insects (e.g. moths) from unmodulated background echoes. Based on their auditory system and wing morphology, horseshoe bats are classified as narrow-space, flutter-detecting foragers: they are specialised to catch slow-flying insects very close to or within vegetation. Horseshoe bats consist of a single genus, Rhinolophus, with 77 recognised species distributed exclusively in the Old World. Although the group is diverse, its morphological uniformity is striking compared to other families. This uniformity is mirrored in the numerous, morphologically similar, and frequently confused pairs of sympatric species[24–26]. Segregation in habitat use and diet have been proposed as major mechanisms for niche differentiation in sympatric bat species[27–30]. However, it remains unclear what promotes niche differentiation in sympatric horseshoe bats, as there are few ecological or behavioural studies (but see), and none of potential sibling species (sensu).
The Mehely’s (Rhinolophus mehelyi Matschie, 1901) and the Mediterranean (R. euryale Blasius, 1853) horseshoe bats can be considered as sibling species (i.e. they are morphologically similar and share a recent common ancestor[32, 33]). R. mehelyi and R. euryale are two cave-dwelling species whose distributions overlap extensively in the Mediterranean basin. Radio-tracking studies show that in allopatric conditions both forage in and along forest edges[34, 35]. A preliminary study suggests that in sympatric conditions they tend to segregate foraging habitats with R. euryale found in more dense woodlands. However, the small sample size and the coarse resolution of locations attained by triangulation of that study, as well as the lack of information on diet, limit any firm conclusion about niche differentiation. R. mehelyi and R. euryale emit similar echolocation signal structures differing only in the frequency of maximum energy, 107 and 104 kHz, respectively[37, 38]. Although broad differences in frequency may facilitate dietary niche differentiation, frequency differences between both species may be too small to allow any dietary differentiation. So far no diet data have been collected for sympatric populations, but moths (Lepidoptera) are both species’ main prey in allopatry[34, 39]. The question remains, how do sibling horseshoe bat species coexist when echolocation signal structure is unlikely to result in differences in diet and their morphology is almost identical?
To address this question we investigated echolocation, wing morphology, diet, and habitat use of R. mehelyi and R. euryale in sympatric conditions during breeding season in the Iberian Peninsula, and compared our results with previous literature in allopatric populations. To make results comparable we focused on data gathered in the Iberian Peninsula during breeding season. Our aim was to ascertain the mechanism(s) of coexistence. We predicted that dietary niche differentiation is not likely to occur but rather that coexistence stems from spatial niche differentiation, with slight differences in wing parameters as the prominent contributing factor. Lower wing loading and aspect ratio values in R. euryale should make it better adapted to forage in more cluttered space, facilitating habitat partitioning.