The Better to Hear (Some of) You With, My Dears
As mentioned in a previous post, many ecosystems and communities contain “ecologically redundant” species. One important question to keep in mind when considering interspecies interactions is that of intraguild competition: how do different species conduct very similar lifestyles without constant struggles over access to prey, shelter, and other essential resources?
This is where niche differentiation comes in. With closely related, sympatric species, detailed data on their behavior often reveals very slight differences that allow the two species to co-exist by minimizing their resource overlap within their particular guild.
Fortunately, the creativity of nature shows no bounds, and innumerable taxa have come up with many methods of niche differentiation. Species might 1)forage at different times of night/day, separating temporally, 2)avoid each other spatially, adopting separate microhabitats, or 3) rely on physical adaptations to specialize their foraging capabilities on separate food items. While morphological adaptation is often put in its own category, it can often accompany the first two as well. As behavior shifts to exploit different resources, morphological adaptation to the divergent lifestyles often results.
This brings us to the puzzling case of two German bat species, Myotis bechsteinii and Myotis nattereri. These two species are sympatric, often occupying the same bat boxes in sequence (they are restless little guys, and colonies move their roosts every few days). They are about the same size—just a very slight difference in mass—and both specialize on insects, have similar temporal foraging patterns, and both hunt by gleaning insects from foliage. Both species have fossil records in the region reaching back to the early Pleistocene. If these two species behave so similarly, then what either 1) drove speciation in the first place, if we choose to assume they evolved in the region, and/or 2) has allowed them to differentiate sufficiently to co-exist for millions of years? It should be noted that they are not sister species: both species are more closely related to other Myotis species than they are to each other.
We have one clue to get us going: M. bechsteinii has slightly longer ears than their sympatric congeners. Actually, they are the Chiropteran chiefs of auricular splendor in Europe, with the longest ear length (relative to body size) of all Myotis on that continent. No small feat, as Myotis is an incredibly diverse genus, with about 90 species globally. (Ok, this is impressive diversity for mammalogists, consider that there are many entire mammal families with less than 10 species total, it might be less impressive for someone studying insects, parasites, birds . . . ok, pretty much any other taxon).
So, recently Siemers and Swift (2006) conducted a study to determine just how these bats are managing to co-exist, and they used the ear length as a starting point. It is known that longer ear length allows bats to detect “prey-generated sound” from activities such as rustling on through leaves. Shorter ear lengths are associated with species that track more stealthy, silent prey. This involves more associative learning, to remember productive spots for prey, and to use echo-cluttering from the foliage to detect insects that are airborne instead of rustling on surfaces.
So, Siemers and Swift did some rather creative work with insects in flight cages, to examine the degree to which the bats responded to moth fluttering in various circumstances (see paper for the details of the experiments and various ways to use moths as bait). They also collected fecal samples, to analyze the insect assemblage in the diet of each species.
The results showed marked differentiation in diet, despite the fact that both species glean insects from foliage and have virtually the same morphology beyond ear length. The majority of M. bechsteinii ‘s diet consisted of moths, crickets, and harvestmen, while M. nattereri appeared to prefer spiders and flies. One taxa that seemed to be consumed equally by both species was the Coleoptera. When we zoom in on the beetles, however, we see some informative divergence: M. bechsteinii prefers noisy terrestrial beetles, while M. nattereri consumes more silent, volant species. Spiders were more predominant in M. nattereri’s diet than M. bechsteinii’s, but again, zooming in on the taxa is interesting: M. bechsteinii preferred noisy cursorial species, while M. nattereri was more likely to consume silent web-builders. To further bolster the results, the fecal analysis was consistent with the behavioral data collected from the exposure to insects in the flight cages.
So, what do we draw from all of this? The take-home message is that it appears that the niche differentiation here is based on sensory ecology. The bats are foraging from the same plants, but are literally “seeing” two entirely different insect assemblages. Echolocation is a masterful tool, and different bat species have found seemingly limitless ways to adjust and specialize their vocalizations and auditory processing to adjust to different habitats, prey bases, and other ecological conditions. Could it be that relying on sensory specialization rather than a more significant morphological adaptation is either “easier” to evolve (they could already all the equipment for echolocating, a slight ear length adjustment is a minor fix) and/or energetically cheaper to maintain?
I found this study fascinating, because you have two species that are almost identical in nearly every feature, and yet they experience their environment and interact with their community in different ways. What do you think of sensory ecology as a mechanism of niche differentiation, do you have another favorite example of this phenomenon? Do you think it qualifies as a separate category of niche differentiation, or should it be seen as an emergent property of one of the traditional categories?
Siemers, B. M. and S. M. Swift. 2006. Differences in sensory ecology contribute to resource partitioning in bats Myotis bechsteinii and Myotis nattereri (Chiroptera: Vespertilionidae). Behav. Ecol. Sociobiol. 59: 373-380.