Acoustic studies of mixed-species groups?

Question

Mixed-species groups are commonly found throughout the animal kingdom, and often documented using visual surveys (for obvious behavioral reasons). However, I am interested in whether we can study mixed-species encounters in acoustic data. Does anyone have resources for this?

There are a couple solid review studies that focus on mixed groups in mammals (Stensland et al 2003) and, more recently, in cetaceans (Syme et al 2021), but these studies exclude any acoustics. I'm aware that studying this phenomenon using acoustics is likely to be pretty complex, but I'm still curious whether anyone has tried to do this or found studies with this focus?

Answer 1

Does "mixed-species chorus" qualify? If so: Structure and dynamics of mixed-species choruses in a tropical anuran assemblage: insights from network analysis (Duarte et al 2021).

Also this paper on mixed-species interactions of monk parakeets (Briceno et al 2019) e.g. group foraging, which includes some mention of vocal interaction.

Answer 2

From my personal research experience... Acoustics studies of marine mammals are complicated by the fact that humans only visually observe a small fraction of their behavior while the animals are at the surface and it can be difficult to acoustically discriminate vocalizations between species in mixed-species encounters.

Many years ago I wanted to know if mixed species schools were more/less likely be to detected using a towed hydrophone during closing-mode shipboard visual line-transect surveys in the Pacific Ocean. These were population-level surveys and my question was related to detecting the presence of groups of dolphins based on detecting any combination of whistles/burst pulses/echolocation clicks (I needed to know more about groups that were NOT vocal!). I was unable to reasonably determine if which species was vocalizing, but instead I looked at the overall acoustic detection of each group. As these sightings were associated with visual observations and group size estimates, I used group size as a co-variate. The overall results were that mixed species schools were consistently more vocal than non-mixed species schools, but that mixed species schools also tended to be larger group sizes. Also, quiet dolphin schools tend to have smaller group sizes in general. There was a bit of variation by species and in some geographic regions. These results were presented at an Acoustical Society of America meeting in 2005 (https://doi.org/10.1121/1.4787441), but were not published.

These are fairly simplistic results-- but using a lot of good data from visually confirmed sightings across an incredibly large geographic area. In the intervening 17 years (!!) our ability to classify dolphin acoustic detections has improved dramatically (it was essentially non-existent at that time!). Eventually as our ability to correctly classify acoustic detection approaches 100%, we should be able to (hopefully!) identify the presence of multiple species in mixed-species groups. Classification methods such as BANTER (see publication here and package here) that allow for assessing the classification scores for acoustic detections may provide a path forward for detecting/classifying mixed species groups (see PlotVotes under 2.4.2 in this BANTER tutorial).

Once we can identify the multiple species in an acoustic encounter--we then have an improved capacity to apply this to scientific questions. Of course, this example I've shown here is for an encounter-- looking at specific calls is even more complicated.

Answer 3

An attempt to find species-specific acoustic cues in mosquito swarms of mixed species of Anopheles gambiae s.s. and Anopheles coluzzii has been made:

The two species of the An. gambiae s.l. complex we worked with (An. coluzzii and An. gambiae s.s.) are African malaria vectors and are undergoing speciation [25]. These species are found in sympatry and mainly mate assortatively. Subtle differences in swarming behaviour between these closely related mosquitoes can minimize hybridization. Female auditory detection of a conspecific swarm of males at long range could increase the female's likelihood of locating and being inseminated by a male of the same species. A female might recognize a species-specific sound signature at long range before males of any other species could hear, chase and mate with her.

But it was not found to be relevant:

First, species-specific cues of swarm sound were found to be weak (electronic supplementary material, figure S4). Second, our behavioural assay did not show any species-specific responses in An. coluzzii females to the swarming sound of An. coluzzii or An. gambiae s.s. males. Third, following our results, we can reject the idea that females use the sound emanating from a swarm to determine whether to avoid entering the swarm of the wrong species, or to join the swarm of the same species, because the female will not hear the swarm before she comes into close proximity of numerous males at the periphery of the swarm.