How much does inner city noise mask bat echolocation?

Question

When traveling in a very busy,very loud city recently, I noticed a few small bats clearly feeding on bugs attracted to a street lamp. I was on a party street, and there was loud music pouring out of multiple buildings near the feeding animals.

I've never seen bats this close to so many humans before, in such an urban environment. I know there are studies of bats using urban environments, but it often comes with a call to add ponds and other natural habitats. You know, areas that can be made quieter than a party street.

My question is, how effective is their echolocation in these noisy areas? Do they have self filtering capabilities? How much is the city around them masking, and therefore impairing, their ability to find food? And possibly, could they be using the light from the street lamp to help aid their foraging efforts?

Answer 1

I would personally say that the short answer is no, bats are not particularly affected by anthropogenic noise, but the jury is still out.

Urban noise tends to be low frequency, below the ultrasonic realm in which bats operate. Most urban noise, from planes, trains and automobiles, will generally be below ~5 kHz (or thereabouts?), while bats use frequencies between 15-150 kHz, so there will generally be little overlap between the two.

However, at least one paper has found localised impacts on bat distribution/activity from simulated playbacks of traffic noise: https://www.sciencedirect.com/science/article/pii/S0269749119365546, and there are a number of other studies that have shown that bats (and other wildlife) will actively avoid roads. One of the issues here though is trying to separate out the effects of noise itself from other reasons that bats might avoid roads, such as exposure to wind/pressure changes, or lack of invertebrate prey (e.g. as set out in this report: http://iene.se/wp-content/uploads/Effects-of-Traffic-Noise-and-Road-Construction-Noise-on-Bats.pdf). As contrast, one paper, which I can't now find the reference for, showed that bats were more affected by tree leaves shaking in the wind (producing ultrasound), than by traffic noise.

One other complicating factor is that different bats will have different levels of response. This is the same for lighting - which will definitely attract some bats to feed on the invertebrates gathered around the lights, while other species will avoid brightly lit areas.

In answer to your final questions, I would argue that the main limitation for bats in urban areas is likely to be habitat/food availability, rather than their ability to echolocate around the area - they will, of course, be out at night when human activity and noise will generally be lower than in the day.

And for the ultimate city bats experience - check out the Youtube videos on the 250,000 free-tailed bats roosting under the Waugh Street road bridge in Houston. Like this one: https://www.youtube.com/watch?v=Goiqpr2Xz6s

Answer 2

Re: Light pollution

Check out this publication:

Laforge, A., Pauwels, J., Faure, B. et al. Reducing light pollution improves connectivity for bats in urban landscapes. Landscape Ecol 34, 793–809 (2019). https://doi.org/10.1007/s10980-019-00803-0

...which looked at bat traffic along corridors of different light levels in an urban setting. Interestingly, they find different results with different species, whereby: "M. daubentonii responded negatively [to high levels of light], P. nathusii had a positive response for low values then a negative response after a threshold radiance value of 20 W.m-2 .sr-1 and E. serotinus responded positively."

For each of the 3 species, they've also mapped out how costly each of the flight corridors were:

They found that the least costly path between habitat patches was of shorter distances when it was darker (at least for M. daubentonii and P. nathusii).

Secondly, a paper from last year compared the effects of Artificial Light at Night (ALAN) across different guilds of European bats, and their summary table below is useful for overviewing positive/negative/neural effects.

Christian C Voigt, Jasja Dekker, Marcus Fritze, Suren Gazaryan, Franz Hölker, Gareth Jones, Daniel Lewanzik, Herman J G A Limpens, Fiona Mathews, Jens Rydell, Kamiel Spoelstra, Maja Zagmajster, The Impact Of Light Pollution On Bats Varies According To Foraging Guild And Habitat Context, BioScience, Volume 71, Issue 10, October 2021, Pages 1103–1109, https://doi.org/10.1093/biosci/biab087