J Comp Physiol A (2015) 201:111–121 DOI 10.1007/s00359-014-0945-8 REVIEW Moth hearing and sound communication Ryo Nakano · Takuma Takanashi · Annemarie Surlykke Received: 17 July 2014 / Revised: 13 September 2014 / Accepted: 15 September 2014 / Published online: 27 September 2014 © Springer-Verlag Berlin Heidelberg 2014 Abstract Active echolocation enables bats to orient notion of moths predominantly being silent. Sexual sound and hunt the night sky for insects. As a counter-measure communication in moths may apply to many eared moths, against the severe predation pressure many nocturnal perhaps even a majority. The low intensities and high fre- insects have evolved ears sensitive to ultrasonic bat calls. quencies explain that this was overlooked, revealing a bias In moths bat-detection was the principal purpose of hear- towards what humans can sense, when studying (acoustic) ing, as evidenced by comparable hearing physiology with communication in animals. best sensitivity in the bat echolocation range, 20–60 kHz, across moths in spite of diverse ear morphology. Some Keywords Co-evolution · Sensory exploitation · eared moths subsequently developed sound-producing Ultrasound · Echolocating bats · Predator-prey organs to warn/startle/jam attacking bats and/or to com- municate intraspecifically with sound. Not only the sounds for interaction with bats, but also mating signals are within Introduction the frequency range where bats echolocate, indicating that sound communication developed after hearing by “sensory Insect hearing and sound communication is a fascinating exploitation”. Recent findings on moth sound communica- subject, where the combination of many classical studies tion reveal that close-range (~ a few cm) communication and recent progress using new technological and molecular with low-intensity ultrasounds “whispered” by males dur- methods provide an unsurpassed system for studying and ing courtship is not uncommon, contrary to the general understanding the evolution of acoustic communication, intraspecific as well as between predator and prey. Sound is fairly easy to quantify allowing for estimates of commu- R. Nakano nication distances and thus inferences about communica- Breeding and Pest Management Division, NARO Institute of Fruit Tree Science, 2-1 Fujimoto, Tsukuba, tion partners. Also, we can simulate sounds and therefore Ibaraki 305-8605, Japan do experiments to test the importance of specific acoustic features in the sound signals. R. Nakano Moths have been particularly attractive because the Department of Biological Sciences, University of Toronto Scarborough, 1265 Military Trail, Toronto, ON M1C 1A4, predator, echolocating bats, is so well defined and the Canada predator–prey interaction restricted to audition. These facts probably explain that the bat–moth model has found its T. Takanashi way into many textbooks as a clear-cut example of co-evo- Department of Forest Entomology, Forestry and Forest Products Research Institute, 1 Matsunosato, Tsukuba, Ibaraki 305-8687, lution, leading to adjustment of sensory physiology as well Japan as behaviors in predator and prey. Several authors beginning with the “father of moth hear- * A. Surlykke ( ) ing physiology”, Roeder (Roeder 1974) and later many Department of Biology, University of Southern Denmark, Campusvej 55, 5230 Odense M, Denmark others, in particular Fullard (Fullard 1998), have written e-mail: [email protected] excellent broad reviews outlining the sensory ecology of 1 3 112 J Comp Physiol A (2015) 201:111–121 acoustic adaptations of many moth species from areas with Pyraloidea, as well as in Drepanoidea the ears are found different selection pressure from a varying number of sym- at the base of the abdomen and have four sensory cells, patric bats. Our aim with this review is not to repeat what A1-A4. In those Sphingidae (Bombycoidea) species, which they already did, but to build upon their data and update can hear, ears are located on the proboscis. Thus, moth ears with many new findings. are highly diverse in placement and morphology, but quite similar in shape of the threshold curve. Even in Hedyloi- dea, the nocturnal sister group to butterflies, the ears at the Moth hearing wing base are tuned to ultrasonic frequencies comparable to other nocturnal Lepidoptera. Ears have evolved independently in many insect groups, The frequency range of hearing is roughly the same in presumably reflecting the fact that with an exoskeleton it all moth groups: the ears are most sensitive to ultrasonic is “easy” to make an ear from mechanoreceptors attach- frequencies with best frequencies ~20–60 kHz for the most ing to the surface (Fullard and Yack 1993; van Staaden sensitive A-cell, A1 (Fig. 1). In general, there is a cor- and Römer 1998). In moths, tympanal hearing organs have relation between size and best frequency, such that large evolved independently at least five times (Minet and Sur- moths are tuned to lower frequencies than smaller moths lykke 2003; Greenfield 2014). In the superfamily Noc- (Surlykke et al. 1999). In moths with two or four sensory tuoidea, the ear is placed on the metathoracic segment and receptors, the A2-A4 cells have higher thresholds, extend- has two sensory cells, A1 and A2, except in the Notodonti- ing the dynamic range of the ear from the approximate dae, where only one A-cell is found. In Geometroidea and 20 dB of each cell. In all moths, where threshold curves Fig. 1 Hearing in moths. Ears and hearing threshold curves for (Tym) in its frame with scolopidia 1–4 viewed from the dorsal cham- moths from three different superfamilies. a Noctuoidea have ears on ber. The lower panel shows threshold curves for female (red), male the metathorax with two sensory cells attached directly to the tympa- (blue), and average (black) Drepana arcuata. (Surlykke et al. 2003). c num (Tym). The lower panel shows individual and average thresholds Some hawkmoths, Sphingidae (Bombycoidea) have ears made of the for the most sensitive A-cell in Axylia putris (Noctuidae) from Den- palp and pilifer mouthparts. In upper left panel the right palp (par) mark. b Drepanoidea, hook tip moths, have ears (upper left, black and of the Death’s head moth, Acherontia atropos, has been deflected white arrows showing location) with internal tympanal membranes to show the pilifer (pir). The scale-plate (asterisk) on the palp func- as a partition wall between the dorsal (dc) and ventral (vc) air cham- tions as tympanum. Threshold curves for A. atropos are shown below bers. Sound presumably enters through the anterior external mem- (adapted from Göpfert et al. 2002) brane (aem). Below representation of the curved tympanic membrane 1 3 J Comp Physiol A (2015) 201:111–121 113 have been determined also for less sensitive sensory cells, produce sounds in the bat frequency range also supports the the best frequency (frequency with the lowest threshold) is notion that moth ears evolved originally to detect bats. If the same for all the A-cells of the ear, which means that moth ears pre-existed sound production, the sensory bias moths are tone-deaf and cannot discriminate between dif- (Ryan et al. 2001) forced males to produce sounds that ferent frequencies (Miller and Surlykke 2001; ter Hofstede would fall into the sensitive frequency range of the females et al. 2011, 2013). The sensitivity at the best frequency is (Nakano et al. 2013; Greenfield 2014). In the following in the range of 25–45 dB SPL. Sensitivity is also corre- section, we will discuss the three main scenarios for sound lated to size, such that larger moths are not only tuned to communication in moths: (1) interspecific acoustic interac- lower frequencies, but also more sensitive (Surlykke et al. tion with bats, (2) long- and short-distance loud intraspe- 1999). Moth species vary greatly in size, and larger moths cific sexual communication, and (3) short-distance whis- should be more conspicuous to bats because they provide pering “private” intraspecific communication. a larger reflective surface for echolocation calls and thus a greater target strength, which will enable echolocating bats to detect them at greater distances than small moths. Moth sound communication The lower thresholds of larger moths compensate for their increased conspicuousness to bats by enabling them to Loud sound production for interspecific interaction also detect bats at greater distances, and relative detection with bats distances are roughly constant across moth sizes: In spite of less sensitive hearing, moths can detect bats around ten Many species from one family, Arctiidae (subfamily Arc- times the distance where bats can detect moths, because tiinae in the family Erebidae), within the superfamily Noc- moths are detecting the outgoing sound, while bats detect tuoidea, have evolved sound-producing organs, tymbals, the small fraction of sound returned as an echo. Thus, on the metathorax. Both males and females react to bat moths can detect bats at ca. 20–100 m, while bats can sounds by emitting intense ultrasonic clicks, either sin- detect moths (1–10 m) (Surlykke et al. 1999; Surlykke and gle clicks or trains of clicks depending upon whether the Kalko 2008). tymbal is smooth or striated with microtymbals. It seems There are several facts strongly suggesting that moth obvious that the clicks increase the survival chance of the ears evolved to detect echolocating bats: (1) the tuning of moth, but also evaluate the distance to the approaching bat all moth ears to bat frequency range in spite of morphologi- based on the repetition rate of the bat calls to elicit clicking cal differences (Fig. 1), (2) the positive correlation between (Fullard et al. 2007b; Ratcliffe et al. 2009). However, the how actively moths fly at night and their auditory sensitiv- exact function of clicks has been much debated. The three ity (ter Hofstede et al. 2008), (3) the absence of intraspe- most likely, non-mutually exclusive, hypotheses are star- cific acoustic communication in most moths (but see Sect.