DOI: 10.5061/dryad.q2bvq83gp
title: Data from: Differences in distress: variance and production of American crocodile (Crocodylus acutus) distress calls in Belize

description: Acoustic communication of American Crocodiles (Crocodylus acutus) is
 relatively understudied. Our overall aim was to determine the acoustic
 structure of wild American Crocodile distress calls, distinguish call
 differences among size classes (hatchling, juvenile, sub-adult, and
 adult), and investigate call production on a gradient of human
 disturbance. American Crocodile distress calls have strong frequency
 modulation and are comprised of multiple harmonics in a downsweeping
 pattern. Measured parameters (total duration, first quartile duration,
 maximal frequency, first quartile frequency, end frequency, slope of first
 quartile, slope of last quartiles) differed significantly among size
 classes (P &lt; 0.05). Hatchling distress calls are higher in
 frequency and strongly modulated, whereas calls produced by sub-adults and
 adults showed little modulation, are lower in frequency, and have greater
 overall duration. Proportion of crocodiles that produced distress calls
 during capture differed by size class and sampling location, particularly
 adult distress calls which are reported here to be produced with
 undocumented frequency. We determined that American Crocodiles of all size
 classes produce distress calls at varying rates among study sites. Our
 results demonstrate that American crocodiles produce distress call more
 frequently at sites with higher anthropogenic activity. Measured call
 parameters of juveniles and hatchling American crocodiles also varied
 among sites in relation to human disturbance. Calls recorded at sites of
 high anthropogenic impact have increased duration and less modulation
 which may adversely affect response to emitted distress calls.
 Proportional and call parameter variances suggest anthropogenic activity
 as a driver for increased call production and alteration of call
 parameters at high human-impacted sites.
description: We recorded American Crocodiles on Ambergris Caye from May –
 August 2015, December 2015 – January 2016, and March of 2016.  Concurrent
 with ongoing population surveys, we collected recordings on Caye Caulker
 in January, March, and August 2016. We collected acoustic recordings from
 BAL June – August 2016. We recorded distress calls <i>ad
 libitum</i> in the field during capture events as recording
 conditions and difficulty of capture determined the actual number of
 recordings collected. Independent of acoustic recording, we noted
 incidence of call production for each individual capture as not all
 captures resulted in call production or successful recording. We collected
 distress call recordings from May 2015 – January 2016 using a Marantz
 PDM661 or Roland R-26 digital recorder coupled with a Senheiser ME67
 shotgun directional microphone. During the March–August 2016 field
 seasons, we employed a Sony Zoom H5 digital recorder with an XY modular
 microphone capsule. We recorded all calls in .wav format at a sampling
 rate of 44.1 kHz and 24 bits per sample. Although equipment differed,
 sampling protocol remained consistent. We held hatchling and juvenile
 crocodiles in hand and collected recordings approximately 50 cm from the
 microphone.  We recorded sub-adult and adult crocodiles from 1–2 m from
 the microphone to ensure safety of personnel. We also recorded any calls
 or responses from nearby conspecifics, noting size class if possible, when
 we recorded distress calls of captured crocodiles.  
 <strong>Sound Analysis    </strong>
 We performed acoustic analysis to determine the structure of
 distress calls for each size class. We analyzed five call per individual,
 in one case only 3 calls were analyzed due to heavy background noise, and
 measured seven acoustic variables, two temporal and five spectral, using
 Raven Pro 1.5 acoustic analysis software (Bioacoustics Research Program,
 2014).  We used spectrographic analysis (window size 1024, overlap 80%) of
 the fundamental frequency to determine maximal frequency
 (F<sub>max</sub>, Hz), frequency at end of first quartile
 (F<sub>1/4</sub>, Hz), and final frequency
 (F<sub>end</sub>, Hz; Fig. 3A).  Using call oscillograms, we
 measured temporal properties for total duration (DT, s) and duration of
 the first quartile (D<sub>1/4</sub>, s; Fig. 3B). Measurement
 windows were drawn around the fundamental frequency and the maximal
 frequency values at the beginning, end, and first quartile were recorded.
 We used frequency and temporal measurements to calculate call modulation
 of the first temporal quartile slope (Slope 1, Hz/s, calculated as
 (F<sub>1/4</sub>−F<sub>max</sub>)/D<sub>1/4</sub>), and the slope of the remaining three temporal quartiles (Slope 2, Hz/s, calculated as (F<sub>end</sub>−F<sub>1/4</sub>)/( DT−D<sub>1/4</sub>)) (Vergne et al. 2012). Concurrent to call measurements, we recorded number of calls produced by each individual for 10, 20, and 30 second intervals as total recording time varied between individual crocodiles.  We began call counts at the first recorded call for each individual We used size designation to organize and analyze distress call recordings by overall size class.  We performed statistical analyses using RStudio version 0.99.902 (RStudio Team, 2015). Our call parameter data did not meet the assumptions of normality or homogeneity of variance (P &lt; 0.05)(Boucher 2017).  Thus, we analyzed call parameter means (DT, D<sub>1/4</sub>,<sub> </sub>F<sub>max</sub>, F<sub>1/4</sub>, Fend, Slope 1, Slope 2) among size classes (hatchling, juvenile, sub-adult, adult) by performing non-parametric Kruskal-Wallis (<i>H</i>) tests. We used Mann-Whitney post-hoc testing with Bonferroni correction to determine pairwise variance of call parameters between size classes following a significant Kruskal-Wallis test (<i>P</i> &lt; 0.05).  We tested size class differentiation by call parameters using a principal component analysis cross-validated by discriminant function analysis which assigned each recorded individual to a size class based on call parameters. To determine variance in number of calls produced (10, 20, 30 second intervals) we performed one-way analysis of variance (ANOVA) tests as these data met assumptions of normality (<i>P</i> &gt; 0.05) and equality of variance (<i>P</i> &gt; 0.05). We analyzed differences in call production by determining the total number of American Crocodiles captured throughout the study for each location and compared them by size class. To account for small sample size, we aggregated data for Caye Caulker and BAL sites.  We performed a 2-sample test for equality of proportions with continuity correction (Newcombe, 2008) on total call production proportions among size classes.  Designed for small sample sizes (&lt; 5), we performed Fisher’s Exact Test (Agresti, 2002) to determine inequality in call production by size classes between Ambergris Caye, and the combined Caye Caulker and BAL sites.  As a compliment to proportional tests, we performed Mann-Whitney tests to determine variance of spectral parameters of hatchling and juvenile calls between Ambergris Caye and aggregated Caye Caulker and BAL sites.