T Chabert’s research while affiliated with French National Centre for Scientific Research and other places

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Figure 1. Distribution of crocodilian juvenile calls within the acoustic space. Each circle represents a single individual. The diameter of disks is proportional to the individual’s body length. Alligatoridae are in dark blue (American alligator) and sky blue (spectacled caiman). Crocodylidae are in red (Nile crocodile), plum (Morelet’s crocodile), and pink (Orinoco crocodile). The acoustic structure of calls was first described using 13 parameters in the frequency and temporal domains (see text for a description of the parameters), further reduced using a principal component analysis into two independent Acoustic Dimensions (AD1 and AD2). We then calculated the mean AD1 and AD2 for each recorded individual, so that each individual could be positioned on the two-dimensional acoustic space. The first axis of the acoustic space (AD1) is mainly related to the distribution of energy among the frequency spectrum (with higher scores meaning wider frequency band). The second axis of the acoustic space (AD2) is mainly related to the pitch (with higher scores indicating higher pitched calls). The first axis of the acoustic space separates out well the American alligator (family Alligatoridae) from the Crocodylidae. Both axes contribute to separate out juveniles based on their individual body length (see text for details). The tree shows phylogenetical relationships between species 8,10,59 . 
Figure 2. Correlations between individual size (total body length) and (a) the calls’ mean pitch, and (b) the calls’ centroid of the frequency spectrum. Each dot represents a single individual. Both acoustic parameters decrease with individual size, underlying that smaller individuals utter higher pitched calls with a wider frequency bandwidth 59 . Alligatoridae are in dark blue (American alligator) and sky blue (spectacled caiman). Crocodylidae are in red (Nile crocodile), plum (Morelet’s crocodile), and pink (Orinoco crocodile). Regression lines are shown for the alligator and the Nile crocodile only. Slopes of the regression lines are not significantly different between both species (see results). 
Figure 3. Map of the study area in the Panhandle, Okavango Delta, Botswana. Numbered dots indicate the position of the 13 crocodile nests mapped in December 2014. White dots correspond to the nine nests where we did the playback experiments in January 2015. 
Figure 4. Spectrograms of experimental calls. ( a ) Call emitted by a Nile crocodile juvenile measuring 35.5 cm (total body length; “small” individual). ( b ) Call emitted by a 64.5 cm Nile crocodile juvenile (“large” individual). ( c ) Synthetic signals; from left to right: high-pitched synthetic pure tone modulated in frequency (SYNTsmall); low-pitched pure tone modulated in frequency (SYNTlarge); unmodulated pure tone (NOFM, control signal). Graphical representation produced with the R package Seewave (Sueur et al. 2008) with spectrograms set to Hanning window and a FFT window length of 512 with 90% overlap. 
Figure 5. Behavioural response of wild breeding female Nile crocodiles to calls of “large” and “small” juveniles. Most females have been tested with both stimuli (the lines on the figure link responses from the same females; those females which have been tested once are represented by single dots). Solid grey lines = females that approached more towards the loudspeaker when challenged with calls from a small juvenile than when tested with calls from a large juvenile. Dashed grey lines = females that responded equally to both experimental signals or approached more towards the loudspeaker when hearing calls from a large juvenile. Positive values on the reaction intensity y-axis mean an approach to the loudspeaker. Negative values mean a retreat (see text for details). 

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Size does matter: crocodile mothers react more to the voice of smaller offspring OPEN
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November 2015

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40 Citations

T Chabert

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A Colin

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Parental care is widespread in Archosaurs (birds, crocodilians, dinosaurs and pterosaurs), and this group provides a useful model for the evolution of parent-offspring interactions. While offspring signalling has been well-studied in birds, the modulation of parental care in crocodilians remains an open question. Here we show that acoustic communication has a key role in the dynamics of crocodilian' mother-offspring relationships. We found embedded information about the emitter's size in juvenile calls of several species, and experimentally demonstrated that Nile crocodile mothers breeding in the wild are less receptive to the calls of larger juveniles. Using synthetized sounds, we further showed that female' reaction depends on call pitch, an important cue bearing size information. Changes in acoustic interactions may thus go with the break of maternal care as well as dispersal of juvenile crocodilians. This process could have characterized other archosaurs displaying rapid early growth such as dinosaurs and pterosaurs.

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Citations (1)

... Spectral cues are also contained in hatchling calls that mediate protective behavioral responses of guarding adults toward groups of young (Chabert et al., 2015). ...

Reference:

Gharial acoustic signaling: Novel underwater pops are temporally based, context‐dependent, seasonally stable, male‐specific, and individually distinctive
Size does matter: crocodile mothers react more to the voice of smaller offspring OPEN
T Chabert

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A Colin

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[...]

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