Matching habitat choice in Azure sand grasshoppers
Camacho, Carlos; Sanabria-Fernández, Alberto; Baños-Villalba, Adrián; Edelaar, Pim (2021), Matching habitat choice in Azure sand grasshoppers, Dryad, Dataset, https://doi.org/10.5061/dryad.qbzkh18dv
This data set includes measurements of substrate use by ground-perching grasshoppers (Sphingonotus azurescens) in relation to body colour before and after experimental manipulation of their original colour. The data were collected between May and September 2017 in an urban mosaic of dark and pale pavements located in an abandoned housing development area near Dos Hermanas (Seville, Spain; 37.306° N, 5.932° E). The analysis presented in this article is based on 80 adult females and 138 adult males.
The sampling area included a grid of four parallel streets (range: 223-335 m long) and one crossing street (395 m long). To detect grasshoppers, we walked up and down the streets sweeping from side to side a 30-cm diameter mesh net attached to a 1.5-m pole, which causes adults to jump up and fly a few meters. These were then caught with the same net. Capture sessions were conducted between 9:00-14:00 and 19:00-21:30 hours.
For each individual, we recorded sex, colour (% blackness), type of substrate on which it was present upon disturbance (dark asphalt or pale sidewalks/parking lots), GPS coordinates, date, time of capture, and temperature conditions (coded as: 1 = cool, 2 = mild, 3 = warm, 4 = hot). Each grasshopper was marked with a unique combination of 2 letters on the posterior part of both forewings using a black permanent marker pen (Staedtler permanent Lumocolor), unless already marked. Marked grasshoppers were not recaptured, because in virtually all cases the letter code could be identified from a distance using binoculars. For visual recaptures, we also recorded the type of substrate upon encounter, coordinates, date, and time of resighting using the same field methods as for first captures.
Upon first capture, individuals were alternatingly assigned to receive a dark (approximately 80% blackness) or pale (approximately 20% blackness) colour that resembled the extremes of the natural cline of colour variation. To experimentally mimic the natural colour of the palest and darkest individuals in the population, we applied a thin layer of white or black water-based (aquarelle) paint on those parts that are most likely to be relevant in self-colour assessment of grasshoppers: circumocular mask, dorsal region between the eyes, cheeks, thorax, femur, tibia, and forewings. Grasshoppers were released after painting close (< 5 m) to their capture site at the border between dark and pale substrates.
To investigate the effect of grasshopper colour on substrate choice, we ran Generalized Linear Mixed Models (GLMM) with a binomial error structure and logit link function. First captures (unpainted individuals) and visual recaptures (resightings of painted individuals) were analysed in separate models to examine the effects of the original and new colour, respectively. For both models, the dependent variable was the type of substrate chosen by grasshoppers, coded as dark asphalt (yes) or pale tiles/cement (no). Further details are provided in the main text of the article.
Description of variables:
DATE Date of capture.
DECIMALTIME Time of capture, in decimal hours.
THERMALCOND Temperature conditions, coded as: 1 = cool, 2 = mild, 3 = warm, 4 = hot.
IDENTITY Grasshopper identity, corresponding to a unique combination of 2 letters
SEX M= Male, F = Female
ORIGINALCOLOUR % blackness, based on a 100-level grey scale.
RESIGHTYN First capture (N) or resighting (Y)
SUBSTRATE Type of substrate on which the individual was found, coded as 1 = dark asphalt, 0 = pale sidewalks/parking lots
TREATMENT Type of colour manipulation
GPSLAT UTM Latitude
GPSLON UTM Longitude
TIMESINCECAPTURE Time since capture, in decimal hours.
Time since manipulation was modelled as binary variable (>48 h vs. ≤48 h) to improve model convergence and avoid overfitting. For model convergence reasons, sampling dates were grouped into 3-day periods. Note that, unlike conventional habitat selection analyses, our analyses do not require the incorporation of the relative availability of each substrate type because we are not interested in evaluating habitat selection in the broad sense, but whether habitat use differs between dark and pale grasshoppers after experimental manipulation of their phenotypes and subsequent release at the border of contrasting substrates.