Miscel.lania Zoologica 20.2 (1997) 1 The use of regression equations for the estimation of prey length and biomass in diet studies of insect ivore vertebrates J. A. Hódar Hódar, J. A,, 1997. The use of regression equations for estimation of prey length and biomass in diet studies of insectivore vertebrates. Misc. Zool., 20.2: 1-10. The use ofregression equations for estirnation ofprey length and biornass in diet studies of insectivore vertebratex- This work presents two series of allometric regression equations for the estimation of arthropod body length and mass from recognizable remains in food samples of vertebrates. The utility of the equations is tested by analysing faeces from a reptile species Tarentola rnauritanica. The application of these equations greatly increases the number of prey for which body length and biomass is estimated. Consequently, the application of regression equations is encouraged for diet studies, although it is also advisable either to previously test the applicability of published equations or to develop new equations if necessary. Key words: Arthropoda, Body length, Body weight, Diet studies, Ecological methods, Regression equations. (Rebut: 26 V 97; Acceptació condicional: 14 VI1 97; Acc. definitiva: 30 X 97) José A. Hódal; Depto. Biología Anirnaly Ecología, Fac. de Ciencias, Univ de Granada, E- 18071 Granada, España (Spain). Financia1 support was provided by a PFPl grant from the Spanish Ministerio de Educación y Ciencia and a grant from DGICYT number PB90-0852. ISSN: 02 1 1-6529 O 1997 Museu de Zoologia 1 Hódar lntroduction which is needed for the application of the equation. An improvement in this procedure Studies on diet analysis are necessary to un- has been the use of equations estimating derstand the ecology of most organisms. Nev- both body length and mass from measure- ertheless, the methodology of diet analysis is ments of characteristic parts of the prey, still often questionable and even inaccurate or recognizable among the remains usual found lax. In birds, for instance, diet is sometimes in a diet study (CALVER& WOOLLER, 1982; D~AZ not directly measured, but assumed from & Dínz, 1990). Despite its considerable utility, morphology, behaviour, or prey availability this procedure is still not widely employed (ROSENBERG& COOPER, 1990). In the best cases, by many researchers. diet studies consist simply of a qualitative The aim of the present study is to demon- and quantitative description of diet compo- strate the utility of these equations in stand- sition. Only recently has there been an in- ard studies of diet analysis of insectivorous creasing interest in more detailed diet infor- vertebrates, and the advantage of their avail- mation, and methodology has improved to ability, even when it is necessary to develop provide the diet description in terms of them. biomass or prey size, in order to evaluate how profitable the diet is to a given animal (ROSENBERG& COOPER, 1990). Insects, and ar- Material and methods thropods in general, deserve special atten- tion in this sense because of the importance Arthropod sampling and laboratory procedure of the latter as food for a variety of animals (and even plants, ZAMORA,1995). One diffi- The methodology followed both in the field culty is, however, that direct measuring and sampling and the lab procedure was the weighing of the prey is usually impossible same as in H~DAR(1996), and will be de- because prey specimens are often fragmen- scribed here only briefly. Arthropods were tary, due to manipulation or digestion by caught during the period 1990-1992, mainly predators. Complete prey can be found only in three different zones of the Guadix-Baza at times, e.g. in studies based on stomach- Basin (Granada province, south-eastern content analysis. However, as these aggres- Spain), and stored in Scheerpeltz preserva- sive techniques imply sacrificing the animal tive. From this collection, 41 groups were studied, researchers tend to avoid their use. selected, both because of their numerical In contrast, non-aggressivetechniques, such significance in the arthropod community as faecal analysis, are widely accepted, but sampled and their role as prey for different offer only prey remains to estimate the prey species of insectivorous birds and reptiles. biometry. Under these circumstances, tech- Selection was made based on both the tax- niques to estimate arthropod length and onomy (mainly Order, following BARRIENTOS, biomass from a variety of morphological 1988) and the morphological characteristics data constitute valuable tools for ecologi- of specimens. In hemimetabolous insects, cal studies. nymphs and imago were pooled, whereas in In the literature, these estimates have been holometabolous, equations were performed performed by means of widely differing pro- with larvae and imago were separated. cedures, restricted at times rather to a spe- Heteroptera was subdivided into "heavy" and cific subject (HERRERA,1978; CALVER& WOOLLER, "slender", the former for insects with heavy 1982; SMILEY & WISDOM,1982). The most com- and wide bodies (e.g. Scuterellidae) and the mon procedures have been estimations from latter for those with slender and light bod- the mean of each taxonomic group (ignor- ies (e.g. Reduviidae). Homoptera did not in- ing the individual weight variance) or the clude Aphidae, Hymenoptera did not include application of regression equations from lin- Formicidae. eal measurements (ROGERSet al., 1976; ROGERS The term OTU (Operational Taxonomic et al., 1977; SCHOENER, 1980; GOWING& RECHER, Unit, sensu SNEATH & SOKAL, 1973) was used to 1984; SAMPLE et al., 1993). In this latter case define these 41 groups. The specimens in- however, the prey remains do not always cluded in the regression calculations were allow measurement of prey body length, arbitrarily selected within the body-size gra- Miscel.lania Zoologica 20.2 (1997) 3 Fig. 1. Examples of the measurements taken on the arthropod bodies. Arthropods are represented without legs for simplicity: 1. Scorpionida; 2. Soliphuga chelicera; 3. Araneae; 4. Araneae chelicera; 5. Lepidoptera larva mandible; 6. Dermaptera; 7. Coleoptera; B. Body length; P. Pronotum or prosoma width; Q. Chelicera; F. Forceps or caudal appendages; H. Head width; E. Elytrum length; M. Mandible. Ejemplos de las medidas tomadas en los cuerpos de los artrópodos. Los ejemplares se representan sin patas por simplicidad. 1. Scorpionida; 2. Quelícero de Soliphuga, 3. Araneae; 4. Quelícero de Araneae; 5. Mandíbula de larva de Lepidoptera; 6. Dermaptera; 7. Coleoptera; B. Longitud del cuerpo; f! Anchura del pronoto o del prosoma; Q. Quelícero; E Forceps o apéndices caudales; H. Anchura de la cabeza; E. Longitud del élitro; M. Mandíbula. 4 Hódar dient of the appropriate OTU, in an effort to els, showing that body mass estimation from cover the gradient as fully as possible. a linear measurement fits an exponential equa- For each individual, the total body length tion better than a lineal or other type of (B) without appendages was measured. measurement (see e.g. SCHOENER, 1980; GOWING Specimens were then dried in an oven at & RECHER, 1984). In the second case, a linear 70°C for 24 h and weighed with an elec- equation InB = a + b (InL) was used to esti- tronic scale (precision 0.01 mg). Measure- mate body length from length of body frag- ments were made mainly with a binocular ments, and this equation was not transformed microscope 10-40x with an ocular microm- because one linear measurement (L1) was es- eter, and sometimes with a digital calliper, timated from another linear measurement (L1) both with 0.05 mm precision. Once weighed, (Dinz & Dinz, 1990). Significance of the regres- specimens were rehumidified, and several sions was corrected with post-hoc Bonferroni body parts were measured, depending on sequential adjustments (RICE,1989). the OTU to which the specimen belonged. The effectiveness of applying these equa- The parts selected were those usually re- tions is demonstrated using a reptile species, maining recognizable and measurable after Tarentola mauritanica, the diet of which was the passage through the digestive tract of studied by means of faecal analysis (J. A. an insectivore (RALPHet al., 1985; MOREBY, Hódar & J. M. Pleguezuelos, in prep.), re- 1988; pers. obs.). These measurements were cording the body fragments from which esti- (fig. 1): H. Maximum head width, including mations of both body weight and length ommatidia; P. Maximum prosoma or pro- were made. Analysis of diet samples followed notum width, except in Scorpionida, at the the usual procedures in these cases: samples eye level; E. Elytron length, measured from were dispersed under a binocular microscope the humeral calus to the tip, in Coleoptera; and the remains identified and measured Q. Chelycera length, in Scorpionida, total (ROSENBERG& COOPER, 1990). length of the pedipalp claw; in Araneae, chelicera claw length; in Soliphuga, cheli- cera jagged part; F. Total length of the for- Results and discussion ceps or caudal appendages, in Dermaptera; M. Length of the jagged part of the mandi- A total of 473 arthropods were used to de- ble, in Orthoptera and Lepidoptera larvae. velop the equation series used here, imply- ing 473 measurements of body weight and Statistical analysis length, and 840 of different body parts. The parameters estimated for the regression equa- Regressions were performed with data trans- tions are shown in the table 1. More than formed to logarithms. This procedure usually 90% of equations have R2 values up to 0.8 normalizes and reduces heteroscedasticityof (80% of the variance explained), and are the data (EDWARDS,1985; ZAR, 1996). The re- highly significant. In fact, only two equa- gression analysis used with the transformed tions were non-significant (p > 0.05) after data was always linear. However, since there sequential Bonferroni correction. It is also were equations estimating body weight from noteworthy that a large number of individu- length measurements. but also equations es- als were not needed to construct significant timating body length from length measure- equations (Dínz & Dínz, 1990; H~DAR,1996).