Analysing body condition: mass, volume or density?

Abstract

Summary 1.Body condition (defined as the relative amount of energy reserves in the body) is an animal trait with strong ecological implications. In some animal taxa (e.g. arthropods), the external volume of the body part in which most nutrients are stored (e.g. abdomen) is used interchangeably with body mass to estimate body condition, making the implicit assumption that abdomen residual volume is a good surrogate of residual mass. However, the degree of correlation between these two measures should largely depend on the density of the nutrients stored. 2. We simulated two food-supplemented experimental groups of animals, each storing a slightly different amount of lipids either in their abdomens or in their entire bodies, and explored (i) how different estimates of condition were able to detect fixed differences between the groups; and (ii) how the amount of lipids stored could affect the outcome of non-intrusive measures of condition on a dichotomous variable (e.g. survival, mating success). We found that density body condition (body mass statistically controlled for structural body size and body volume) has much greater power to detect differences between experimental groups or effects on binary response variables than do classic mass/size or volume/size condition indices. 3. Using data on Lycosa tarantula (L.), a burrowing wolf spider, we report dramatic differences among these three indices in their ability to detect sex differences in the effect of feeding treatment on body condition at maturity. In particular, a plot of residual mass against residual volume reflecting nutrient density suggests that poorly fed spiders are nutritionally unbalanced, since well-fed spiders invest in nutrients of very different density. 4. Furthermore, using data on Scathophaga stercoraria (L.), the yellow dung fly, we found that an index of density condition was better at distinguishing condition differences among three populations than were mass or volume condition estimates alone. 5. We propose that including these three surrogates of condition (mass, volume and density) will substantially improve the accuracy of non-intrusive estimates of body condition, thus providing more powerful tools with direct application in a wide range of disciplines.

Body density and diving gas volume of the northern bottlenose whale (Hyperoodon ampullatus)

“…Diving lung volume is an important parameter in respiratory physiology because it affects the available oxygen store for aerobic metabolism during dives (Ponganis, 2012) and predicted levels of nitrogen absorption (Hooker et al, 2012) during a dive. Because lipids are less dense than other non-gas body components, body density is strongly affected by lipid store body condition (Biuw et al, 2003; Fields et al, 2005; Moya-Laraño et al, 2008). For marine divers, variations in net animal buoyancy driven by body density and diving gas volume lead to differences in swimming and gliding patterns during dives (Sato et al, 2002, 2003; Miller et al, 2004; Nousek-McGregor et al, 2014).…”

Does selective logging stress tropical forest invertebrates? Using fat stores to examine sublethal responses in dung beetles

“…Body fat content was extracted using 200 ml of n ‐hexane heated under reflux for 4 hr (18–22 cycles/hr) at 63–65°C. We opted to use Soxhlet fat extraction as it has been suggested as a reference for evaluation of other extraction methods (Tzompa‐Sosa et al., 2014; Wang et al., 2010) and has been previously used to assess the body fat content of dung beetles and other insects (Edwards, Division, & Box, 1988; Hart & Tschinkel, 2012; Moya‐Laraño et al., 2008; Tzompa‐Sosa et al., 2014). …”

Paternal Care Decreases Foraging Activity and Body Condition, but Does Not Impose Survival Costs to Caring Males in a Neotropical Arachnid

“…Between January and March 2009, we sampled 93 Iporangaia males in the field and, using an electronic caliper (precision of 0.01 mm) and an electronic scale (precision of 0.001 g), we took the following measurements from each individual: (a) dorsal scute length ( DSL ), (b) total body length ( TBL ), (c) body width ( BW ) at the widest portion of the opisthosoma, (d) body height ( BH ) at the highest portion of the opisthosoma, and (e) total body mass ( TBM ). The dorsal scute (or carapace) is a rigid structure that does not change in size with food acquisition and can be classified as a structural body size measure ( sensu [38]). The last five opisthosomal tergites, on the other hand, are not fused, but rather connected by a highly elastic membrane, allowing body expansion after a meal.…”