The size-grain hypothesis: a phylogenetic and field test

Abstract.  1. The size-grain hypothesis predicts that environmental rugosity results in positive allometric scaling of leg length on body length because of changes in locomotion costs.
2. The scaling of leg length and body length in ants was re-examined using phylogenetic independent contrast methods, and the allometric relationship found by Kaspari and Weiser ( Functional Ecology , 13 , 530–538, 1999) was supported.
3. The size-grain hypothesis was tested further by comparing the body sizes of ants from areas of contrasting habitat complexity in two different savanna habitats. No support for the size-grain hypothesis was found. Small body size classes were no more speciose in the rugose than in the more planar environment, and small ants were more abundant in the planar environment.     

Habitat complexity facilitates coexistence in a tropical ant community

The role of habitat complexity in the coexistence of ant species is poorly understood. Here, we examine the influence of habitat complexity on coexistence patterns in ant communities of the remote Pacific atoll of Tokelau. The invasive yellow crazy ant, Anoplolepis gracilipes (Smith), exists in high densities on Tokelau, but still coexists with up to seven other epigeic ant species. The size-grain hypothesis (SGH) proposes that as the size of terrestrial walking organisms decreases, the perceived complexity of the environment increases and predicts that: (1) leg length increases allometrically with body size in ants, and (2) coexistence between ant species is facilitated by differential habitat use according to body size. Analysis of morphological variables revealed variation inconsistent with the morphological prediction of the SGH, as leg length increased allometrically with head length only. We also experimentally tested the ability of epigeic ants in the field to discover and dominate food resources in treatments of differing rugosity. A. gracilipes was consistently the first to discover food baits in low rugosity treatments, while smaller ant species were consistently the first to discover food baits in high rugosity treatments. In addition, A. gracilipes dominated food baits in planar treatments, while smaller ant species dominated baits in rugose treatments. We found that the normally predictable outcomes of exploitative competition between A. gracilipes and other ant species were reversed in the high rugosity treatments. Our results support the hypothesis that differential habitat use according to body size provides a mechanism for coexistence with the yellow crazy ant in Tokelau. The SGH may provide a mechanism for coexistence in other ant communities but also in communities of other terrestrial, walking insects that inhabit a complex landscape.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Allometry of the limb bones of mammals from shrews (Sorex) to elephant (Loxodonta)

Measurements have been made of the principal leg bones of 37 species representing almost the full range of sizes of terrestrial mammals. The lengths of corresponding bones tend to be proportional to (body mass)^0·35 and the diameters to (body mass)^0·36, except in the family Bovidae in which the exponents for length are much nearer the value of 0·25 predicted by McMahon's (1973) theory of elastic similarity. Comparisons are made between mammals of similar size belonging to different orders.     

The size–grain hypothesis: do macroarthropods see a fractal world?

Abstract.  1. In the size–grain hypothesis (a) long legs allow walking organisms to step over gaps and pores in substrate but prohibit them from entering those gaps; (b) the world is more rugose for small organisms; and (c) the relative cost of long legs increases as organisms grow smaller. The hypothesis predicts a positive allometry of leg length ( = mass ^b where b > 0.33 of isometry), a pattern that robustly holds for ants.
2. Toward testing for leg length allometries in other taxa, arthropods were extracted from the Panama leaf litter and measured. Three common taxa (spiders, diplopods, Coleoptera) yielded b s that exceeded 0.33 while three others (Acarina, Pseudoscorpiones, and Collembola) did not. The exponent b tended to increase ( P = 0.06, n = 7) with an arthropod taxon's average body mass.
3. Since leg length in cursorial organisms tends toward isometry in very small and very large taxa (i.e. mammals) this suggests that the size–grain hypothesis may best apply at a transition zone of intermediate body mass: the macroarthropods.
4. Body length was a robust predictor of mass in all groups despite variation in shape.     

Environmental rugosity, body size and access to food: a test of the size-grain hypothesis in tropical litter ants

In terrestrial walking organisms, long legs help to decrease the cost of running, allowing animals to step over environmental interstices rather than walking through them. However, long legs can complicate the infiltration of these interstices, which may contain food sources and refugia. Since the number of environmental interstices perceived by an organism (rugosity) increases as it body size decreases (size-grain hypothesis, SGH), natural selection should favor proportionally smaller legs with decreasing body size. Recent work demonstrated that ants fit this hypothesis. We experimentally tested the assumption of the SGH that small ants, which have proportionally smaller legs than larger ants, are more successful in exploring environmental interstices because they can easily penetrate them. We examined the ability of tropical litter ant species with different body sizes to access food baits in 'landscapes' (=plots) with different levels of rugosity and food exposure. In the first experiment, three levels of landscape rugosity were defined by manipulating the density of leaf litter placed on the ground plots: a) plain landscape: no litter fall, b) intermediate rugosity (∼0.5 kg of litter fall) and c) high rugosity (∼1 kg). In a second experiment, food baits were in plain landscapes, exposed or covered by leaf litter. The body lengths of ants that first accessed food baits ranged from 1.5 to 12 mm. Ants that first reached food baits in the most rugose landscapes were ∼40% smaller than ants that first found baits in plain landscapes. Smaller ants were also the first to access covered food. The application of a phylogenetic comparative method suggested the same patterns. We conclude that these results support the size grain hypothesis. Environmental rugosity might have operated as a selective force to shape the morphological characteristics of litter ant species.     

Allometry between leg and body length of insects: lack of support for the size–grain hypothesis

• 1 The size–grain hypothesis ( Kaspari & Weiser, 1999 ) states that (1) as organisms decrease in size, they perceive their environment as being more rugose; (2) long legs allow organisms to step over obstacles but hinder them from entering small gaps; and (3) as the size of an organism decreases, the benefits of long legs begin to be outweighed by the costs of construction. Natural selection should therefore favour proportionally longer legs in larger organisms, thereby leading to a positive allometry between leg and body length (scaling exponent b > 1).
• 2 Here we compare the scaling exponent of leg‐to‐body length relationships among insects that walk, walk and fly, and predominantly fly. We measured the lengths of the hind tibia, hind femur, and body length of each species.
• 3 The taxa varied considerably in the scaling exponent b. In seven out of ten groups (Formicidae, Isoptera, Carabidae, Pentatomidae, Apidae, Lepidoptera, Odonata adult), b was significantly greater than one. However, there was no gradual decrease in b from walking to walking/flying to flying insects.
• 4 The results of the present study provide no support for the size–grain hypothesis. We propose that leg length is not only affected by the rugosity of the environment, but also by (1) functional adaptations, (2) phylogeny, (3) lifestyle, (4) the type of insect development (hemimetabolism or holometabolism), and (5) constraints of gas exchange.

     

Effect of body size, temperature, and salinity on the routine metabolism of larval and juvenile spotted seatrout

Routine oxygen consumption rates of young spotted seatrout Cynoscion nebulosus (Sciaenidae) were measured over a range of temperatures (24, 28, 30 and 32° C) and salinities (5, 10, 20, 35 and 45). Larvae and juveniles, 4·1–39·5 mm standard length ( L _S), ranging several orders of magnitude in dry body mass were used to estimate the mass–metabolism relationship. Oxygen consumption (μl O₂ larva⁻¹ h⁻¹) scaled isometrically with body mass for larvae <5·8 mm L _S(phase I, slope = 1·04) and allometrically thereafter (phase II, slope = 0·78). The inflection in the mass–metabolism relationship coincided with the formation of the hypural plate and an increase in the relative tail size of larvae. Salinity did not have a significant effect on routine metabolism during phase I. Temperature and salinity significantly affected routine metabolism during phase II of the mass–metabolism relationship. The effect of salinity was temperature dependent, and was significant only at 30° C. Response surfaces describing the environmental influences on routine metabolism were developed to provide a bioenergetic basis for modelling environmental constraints on growth.     

Characteristics of egg and larval production in captive bluespotted gobies

Spawning of the Hawaiian coral-reef goby Asterropteryx semipunctata was diurnal, occurring at various times throughout the day. Mean length of eggs deposited in nests was 0·76 mm (range 0·67–0·84); mean egg width was 0·47 mm (range 0·41–0·52). Clutch size varied from 296 to 1552 eggs (mean=886±309), and was independent of standard length, total body weight, and body condition. Mean relative clutch size was 1·59 eggs mg^-1 total body weight (range 0·84–2·43). Clutches hatched 4–5 nights after being deposited in a nest. Mean notochord length of newly-hatched larvae was 1·88 mm (range 1·60–2·04). The minimum period of time that elapsed between egg deposition and subsequent growth of a new batch of oocytes to spawning size was 5–6 days, providing a reasonable estimate of minimum spawning interval. Compared with other gobiids, tropical species tend to have shorter incubation periods, smaller eggs and smaller larvae at hatching.     

Elemental stoichiometry of Drosophila and their hosts

1. Nitrogen (N) and phosphorus (P) availabilities are important ecological determinants of resource use in nature. Despite the wide range of hosts used by species of the genus Drosophila , elemental composition of natural resources of these flies has never been investigated.
2. Total body N and P contents were determined in seven species of wild-caught Drosophila , their natural hosts, and artificial diets routinely used to rear these flies in the laboratory. The flies tested included D. hydei, D. arizonae, D. simulans and D. pseudoobscura collected from rotting fruit (melons), and the cactophilic D. nigrospiracula, D. mojavensis and D. pachea collected from their specific host plants, Saguaro, Organpipe and Senita cactus, respectively.
3. Natural hosts varied in elemental composition, with fruit showing higher N (2·8–4·3% dry mass) and P (0·50–0·67%) levels compared with cacti (0·5–1·6% N; 0·01–0·29% P). No consistent differences in N and P levels were found between healthy and necrotic cactus tissue.
4. Total body N and P also varied among Drosophila species. This variation mirrored the levels of N and P found in the respective hosts and laboratory diets. N:P ratios were consistently lower in female flies compared with conspecific males suggesting phosphorus demands during oogenesis are high.
5. Potential mechanisms by which Drosophila deal with N or P limitation in nature are discussed.     

Size–abundance relationships in Florida ant communities reveal how ants break the energetic equivalence rule

1. Ants are among the most abundant terrestrial organisms, yet little is known of how ant communities divide resources because it is difficult to measure the number of individuals in colonies and the density of colonies. 2. The body size–abundance relationships of the ants of five upland ecosystems in Florida were examined. The study tested whether abundance, energy use, and total biomass were distributed among species and body sizes as predicted by Damuth's energetic equivalence rule. Estimates of average worker body size, colony size, colony mass, and field metabolic rates were used to examine the relationships among body sizes, energy use, and total biomass. 3. Analyses revealed significant variation in energy use and did not support the energetic equivalence hypothesis. Specifically, the energy use and total standing biomass of species with large workers and colonies was much greater than smaller species. 4. These results suggest that larger species with larger colonies account for a disproportionate fraction of the total abundance and biomass of ants. A general model of resource allocation in colonies provides a possible explanation for why ants do not conform to the predictions of the energetic equivalence rule and for why ants are so abundant.     

Influence of pregnancy on the thermal biology of the lizard, Sceloporus jarrovi: why do pregnant females exhibit low body temperatures?

1. Selected body temperatures of female lizards, Sceloporus jarrovi , were measured on a photothermal gradient during late pregnancy and again when postpartum, and pregnant females were subjected to one of three fluctuating temperature regimes that simulated body temperatures of (1) pregnant females, (2) postpartum females or (3) allowed normal thermoregulation.
2. Overall, females selected lower body temperatures when pregnant (mean = 32·0°C) than when postpartum (mean = 33·5°C).
3. Females regulated body temperature more precisely when pregnant than when postpartum as judged by their smaller variances in body temperature throughout the day.
4. When pregnant, females selected a lower mean maximum body temperature (mean: pregnant = 32·8°C; postpartum = 34·5°C) than when postpartum, but selected mean minimum body temperatures did not differ.
5. None of the experimental temperature treatments was detrimental to pregnant females. Female body length increased during pregnancy but the rate of increase did not differ among treatments. Moreover, length-adjusted body mass of postpartum females did not differ among treatments.
6. Pregnant females that experienced postpartum body temperatures produced neonates that were smaller in body mass and length than pregnant females that experienced pregnant body temperatures and females that were allowed to thermoregulate.
7. For neonates resulting from the postpartum body temperature treatment, the disparity in the body length, but not mass, was still observed at 9 days of age, although survival and growth of neonates was high and did not differ among treatments.
8. The results demonstrate that pregnant females could maintain higher postpartum body temperatures without compromising their physical condition, but select relatively low body temperatures, presumably to avoid decrements in offspring fitness.     

The influence of sex and body size on food habits of a giant tropical snake, Python reticulatus

1. In many animal species, dietary habits shift with body size, and differ between the sexes. However, the intraspecific range of body sizes is usually low, making it difficult to quantify size-associated trophic shifts, or to determine the degree to which sex differences in diet are due to body-size differences. Large snakes are ideal for such a study, because they provide a vast range of body sizes within a single population.
2. More than 1000 Reticulated Pythons ( Python reticulatus ) from southern Sumatra were examined, with specimens from 1·5 to > 6 m in snout–vent length, and from 1 to 75 kg in mass. Females attained much larger body sizes than did conspecific males (maxima of 20 vs 75 kg, 5 vs 7 m), but had similar head lengths at the same body lengths.
3. Prey sizes, feeding frequencies and numbers of stomach parasites (ascarid nematodes) increased with body size in both sexes, and dietary composition changed ontogenetically. Small snakes fed mostly on rats, but shifted to larger mammalian taxa (e.g. pangolins, porcupines, monkeys, wild pigs, mouse deer) at 3–4-m body length.
4. Adult males and females showed strong ecological divergence. For some traits, this divergence was entirely caused by the strong allometry (combined with sexual size dimorphism), but in other cases (e.g. feeding frequency, dietary composition), the sexes followed different allometric trajectories. For example, females shifted from rats to larger mammals at a smaller body size than did conspecific males, and feeding frequencies increased more rapidly with body size in females than in males. These allometric divergences enhanced the degree of sex difference in trophic ecology induced by sexual size dimorphism.     

Does body storage act as a food-availability cue for adaptive adjustment of egg size and number in Daphnia magna?

1. The hypothesis that body storage is used by daphniids as a physiological cue for adaptively adjusting egg size and number to food availability was tested.
2. Egg size and number were examined to see whether they are related to individual variation in body storage independent of maternal size, genotype and food level. Egg mass and brood size (number of eggs in a brood) were compared to somatic mass, all adjusted to maternal body length, at two food levels in two parthenogenetic clones of Daphnia magna .
3. The prediction that adjusted brood size should increase with body storage, whereas adjusted egg mass should decrease with increasing body storage, was not fulfilled as seven of eight comparisons failed to fit the expectation.
4. It is concluded that body storage is probably not the food‐availability cue used by daphniids to control intrabrood resource allocation. Other possibilities, such as chemical cues emitted by food organisms and by coexisting cladocerans, are briefly discussed.     

Allometry of territory size and metabolic rate as predictors of self-thinning in young-of-the-year Atlantic salmon

1. Self-thinning is a progressive decline in population density caused by competitively induced losses in a cohort of growing individuals and can be depicted as: log₁₀ (density) = c − β log₁₀ (body mass).
2. In mobile animals, two mechanisms for self-thinning have been proposed: (i) the space hypothesis predicts that maximum population density for a given body size is the inverse of territory size, and hence, the self-thinning slope is the negative of the slope of the allometric territory-size relationship; (ii) the energetic equivalence hypothesis predicts that the self-thinning slope is the negative of the slope of the allometric metabolic rate relationship, assuming a constant supply of energy for the cohort.
3. Both hypotheses were tested by monitoring body size, population density, food availability and habitat for young-of-the-year Atlantic salmon ( Salmo salar ) in Catamaran Brook, New Brunswick. The results were consistent with the predictions of the space hypothesis. Observed densities did not exceed the maximum densities predicted and the observed self-thinning slope of −1·16 was not significantly different from the slope of −1·12, predicted by the allometry of territory size for the population under study.
4. The observed self-thinning slope was significantly steeper than −0·87, predicted by the allometry of metabolic rate, perhaps because of a gradual decline in food abundance over the study period. The decline in density was more rapid in very shallow sites and may have been partly caused by a seasonal change in water depth and an ontogenetic habitat shift rather than solely by competition for food or space.
5. The allometry of territory size may be a useful predictor of self-thinning in populations of mobile animals competing for food and space.     

Escape latency is size independent in grey mullet

Using grey mullet Mugil cephalus as a model species, the hypothesis that escape latency increases with body size was tested. Minimum escape latency was c . 10 ms (mean ± s . d . 18·1 ± 13·7 ms; range 8–72 ms) and was independent of body size.     

Nitrogen and phosphorus content of some temperate and tropical freshwater fishes

The nitrogen and phosphorus content of two temperate fishes, Rutilus rutilus and Perca fluviatilis , and six tropical fishes, Oreochromis niloticus , Cichla monoculus , Serrassalmus rhombeus , Plagioscion squamosissimus , Prochilodus brevis and Hoplias malabaricus , were investigated to test the hypothesis that variation in body P content and N:P ratio is related to body size. Regressions of %P and N:P ratios against fish size (length and mass) confirmed the hypothesis for P. fluviatilis and P. squamosissimus , suggesting that body size is an important factor driving body P content and N:P ratios in some fishes. Moreover, significant increases in %N and N:P ratio with body size was found for H. malabaricus , a common piscivorous fish of the Neotropics. Interspecific variation in %P and N:P ranged two-fold and significant differences ( P < 0·05) were found among the tested species. The mean ± s . d . elemental content across all fishes ( n = 170) was 10·35 ± 1·29% for N and 3·05 ± 0·82% for P, while the N:P ratio was 8·00 ± 2·14. Data on fish body nutrient content and ratio will improve parameterization of bioenergetics and mass balance models and help clarify the role of fishes in nutrient cycles in both temperate and tropical freshwaters.     

Specificity and host predictability: a comparative analysis among monogenean parasites of fish

1. This article compares generalist (parasite species found on two or more host species) and specialist (found on only one host species) monogenean parasite species of fish. The reduction of the host range – that is an increase in host specificity – may correspond with a better adaptation of the parasite to a more predictable host environment. A more predictable environment may allow the parasite species to develop specific adaptations.
2. We assume that the more predictable host environment can be evaluated by host body size, since numerous life-traits, such as longevity, are positively correlated with size.
3. We found that specialist parasites parasitize larger hosts species than generalist parasites. We also found a good relationship between host body size and parasite body size for specialist parasite species.
4. An adaptation to the mechanical problems encountered in the host's gill chamber may lead to an increase in parasite body size. The infection of a larger part of the host population in order to decrease the chances of local extinction due to fluctuations of host abundance may be another adaptive mechanism.
5. We found a negative correlation between parasite body size and prevalence for generalist parasite species. This relationship disappeared when using the comparative method controlling for phylogeny, which proved that it was a phylogenetic effect.     

Description of a new Lake Tanganyikan cichlid fish of the genus Cyprichromis(Perciformes: Cichlidae) with a note on sexual dimorphism

Cyprichromis coloratus , a new cichlid species, is described based on 41 type specimens from the Zambian coast of Lake Tanganyika. It differs from Cyprichromis microlepidotus by having fewer scales in the longitudinal line (41–44 v . 59–70 in C. microlepidotus ) and upper lateral line (31–36 v . 47–59), from Cyprichromis leptosoma by more dorsal‐fin spines (14–15 v . 11–13 in C. leptosoma ) and longitudinal line scales (41–44 v . 38–40), from Cyprichromis pavo by fewer dorsal‐fin spines and soft rays (29–31, usually 30, in total v . 30–32, usually 31, in C. pavo ), and from Cyprichromis zonatus by shallower body depth [24·0–28·5 (26·4 ± 1·1)% of standard length v . 24·5–30·5 (28·4 ± 1·3)% in C. zonatus ] and smaller eye [eye length 25·2–30·9 (27·2 ± 1·1)% of head length v . 27·9–33·3 (30·2 ± 1·2)%]. The new species is also separable from C. microlepidotus , C. pavo and C. zonatus by absence of distinct small spots on the body ( v . pearl grey and yellowish small spots along scale rows on body at least in males of C. microlepidotus and C. pavo ), its distinct dichromatism (particularly on the caudal fin) between males of the same population ( v . dichromatism absent in C. zonatus ) and absence of vertical bands ( v . three or four distinct vertical bands beneath dorsal fin base in live males of C. zonatus ). Cyprichromis coloratus n. sp. also exhibits sexual dimorphism, females having a larger head and males having longer pelvic fin, differences which are discussed in relation to reproductive behaviour. A key to the five species of Cyprichromis is included. A lectotype is designated for Cyprichromis leptosoma .     

Interannual and regional variations in body length, weight and energy content of age-0 Pacific herring from Prince William Sound, Alaska

Age-0 Pacific herring were surveyed in October of 4 years in a large northern Gulf of Alaska estuary, to determine the range of variations in length, weight and whole body energy content (WBEC). These parameters reflect their preparedness for surviving their first winter's fast. During the surveys there were distinct regional and interannual variations in all three parameters for individual groups of herring in Prince William Sound. Likewise, with each collection there was typically a large range of size and WBEC values. The average standard length was (±S.D.) 80±13 mm (range=40–118), the mean whole body wet weight was 5·7±3·0 g (range=0·7–29·2) and the average WBEC of all age-0 herring captured, regardless of year or site (n=1471), was 5·4± 1·0 kJ g⁻¹ wet weight (range=2·4–9·4). The large range of WBEC and size indicates that age-0 herring at different capture sites were not all equally prepared for surviving their first winter.     

Interspecific allometry of population density in mammals and other animals: the independence of body mass and population energy-use

Global regressions of ecological population densities on body mass for mammals and for terrestrial animals as a whole show that local population energy-use is approximately independent of adult body mass—over a body mass range spanning more than 11 orders of magnitude. This independence is represented by the slope of the regressions approximating –0.75, the reciprocal of the way that individual metabolic requirements scale with body mass. The pattern still holds for mammalian primary consumers when the data are broken down by geographic area, by broad habitat-type and by individual community. Slopes for mammalian secondary consumers are also not statistically distinguishable from –0.75. For any given body mass temperate herbivores maintain on average population densities of 1.5 to 2.0 times those of tropical ones, though slopes do not differ. Terrestrial animals of all sizes exhibit approximately the same range of population energy-use values. These results agree with those reported for population energy-budgets. It is suggested that rough independence of body mass and the energy-use of local populations is a widespread rule of animal ecology and community structure.