Ecological correlates of body size and egg size in parasitic Ascothoracida and Rhizocephala (Crustacea)

In order to identify key factors in the evolution of life history traits in Ascothoracida and Rhizocephala (two groups of crustacean parastes of invertebrates), comparative analyses were performed using phylogenetically independent contrasts. Among 59 ascothoracidan species, latitude correlated positively with body size, whereas there was no relationship between water depth and body size. Body size also correlated strongly with egg size; however, once corrected for body size, egg size was not related to either latitude or water depth. Among 91 rhizocephalan species, neither latitude nor water depth correlated with body size. However, host species of larger sizes harboured larger species of rhizocephalan parasites. Egg size of rhizocephalans did not correlate with body size, and was not influenced by either latitude or water depth. The patterns observed in this study show both differences from an similarities to those reported for other groups of crustacean parasites, and suggest that adaptations to similar selective pressures are not always identical among distantly-related taxa.     

The evolution of life-history traits in parasitic and free-living platyhelminthes: a new perspective

Parasite life histories have been assumed to be shaped by their particular mode of existence. To test this hypothesis, we investigate the relationships between life-history traits of free-living and parasitic platyhelminthes. Using phylogenetically independent contrasts we examine patterns of interspecific covariation in adult size, progeny volume, daily fecundity, total reproductive capacity, age at first reproduction and longevity. The correlations obtained indicate a similar causal chain of life history variations for free-living and parasitic platyhelminthes. These results suggest that increased longevity favours delayed reproduction. Furthermore, growth pattern determines adult body size and age at maturity. For platyhelminthes, whether free-living or parasitic, the total reproductive capacity is found to be directly determined by the size of the worm. Within this group the parasitic way of life does not seem to influence the basic patterns of life history evolution. Received: 20 September 1997 / Accepted: 1 March 1998     

Insectivore life histories: further evidence against an optimum body size for mammals

1. This study tests a model of the relationship of body mass to reproductive power (the rate of conversion of energy from the environment to an organism's offspring). Specifically tested is the prediction that the regression of life-history variables on body size will change slope and sign about an 'optimum' body mass of 100 g.
2. Life-history data from the mammalian order Insectivora have been collated and analysed using a phylogenetic comparative method to test this prediction.
3. The analyses showed little evidence for significant changes in slope or sign around 100g body mass, or other possible optimal body masses, contradicting the predictions of the model. These findings agree with those of similar analyses on life-history variables of bats.     

Life history tactics of darters (Percidae: Etheostomatiini) and their relationship with body size, reproductive behaviour, latitude and rarity

Patterns of covariation of life history traits of darters in the genus Etheostoma are reviewed. The primary pattern is associated with body size. Large darters grow faster, mature at a larger size, produce bigger clutches, and have longer reproductive and life spans, and shorter spawning seasons, than do small darters. When the effects of size are removed statistically, the dominant secondary pattern matched the r-K continuum from fast-growing, short-lived, primarily semelparous species with many small ova and a high reproductive effort (r-species) to slow-growing, long-lived, iteroparous species with few large ova and a low reproductive effort (K-species). Variation in life history traits is also influenced by reproductive behaviour, latitude, and rarity (as measured by geographic range). There are significant differences in the primary and secondary life history patterns among reproductive guilds. Latitude and rarity are not correlated with these primary and secondary patterns. Instead, they account for variation of tertiary patterns. Rare species may not match the reproductive performance of more common and widely distributed species. Future studies of life history traits in darters should focus on species whose reproductive behaviour differs from that of the species reviewed in this study, and on the demographic characteristics of rare or declining populations.     

The evolution of body size in birds. I. Evidence for non-random diversification

The hypothesis that evolution of body size in birds was a random process coupled with an absolute lower boundary on body mass was tested using data on 6217 species of extant birds. The test was based on the fact that subclades within birds that have body masses much larger than this minimum should not have skewed log body mass distributions, while clades close to this boundary should. Bird species were classified into 23 orders suggested by Sibley and Monroe (1988). Thirteen orders that had average log body masses greater than the average for all birds had significantly skewed log body mass distributions. This is inconsistent with the hypothesis that evolution of body size in birds is random, but is constrained only at the smallest body masses. Most orders of birds cannot be considered random samples from the parental distribution of all birds. When the pattern of body mass evolution in birds is reconstructed using an estimate of the phylogenetic relationships among orders, there are many more instances where a large taxon putatively originated from a smaller one than vice versa. The non-random nature of body mass evolution in birds is consistent with models that postulate that evolution is constrained by the ability of individuals to turn resources into offspring.     

Ecological correlates of seed size in the British flora

1. The association between seed size and habitat shade within the British flora was investigated using a data set of seed masses, life histories and quantitative measures of habitat shade for 504 species; the association between seed size and seed longevity was investigated using a data set of seed masses, life histories and seed longevities for 301 species.
2. The data were analysed using the method of phylogenetically independent contrasts (PICs) calculated using the software package CAIC (Comparative Analysis by Independent Contrasts).
3. Seed mass was found to be positively correlated with habitat shade and negatively correlated with seed longevity, after variation owing to life history had been accounted for.     

Microbiome analysis among bats describes influences of host phylogeny, life history, physiology and geography

Metagenomic methods provide an experimental approach to inform the relationships between hosts and their microbial inhabitants. Previous studies have provided the conceptual realization that microbiomes are dynamic among hosts and the intimacy of relation between micro- and macroorganisms. Here, we present an intestinal microflora community analysis for members of the order Chiroptera and investigate the relative influence of variables in shaping observed microbiome relationships. The variables ranged from those considered to have ancient and long-term influences (host phylogeny and life history) to the relatively transient variable of host reproductive condition. In addition, collection locality data, representing the geographic variable, were included in analyses. Results indicate a complex influence of variables in shaping sample relationships in which signal for host phylogeny is recovered at broad taxonomic levels (family), whereas intrafamilial analyses disclosed various degrees of resolution for the remaining variables. Although cumulative probabilities of assignment indicated both reproductive condition and geography influenced relationships, comparison of ecological measures among groups revealed statistical differences between most variable classifications. For example, ranked ecological diversity was associated with host phylogeny (deeper coalescences among families were associated with more microfloral diversity), dietary strategy (herbivory generally retained higher diversity than carnivory) and reproductive condition (reproductively active females displayed more diverse microflora than nonreproductive conditions). Overall, the results of this study describe a complex process shaping microflora communities of wildlife species as well as provide avenues for future research that will further inform the nature of symbiosis between microflora communities and hosts.     

Maternal body size as an evolutionary constraint on egg size in a butterfly

Genetic and developmental constraints have often been invoked to explain patterns of existing morphologies. Yet, empirical tests addressing this issue directly are still scarce. We here set out to investigate the importance of maternal body size as an evolutionary constraint on egg size in the tropical butterfly Bicyclus anynana, employing an artificial two-trait selection experiment on simultaneous changes in body and egg size (synergistic and antagonistic selection). Selection on maternal body size and egg size was successful in both the synergistic and the antagonistic selection direction. Yet, responses to selection and realized heritabilities varied across selection regimes: the most extreme values for pupal mass were found in the synergistic selection directions, whereas in the antagonistic selection direction realized heritabilities were low and nonsignificant in three of four cases. In contrast, for egg size the highest values were obtained in the lines selected for low pupal mass. Thus, selection on body size yielded a stronger correlated response in egg size than vice versa, which is likely to bias (i.e., constrain), if weakly, evolutionary change in body size. However, it seems questionable whether this will prevent evolution toward novel phenotypes, given enough time and that natural selection is strong. Correlated responses to selection were overall weak. Egg and larval development times tended to be associated with changes in maternal size, whereas variation in pupal development times weakly tended to follow variation in egg size. Lifetime fecundity was similar across selection regimes, except for females simultaneously selected for large body mass and small egg size, exhibiting increased fecundity. Multiple regressions showed that lifetime fecundity and concomitantly reproductive investment were primarily determined by longevity, as expected for an income breeder, whereas egg size was primarily determined by pupal mass. Evidence for a phenotypic trade-off between egg size and number was weak.     

Comparative body size relationships in pocket gophers and their chewing lice

In this paper, we use the method of independent contrasts to study body size relationships between pocket gophers and their chewing lice, a host-parasite system in which both host and parasite phylogcnies are well studied. The evolution of body size of chewing lice appears to be dependent only on the body size of their hosts, which confirms the 1991 findings of Harvey and Keymer. We show that there is a positive relationship between body size and hair-shaft diameter in pocket gophers, and that there is also a positive relationship between body size and head-groove width in chewing lice. Finally, we show a positive relationship between gopher hair-shaft diameter and louse head-groove width. We postulate that changes in body size of chewing lice are driven by a mechanical relationship between the parasite's head-groove dimension and the diameter of the hairs of its host. Louse species livingon larger host species may be larger simply because their hosts have thicker hairs, which requires that the lice have a wider head groove. Our study of gopher hair-shaft diameter and louse head-groove dimensions suggest that there is a 'lock-and-key' relationship between these two anatomical features.     

Life history evolution in cichlids 2: directional evolution of the trade-off between egg number and egg size

The negative relationship between offspring number and offspring size provides a classic example of the role of trade-offs in life history theory. However, the evolutionary transitions in egg size and clutch size that have produced this negative relationship are still largely unknown. Since body size may affect both of these traits, it would be helpful to understand how evolutionary changes in body size may have facilitated or constrained shifts in clutch and egg size. By using comparative methods with a database of life histories and a phylogeny of 222 genera of cichlid fishes, we investigated the order of evolutionary transitions in these traits in relation to each other. We found that the ancestral large-bodied cichlids first increased egg size, followed by a decrease in both body size and clutch size resulting in the common current combination of a small-bodied cichlid with a small clutch of large eggs. Furthermore, lineages that deviated from the negative relationship between clutch and egg size underwent different transitions in these traits according to their body size (large bodied genera have moved towards the large clutch/small egg end of the continuum and small bodied genera towards the small clutch/large egg end of the continuum) to reach the negative relationship between clutch size and egg size. Our results show that body size is highly important in shaping the negative relationship between clutch size and egg size.     

Effects of body size, age and maturity stage on diet in a large shark: ecological and applied implications

Ontogenetic diet shifts are a widespread phenomenon among vertebrates, although their relationships with life history traits are poorly known. We analyzed the relative importance of body size, age and maturity stage as determinants of the diet of a marine top predator, the copper shark, Carcharhinus brachyurus, by examining stomach contents using a multiple-hypothesis modeling approach. Copper sharks shifted their diet as size and age increased and as they became sexually mature, incorporated larger prey as they grew, and had a discrete shift in diet with body size, with only individuals larger than ≈200 cm total length able to prey on chondrichthyans. Body size was the most important trait explaining the consumption of chondrichthyans, while age determined the consumption of pelagic teleosts. Pelagic teleosts were consumed mostly by medium-aged sharks, a result, probably, of a risk-reducing feeding strategy at young ages coupled with either a senescence-related decline in performance or a change in sensory capabilities as sharks age. Copper sharks of all sizes were able to cut prey in pieces, implying that gape limitation (i.e., the impossibility of eating prey larger than a predator’s mouth) did not play a role in producing the diet shift. Our results suggest that, contrary to the current practice of setting minimum but not maximum size limits in catches, any plan to conserve or restore the ecological function of sharks, through their predatory control of large prey, should aim to maintain the largest individuals.     

Sexual selection uncouples the evolution of brain and body size in pinnipeds

The size of the vertebrate brain is shaped by a variety of selective forces. Although larger brains (correcting for body size) are thought to confer fitness advantages, energetic limitations of this costly organ may lead to trade-offs, for example as recently suggested between sexual traits and neural tissue. Here, we examine the patterns of selection on male and female brain size in pinnipeds, a group where the strength of sexual selection differs markedly among species and between the sexes. Relative brain size was negatively associated with the intensity of sexual selection in males but not females. However, analyses of the rates of body and brain size evolution showed that this apparent trade-off between sexual selection and brain mass is driven by selection for increasing body mass rather than by an actual reduction in male brain size. Our results suggest that sexual selection has important effects on the allometric relationships of neural development.     

Life history evolution in cichlids 1: revisiting the evolution of life histories in relation to parental care

Empirical links between egg size and duration of parental care in fishes have generated a considerable amount of theory concerning life history evolution. However, to date, this link has not been investigated in relation to other important life-history traits such as clutch size and body size, or while controlling for shared ancestry between species. We provide the first phylogenetically based tests using a database with information on egg size, clutch size, body size and care duration in cichlid fishes (Cichlidae). Multiple regression analyses, based on independent contrasts on both the species and the genus level, showed that clutch size is the variable most closely related to duration of care. This pattern appeared to be driven by post-hatch care relationships. Our results show that, contrary to expectation, there is no positive link between egg size and care duration in Cichlidae. Instead, greater reproductive output through increased clutch size investment appears to have coevolved with greater care of offspring. We suggest that re-evaluation of the generality of current models of the evolution of egg size under parental care in fishes is needed.     

Continentality affects body condition and size but not yearly reproductive output in female European hares (Lepus europaeus)

Life history in reproductive pattern in hares are affected by ambient temperature. We hypothesized that European hares dwelling in areas of higher energy demands would have larger body sizes, larger fat depots and a delayed first reproduction. To test this assumption we compared yearly reproductive output as well as age, body size, body weight and body condition of female European hares from Belgium (temperate oceanic climate) and Lower Austria (temperate continental climate). Our results reveal that there was no effect of study site on annual reproductive output in female European hares. However, adult female hares from Belgium were significantly smaller and had significantly lower body condition in late autumn compared to the Lower Austrian sample, although Belgian individuals were actually older than Lower Austrians. These findings suggest that females in Belgium are more under an r-selection regime whereas Lower Austrian females might be more under K-selection within the r-K-continuum.     

Covariation of sexual size dimorphism and adult sex ratio in parasitic nematodes

Females are larger than males in most invertebrate taxa, a phenomenon believed to result from the pressures exerted on female body size by size-dependent fecundity. Male-male competition, which can act on male body size, is not thought to play as important a role in the evolution of sexual size dimorphism in invertebrates as it apparently does in some vertebrate groups. Here, using a comparative approach, the relationship between sexual size dimorphism and adult sex ratio is examined across 46 natural populations (41 species) and 30 experimental populations (21 species) of parasitic nematodes. If male-male competition via physical contests is important, relative male size should increase as the sex ratio becomes less female-biased. This is exactly what was found in the analyses, where residuals of male size regressed on female size were used as measures of sexual size dimorphism. This result was independent of any phylogenetic influences, and was obtained for both natural and experimental nematode populations. In addition, there was no evidence of any Allometric relationship between male and female body size. The average ratio of male size to female size was roughly constant across all species and independent of body size. The results are consistent with a role for male-male competition in explaining specific deviations from the average ratio of male to female body size, suggesting a significant role for sexual selection in the evolution of nematode body sizes.     

Environment-dependent genetic correlations between development time and body mass in a scorpionfly

Development time and body mass at maturation are two important fitness traits fundamental for our understanding of life history theory. Generally, fast development is associated with small adult body mass, as it will take longer to grow large. However, the strength of this trade-off may depend on average food availability, as the potential benefit of long development will depend on the rate of food intake. Here, I report results of a food manipulation experiment during larval development of the scorpionfly Panorpa cognata (Insecta, Mecoptera). Development time showed considerable genetic variation, yet food level had no influence and there was a strong genetic correlation in development time across environments. As expected, larval and adult body weight was significantly affected by food availability. Furthermore, body mass was influenced by a highly significant genotype-by-environment interaction. The reaction norm for body mass in response to food treatment was much stronger in families with long development time compared with rapidly developing genotypes. This effect was accompanied by a shift in the genetic correlation between development time and body size when comparing the two food levels. Specifically, the genetic correlation between body mass and development time changed from being positive at high food levels to a negative genetic correlation at low food levels. These results are consistent with other empirical findings demonstrating a similar shift in genetic correlations between body mass and development time when comparing favourable and unfavourable environmental conditions.     

Evolution of conspicuous colouration, body size and gregariousness: a comparative analysis of lepidopteran larvae

Protective colouration in animals includes camouflage (i.e., crypsis), that decreases the risk of detection, and conspicuous colouration, which is often used in combination with chemical defences to deter predators from attacking. Experiments have shown that the efficacy of conspicuous colouration increases with increasing size of pattern elements and larger body size. Prey species that have acquired avoidance inducing colouration therefore may be exposed to selection for larger body size, and such colouration may more easily evolve in large than in small prey species. Here we test for a difference in body size between species with different colouration modes and perform a comparative analysis based on phylogenetically independent contrasts to examine if evolutionary shifts in colour pattern have been associated with evolutionary changes in body size, using data for 578 species of moths. Larval body size did not differ between species with signalling and non-signalling larvae, and results from the comparative analysis suggest that these two traits have not evolved in parallel. The lack of association between evolutionary changes in colouration and body size may reflect a confounding influence of lifestyle, because evolutionary shifts from solitary to group-living larvae were associated with decreased larval body length and adult wing span. Because evolutionary changes in larval body size were associated with evolutionary changes in adult wing span the predicted association between colouration and size may have been confounded also by conflicting selection on body size in larvae and adults.     

Sexual size dimorphism in caecilian amphibians: analysis, review and directions for future research

Sexual dimorphism, widespread in the animal kingdom, describes differences between the sexes in size, shape and many other traits. Sexual size dimorphism (SSD) plays a significant role in understanding life history evolution and mating systems. The snakelike morphology of limbless caecilian amphibians lacking obvious secondary sexual characters (in contrast to frogs and salamanders) impedes accurate intrasexual comparisons. In this study, sexual size dimorphism in the oviparous caecilian Ichthyophis cf. kohtaoensis, a phylogenetically basal caecilian, was analysed. Females were larger in all body and head characters tested. However, when adjusted to body size (total length), females differed only in their cloacal shape. Clutch volume was positively correlated to female body size, thus female fecundity increased with body size supporting the hypothesis of a fecundity-selected SSD in the oviparous Ichthyophis cf. kohtaoensis. A review of the present SSD data for caecilians shows that many species are monomorphic for body size but show dimorphism in head size, while other species demonstrate female-biased SSD. Male-biased SSD has not been reported for caecilians. To understand life history evolution in caecilians, further studies on the reproductive biology of other taxa are urgently needed, in particular for rhinatrematids and uraeotyphlids. New data will allow phylogenetically controlled comparative analyses to fully explore the pattern of SSD among caecilian lineages.