Species richness in agamid lizards: chance,body size,sexual selection or ecology?

Why does species richness vary so greatly across lineages? Traditionally, variation in species richness has been attributed to deterministic processes, although it is equally plausible that it may result from purely stochastic processes. We show that, based on the best available phylogenetic hypothesis, the pattern of cladogenesis among agamid lizards is not consistent with a random model, with some lineages having more species, and others fewer species, than expected by chance. We then use phylogenetic comparative methods to test six types of deterministic explanation for variation in species richness: body size, life history, sexual selection, ecological generalism, range size and latitude. Of eight variables we tested, only sexual size dimorphism and sexual dichromatism predicted species richness. Increases in species richness are associated with increases in sexual dichromatism but reductions in sexual size dimorphism. Consistent with recent comparative studies, we find no evidence that species richness is associated with small body size or high fecundity. Equally, we find no evidence that species richness covaries with ecological generalism, latitude or range size.     

A phylogenetic analysis of body size evolution in the Anolis roquet group (Sauria: Iguanidae): character displacement or size assortment?

The important role that competition plays in structuring communities is well documented; however, the role of competition in an evolutionary context remains unclear. Evolutionary investigations into the role of competition have often focused on the process of character displacement, and a good example of this is the evolution of body size in the Anolis lizards of the Caribbean islands. Previous work on the A. roquet species group has taken a phylogenetic approach and concluded that patterns of body size differences are not caused by character displacement but are a result of size assortment. Using a phylogenetic reconstruction based on the sequence of the cytochrome b gene (cyt-b) and ancestral character-state reconstruction methods, we investigated the roles of character displacement and size assortment. Our results indicated that size assortment alone was insufficient to explain the observed patterns of body size differences. Furthermore, we found that change in body size was associated with a change in allopatry/sympatry, thus supporting the character-displacement hypothesis. We conclude that patterns of body size differences in the A. roquet species group appear to be the result of a combination of character displacement and size assortment because character displacement was only found to be possible on three occasions.     

Egg size in relation to body size in rotifers: an indication of reproductive strategy?

Egg sizes and body sizes of 43 egg-bearing rotifer species of numerous, mostly tropical, general have been recorded. Larger absolute egg volumes have been found for larger rotifer species, but the increase was lower than expected in proportion to body size, i.e. the relative egg volume decreased with increasing body size. Obviously the relative investment per offspring is smaller in larger rotifer species.     

Yolk platelets in artemia embryos: are they really storage sites of immature mitochondria?

We have used semi-quantitative polymerase chain reaction (PCR) technology to determine the mitochondrial DNA (mtDNA) content of yolk platelets isolated from embryos of the brine shrimp, Artemia franciscana, and ultrastructural analysis of yolk platelet formation to determine whether these organelles contain mitochondria as reported previously. Using six different isolation and purification protocols, we found one yolk platelet preparation to be devoid of mtDNA, while four yolk platelet preparations contained mtDNA ranging from 16.4 to 85 pg/10(6) yolk platelets. One preparation contained 600 pg mtDNA per 10(6) yolk platelets. Based on our PCR analyses, the mtDNA component of Artemia yolk platelets represented 0.16-4.5% of the total DNA isolated from the platelets. We calculated that Artemia yolk platelets contain, on average, approximately 1.78 molecules of mtDNA/platelet. Direct analysis of mtDNA in "free" mitochondria isolated from yolk platelet-free preparations of Artemia embryos and newly hatched larvae yielded 0.76-0.80 ng/animal. Based on these values, the mtDNA content of yolk platelets was approximately 0.2% of total mtDNA in Artemia embryos. Microscopic analysis of yolk platelet formation during oogenesis in Artemia failed to show the inclusion of mitochondria during the assemblage of yolk platelets. The "mitochondria-like" structures that appear in yolk platelets during their utilization lack the well defined inner and outer membranes characteristic of mitochondria making it unlikely that the yolk platelet inclusions are mitochondria. Our results from PCR technology and ultrastructure analysis demonstrate that mtDNA in yolk platelets of Artemia franciscana embryos is a minor component of the total mtDNA in the embryo, and they fail to support the notion that yolk platelets in Artemia are a major source of immature mitochondria for development.     

Evolution of body size in food webs: does the energetic equivalence rule hold?

The energetic equivalence rule (EER), which is derived from empirical observations linking population density and body size and from the allometric law linking metabolism and body size, predicts that the amount of energy used by the various species should be independent of body size. Here, we examine this hypothesis using a model that allows entire food webs to emerge from coevolution of interacting species. Body size influences both individual metabolism and interactions among species in the model. Overall, population density does decrease with body size roughly following a power law whose exponent is variable. We discuss this variability in the light of empirical data sets. The emerging relationship between the flux of resources exploited by the various species and their body size follows a decreasing power law, thus contradicting the EER. Our model emphasizes the importance of considering the influence of body size on species interactions in attempting to explain large-scale patterns related to body size.     

Seeing the trees for the wood: random walks or bounded fluctuations of population size?

Summary It is often claimed that the fluctuation of numbers in field populations is fundamentally different from random walks of densities, in that population size is kept between certain positive limits. To test this hypothesis patterns of fluctuation in field populations were compared with random walks of density of about the same duration. It was found that the boundaries (Log-Range) between which numbers fluctuate in field populations increase with time to about the same extent as in comparable random walks of density. Moreover, deviations of the trend of numbers over years (Average lnR) from zero trend in populations of 62 (carabid) species were just those expected for simulated random walk runs, with the median value of Var(lnR), and different values for mean population size that cover the possible range of survival times for these species. This means that the null hypothesis that in the field numbers would fluctuate as random walks of densities could not be rejected. Although it is not very probable that field populations fluctuate exactly like random walks of densities, random walk models appear to mimic the fluctuation patterns of field populations sufficiently closely to explain what happens in nature, and to deny the need for regulation. The same conclusion was drawn in earlier studies where statistical tests were applied to fluctuation patterns of field populations (Den Boer and Reddingius 1989; Den Boer 1990a). Random walks of densities do not exclude the possibility that local populations can persist for some centuries.Communication No. 435 of the Biological Station Wijster     

Are there body size implications for the success of globally introduced land birds?

Verbal models have hypothesized a relation between body size and the successful introduction of animal species. This relation is largely based on studies of intrinsic rate of increase in what have been termed "colonizing" species. From these studies it has been predicted that introduction success should be negatively correlated with body size across taxa but positively correlated within closely related taxa. T examine this relation for globally introduced land birds. Introduced land birds are. on average, larger bodied than extant land bird species. Across species, families, and higher family nodes, global introduction success is significantly related to decreasing body size. However, within taxa there is a significant positive relationship between introduction success and body size. I discuss possible explanations for the observed relations and conclude that an indirect but genuine relationship between the introduction success of land birds and their body size is currently the most plausible.     

Herd size in large herbivores: encoded in the individual or emergent?

In large mammalian herbivores, the increase of group size with habitat openness was first assumed to be an adaptive response, encoded in the individual. However, it could, alternatively, be an emergent property: if groups were nonpermanent units, often fusing and splitting up, then any increase of the distance at which animals perceive one another could increase the rate of group fusion and thus mean group size. Dynamical models and empirical data support this second hypothesis. This is not to say that adaptive modifications of mean herd size cannot occur. However, this changes the way in which we can envisage the history of gregariousness in large herbivores during the Tertiary.     

Species richness among birds: body size,life history,sexual selection or ecology?

Why do some avian families contain so many more species than other families? We use comparisons between sister taxa to test predictions arising from six explanations to this puzzle: that differences between families are due to chance, body size, life history, sexual selection, intrinsic ecological factors or extrinsic abiotic factors, respectively. In agreement with previous analyses, we find no support for the idea that differences in species richness are simply due to chance. However, contrary to most previous work, we also find no support for the hypotheses that high species richness is correlated with small body size and fast life history. Rather, high species diversity is strongly associated with pronounced plumage dichromatism, generalist feeding habits and good dispersal capabilities as well as large and fragmented geographical ranges. In addition, all of these relationships are robust to the removal of the two most speciose avian lineages, the Ciconiiformes and the Passeriformes. The supposed relationships between species richness and both body size and life history are, however, due to phylogenetic non-independence. Together with previous work showing that differences between avian lineages in extinction risk are associated with variation in body size and life history, these results indicate that extinction rates and speciation rates are not necessarily determined by the same factors. Hence, high extinction rates are not inevitably associated with low speciation rates. Extinction-prone lineages may, in fact, have a high rate of speciation. In such lineages a high proportion of ''vulnerable'' species would be a natural, ongoing phenomenon.     

Optimal body size and energy expenditure during winter: why are voles smaller in declining populations?

Winter is energetically challenging for small herbivores because of greater energy requirements for thermogenesis at a time when little energy is available. We formulated a model predicting optimal wintering body size, accounting for the scaling of both energy expenditure and assimilation to body size, and the trade-off between survival benefits of a large size and avoiding survival costs of foraging. The model predicts that if the energy cost of maintaining a given body mass differs between environments, animals should be smaller in the more demanding environments, and there should be a negative correlation between body mass and daily energy expenditure (DEE) across environments. In contrast, if animals adjust their energy intake according to variation in survival costs of foraging, there should be a positive correlation between body mass and DEE. Decreasing temperature always increases equilibrium DEE, but optimal body mass may either increase or decrease in colder climates depending on the exact effects of temperature on mass-specific survival and energy demands. Measuring DEE with doubly labeled water on wintering Microtus agrestis at four field sites, we found that DEE was highest at the sites where voles were smallest despite a positive correlation between DEE and body mass within sites. This suggests that variation in wintering body mass between sites was due to variation in food quality/availability and not adjustments in foraging activity to varying risks of predation.     

Precopulatory mate assessment in relation to body size in the earthworm Lumbricus terrestris: avoidance of dangerous liaisons?

In the earthworm Lumbricus terrestris L., mating occurs on the soil surface, but partners remain anchored in their burrow andmating is preceded by repeated mutual burrow visits betweenneighbors. This study focuses on body size as one possibletrait that earthworms may assess during these burrow visits.Size-related mate choice is predicted to result in size-assortativemating, which we found in one field sample (n = 90 pairs),but not in a second (n = 102). We discovered that when mates separate, one of them can be pulled out of its burrow. Thiswas more likely for small individuals or those mating acrosswide distances. In a subsequent greenhouse experiment, we allowedfocal individuals to mate with two neighbors of different sizes.Relative size affected neither mating rate nor primary preference,but focals mated sooner with the same-sized neighbor than witha differently sized one. Small focals visited large neighborsmore often than small ones. We conclude that size influencesmate choice as well as the outcome of mating and discuss howthe "tug-of-war" that ends a mating contributes to this result.Precopulatory visits may involve assessment as well as enticementto lure the partner closer to the individual's own burrow,in order to minimize the risk when mating with a partner thatis large or far away.     

Composition of the body mass overshoot in European barn owl nestlings (Tyto alba ): insurance against scarcity of energy or water?

European barn owl chicks (Tyto alba) show a body mass overshoot prior to fledging that has been predicted to serve as an energy reservoir during periods of stochastic food availability. However, the composition of the mass overshoot has heretofore not been directly examined in nestlings of this or any other species displaying a body mass overshoot during growth (e.g., raptors and seabirds). To experimentally determine whether the overshoot in body mass in juvenile European barn owls (Tyto alba) may act as an energy reservoir, we compared the body composition of owl chicks raised on an ad libitum diet to those fed a restricted diet designed to eliminate the overshoot. Chicks raised on the two diets were also compared for differences in maturation of diverse functions (e.g., locomotion) and tissues (e.g., skeletal development). Contrary to expectations, our results on body composition in juvenile barn owls indicate that the mass overshoot prior to fledging is primarily comprised of an increased water compartment. Thus, we suggest that the mass overshoot in owls (and possibly in other species) does not serve as an energy reservoir but, rather, may function as an insurance against dehydration when hot in-nest conditions force chicks to rely on evaporative cooling: temperatures in barn owl nests can reach up to 43 degrees C. We found no significant differences in maturation indexes between diet treatments at the time of fledging.     

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.     

Does individual personality predict male mating preference for female body size in the Trinidadian guppy?

Within populations, individual animals vary considerably in their behaviour, including mate choice and personality. There is mounting interest in the potential covariation between these two behaviours within individuals, such that personality would influence mate choice. We experimentally tested this proposition under controlled laboratory conditions using male Trinidadian guppies (Poecilia reticulata) as a model study system. We assayed repeatedly the mating preference of individual males for females based on their body size. Additionally, we assayed repeatedly two ecologically relevant personality traits in males, namely exploration of a novel environment and boldness under a simulated predation threat. Finally, we analysed whether male mating preference and personality traits were repeatable, and tested whether the personality of individual males was correlated (covaried) with their mating preference scores. Although all but one of the measures of exploration and boldness behaviour were repeatable over time, male mating preference scores were not repeatable. Measures of male exploration and boldness were not inter-correlated among individuals, suggesting the absence of a behavioural syndrome between exploration and boldness. Unexpectedly, males did not exhibit on average a significant mating preference for larger females over smaller ones; they chose randomly between the paired stimulus females. Overall, we found no compelling evidence for a relationship between individual personality traits and mating preference in male guppies, suggesting that personality does not predict mate choice, at least in our study population and under our experimental conditions. We discuss potential factors, other than male personality and body length, that might maintain inter-individual variation in male mating preferences in the guppy in the wild.     

Is bigger really bigger? Differential responses to temperature in measures of body size of the mosquito,Aedes albopictus

When confronted with variation in temperature, most ectotherms conform to a growth rule that "hotter is smaller". This phenomenon can have important implications on population dynamics, interactions with other species, and adaptation to new environments for arthropods. However, the impact of other environmental factors and genetics may affect that general rule. Furthermore, most studies measure a single body part, and do not examine how temperature and other factors alter the allometric relationship between measurements of growth. In this study, we test the hypothesis that temperature and nutrition, while strongly affecting growth, do not change the allometric relationship between body mass and wing length in the mosquito Aedes albopictus. We tested this hypothesis by growing larval mosquitoes from two populations at five temperatures with three food levels. Contrary to our hypothesis, we find that temperature has a profound effect on allometry, with higher temperatures resulting in mosquitoes with shorter wings and greater body mass, and that the effects of temperature are dependent upon available food and population origin. We therefore reject our hypothesis and propose several testable mechanisms that will provide greater insight into the relationship between temperature, food, and measures of growth.     

What drives the evolution of body size in ectotherms? A global analysis across the amphibian tree of life

Aim

The emergence of large-scale patterns of animal body size is the central expectation of a wide range of (macro)ecological and evolutionary hypotheses. The drivers shaping these patterns include climate (e.g. Bergmann's rule), resource availability (e.g. ‘resource rule’), biogeographic settings and niche partitioning (e.g. adaptive radiation). However, these hypotheses often make opposing predictions about the trajectories of body size evolution. Therefore, whether underlying drivers of body size evolution can be identified remains an open question. Here, we employ the most comprehensive global dataset of body size in amphibians, to address multiple hypotheses that predict patterns of body size evolution based on climatic factors, ecology and biogeographic settings to identify underlying drivers and their generality across lineages.

Location

Global.

Time Period

Present.

Major Taxa Studied

Amphibians.

Methods

Using a global dataset spanning 7270 (>87% of) species of Anura, Caudata and Gymnophiona, we employed phylogenetic Bayesian modelling to test the roles of climate, resource availability, insularity, elevation, habitat use and diel activity on body size.

Results

Only climate and elevation drive body size patterns, and these processes are order-specific. Seasonality in precipitation and in temperature predict body size clines in anurans, whereas caecilian body size increases with aridity. However, neither of these drivers explained variation in salamander body size. In both anurans and caecilians, size increases with elevational range and with midpoint elevation in caecilians only. No effects of mean temperature, resource abundance, insularity, time of activity or habitat use were found.

Main Conclusions

Precipitation and temperature seasonality are the dominant climatic drivers of body size variation in amphibians overall. Bergmann's rule is consistently rejected, and so are other alternative hypotheses. We suggest that the rationale sustaining existing macroecological rules of body size is unrealistic in amphibians and discuss our findings in the context of the emerging hypothesis that climate change can drive body size shifts.