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33.1C: Limits on Animal Size and Shape - Biology

33.1C: Limits on Animal Size and Shape - Biology


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Animal shape and body size are influenced by environmental factors as well as the presence of an exoskeleton or an endoskeleton.

Learning Objectives

  • Explain how the environment and skeletal structure can put limits on the size and shape of animals

Key Points

  • Aquatic animals tend to have tubular shaped bodies ( fusiform shape) that decrease drag, enabling them to swim at high speeds.
  • Terrestrial animals tend to have body shapes that are adapted to deal with gravity.
  • Exoskeletons are hard protective coverings or shells that also provide attachments for muscles.
  • Before shedding or molting the existing exoskeleton, an animal must first produce a new one.
  • The exoskeleton must increase thickness as the animal becomes larger, which limits body size.
  • The size of an animal with an endoskeleton is determined by the amount of skeletal system required to support the body and the muscles it needs to move.

Key Terms

  • fusiform: shaped like a spindle; tapering at each end
  • exoskeleton: a hard outer structure that provides both structure and protection to creatures such as insects, Crustacea, and Nematoda
  • apodeme: an ingrowth of the arthropod exoskeleton, serving as an attachment site for muscles
  • endoskeleton: the internal skeleton of an animal, which in vertebrates is comprised of bone and cartilage

Limits on Animal Size and Shape

Animals with bilateral symmetry that live in water tend to have a fusiform shape: a tubular shaped body that is tapered at both ends. This shape decreases the drag on the body as it moves through water and allows the animal to swim at high speeds. Certain types of sharks can swim at fifty kilometers an hour, while some dolphins can swim at 32-40 kilometers per hour. Land animals frequently travel faster (although the tortoise and snail are significantly slower than sharks or dolphins). Another difference in the adaptations of aquatic and land-dwelling organisms is that aquatic organisms are constrained in shape by the forces of drag in the water since water has higher viscosity than air. However, land-dwelling organisms are constrained mainly by gravity; drag is relatively unimportant. For example, most adaptations in birds are for gravity, not for drag.

Most animals have an exoskeleton, including insects, spiders, scorpions, horseshoe crabs, centipedes, and crustaceans. Scientists estimate that, of insects alone, there are over 30 million species on our planet. The exoskeleton is a hard covering or shell that provides benefits to the animal, such as protection against damage from predators and from water loss (for land animals); it also provides for the attachments of muscles. As the tough and resistant outer cover of an arthropod, the exoskeleton may be constructed of a tough polymer, such as chitin, and is often biomineralized with materials, such as calcium carbonate. This is fused to the animal’s epidermis. Ingrowths of the exoskeleton called apodemes function as attachment sites for muscles, similar to tendons in more advanced animals. In order to grow, the animal must first synthesize a new exoskeleton underneath the old one and then shed or molt the original covering. This limits the animal’s ability to grow continually. It may limit the individual’s ability to mature if molting does not occur at the proper time. The thickness of the exoskeleton must be increased significantly to accommodate any increase in weight. It is estimated that a doubling of body size increases body weight by a factor of eight. The increasing thickness of the chitin necessary to support this weight limits most animals with an exoskeleton to a relatively-small size.

The same principles apply to endoskeletons, but they are more efficient because muscles are attached on the outside, making it easier to compensate for increased mass. An animal with an endoskeleton has its size determined by the amount of skeletal system it needs in order to support the other tissues and the amount of muscle it needs for movement. As the body size increases, both bone and muscle mass increase. The speed achievable by the animal is a balance between its overall size and the bone and muscle that provide support and movement.


Body size and species richness

The body size-species richness distribution is a pattern observed in the way taxa are distributed over large spatial scales. The number of species that exhibit small body size generally far exceed the number of species that are large-bodied. Macroecology has long sought to understand the mechanisms that underlie the patterns of biodiversity, such as the body size-species richness pattern.

This pattern was first observed by Hutchinson and MacArthur (1959), [1] and it appears to apply equally well to a broad range of taxa: from birds and mammals to insects, bacteria (May, 1978 [2] Brown and Nicoletto, 1991 [3] ) and deep sea gastropods (McClain, 2004 [4] ). Nonetheless, its ubiquity remains undecided. Most studies focus on the distribution of taxonomic fractions of largely non-interacting species such as birds or mammals this article is primarily based on those data.


Introduction

Most eukaryotic cells have a single nucleus in which a nuclear envelope (NE) separates the chromosomes from the cytoplasm. The NE (Fig. 1) is made of a double membrane that is perforated by nuclear-pore complexes (NPCs). The outer nuclear membrane, which is continuous with the ER, connects with the inner nuclear membrane at the curved membrane regions that surround each NPC. In metazoans, the NE also contains a meshwork of proteins, which are collectively called the nuclear lamina, that underlies the inner nuclear membrane and interacts with portions of the chromatin. The nuclear lamina is made predominantly of intermediate filaments called lamins, of which there are two main types: type A and type B (for a review, see Dechat et al., 2008). In mammals, there are two major A-type lamins, lamin A and lamin C, which are generated by alternative splicing of the LMNA gene. There are also two major B-type lamins, lamin B1 and lamin B2, each encoded by its own gene. Lamin A and the two lamin Bs have a C-terminal domain that is lipid modified (farnesylated), thereby promoting their attachment to the inner nuclear membrane. This domain is missing in lamin C and is normally removed by proteolytic cleavage in lamin A. In addition to their location at the nuclear periphery, lamins are also present in the nucleoplasm (Dechat et al., 2008).

The nuclear lamina also contains a variety of lamin-associated proteins and other proteins that are embedded in the inner nuclear membrane (reviewed by Wilhelmsen et al., 2006). The nuclear lamina might also play a role in the internal organization of the nucleus. For example, within the nucleus, chromosomes are organized in chromosome territories: rather than being distributed throughout the nucleus, each chromosome appears to occupy a discrete region, or territory (for a review, see Cremer et al., 2006). In addition, heterochromatic chromosomal regions, which are chromosome domains that are transcriptionally inactive, tend to localize at the nuclear periphery (Akhtar and Gasser, 2007). It is likely that the nuclear lamina contributes to these types of intranuclear chromosome organization. Unicellular eukaryotes and plant cells do not have lamins, although they might have proteins that function as a nuclear lamina.

During mitosis, a parent cell gives rise to two daughter cells, each with its own nucleus. Two main strategies have evolved to successfully carry out this task: open mitosis and closed mitosis (Fig. 2). Open mitosis occurs in most eukaryotic cells, whereas closed mitosis occurs in certain species of fungi. In open mitosis, the NE disassembles early in mitosis, allowing microtubules that emanate from cytoplasmic centrosomes to contact the chromosomes and promote their segregation (reviewed by Prunuske and Ullman, 2006). At the end of open mitosis, the NE reassembles around the two segregated DNA masses to form the two daughter nuclei. In closed mitosis, the NE does not disassemble and chromosome segregation takes place entirely within the confines of the nucleus. This strategy works in cell types where the centrosome equivalents, known as spindle-pole bodies, are embedded in the NE, allowing microtubules to associate with chromosomes without the need for NE disassembly. Some organisms, such as Aspergillus nidulans, undergo semi-open mitosis, in which the partial disassembly of the NPCs creates large holes in the NE, but the envelope itself does not completely disassemble (for a review, see De Souza and Osmani, 2007).

In all forms of mitosis – open, closed or semi-open – the NE undergoes dramatic structural changes. During open mitosis, the NE has to reform around all of the chromosomes, rather than around a subset of chromosomes, and it then must expand to attain its final size and shape. During closed mitosis, the NE expands to accommodate the movement of the segregating chromosomes, and it must then get cleaved, resealed and restructured to form two round daughter nuclei. In the past few years, numerous studies have shed light on the molecular mechanisms that underlie these processes. To understand how these nuclear gymnastics take place, we first discuss the factors that contribute to nuclear shape and size. We then examine how the NE reassembles to form a nucleus with the proper attributes following mitosis.

The nuclear envelope. The NE is an integral part of the ER-membrane network (in blue-green). The inner nuclear membrane (INM) and outer nuclear membrane (ONM) connect at sites of NPCs (green barrels) where the membrane curves as it surrounds the NPC. The ONM is continuous with the peripheral ER. The NE contains a variety of proteins that are embedded in the INM (purple) or the ONM (light blue). Most ONM proteins are also found in the peripheral ER. INM proteins can interact with the underlying nuclear lamina (dark blue), with ONM proteins or with chromatin (red), often through linker proteins (yellow). For a detailed description of the various proteins associated with the NE see recent reviews (Crisp and Burke, 2008 Guttinger et al., 2009).

The nuclear envelope. The NE is an integral part of the ER-membrane network (in blue-green). The inner nuclear membrane (INM) and outer nuclear membrane (ONM) connect at sites of NPCs (green barrels) where the membrane curves as it surrounds the NPC. The ONM is continuous with the peripheral ER. The NE contains a variety of proteins that are embedded in the INM (purple) or the ONM (light blue). Most ONM proteins are also found in the peripheral ER. INM proteins can interact with the underlying nuclear lamina (dark blue), with ONM proteins or with chromatin (red), often through linker proteins (yellow). For a detailed description of the various proteins associated with the NE see recent reviews (Crisp and Burke, 2008 Guttinger et al., 2009).


Arguments based on elastic stability and flexure, as opposed to the more conventional ones based on yield strength, require that living organisms adopt forms whereby lengths increase as the ⅔ power of diameter. The somatic dimensions of several species of animals and of a wide variety of trees fit this rule well.

It is a simple matter to show that energy metabolism during maximal sustained work depends on body cross-sectional area, not total body surface area as proposed by Rubner (1) and many after him. This result and the result requiring animal proportions to change with size amount to a derivation of Kleiber's law, a statement only empirical until now, correlating the metabolically related variables with body weight raised to the ¾ power. In the present model, biological frequencies are predicted to go inversely as body weight to the ¼ power, and total body surface areas should correlate with body weight to the ⅝ power. All predictions of the proposed model are tested by comparison with existing data, and the fit is considered satisfactory.

In The Fire of Life, Kleiber (5) wrote "When the concepts concerned with the relation of body size and metabolic rate are clarified, . . . then compartive physiology of metabolism will be of great help in solving one of the most intricate and interesting problems in biology, namely the regulation of the rate of cell metabolism." Although Hill (23) realized that "the essential point about a large animal is that its structure should be capable of bearing its own weight and this leaves less play for other factors," he was forced to use an oversimplified "geometric similarity" hypothesis in his important work on animal locomotion and muscular dynamics. It is my hope that the model proposed here promises useful answers in comparisons of living things on both the microscopic and the gross scale, as part of the growing science of form, which asks precisely how organisms are diverse and yet again how they are alike.


Effects of population size

Over long periods of time, genetic variation is more easily sustained in large populations than in small populations. Through the effects of random genetic drift, a genetic trait can be lost from a small population relatively quickly (see biosphere: Processes of evolution). For example, many populations have two or more forms of a gene, which are called alleles. Depending on which allele an individual has inherited, a certain phenotype will be produced. If populations remain small for many generations, they may lose all but one form of each gene by chance alone.

This loss of alleles happens from sampling error. As individuals mate, they exchange genes. Imagine that initially half of the population has one form of a particular gene, and the other half of the population has another form of the gene. By chance, in a small population the exchange of genes could result in all individuals of the next generation having the same allele. The only way for this population to contain a variation of this gene again is through mutation of the gene or immigration of individuals from another population (see evolution: Genetic variation in populations).

Minimizing the loss of genetic variation in small populations is one of the major problems faced by conservation biologists. Environments constantly change, and natural selection continually sorts through the genetic variation found within each population, favouring those individuals with phenotypes best suited for the current environment. Natural selection, therefore, continually works to reduce genetic variation within populations, but populations risk extinction without the genetic variation that allows populations to respond evolutionarily to changes in the physical environment, diseases, predators, and competitors.


Evolution and Ecology of Species Range Limits

Species range limits involve many aspects of evolution and ecology, from species distribution and abundance to the evolution of niches. Theory suggests myriad processes by which range limits arise, including competitive exclusion, Allee effects, and gene swamping however, most models remain empirically untested. Range limits are correlated with a number of abiotic and biotic factors, but further experimentation is needed to understand underlying mechanisms. Range edges are characterized by increased genetic isolation, genetic differentiation, and variability in individual and population performance, but evidence for decreased abundance and fitness is lacking. Evolution of range limits is understudied in natural systems in particular, the role of gene flow in shaping range limits is unknown. Biological invasions and rapid distribution shifts caused by climate change represent large-scale experiments on the underlying dynamics of range limits. A better fusion of experimentation and theory will advance our understanding of the causes of range limits.


THE GEOGRAPHIC RANGE: Size, Shape, Boundaries, and Internal Structure

AbstractComparative, quantitative biogeographic studies are revealing empirical patterns of interspecific variation in the sizes, shapes, boundaries, and internal structures of geographic ranges these patterns promise to contribute to understanding the historical and ecological processes that influence the distributions of species. This review focuses on characteristics of ranges that appear to reflect the influences of environmental limiting factors and dispersal. Among organisms as a whole, range size varies by more than 12 orders of magnitude. Within genera, families, orders, and classes of plants and animals, range size often varies by several orders of magnitude, and this variation is associated with variation in body size, population density, dispersal mode, latitude, elevation, and depth (in marine systems). The shapes of ranges and the dynamic changes in range boundaries reflect the interacting influences of limiting environmental conditions (niche variables) and dispersal/extinction dynamics. These processes also presumably account for most of the internal structure of ranges: the spatial patterns and orders-of-magnitude of variation in the abundance of species among sites within their ranges. The results of this kind of “ecological biogeography”need to be integrated with the results of phylogenetic and paleoenvironmental approaches to “historical biogeography”so we can better understand the processes that have determined the geographic distributions of organisms.


What Limits Cell Size?

The primary limitation on the size to which a single cell can grow is a mathematical principle called the surface to volume ratio. As the size of a three-dimensional object grows, its volume increases more rapidly than its surface does, which causes metabolic problems for cells. Additionally, the amount of cytoplasm the nucleus can contain and the structural limitations on the cell prevent them from being larger as well.

Cells are discrete metabolic units. They must be able to take resources in and expel waste and energy. The only place a cell can do this is along the thin, skin-like membrane surrounding it. As the volume of the cell increases in size, it must acquire and expel more substances however, because the volume grows more quickly than the surface area, there is a limit to the amount of diffusion that can take place into or out of a cell.

The nucleus of a cell is essentially a small sphere within a larger sphere. Because the nucleus must become larger to control a larger cell, the nucleus is also susceptible to the problem of surface to volume ratio. This limits the size of the nucleus, which in turn, limits the size of the entire cell.

While the outer membrane of a cell protects the cell well on a microscopic level, large cells would require exceptionally thick membranes. As these membranes thicken enough to hold larger cells, they suffer from decreased permeability.


Cell Differentiation [back to top]

Multicellular organisms have another difference from unicellular ones: their cells are specialised, or differentiated to perform different functions. So the cells in a leaf are different from those in a root or stem, and the cells in a brain are different from those in skin or muscle. In a unicellular organism (like bacteria or yeast) all the cells are alike, and each performs all the functions of the organism.

Cell differentiation leads to higher levels of organisation:

A multicellular organism like a human starts off life as a single cell (the zygote), but after a number of cell divisions cells change and develop in different ways, eventually becoming different tissues. This process of differentiation is one of the most fascinating and least-understood areas of modern biology. For some organisms differentiation is reversible, so for example we can take a leaf cell and grow it into a complete plant with roots, stem, leaf and vascular tissue. However for humans and other mammals differentiation appears to be irreversible, so we cannot grow new humans from a few cells, or even grow a new arm.


FITTING PIECES OF THE PUZZLE

Current theories of nonadaptive or adaptive plasticity of body size in response to temperature are relatively simple, in that each focuses on only one or two of the mechanisms by which temperature can influence the life history. In reality, most variables are affected by temperature, and optimal reaction norms for age and size at maturity depend on the relative strengths of these thermal effects. Atkinson (1994) suggested that three thermal effects in particular were key to understanding temperature-size relationships: thermal constraints on maximal body size, thermal sensitivities of growth rate, and thermal sensitivities of juvenile survivorship. To this list, we add thermal effects on the frequency of reproduction and the survivorship of adults, which have not received serious consideration from life historians (but see Charnov and Gillooly, 2004). Because reproduction is typically less frequent in colder environments, natural selection could favor a larger body size to enhance fecundity at each reproductive episode. For similar reasons, a larger size at maturity might be adaptive if the survivorship of adults is lower in colder environments ( Stearns and Koella, 1986). Finally, a larger body size could enable individuals to produce larger offspring or to provide better parental care, which are thought to be adaptive in colder environments ( Perrin, 1988 Yampolsky and Scheiner, 1996). Like many hypotheses in evolutionary ecology ( Quinn and Dunham, 1983), these mechanisms are not mutually exclusive therefore, some or all of them could contribute to an explanation for the temperature-size rule. Moreover, the relative importance of each mechanism probably varies among species. By combining these mechanisms in a single theory, one can achieve a deeper understanding of the relationships among temperature, growth rate and body size.

When developing a multivariate theory of temperature-size relationships, biologists should focus on the developmental reaction norm because this approach forces one to consider the coevolution of thermal reaction norms for growth rate and size at maturity. Allometric and thermal effects on growth rate can be modeled by including well-established functional constraints, such as tradeoffs associated with acquisition, allocation, and specialization. The natural variation in temperature and the covariations between temperature and other environmental variables must be considered because they play important roles in the coevolution of growth rate and body size. Because current theory describes how the thermal environment shapes the optimal reaction norm for growth rate, modeling the developmental reaction norm could reveal why specific temperature-size relationships have evolved in specific environments. Such a breakthrough is needed if we are to understand not only why most species follow the temperature-size rule, but also why certain species do not.

Table 1. Thermal sensitivities of anabolism and catabolism in ectotherms



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