Ecology

Ecology, the study of an organism's relationship to its surroundings, consists of several distinct areas, all of which have their own specific approaches and methods. Ecophysiology deals with physiological mechanisms, evolutionary ecology is concerned with life history-related, fitness-relevant, and population-genetic aspects, behavioral ecology looks at how the animal deals with its surroundings, and community ecology asks how groups of species can live together. This text will approach how mammals are able to adapt to extreme conditions and will also consider spatial and temporal distribution, predator-prey relationships, and relationships between species forming similar niches.

Mammals are endotherms. This means that they have to put a great deal of energy into regulating their body temperatures. In a cold environment, there are several strategies to deal with those challenges: mammals, contrary to reptiles, are often capable of developing a rather thick isolatory tissue (subcutaneous fat) plus thick fur. This option is not open to reptiles due to the fact that they need to retain the high thermal conductancy of their integument in order to heat themselves by exposure to sun rays. Small mammals, however, have this capability in a lesser extent than larger ones. An additional option for larger species is migration.

Another mammalian adaptation is known as non-shivering thermogenesis (NST), the burning of so-called brown fat, a special tissue rich in mitochondria and often deposited around the neck or between the shoulder blades. The most effective way of dealing with the challenge of a cold environment is torpor, the reduction of one's body temperature and basal metabolic rate, in some species to around or even slightly below 32°F (0°C). For example, the Arctic ground squirrel (Sper-mophilus parryii) goes down to a startling body temperature of 28°F (-2°C). Daily torpor, or larger periods of hibernation, can be found in members of at least five placental and two marsupial orders. The largest species found with "real torpor," lowering their body temperatures by at least 50°F (10°C), are badgers (both the American and the Eurasian species—in the latter case it was found in an individual of 27 lb [13 kg] body weight). Bears also become dormant in winter, but their body temperature is lowered only by about 41°F (5°C), and their physiological mechanisms are different from those of the smaller species.

How do mammals deal with desert conditions? Deserts are not only characterized by extreme temperatures (hot and cold, many small species thus also exhibit daily torpor), but also by arid conditions and a low biodiversity. One strategy for smaller mammals to deal with this low productivity again is het-erothermy, to reduce basal metabolism and thus economize on one's energy demand. Large mammals such as camels can also store heat quite effectively in their large bodies; camels can increase their body temperatures under heat-stress up to 106°F (41°C) during the day, and lower it to around 93°F (34°C) at night. This can save about 12,000 kJ and 1.3 gal (5 l) of sweat. Water balance in many species of desert-dwelling mammals is improved by a counter-current system in their nasal conchae, and by recycling water in the kidneys. Kangaroo rats (Dipodomys) are among those species that regularly live without the need to drink open water; they are able to survive on the water content of their food (seeds), and from oxidation. Grazing mammals can improve their water balance by feeding at night, because grass species are rather rich in tissue fluid at that time. Carnivores extract water from vertebrate prey—even fennecs can keep their water balance simply by feeding on mice. The effectiveness of desert mammals is increased by behavioral adaptations, such as regularly retreating into burrows or shady areas where it's not only cooler but also more humid.

An even more challenging habitat for mammals is extreme high altitude. While desert means cold plus dry plus food scarcity, high altitude means desert plus low oxygen. Thus, physiological adaptations found in mammals at high altitudes include all those just discussed, plus specific ones to improve gas exchange (lung tissue and blood capillaries becoming more intricate), better oxygen transport in the blood, and better oxygen-dissociative capabilities from hemoglobin to body tissues. Also, smaller mammals in arid or mountainous habitats often retreat into underground burrows.

Living underground continually, or at least for a large part of the animal's daily activities, is as challenging an environment as the one they might escape from. Members of at least 11 families (marsupials, insectivores, and rodents) have adopted a subterranean lifestyle with two different methods of digging: hand digging, as performed by moles and the marsupial mole, mostly in loose soils, or tooth digging, performed by rodents in hard substrates. The environment in an underground tunnel has several specific

Kangaroo Rat Nasal Conchae
Migrating herds of blue wildebeest (Connochaetes taurinus), Paradise Plains, Masai Mara. (Photo by Animals Animals ©Rick Edwards. Reproduced by permission.)

characteristics. It is often hot, rather humid, and carries more carbon dioxide and less oxygen than fresh air does. Most subterranean species thus are rather small (the largest reaching a few kilograms only), sparsely furred, have a low resting metabolic rate, and have a more effective cardiac and respiratory system (for example, myoglobin density and cap-illarization in muscle tissue is higher, hemoglobin has a higher oxygen affinity, hearts are bigger and more effective, and the animals are more tolerant of hypoxia than related, non-burrowing species). Another way of dealing with high carbon dioxide concentrations seems to be excreting bicarbonates via urine, which is achieved by a high concentration of calcium-ion (Ca2+) and magnesium-ion (Mg2+) in the urine. Subterranean mammals also encounter a unique sensory environment. Their preferred mode of communication, even in small species, is low-frequency acoustics, as they are rather insensitive to high frequencies and vibratory communication. Some mole rats (Cryptomys hottentotus, Spalax ehrenbergi) are also capable of magnetic field orientation.

There are, especially for smaller mammals, other ecological factors at least as important (if not even more so) that also determine which activity patterns (being diurnal, nocturnal or crepuscular, being active in short or long bouts, etc.) are most adaptive under given situations. One of these factors is predation. Even larger species, such as kangaroos, tend be active at times when their predators are less likely to attack. In small mammals, predator-prey relationships are perhaps even more decisive. This holds true not only for prey species but also for the predators. Small mammalian predators such as weasels, mongooses, or small dasyurids are potential prey to other raptors and larger mammals themselves. On the other hand, being of small body size means that the energy demands and constraints are particularly severe. Thus, they have to term their activity patterns much more carefully than larger species. Being potential prey puts a heavy ecological load on all smaller mammals. Being active at times of low predator activity (dusk and dawn, when most diurnal raptors are no longer active and many owls and mammals not yet active) is one possibility of escape. Being active in a synchronized way provides safety in numbers (the dilution effect), and predation stress can explain ecologically the often dramatic suddenness in the onset of activity. It is also interesting to note that the onset of activity, in most species, is more fixed by internal factors—termination, however, is more variable.

Besides predation, inter- and possibly intra-specific competition also must be considered as influencing activity. Being active at different times of day or using different parts of a habitat at different times can raise the possibility of niche separation, as has been shown in communities of Gerbillus as well as heteromyid species. The behaviorally or ecologically

Dominant Mammals
The black-footed ferret (Mustela nigripes) takes over abandoned prairie dog (Cynomys sp.) burrows. (Photo by © D. Robert & Lorri Franz/Cor-bis. Reproduced by permission.)

dominant species in these communities regularly excludes the others from the "best" temporal niche. Similarly, intra-spe-cific competition often drives subordinate individuals into other temporal niches and is often the first step in excluding an individual from a group or litter, long before agonistic expulsion is to be seen.

In general, there is a clear relationship between body size and activity patterns of mammals: the smaller the species, the more likely it is to be nocturnal. This is confirmed for both herbivores and carnivores. Being nocturnal offers better protection from being detected by predators as well as competitors (which is a more important factor when belonging to a small species). In small carnivores, additionally, the effect of finding more prey during the night further enhances this preference. There are, however, exceptions to this rule. Micro-tine rodents are characterized by very short, ultradian activity patterns, which are very adaptive to the present ecological conditions. Insectivorous and gregarious small mongooses are diurnal, and tree squirrels are all diurnal.

Predators and prey—are they influencing each other's population biology? The answer to this question is as variable as the species and ecosystems studied to answer it: population cycles of voles, lemmings, and snowshoe hares, mostly in a 3-5 year period, have long been suggested to be driven by specialist predators. However, controlled removal of weasels (Mustela nivalis) in one study did not prevent population crashes of field voles, nor did it influence population dynamics at any other stage of the cycle. Also, for several populations of snowshoe hares, both cyclic and non-cyclic ones, predators (weasels, mink, bobcats, lynx, coyotes, and several birds of prey) were the most frequent cause of death for radio-collared hares, and hare population cycles did heavily influence the reproduction, mortality, and movement of the predators. Nevertheless, at or near peak densities, predator activity appeared to have almost no influence on hare density. At lowest density, no influence was evident either, provided that enough cover was available for the hares to retreat into. Survival of hares was directly related to good cover and good feeding conditions. Juvenile and malnourished individuals were more at risk not only due to predators but also due to hard winters.

Predators That Eat Hares

Caribou (Rangifer tarandus) eat a wide variety of vegetation, which enables them to survive through harsh winters. Much of their native land has been taken over for human use, but large areas have been protected by the government to preserve their habitiat. (Photo by © Annie Griffiths Belt/Corbis. Reproduced by permission.)

Caribou (Rangifer tarandus) eat a wide variety of vegetation, which enables them to survive through harsh winters. Much of their native land has been taken over for human use, but large areas have been protected by the government to preserve their habitiat. (Photo by © Annie Griffiths Belt/Corbis. Reproduced by permission.)

Are Hippopotamus Are Endotherms
An adult hippopotamus (Hippopotamus amphibius) feeds on grass on the banks of the Chobe River in Botswana. Hippopotamuses usually feed at night, but on relatively cool mornings they may wander out of the water to feed on nearby grass. (Photo by Rudi van Aarde. Reproduced by permission.)

In a large, comparative, long-term study of many species of predators and prey (wolf and lynx, weasels and stoats, and raptors and owls, and prey from European bison and moose down to amphibians and shrews), it was found that the largest species were barely influenced by predators at all, that amphibians were mostly influenced in their population densities by weather, and that predators in general could neither influence prey densities nor fluctuations. There were, however, a few exceptions: lynx were able to limit roe deer densities below carrying capacity, and both wolves and lynx were obviously able to influence population densities of roe and red deer. The reason might be that, contrary to most other predator-prey systems, both predators are smaller and have a higher reproductive rate than their prey. In those cases, predators might be able to react (numerically, by means of litter size and survival) more rapidly to changing conditions than their prey does. In many ecosystems, both temperate and tropical, large species of prey migrate and thus leave the areas with highest predator activity. Both migrating gnu and caribou, for example, have been shown to lower predation risk by this strategy. Comparison of migrating and nonmigrating ungulates in the Serengeti, following a severe decline of buffalo (the largest nonmigrating herbivore there) and large predators to poaching, led to astonishing results: topi (Damaliscus lunatus), impala (Aepyceros melampus), Thomson's gazelle (Gazella thomsonii), and warthog (Phacochoerus aethiopicus) seem to be predator-controlled. The red hartebeest (Alcelaphus buselaphus), a close relative of topi (but one with a different feeding style) seem to be regulated by intra-specific competition, and giraffe (Giraffa camelopardalis) and waterbuck (Kobus ellipsiprymnus) declined due to poaching. At least in Thomson's gazelle, but probably also others, the reason for the influence of predators on population performance seem to be more complex than simple mortality. Vigilance and flight increase, whenever predator pressure increases, and this of course affects all individuals, not only the unlucky ones being killed. These costs of anti-predator behavior (avoiding potentially dangerous feeding habitats, spending time alert or on the move, etc.) have to be carried by all members of the population, and they do not bring any benefit to the predators (contrary to the "direct costs" of killed animals). This example demonstrates again the complexity of the whole issue.

How can communities, groups of several or even many species, live together? The ecological term "guild" defines a group of species that use the same resources in a comparable way. Thus we would expect them to compete for these resources, and either ecological displacement or niche separation, at least along one or a few niche axes, should occur. In many guilds of species, a recurring phenomenon known as character displacement exists. This means that in areas where two or more competing species occur (sympatric occurrence), at least one trait should differ more pronouncedly than between populations of the same species in non-overlapping (al-lopatric) habitats. One example: the ermine, a small weasel, is smaller in Ireland than in Great Britain, where an even smaller species, the least weasel (M. nivalis) occurs sympatri-cally. Guilds of carnivores have been studied in many countries, and in many cases character displacement is evident. In areas of sympatry between two small cat species of South America, the margay (Leopardus wiedii) and the jaguarundi (Herpailurus yaguarondi), the margay is more aboreal. Degree of arboreality is also a frequent pattern in separate primate species, both in guilds of guenons and between sympatric lorisids such as the angwantibo (Arctocebus) and the potto (Per-odicticus) in tropical Africa. In the case of the lorisids, one is a smaller, more slender-built species using thinner branches and the upper canopy, while the other is larger and more stoutly built, using the lower, thicker branches.

What Are The Margey Pradater
Female African lions (Panthera leo) are predators in their ecosystem. (Photo by David M. Maylen III. Reproduced by permission.)
Guilds Mammals Animals
New species of animals are still being discovered. Shown here is the recently described species of mouse lemur (Microcebus griseorufus). (Photo by Harald Schütz. Reproduced by permission.)

Carnivore communities are special in that direct, aggressive competition can be observed. Nevertheless, there are many axes along which species can separate. One is body size, which directly relates to prey size. In African savannas, lions, leopards, hyenas, hunting dogs, cheetahs, and jackals all coexist, and direct competition for similar-sized prey is almost fully restricted to cheetahs versus hunting dogs. This relatively peaceful coexistence is due to many factors: social hunting allows some species to take prey of much larger size than their own; the leopard is more arboreal than the other predators; and some predator species migrate to follow their prey while others don't. In sympatric carnivore species of similar body size, gape size (the ability to open one's mouth more or less widely) often acts as a separating axis (as in the guild of cats in South America, mentioned above). Comparative studies of carnivore guilds in Israel (13 species, 4 families), the British Isles (5 species of mustelids), and East Africa (3 species of jackals) found that there is regular separation, apart from body size, in terms of degree of cursorial locomotion, in diameter or shape of canine teeth, and in skull length. Direct overlap of all these niche parameters mostly occurred when a newly introduced species such as

American mink (Mustela vison) or a recently immigrated one, such as the striped jackal (Canis adustus) in East Africa, was present. In those cases, character divergence and niche shifts or niche compression did become evident, and mostly led to displacement in one species (European mink [Mustela lutreola], sil-verback jackal [Canis mesomelas]).

A guild of granivorous (seed-eating) rodents was studied in the Sonora Desert of Arizona. One cricetid and four het-eromyid species of different sizes did occur in this community, and all appeared to eat seeds of the same plant species (except for one large species eating a somewhat larger number of insects and one small species eating more seeds of one particular bush). A remarkable difference, however, was found in the spatial arrangement of their feeding places. The large species, a kangaroo rat, mostly exploited patches rich in seeds, such as near rocks or in depressions in the soil. The small species, a pocket mouse, also collected seeds from patches but only in about 6.6% of observed feeding bouts. For the rest, seeds were collected in a more systematic way, while searching in a "sauntering" manner. Both species used olfactory cues to search for food. However, as the kangaroo rats move bipedally in a rapid hop, and thus can easily move from one patch to the other, the smaller species walk quadrupedally. These differences in locomotion not only carry different energetic costs but also allow the animals to make use of olfactory gradients (gradual increases/decreases of concentration) differently: the faster an animal moves, the easier it can detect an olfactory gradient when a larger patch of seeds exudes some stronger smell. The slow-moving pocket mouse, on the other hand, can sniff out individual grains.

Desert Mouse Burrow

A four-striped grass mouse (Rhabdomys pumilio) sheltering in its burrow from the heat of the day. When temperatures above ground reach to 108°F (42°C), temperature inside the burrows will remain below 77°F (25°C). Such sheltering enables a variety of mammals to survive in desert and semi-arid regions throughout the African continent. (Photo by Rudi van Aarde. Reproduced by permission.)

A four-striped grass mouse (Rhabdomys pumilio) sheltering in its burrow from the heat of the day. When temperatures above ground reach to 108°F (42°C), temperature inside the burrows will remain below 77°F (25°C). Such sheltering enables a variety of mammals to survive in desert and semi-arid regions throughout the African continent. (Photo by Rudi van Aarde. Reproduced by permission.)

Rudi Van Aarde
A black rhinoceros (Diceros bicornis) browses on low-growing shrubs. (Photo by Rudi van Aarde. Reproduced by permission.)

The most obvious and oft-cited guilds of larger mammals certainly are ungulate communities. Many studies have been made on groups of herbivore species both in temperate and tropical areas. Despite the fact that up to at least six species of native ungulates can coexist in temperate climes and more than 20 in some African savannas, it is surprising that most studies do not find obvious competition effects. Contrary to carnivores, where inter-species killing and direct competition over food are regular features, there is practically no evidence of direct aggressive competition. Even indirect, long-term effects on population density caused by the changing number of another species is mostly absent. Things only change as soon as newly introduced species come into the community. Thus it was found that feral muntjacs (Muntiacus) in Great Britain severely competed with roe deer (Capreolus). When cattle were introduced into an area where black-tailed deer (Odocoileus) were numerous, the deer retreated into other habitats, which cattle avoided. Competition between species of deer was documented in communities of deer in New Zealand, where all three species (red [cervus elaphus], fallow [Dama dama], and white-tailed deer [Odocoileus virginianus]) had been introduced. Thus it seems that guilds of ungulates having a long (co-) evolutionary history together can coexist, probably because there are enough dimensions along which to separate. The most famous example is the grazer-browser division or, more accurately, the division into concentrate selectors, bulk-feeders, and intermediate feeders. Apart from selecting leaves, grass, or something in between, there are subdivisions in each set of species. Height preference is also a separating criterion for grazers: some species, such as zebra, feed on high and dry or lignified grass, while others feed only on lower, mostly fresh plants. Another separating axis is bite size, which is determined by jaw/snout size and tooth rows. Animals with a broader mouth are less selective in feeding. Body and gut size are also important criteria in deciding what to forage and how to digest. The smaller species have a higher energy demand and specialize on high quality leaves; larger species feed on lower quality grass such as stems or leaf sheaths, or older plants. The reason for this is that larger species, with larger fermentation chambers in their guts, can digest cell walls more effectively and need less energy per unit of body mass. Equids are better able to extract energy from large amounts of fiber-rich food, and ruminants do better with restricted food mass. Equids have a lower reproductive rate than ruminants but are better able to defend themselves against predators due to their sociality. Often in savannas there is a succession of different ungulate species foraging on the same spot, one after the other: zebras (Equus) start by eat-

Keren Corbis
The bamboo that the panda (Ailuropoda melanoleuca) eats provides it with very little nutrition, but it is available year round. (Photo by © Keren Su/Corbis. Reproduced by permission.)
Keren Corbis

The capped langur (Trachypithecus pileatus) is one of the rarest primates and lives in the trees of Bhutan. (Photo by Harald Schütz. Reproduced by permission.)

Seals Plastic Ecology

Humans have an effect on other mammals' ecology. Here an Antarctic fur seal (Arctocephalus gazella) has plastic packaging around its neck. (Photo by Paul Martin/Nature Portfolio. Reproduced by permission.)

ing the high, old, dry grass; wildebeest (Connochaetes) follow and eat the lower grass plants, but still select on the level of whole plants; Thomson's gazelle, with their narrow snouts, select only the freshest parts in the middle of grass plants; and kongoni feed on the long, lignified stalks that remain. Migrating versus non-migrating is another axis of niche separation. This is the famous Bell-Jarman principle first described for the Serengeti and other East African ecosystems, which allows up to eight species of grazers and about 20 species of ruminants in total to coexist.

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Responses

  • FRANK
    What structures for movement do blackfooted ferrets have?
    6 years ago
  • nora
    How do african striped grass mouse protect itself from predation?
    6 years ago

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