Alcock Chapter 8

Choosing Where to live

Habitat Selection

• The rule that certain species live in particular places applies to all groups of animals—presumably because the opportunities for successful reproduction are much better in habitat A than in habitat B for members of species X.

• The importance of access to appropriate habitat has be dramatically illustrated by the link between habitat destruction and the declining populations of certain animals.

Habitat Selection (cont’d)

European Great Tit

• Given the importance of being able to breed in specific habitat types, we would expect animals to have evolved strong preferences for some places over others, even if they are capable of reproducing in a variety of environments (p. 242).

• The European great tit, for example, can nest either in mixed woodland or in hedgerows. But the birds prefer mixed woodland to hedgerows, as demonstrated by the shifts made by hedgerow birds into woodland sites upon the experimental removal of breeding pairs from favored habitat (p. 242).

Habitat Selection (cont’d)• If habitat preferences are adaptive, then individuals that

are able to fulfill their preferences ought to leave more descendants than those unable to acquire prime real estate.

• This is true for European great tits.• It is also consistent with the finding that in many species

some individuals occupy SOURCE HABITATS (where the population grows), while others are relegated to SINK HABITATS (where the population declines).

• Poor-quality SINK habitats are generally used by competitors who are unable to insert themselves into superior source habitats, often because they are excluded by older, more accomplished opponents and so must make the best of a bad situation elsewhere.

Costs and Benefits of Dispersal

• Dispersing individuals not only have to pay energetic developmental and travel costs, but are also more often exposed to predators—all of which raises the question: Why are animals so often willing to leave home even when this means leaving a familiar, resource-rich location.

Dispersal (cont’d)

• Male mammals typically disperse greater distances than females.

• The usual rule is that males, not females, fight with one another for access to mates and therefore loser males will find it advantageous to move away from same-sex rivals that they cannot subdue.

Dispersal cont’d)

• In Belding’s ground squirrel, young males travel about 150 m. from the safety of their mother’s burrow, whereas young females usually settle down only 50 m. or so from the burrow in which they were born (Fig. 8.10, p. 250) .

• Dispersal by juvenile animals of many species may be an adaptation against inbreeding depression.

• When two closely related individuals mate, the offspring they produce are more likely to carry damaging recessive alleles in double doses than are offspring produced by unrelated pairs.

• The risk of associated genetic problems should in theory reduce the average fitness of inbred offspring, and high juvenile mortality does indeed occur in inbred populations.

Effects of In- breeding

Include… • Elevated incidence of recessive genetic diseases;• Reduced fertility both in litter size and in sperm viability; • Increased congenital defects such as heart defects & cleft palates; • Fluctuating asymmetry (such as crooked faces, or uneven eye placement

and size);• Lower birthweight ;• Higher neonatal mortality; • Slower growth rate; • Smaller adult size; and • Loss or reduced immune system function.

Belding’s Ground Squirrel and White-Footed Mice

Dispersal in White-Footed Mice

• The risk of associated genetic problems should, in theory, reduce the average fitness of inbred offspring, and high juvenile mortality does occur in inbred populations of many animals.

• When inbred and non-inbred white-footed mice were experimentally released into a field from which their ancestors had been captured, the inbred mice survived only about half as well as the outbred ones (p. 250 and Fig. 8.11, p. 251).

Inbreeding Depression (cont’d)• Even if inbred mice managed to reach adulthood, they were

less likely to reproduce than outbred mice (about 2x less likely) (Fig. 8.11, p. 251)

• Female Belding’s ground squirrels may remain near their birth areas because their reproductive success depends on possession of a territory in which to rear young.

• Female ground squirrels that remain near their birthplace enjoy assistance form their mothers in defense of their burrows against rival females (p. 251).

• So the benefits of remaining on familiar ground are greater for females than for males.

Male Lion Dispersal

• Lions live in large groups or prides from which maturing young males disperse.

• In contrast, the daughters of the resident lionesses usually spend their entire lives where they were born (see Fig. 8.12, p. 251).

• The sedentary females benefit from their familiarity with good hunting grounds and safe breeding dens in their natal (birth) territory among other things.

Lion Pride

Male Lion Dispersal

• The departure of many young male lions often coincides with the arrival of a new, mature , dominant male (sometimes more than one)-- the new pride “owner”--who violently displaces the previous pride master and chases off subadult males.

• Yet, even if young males are not evicted after a pride takeover, they often leave anyway, without any coercion from adult males, and without ever having attempted to mate with female relatives.

Lions (cont’d)

• Mature males (pride owners) sometimes disperse after several years with their pride.

• By doing this the dominant males can attempt take over another pride of females at the time when their daughters in the first pride are becoming sexually mature (p. 251-52).

• So proximate inhibitions against inbreeding apparently exist in lions and cause males to leave home.

Lions (cont’d) dead male lion

• Ultimately, dispersing males may gain by mating with nonrelatives, even though the timing of their departure from their birthplace is not always under their control.

• Take a look at the data in Fig. 8.12 bar graphs: What happens most often to males who disperse? How often to males transfer to (take over) another pride? What is the female survival rate compared to male survival rate? Why do you think this situation works in lions?

Figure 8.12 (in short)• Stayed in pride (F)------------------------------------• Formed new pride (F) ----------• Transferred pride (M)**• Nomads (M)********• Left area (F)- (M)*********** Died (F)---- (M)**** Disappeared (F)— (M)******** Percentage of Lions = 0--------20--------40--------60---------80

Migration Arctic Tern

• A familiar, yet amazing, form of dispersal is migration, which involves movement away from and subsequent return to the same location on an annual basis (p. 252).

• Many living birds, mammals, fishes and sea turtles migrate.• Nearly half of all breeding birds of North America are migrants

that take off in the fall for a trip to Mexico, Central America or South America for the winter, only to return in the spring.

• Note: Fig. 8.13, p. 252, describing the Arctic tern which flies from the Arctic to the Antarctic and back each year.

Migration Poses a Historical Problem

• If sedentary species were ancestral to migratory ones (the probably were), how could the ability to travel 1000s of miles each year to specific destination have evolved?

• A possible answer: many bird species in the tropics engage in short-range “migrations” of dozens to hundreds of miles, with individuals moving up and down mountainsides or from one region to another adjacent to it (p. 253).

The Three-Wattled Bellbird (Fig. 8.14)

• The three-wattled bellbird has an annual migratory cycle that takes it from its breeding area in the mid-elevation mountainous forests of north-central Costa Rica to lowland forests on the Atlantic side of Nicaragua, then to the coastal forests on the Pacific side of southwestern Costa Rica, from which the bird returns to its mountain breeding area (review Fig. 8.14, p. 253).

Three-Wattled Bellbird

•The distances traveled by migrating bellbirds are substantial (up to 200 km) but not exceptional.

*Note 3 wattle strands hanging from male’s head.

Short-Range Migrants (cont’d)• Short-range migrants occur in nine families of songbirds

believed to have originated in the tropics.• Of these nine families, seven also include long-distance

migrants that move 1000s of km. from tropical to temperate regions.

• The co-occurrence of short-range and long-distance migrants in these seven families suggests that short-range migration preceded long-distance migration, setting the stage for the further refinements needed for impressive migratory trips of some species.

• Long-distance migrants are most likely descended from species the moved far less on an annual basis in the past.

The Thrush Genus,

Catharus

• One genus of birds, the Catharus thrushes, may shed some light on this theory of avian migration.

• Fig. 8.15, p. 254, describes Catharus thrushes that have 12 species, 7 of which are resident in areas from Mexico to South America; the other 5 are migratory species that travel between spring/summer breeding areas in North America and winter zones to the south, especially in South America.

Catharus thrushes

• These observations suggest that the ancestors of the migratory Catharus species lived in Mexico or Central America where they were likely non-migratory.

• The distribution of Catharus species suggest that migratory species evolved at least three times, with subtropical or tropical resident species giving rise to migratory lineages.

Catharus Thrushes

Costs of Migration• The costs of long-distance migration are not

trivial.• Extra weight: the migrant has to gain weight in

order to build up energy reserves for the trip.• Some migrant songbirds nearly double their body

weight prior to their long flights.• The danger: Even moderate fuel loads

dramatically alter the take-off angles of individuals startled by a predator, almost certainly increasing the chance that the predator will catch the bird.

Costs of Migration (cont’d)

Red knot• It is also true that fully “loaded” (maximum weight) red knots,

once under way, actually fly more efficiently in terms of turning fuel into wing power than when they are at their non-migration body weight.

• Yet flight, not matter how efficient, still costs calories and there is always the chance that migrant will run out of gas before reaching its destination—not a fitness-enhancing event.

• An optimality approach to migration generates the prediction that migrants will evolve tactics that reduce the costs of the trip.

• For example, does the V-formation adopted by many large birds when migrating actually save energy? Is it an effective adaptation?

Optimizing the Tactics of Migration

• How to reduce the energy cost of long-distance migration!?!• The V-formation in flight: Fig. 8.17, p. 255, presents data to

show that pelicans flying in a V-formation can save significant energy.

• Research on the V-formation flying of pelicans (which had been imprinted on an ultralight aircraft so scientists could monitor them in detail) is very informative.

• Data on the pelicans’ heart rates and wing beat frequencies revealed that birds flying alone had to work harder than those flying in V-formation (review Fig. 8.17, p. 255).

V-Formation Flying

The overall energy savings that pelicans can save by taking advantage of the updrafts created by the wing beats of the companions was about 11-14%--not a trivial sum during long-distance travel.

Weight/Fuel Supply and Migration Distance

• The red-eyed vireo, a small songbird , migrates for the winter from the eastern USA to the Amazon basin of So. America .

• This vireo has two options: cross the Gulf of Mexico (the shorter distance, but potentially fatal if the flyer runs short on fuel); or stay close to land, moving along the coast of Texas to Mexico and then proceeding south.

• Scientists predicted that red-eyed vireos with low fat reserves would opt for the land route; while those with adequate fat would fly over the sea.

The Red- Eyed Vireo

• Migrant birds appear to have evolved an unconscious sensitivity to the mortality risks of their migratory decisions.

• The trans-Gulf flight is shorter but vireos that can’t make it all the way to Venezuela die on route.

• Researchers caught migrating vireos in the fall along the Alabama coast and weighed them (p. 256).

Red-Eyed Vireos (cont’d)

• Birds with less than 5 grams of body fat indicated that they would take the longer land route; birds with 5 g of more of body fat appeared to opt for the for the more direct, overwater route (see Fig. 8.18, p. 256).

• Alcock suggests that red-eyed vireos have evolved an unconscious sensitivity to the mortality risks of the migratory decisions (p.256).

Red-Eyed Vireo Migration

During winter, Red-eyed Vireos are most numerous in the northern Amazon basin where they feed primarily on fruits. As migration time nears, they begin to consume more insects. Whether or not a bird crosses the Gulf appears diet-related. Birds with substantial fat reserves are more likely to attempt a crossing; individuals lacking adequate reserves tend to orient northwestward and follow the coast.

Benefits of Migration• Migration is risky—so what ecological conditions might elevate

the benefits of migration enough to outweigh the risks, leading to the spread of migratory abilities by natural selection (p. 258).

• For migratory songbirds in the Americas, the answer probably lies in the immense populations of protein-rich insects that appear in the northern United states and Canada in the summer, when long days fuel the growth of plants on which the insects feed (note the previous slide’s progression of the insect “bloom” in the red-eyed vireo northern spring migration).

• Also the many hours of summer daylight mean that migrant songbirds can search for food longer each day than can tropical bird species (which have only about 12 hours).

Blackpoll Warblers

• Blackpolls fly nonstop in the fall from eastern Canada all the way to South America (see Fig. 8.19, p. 257), over 3000 km of ocean.

• A safer passage would be along the east coast of the USA, then along the shoreline down along the Gulf states into Mexico and Central America (or by island hopping south from Florida and across the Caribbean to So. America).

• But the sea route is half as long as a land-based route and blackpolls caught on Atlantic and Caribbean Islands are typically in good condition upon arrival, indicating their capacity to make long nonstop ocean crossings. (p. 257).

The Blackpoll Warbler

• At first glance, a blackpoll that selects its fall migratory route (southward) would seem to have a death wish; yet blackpolls commonly appear on islands and the Atlantic and the Caribbean in good condition when they arrive, demonstrating their capacity to make long, nonstop ocean crossings (p. 256-57).

• Why chose this route? (1) the sea route from Nova Scotia to Venezuela is half as long as a land-based trek; (2) there are few predators on the way, and (3) the birds use wind currents to lessen the energy costs—so overall it’s an effective adaptation.

The Blackpoll’s Return

• QUESTION? When blackpoll warblers return to Canada from S. America in the spring, they do not retrace their fall oceanic migratory path, but instead travel mostly over land. WHY?

Swainson’s Thrush

• Discussion Question 8.6 (p. 257): Swainson’s thrush breeds in a large band right across N. America.

• Those birds that live in the northwestern part of N. America do not all follow the same migratory route.

• Some birds go right down the Pacific coast and winter in Central America; most others travel all the way to the eastern part of N. America before flying south to winter in S. America (review Fig. 8.20, p. 258).

• QUESTION: what could account for these two different behaviors? What may be going on here?

Swainson’s Thrush Summer and Winter Distribution

What hypothesis is suggested? Is something new going on here? If so what? And why?

Other Factors in Migration

• Food is not the only factor favoring migration (p. 259).• On the savannas of East Africa, wildebeests, zebras, and

gazelles move from south to north and back again yearly.• The move north appears to be triggered by the dry season,

while the onset of rains sends the herds south again.• It might seem that the herds are tracking grass production.,

which is dependent on rainfall.• But researchers have discovered that the most important

factor sending the animals north is the decline water supplies and an increase in the saltiness of water in drying rivers and waterholes.

• If one knows the salinity of the water, one can predict when the animals will march north.

Wildebeest Migration in East Africa

A rivers and waterholes shrink in the south, they become more salty thereby stimulating the wildebeestsnorthern trek seeking better water as well as plentiful grass (p. 259).

Territorial Contests• Studies of territoriality have found that male winners in the

competition for territories gain substantial indirect and direct reproductive benefits (p. 267).

• Given this evidence, it seems odd that when a territory holder is challenged by a male rival, the owner almost always wins the contest (usually within a few seconds).

• Why do intruders give up so quickly? • One notion is that males that succeed in acquiring territories

have some sort of non-arbitrary advantage over others that translates into superior resource-holding power—such as larger body size (p. 269).

• In fact, in species ranging from rhinoceroses to fiddler crabs to wasps, territorial individuals are relatively large individuals, permitting them to drive smaller rivals away.

Territoriality

• Animals defend territory for many reasons: breeding areas, food resources, nesting sites, access to sun or shade, areas to attract mates, water, etc.

• Defense of space is very widespread, occurring in diverse animal species; yet many animals including honey bees and monarch butterflies ignore or tolerate their fellows (p. 264).

• A cost-benefit analysis of territoriality requires that we consider the disadvantages and advantages.

Territoriality(Defending a Territory)

• For male side-blotched lizards (a common species in the American West) the value of a territory is a function of the number of rocks it contains.

• Abundant rocks attract females which bask on the rocks or seek shade from them; good rocks attract females which like to sun on them.

• So when researchers removed valuable rocks from male territories, male lizards expanded their territories, presumably to include new rocks to make up for the ones lost (p. 243-44; see also Fig. 8.2).

Side-Blotched Lizards

male competitors, too; so males compete for the best rock-strewn areas.

Male Side-Blotched Lizards

• In contrast, when rocks were added, male lizards contracted their territorial borders to a smaller size—less area to defend (but still with a sufficient number good rocks to attract females—as well as male competitors).

• In either case males were able to acquire about one mate on average.

• These male lizards possess sufficient flexibility to adjust their territorial size when a change occurs in territorial quality.

Being Territorial: Costs and Benefits

• Many animals, including honey bees and monarch butterflies, ignore or tolerate their fellows (p. 264).

• So why spend time and energy to be territorial?

• A cost benefit analysis requires that we consider the disadvantages of territorial defense, one of the most obvious of which is the time cost of the behavior.

Costs & Benefits of being Territorial

• A territorial surgeonfish (above photo) chases away rivals from its algae-rich territory on a Samoan reef an average of 1900 times each day--an extraordinary amount of time and energy expended (p. 264).

• The wear and tear of these territorial actions, including actual fighting in many species, can shortened life.

• In addition to the risks of injury and exhaustion, other damaging effects can arise indirectly from the underlying problems (e.g. stress, too little food, bad health, etc.).

Underlying Mechanisms of Territorial Aggression

Yarrow’s spiny lizard

• In species in which testosterone promotes territorial defense, the effects of the hormone may exact a toll as it may increase the activity level of males, even if they are not actually fighting, yet they suffer all the same.

• Experiments with Yarrow’s spiny lizard illustrate this effect (p. 264-65; read caption for Fig. 8.2, p. 265): which males (the implanted ones or the controls) disappeared at a faster rate and why?

• Was there an environmental variable that influenced the outcomes? Was food involved?

The Case of the American Redstart

• These warblers compete for territories in the non-breeding season, when they are wintering in Central America and on Caribbean Islands.

• In Jamaica, males tend to occupy black mangrove forests along the coast, while females are more often found in second-growth scrub inland.

• Heavier males in mangrove habitat attack intruding females and younger males, forcing them into second-rate habitats (p. 265).

Redstarts (cont’d)

• Males in the mangroves retained their weight over the winter, whereas redstarts in the inferior scrub habitat generally lost weight.

• What reproductive advantage is obtained by male redstarts behaving this way ? (review p. 286 text and Figures 8.27 and 8.28, which provide data to explain this behavior).

Resource Holding Power

• Why do residents (territory holders) usually repel intruders? (p. 269)

• In species ranging from rhinos to fiddler crabs to wasps, territorial (holding) individuals are relatively larger.

• But in the case of territorial male dameselflies its body fat that wins—this species uses non-contact aerial dogfights to establish mating territories. It’s a war of attrition—the ones with least energy (least body fat) are defeated.

Territoriality

• Animals defend territory for many reasons: breeding areas, food resources, nesting sites, access to sun or shade, areas to attract mates, water, etc.

• Defense of space is very widespread, occurring in diverse animal species; yet many animals including honey bees and monarch butterflies ignore or tolerate their fellows (p. 264).

• A cost-benefit analysis of territoriality requires that we consider the disadvantages and advantages.

The Case of the Side- Blotched Lizard

• The side-blotched lizard is a common in the US West.• The value of a territory for this lizard is a function of the

number of rocks the territory contains (p. 243-44).• The abundance of rocks attracts females that use this

resource to bask in the sun or seek shade/cover; however, territories that attract females also attract male competitors.

• Experimenters altered male territories by adding and removing rocks.

• Fig. 8.2, p. 244, shows that males expanded their territories when valuable rocks were removed (presumably to locate more suitable rocks); while males that received extra rocks contracted their territorial borders. Why?

• What happened in the end?

Territoriality: Costs vs. Benefits

Samoan surgeonfish

• Disadvantages include time devoted to one pursuit, energy expended, risk of exhaustion, wear and tear on the body, and risk of injury or death.

• A territorial surgeonfish chases rivals away from its algae-rich turf on a Samoan reef an average of 1900 times each day.

• The wear and tear of this kind of territorial behavior as well as actual fighting can shorten an animal’s lifespan.

The Case of Yarrow’s Spiny Lizard

• The data in Fig. 8.25, p. 265, are interesting.• In this experiment some males of Yarrow’s spiny

lizard received a testosterone implant.• These males spent much more time moving about

than did control males (no testosterone implant).• What happened to the testosterone-implanted

males that DID NOT not receive a food supplement?• What happened to the testosterone-implanted

males that DID receive a food supplement?• What was the conclusion of the researchers?

The Case of the American Redstart

• These warblers compete for territories during the non-breeding season, when they are on their tropical wintering grounds in Central America and the Caribbean (p. 265).

• In Jamaica, males tend to occupy black mangrove forests along the coast, while females are more often found in second-growth scrub inland.

• Researchers observed that heavier males in mangrove habitat attack intruding females and younger males, apparently forcing them into second-rate habitats.

• It was found that redstarts living in mangroves retained their weight over the winter, whereas birds in the apparently inferior scrub habitat generally lost weight.

Redstarts (cont’d)

• Probably because they have more energy reserves, territory holders in mangroves leave their winter grounds to fly north sooner than birds living in second-growth scrub (p. 266).

• Review p. 266 and Figures 8.27 and 8.28.• What are the advantages to redstarts than occupied the

better winter territories (mangrove forests)?• What were the final results? Why does this behavior clearly

have a selective advantage?

Territorial Contests• Studies of territoriality have found that male winners in the

competition for territories gain substantial indirect and direct reproductive benefits (p. 267).

• Given this evidence, it seems odd that when a territory holder is challenged by a male rival, the owner almost always wins the contest (usually within a few seconds).

• Why do intruders give up so quickly? • One notion is that males that succeed in acquiring territories

have some sort of non-arbitrary advantage over others that translates into superior resource-holding power—such as larger body size (p. 269).

• In fact, in species ranging from rhinoceroses to fiddler crabs to wasps, territorial individuals are relatively large individuals, permitting them to drive smaller rivals away.

Territorial Contests (cont’d)

• The case of the African widowbird (p. 269, Fig. 8.30) does not involve larger body size.

• In this species, males with larger redder shoulder patches are more likely to hold territories than are rivals with smaller duller epaulets.

• The less gaudy males become “floaters,” hanging around the territories or other males, conceding defeat whenever challenged, but ready to assume control of vacant territories should a resident male disappear.

The African Red-Shouldered Widow Bird (cont’d)

• Experimenters found that when resident males complete with floaters for food in captivity, residents usually win, even though they have been removed from their breeding territories for the lab.

• This happens even if the dominant males red epaulets have been painted black (Fig. 8.30, p. 269).

• This suggests that some intrinsic (yet-to-be-discovered) feature of certain males other than body weight, which is advertised by size and color of their epaulets but can be expressed in the absence of this signal, enables these males to win fights over others.

Do Territorial Owners Fight Harder? (Another Hypothesis)

• Another hypothesis to explain resident success (established territory holders) in defending their turf: when the fitness payoff for holding a territory increases over time, owners have more to fight for than intruders (p. 270-71; Fig. 8.32).

• On the other had, as territory holders age, their opportunities for future reproductive success fall, but, being established territory holders, the time and energy spent in expensive or risky contests also decreases.

• Overall, life history in some cases may boost the net gain from territorial defense by elderly, but still effective territory owners.

Aged Territory Owners• Not only do the costs of territorial behavior decline for aged

territory holders, but the benefits of territorial possession may grow over time because of the nature of interactions among territorial neighbors (p. 270-71).

• An aggressive newcomer initially has to spend a lot of time dealing with boundary disputes with neighbors, but once territorial borders have been settled, everyone calms down, producing what has been labeled the dear enemy effect.

• Take an African lizard, the mean distance at which territorial males will charge a familiar neighbor is less than 1/5 that for strangers; so when chasing a neighbor, the resident pursues him only for a few centimeters, whereas he chases unfamiliar intruders for over a meter on average.

The Dear Enemy Effect• Once a territory owner and his neighbors have learned who is

whom, they don’t need to expend time and energy in lengthy chases.

• However, if a resident is ousted, the new resident must fight intensely for a time with his neighbors in order to get their territorial borders settled.

• An established resident has more to gain by holding onto his turf than the new intruder can secure by acquiring it, since any new boy on the black will still have to deal with his unfamiliar neighbors even after he has ousted the previous resident—and this takes a lot of time, energy expense, and is risky (p. 271).

Dear Enemy Effect (cont’d)

• A logical extension of the dear enemy hypothesis is that neighbors may find it advantageous to combine forces to repel an intruder that might otherwise displace one of them.

• Such a displacement could very well result in the remaining residents left to cope with a rambunctious newcomer (and that will require time, energy and risk on the part of the established territory owners).

• This factor is believed to be behind the occasional coalitions between two neighboring males in the territorial fiddler crab, Uca mjoebergi.

Male Fiddler Crabs

• Occasionally a male U. mjoebergi will leave his burrow in a mudflat and join a neighbor in fending off a wandering male that has challenged that neighbor.

• Typically the helper intervenes when the encroaching intruder is larger than his “dear enemy” neighbor but smaller than the helper.

• The two-on-one game works 88% of the time, whereas a single defender wins only 71% of the time.

• The benefit the helper gets is in the time and energy savings (and injury risk) that come from having a familiar neighbor as opposed to an aggressive newcomer—males that know each other almost never fight.

Conclusion

• We can predict that when a newcomer is permitted to claim a territory from which the original resident has been temporarily removed, the likelihood of the replacement resident winning a fight against the original resident will be a function of how long the replacement has occupied the site (and may have developed “dear enemy” relationships with neighboring males who male assist in repelling challengers).

• This experiment hs been done with birds such as the red-winged blackbird as well as with some insects and fishes.

Summary

• Please review the five summary paragraphs on pages 272 and 273.