1) Animals evolve preferences for certain habitats over others because reproductive success is often higher in some habitats than others. 2) Habitat selection is illustrated by European great tits preferring woodlands over hedgerows for nesting. Individuals that can acquire preferred habitats tend to leave more descendants. 3) Male mammals generally disperse farther than females to avoid inbreeding and competition with other males for mates. Dispersal distances in species like Belding's ground squirrels and behaviors in species like lions also reflect efforts to avoid inbreeding.

1. Alcock Chapter 8
Choosing Where to live

2. 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.

3. 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).

4. 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.

5. 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.

6. 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.

7. 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.

8. 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.

9. Belding’s Ground Squirrel and White-Footed
Mice

10. 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).

11. 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.

12. 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.

13. Lion Pride

14. 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.

15. 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.

16. 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?

17. 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

18. 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.

19. 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).

20. 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).

21. 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.

22. 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.

23. 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.

24. 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.

25. Catharus Thrushes

26. 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.

27. 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?

28. 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).

29. 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.

30. 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.

31. 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).

32. 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).

33. 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.

34. 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).

35. 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).

36. 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.

37. 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?

38. 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?

39. Swainson’s Thrush
Summer and Winter
Distribution
What hypothesis is
suggested? Is
something new
going on here? If
so what? And why?

40. 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.

41. Wildebeest Migration in
East Africa
A rivers and waterholes
shrink in the south, they
become more salty
thereby stimulating the
wildebeests
northern trek seeking
better water as well as
plentiful grass (p. 259).

42. 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.

43. 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.

44. 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).

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

46. 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.

47. 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.

48. 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.).

49. 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?

50. 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).

51. 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).

52. 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.

53. 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.

54. 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?

55. 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.

56. 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?

57. 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.

58. 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?

59. 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.

60. 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.

61. 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.

62. 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.

63. 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.

64. 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).

65. 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.

66. 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.

67. 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.

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