BIO
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Behavioral Ecology
Lecture Notes I: Introduction
and Definitions
Behavioral Ecology:
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brings together the disciplines of ecology & ethology & emerged
from schools of thought developed primarily in the 1960s & 1970s
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several individuals contributed to the early development of behavioral
ecology as a scientific discipline
1 - J.H.
Crook & David
Lack - pioneered comparative approach
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linked social organization of birds & primates to ecological factors
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Crook (1964) - compared social organization & ecology of 90 species
of weaver birds
(a family of birds that occurs primarily in Africa & Asia; see the
photo of a male Cape Sparrow, Passer melanurus to the right):
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Species of Evergreen forests:
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Primarily insectivorous
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Socially monogamous
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Defend large territories
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Savannah-dwelling species:
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Granivorous
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Nest in large colonies
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Usually polygynous
Source: http://levsen.org/kenya/wildlife/birds/birds.html
Differences in the ecology & behavior of weaver
birds occupying these different
areas are likely related to:
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Food dispersion
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Forest insectivores - food supply is scattered, unpredictable (but not
ephemeral), & not superabundant (food is 'economically defendable')
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reproductive success limited by ability of parents to provide food for
nestlings. With contributions of both parents essential, monogamy is favored.
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Savannah (granivorous) species - ephemeral & locally superabundant
food supply (not 'economically defendable') plus individuals may benefit
by following others to good feeding sites
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Influence of predation
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Forest species - territorial, so best 'defense' against predators may be
cryptic nests and spacing of nests
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Savannah species - non-territorial, so best 'defense' may be to nest in
colonies
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polygyny is probably related to risk of predation. Males defend 'safe'
nest sites & the number of mates obtained by males depends on number
& quality of nest sites defended.
Comparative approach:
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Strengths - focused attention on role of ecological factors (such
as food supply and predators) on social organization & showed that
many characteristics may be inter-related & influenced by similar ecological
factors
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Weaknesses - difficult to disentangle cause & effect, e.g.,
correlation between group size & diet . . . is it because of food (seeds
exploited more efficiently by flocks) or predators (individuals in larger
groups may have reduced predation risk)?
2 - W.D.
Hamilton
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ideas of kin selection & inclusive fitness
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From an evolutionary standpoint, the trouble with altruism is that if an
altruistic behaviour is costly for example, dying for someone else
then the genes that promote it should quickly disappear from the
population. Yet examples of self-sacrifice abound in animal societies.
The most conspicuously selfless are the social insects, the ants, bees
and wasps in which most individuals work tirelessly for the good of the
colony and never reproduce themselves. How can such behavior be explained?
W.D. Hamilton argued that such extreme altruism is most likely to evolve
if, by sacrificing themselves, individuals increase their "inclusive fitness"that
is, the proportion of their genes carried by others in the population.
"Hamilton's rule" is the mathematical formulation of this; put crudely,
it amounts to the idea that you can die for close kin and still spread
your genes, since close kin have many genes in common with you.
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selection acts at the level of the gene
3 - John
Maynard Smith
-
ESS
& game theory (adapted from work of John
Nash - 'A Beautiful Mind')
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see A
brief introduction to game theory
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Example - Hawks & Doves (Krebs and Davies 1993):
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Consider a 'game' with 2 strategies: Hawks (always fight to injure &
kill opponents, & risk injuring themselves) and Doves (display but
never fight):
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When a Hawk meets a Hawk, assume a win (W, or benefit) half the time and
an injury (C, or cost) half the time.
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Hawks always beat Doves.
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Doves always retreat immediately against Hawks.
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When a Dove meets a Dove, assume a win (W) half the time (via better displays!)
& retreat half the time (at no cost).
|
In encounter with: |
| Payoff to: |
|
Hawk
|
Dove
|
|
Hawk
|
1/2 (W - C)
|
W
|
|
Dove
|
0
|
W/2
|
|
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Which strategy (Hawk or Dove) is an ESS?
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Can Hawks invade a population of Doves? Is the payoff for a Dove against
Dove > the payoff for a Hawk against Dove? Or, in other words, is W/2 >
W? No, so Hawks can invade a population of Doves.
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So, is the Hawk strategy an ESS (a strategy that, if adopted by all, cannot
be 'invaded' by another strategy)? Is the payoff for a Hawk against Hawk
> payoff for a Dove against Hawk? Or, is 1/2 (W - C) > 0? Answer: it depends
on the values of W and C.
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If W > C then payoff to Hawks will be positive and Hawk is an ESS.
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If W < C then payoff to Hawks will be negative and neither Dove nor
Hawk will be 'stable' (Hawks will always invade a population of Doves until
Hawks become so frequent that they encounter each other frequently. Doves
can then invade a population of Hawks because Hawks tend to damage each
other too much. A population of all Hawks (where W < C) would go extinct.
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So, the behavior that evolves depends on the nature of the interactions.
4 - E.O.
Wilson (See The
Guardian Profile: Edward O Wilson)
Behavioral Ecology has 2 basic themes:
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Natural selection maximizes gene survival, and individuals (temporary vehicles
for genes) should behave in ways that maximize inclusive
fitness.
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Optimal behavior needed to maximize inclusive fitness will depend on both
the behavior of other individuals & ecological circumstances.
Evolution & Behavior - Basic principles
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Proximate vs. Ultimate causation:
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Proximate = explanations of behavior based on immediate causation
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Ultimate = evolutionary approach; why proximate mechanisms occur &
why (based on fitness) organisms respond as they do.
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Examples - How would you answer these questions from proximate and
ultimate points of view?:
Source: http://landscape.acadiau.ca/herpatlas/photopages/jimpeep1.html
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Examples of proximate questions:
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How does a particular behavior develop in an individual?
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What stimuli elicit the behavior(s)?
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What are the genetic, physiological, and anatomical factors that influence
behavior & how do they operate?
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Examples of ultimate questions:
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What is the adaptive significance of a particular behavior?
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Does a particular behavior maximize fitness?
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Why do other species exhibit similar or different behavior?
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Behavioral ecology is concerned largely with interpreting behavior
in ultimate terms.
But, can natural selection influence behavior?
Natural selection can only work on genetic differences. So, for behavior
to evolve (Krebs and Davies 1993):
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there must be, or must have been in the past, behavioral 'variation' in
the population,
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this variation must be, or have been, heritable, and
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some behaviors must confer greater reproductive success than others.
What is heritability?
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the proportion of phenotypic variance attributable to genotypic variance
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permits selection to be effective in generating behavioral change
To what extent is behavior heritable? Here are some examples:
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Møller, A.P. 2002. Parent-offspring
resemblance in degree of sociality in a passerine bird. Behav. Ecol.
Sociobiol. 51:276-281.
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Moller estimated heritability of two measures of sociability in Barn Swallows.
Offspring resembled their parents (1) with respect to their choice of colony
size, and (2) with respect to their choice of distance from the nearest
neighbor. Heritability was considered equal to the slope of midparent -
offspring regression and yielded h = 0.469 for colony size, and h = 0.345
for nearest neighbour. Genetic effects on sociability was confirmed by
cross-fostering experiments.
Can behavior influence fitness?
Fitness:
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the ultimate currency by which all behavior can be evaluated
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a measure of the selective quality of genes, i.e., their reproductive success
More precisely, FITNESS is a number that, when multiplied by the frequency
of a gene or genotype in one generation, gives the gene frequency in the
next generation.
For example:
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FITNESS = 0, no descendants (no representation of genes in next
generation)
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FITNESS = 1, frequency of genes in next generation = frequency in
preceding one
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FITNESS < 1, frequency of genes declines in next generation
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FITNESS > 1, frequency of genes increases in next generation
In general, FITNESS is a measure of success in 'projecting' copies of one's
genes into succeeding generations. 'One's genes' are obviously present
in oneself BUT are also present in relatives! Thus, what's important is
not just fitness but, rather, inclusive fitness. INCLUSIVE FITNESS is determined
by one's personal fitness plus the indirect effects of the fitness of relatives.
To make things more complicated, however, not all relatives are equally
'valuable' when it comes to inclusive fitness.
Relatedness:
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r = coefficient
of genetic relatedness = the probability that a particular allele,
present in one individual, is also present in another individual because
of their descent from a common ancestor
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r also equals:
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the summed probabilities of all genes within an individual and, so, the
proportion of genotype in 2 individuals that are identical because of their
common descent
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0.5 to the Lth power (where L = the number of generation links between
the two individuals concerned)
Source: http://www.taumoda.com/web/PD/library/kin.html
Source: http://www.science.mcmaster.ca/psychology/psych1a6/1aa3/EvoPsych/lec5-1.htm
EXAMPLES:
Identical twins - r = 1
Unrelated individuals - r = 0
Parent & offspring - r = 0.5
Full siblings - r = 0.5
Half siblings - r = 0.25
Uncles/aunts & nieces/nephews - r = 0.25
Cousins - r = 0.125
So, FITNESS can be influenced by one's behavior plus the effect
of one's behavior on all relatives (with the importance of each relative
devalued in proportion to degree of relatedness). Of course, an obvious
question that comes to mind is: how does an organism 'know' its relatives
(and, if known or recognized, the degree of relatedness)?
KIN
RECOGNITION is the ability to identify relatives. More precisely,
it is the differential treatment of members of the
same species in a way that depends on their genetic relatedness to
the discriminating individual. Such recognition, if it occurs, permits
kin
selection (selection of genes because of their effect in favoring
the reproductive success of relatives other than offspring).
Mechanisms of kin recognition:
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phenotype matching - the process by which individuals inspect certain characteristics
of their relatives of social partners and use those characteristics as
a recognition guide when encountering unknown individuals.
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location
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association/familiarity
| Kissing cousin or close kin? Belding's ground
squirrels (Spermophilus beldingi) sniff to learn who is family &
who's not. More remarkably, they determine in a matter of seconds who is
close-enough kin to risk their lives helping -- and perhaps even whether
they are too closely related to for mating. Mateo (2002) examined potentially
risky nepotistic behaviors. "The word nepotism originally meant favoritism
shown to nephews, but nepotism in Belding's ground squirrels is limited
to mothers, sisters and daughters. Nieces, nephews and more distant kin
are treated like outsiders," Mateo says. She described the risks and benefits
of nepotism in a squirrel world where territorial defense and predator-avoidance
can spell the difference between finding the next meal or becoming one:
o Because male Belding's squirrels leave their natal territory
after weaning, colonies of burrows are inhabited by related females of
various ages (and a few young males that have yet to leave).
o Cooperative territorial defense is important to preserve
nest sites and to keep infanticidal adults away from vulnerable young,
but fighting can cost injuries or even lives. Risking life and limb for
distant kin or unrelated animals doesn't make evolutionary sense.
o Sounding the alarm when a predator approaches is even
more risky, making a helpful squirrel the first target of a coyote, for
example, while the others run for cover. Belding's ground squirrels are
more likely to give the alarm if they can help close kin (even if they
are eaten, nearly identical copies of their genes are hiding in nearby
holes) but they need a precise measurement of relatedness to weigh the
benefits of risky behavior.
Without harming the squirrels, she took a variety of scent-gland
samples, transferred the scents to coded plastic cubes, and placed the
cubes at the entrances of individual burrows. Then she waited for curious
squirrels to emerge from their burrows -- and sniff. If a squirrel spent
a long time analyzing the presented scent, that meant it had detected a
less-related or totally foreign odor compound -- in keeping with a well-established
standard in olfactory behavior studies. A quick sniff would be all a squirrel
needed to recognize the smell of very close kin.
|
And that's what happened, with a few exceptions. For
the most part, scents the squirrels spent the most time sniffing turned
out to have come from distantly related or unrelated squirrels. The scents
they recognized immediately had come from very close kin, the ones they
should be helping to defend territories or evade predators. The more distant
the relationships and the greater the differences in genetic make-ups,
the more time was spent sniffing and evaluating scent compounds. "The sensitivity
and discrimination of their olfactory apparatus is astounding," Mateo said.
"They're like furry gas chromatography machines."
"When other investigators conducted scent studies and
asked women to sniff T-shirts that had been worn by men either genetically
similar or dissimilar to them, most women preferred the shirts worn by
men with dissimilar genes," Mateo notes. "We should not be surprised to
find kin-recognition abilities in a cross section of species, including
humans." - Cornell
News Service
|
FITNESS & INCLUSIVE
FITNESS are often difficult to measure directly in terms of the production
of offspring (& the reproductive success of those offspring). As a
result, investigators in most field studies must settle for indirect measures
of fitness, e.g., number of eggs laid, number of young fledged or weaned,
access to good quality habitats, foraging efficiency, and so on.
What does 'evolution' do with behavior?
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Should select for behaviors that enhance fitness. But, will behaviors always
be optimal?
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Possible 'obstacles' to optimal behavior & fitness (Barash 1982):
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Mutation
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Linkage - genes beneficial in some way may be linked to a gene that tends
to reduce fitness. So, to get the benefit of one gene, an organism must
withstand the liability of the other.
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Pleiotropy
- single alleles have multiple effects. So, if an allele influences traits
X, Y, & Z, with X being an optimum phenotype, there's no reason to
assume Y & Z are also optimum
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Variable environments - difficulty of achieving optimal behavior varies
in proportion to variability of the environment
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Evolutionary
lag - individuals adapted to past conditions are not necessarily adapted
to present conditions
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Phylogenetic
inertia - 'evolutionary baggage'; resistance to acquisition of adaptive
characteristics due to prior evolutionary history (e.g., flightless birds
on many islands)
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Sex - 're-shuffles' genes
| Does phylogenetic inertia explain the evolution of
ineffective antipredator behavior in a sunfish-salamander system? (Sih
et al. 2000)
The streamside salamander, Ambystoma barbouri,
exhibits ineffective antipredator behavior (high emergence rate from refuge,
and high activity while out of refuge) and thus suffers heavy predation
in stream pools with sunfish. A. barbouri evolved relatively recently
from an ancestor that closely resembled a sister species, A. texanum,
which breeds in fishless, ephemeral ponds. Sunfish thus represent a relatively
new selection pressure for A. barbouri. Phylogenetic inertia predicts
that (1) A. texanum should be very poor at coping with fish and
(2) because it has only recently been exposed to fish, A. barbouri
should still be poor at avoiding fish, but due to its recent exposure to
fish, A. barbouri should be better than A. texanum at coping
with sunfish. Experimental results provided mixed support for these predictions.
As predicted, A. texanum suffered heavy sunfish predation. Compared
to A. texanum, A. barbouri showed a greater tendency to initiate
alarm moves that enhanced escape success from fish. However, in both the
presence and absence of fish, A. barbouri showed higher emergence
rates from refuge and higher movement while out of refuge than A. texanum.
These behaviors tend to increase exposure to sunfish, i.e., for these key
behaviors, A. barbouri apparently evolved in the wrong direction
as far as fish predation is concerned. Due to these offsetting effects
(increased exposure to fish, increased escape success), A. barbouri
is no better at surviving with sunfish than A. texanum. A possible
explanation for the high activity of A. barbouri is its use of highly
ephemeral habitats (relative to A. texanum) that favor the evolution
of higher activity, feeding, and developmental rates for A. barbouri
relative
to A. texanum. |
Streamside Salamander (A.barbouri)
Photo by Eric
Williams
Smallmouth Salamander (A. texanum)
Photo by Robert
Rold
|
Examples of behaviors that may be important in terms of maximizing
fitness:
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Feeding behavior:
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How & where to search for food?
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What type of food to eat?
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Forage alone or in a group?
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Sexual behavior:
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How to search for a mate?
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How to choose a mate?
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Mating strategy (e.g., engage in extra-pair copulations)?
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Territorial behavior:
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Defend territory or not?
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If so, how large a territory to defend?
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Where to establish a territory?
Literature Cited:
Barash, D. P. 1982. Sociobiology and behavior, second
ed. Elsevier, New York.
Crook, J. H. 1964. The evolution of social organisation
and visual communication in the weaver birds (Ploceinae). Behaviour Suppl.
10:1-178.
Krebs, J.R. and N. B. Davies. 1993. An introduction to
behavioural ecology, third ed. Blackwell Scientific Publications, London.
Mateo, J.M. 2002. Kin-recognition abilities and nepotism
as a function of sociality. Proceedings of the Royal Society of London
B 269:721-727.
Sih, A., L.B. Kats, and E.F. Maurer. 2000. Does phylogenetic
inertia explain the evolution of ineffective antipredator behavior in a
sunfish-salamander system? Behavioral Ecology and Sociobiology 49:48-56.
Useful links:
Chaos, cheating
and cooperation: potential solutions to the Prisoner's Dilemma
Costly
signaling & the handicap principle
Evolution
of Behavior
Honest
signals in biology: From Zahavi's handicap principle...
Overview
of Kin Recognition
Prisoner's
Dilemma
Prisoner's
Dilemma: An Interactive Game
Red
Queen Hypothesis
Sexual
selection
Strategy
and Conflict: An Introductory Sketch of Game Theory
The
Red Queen
Behavioral Ecology Lecture Notes II - Foraging
behavior
Back
to Behavioral Ecology Syllabus
