Behavioural Adaptations for
Survival
BIOL 3100
Mobbing behaviour
• When a predator is spotted, birds, like
chickadees, will give a specific alarm call
or mobbing call.
• When heard, all individuals present come
in to investigate and attack the intruder
(hawk, owl)
When played back chickadee mobbing calls and other chickadee vocalizations,
significantly more birds approach the speaker and display mobbing behaviour
during mobbing call playback
Thus, chickadee mobbing calls carry meaning for at least 10 other avian
species
Variation in the mobbing call signifies different
predators
The different warnings help flock
mates grasp the relative threat of the intruder
Flying owls, hawks and
flacons provoke a highpitched “seet” call
Small raptors, like saw
whet owls and American
kestrels (which are the
biggest threat) provoke
more “dees” at the end of
the call and elicit a larger
Perched
predators evoke
a loud
“chickadee” call
Crazier still:
Same behaviour is exhibited by gulls when a predator,
such as a raptor (or researcher) is present
A student of Niko Tinbergen’s, Hans Kruuk, was interested in examining the
ultimate causes of mobbing behaviour in black-headed Gulls
Question: Is mobbing an adaptive behaviour and a product of natural
selection?
Hypothesis: Mobbing distracts certain predators, reducing the chance that they
would find the mobbers’ offspring, boosting the fitness of mobbing parent gulls
So, if mobbing behaviour is adaptive, why don’t 100% of the parents mob
predators 100% of the time? In other words, why doesn’t the mobbing
phenotype “win out”
3 constraints on adaptive perfection
1) Failure of appropriate mutations to occur.
Evolutionary constraints on adaptive perfection can
arise from failure of appropriate mutations to occur,
which will prevent selection from keeping up with
environmental change. Thus, maladaptive or
nonadaptive traits may persist. (e.g. ground squirrels in
the arctic, moths)
2) Pleiotropy. Most genes have multiple developmental
effects, not all of which are positive. If overall negative,
selection will act against that gene; if overall positive,
selection will maintain the trait, though there may still
be negative consequences.
3) Coevolution. Interaction of different species that
influence each others fitness. Each species changes in
response to selection pressure imposed by the other,
resulting in an evolutionary arms race.
So back to the gulls…
• There will be constraints on adaptive
perfection, meaning that we should
observe variation in mobbing behaviour
• In addition, if the trait is adaptive, then the
trait either (a) spread through the
population in the past and has been
maintained by natural selection or (b) is
currently spreading relative to alternative
traits
• Thus, mobbing should confer a fitness
advantage (i.e., individuals that mob
should have higher reproductive success)
One approach to test whether a trait is adaptive is to take a cost-benefit
approach in which different phenotypes are analyzed in terms of fitness costs
and benefits
Benefit: Mobbing prevents offspring from getting eaten, predators forced to
spend more time searching for prey
Costs: Time and energy expended, getting eaten yourself by the predator
California ground squirrels react to hunting rattlesnakes by gathering around it and
shaking their tails vigorously to encourage it to depart before being physically
attacked by the squirrels.
Also have a pretty cool adaptation we’ll talk about on Friday…
Species from different evolutionary lineages that live in similar environments and
are subject to similar selective pressures may evolve similar traits through
convergent evolution
In other words, different taxa may find the same solution to the same problem.
Though the squirrels do have some antivenom, there are costs to being bitten by a
snake
Hypothesis: Squirrels should adjust behaviour based on the degree of threat.
Test: When rattles of different sized snakes were played back, the squirrels
approached the big snake rattle sound more cautiously than the small snakes.
Examining the costs and benefits of
antipredator behaviour
Dilution Effect
Observation: Butterflies
aggregate in large, densely
packed groups around mud
puddles on tropic riverbanks,
where they suck up minerals
from the soil.
Hypothesis: Bigger groups are
more likely to attract avian
predators, but cost offset
decreased chance that any one
individual will be eaten
Test: Examine probability of
capture in relation to group size
… in this case, the benefits of
joining a larger groups
substantially outweigh the
costs
Dilution Effect
Is this the reason that mayflies all
emerge at the same time so as to
effectively lodge in your teeth
while biking (or eaten by trout
when they emerge from water)?
Test: Use nets to capture cast-off
skins of mayflies, as well as dead
female mayflies, which die
naturally after laying their eggs,
unless they’re eaten first…
Sweeny & Vannote
Anti-predator behaviour can also be exhibited through direct attacks on potential pred
Have you ever stepped on a wasp nest?
Sawfly larvae cluster in balls of ~10 individuals where they feed on eucalyputus
leaves, which contain resinous, toxic oils, which they store in special sacs and
regurgitate onto attacking ants or birds.
If you can’t fight – hide!
Canyon treefrogs rely on
camouflage for defense from
predators, which means they need
to pick the right rocks on which to
cling tightly and be completely
stationary.
Cryptic colouration depends on background selection. This Australian thorny
devil is very well camouflaged when stationary against bark and debris, but
highly conspicuous on a road.
The classic example of the peppered moth, Biston betula
The classic example of the peppered moth, Biston betula
In Great Britian and the US, the melanic form of the peppered moth was once
extremely rare, but almost completely replaced the salt-and-pepper form from
1850-1950.
As industrial soot darkened the colour of forest tree trunks in urban areas the
whitish moths became more conspicuous and were subsequently eaten,
removing those genes
And yes, this experiment is still valid.
Observational study: examining
the natural frequency of black and
white morphs in response to
changing conditions
Experimental study 1: Place moths on different trees (and on different
locations on trees) and examine predation rates.
Moths of both types were less likely to be found and removed by birds
on limb joints, but overall,
Orientation is also important.
Catocala relicta moths usually
perch head-up with with whitish
forewings over its body on white
birch and other light-barked trees.
Is this behaviour adaptive?
If so, predators should overlook
moths more often when they
perch on their favorite substrate,
in their preferred orientation.
Experimental study 2: Images of moths on different backgrounds and in
different resting positions are shown to a captive blue jay, which is rewarded
for detecting a moth.
Jays saw moths 10-20% less often when the moths were on light-coloured
bark. Birds were especially likely to overlook moths when they were oriented
head-up on birch bark.
Some species, like these grasshoppers (B) use large black and white
patches to disrupt predators’ edge detectors, creating false edges. On the
other hand, the black horse lubber grasshopper (C) emphasizes their
outline to make them more conspicuous (aposomatic colouration)
California ground squirrels chew on cast rattlesnake skins then lick their fur. Wh
When given captive
rattlesnakes a chance to
investigate filter paper,
some of which had been
rubbed on a squirrel then
subsequently rubbed on
snake skin, and other
paper that just had been
rubbed on a squirrel – the
rattlesnake spent twice as
much time inspecting pure
squirrel-scented targets.
But sometimes it pays to be conspicuous...