Living in Groups Flashcards
Predator dilution/diluting the risk of attack: theory
- Old view: group-living evolves through benefits to populations/species
- New view: group-living evolves because of its benefits to individuals
- GC Williams and WD Hamilton: individuals associate with others to reduce their own risk of attack
- Alone: chance of being the victim from a predator = 1
- In a group of N individuals: chance of being victim = 1/N
Diluting the risk of attack: evidence from water skaters & fish
Diluting the risk of attack: evidence from horses & blood-sucking flies
Predator swamping: synchrony in time
- Mayflies in North America emerge synchronously each year
- During a two-week period from late May to early June, larvae transform into winged adults and rise up from the water’s surface
- Adults mate, lay eggs, and all die within an hour or so
- Face predation from dragonflies, bats, and birds
- Individual mayflies are safest from predation on days when more adults emerge
Selfish herds
- Just as a mayfly emerging synchronously benefits from being in a group, individuals in the middle of a group may benefit more than individuals at the edges of the group
- WD Hamilton: The “selfish herd” effect
- Individuals in a group should approach others to reduce their ‘domain of danger,’ perhaps explaining continuous movement seen in swarms
- Domain of Danger: the space surrounding an individual that is closer to it than any other members of the group
Selfish herds: trade-offs in herd location
- Position in a group may represent a
trade-off between foraging benefits and predation risk - Front positions can be the first to encounter food
- Front positions are more vulnerable!
- Hungry individuals tend to occupy
positions at the front of a group (Krause
1993) - Satiated individuals tend to seek more
central positions (Bumann et al. 1997)
Predator confusion
Individuals in large groups may benefit from predators being unable to focus on a single target (Imagine someone tossing one ball at you versus 10 balls!)
Communal defense: improved vigilance
Groups detect predators sooner than individuals (because individuals can take turns being on guard)
Trade-off: scanning vs. foraging
- Imagine an ostrich
- Can scan for predators 100% of the time: won’t die by lion, but will die by hunger
- Can forage 100% of the time: won’t die by hunger, will die by lion
- What is the best allocation of time for scanning vs foraging?
- In a group, everyone occasionally scans/occasionally forages
- If anyone cheats and reduces scanning, they will benefit!
- But if everyone cheats, everyone will die
- Vigilance in groups involves a “game,” and the best strategy depends on what everyone else is doing
- An Evolutionary Stable Strategy (ESS): the vigilance strategy which, when adopted by the majority of the population, cannot be bettered by any alternative vigilance strategy
Precise details differ depending on various parameters—like relative risk of starvation and predation—but generally vigilance/individual declines with increasing group size
Group living and foraging: better food finding
Optimal group size
Do animals actually live in optimally-sized groups?
But even if we could account for all of these factors and generate a predicted optimal size, would we predict animals to live in optimally-sized groups?
- Imagine an optimal group with 7 individuals
- A new individual comes along
- Should it join the group or form its own group?
- Join the group! Individual net benefit in group size of 8 is still higher than in group size of 1
- How many individuals will the group have before new individuals should STOP joining?
- 14 individuals
- At this point, a newcomer would do better
alone - Unless groups of optimal size can prevent newcomers from joining, groups in nature will often be larger than the optimum because of benefits to new individuals that join
- One thing about this graph: it treats the benefits to all individuals in the group as the same
Overview of traffic lanes in army ants
- Army ant swarms of up to 200,000 individuals can form spectacular trails up to 20m wide and 100m long.
- In these swarms, ants leaving the nest predominantly use the margins (or edges) of the trail, and those that are returning with food items predominantly use the center.
- How do they organize this remarkably complex group movement?
Mechanisms of traffic lanes in army ants
- 1) Ants detect pheromone trails with their two antennae, always move towards the antennae detecting the stronger pheromone concentration
- When in the middle of their respective trail, there’s almost no difference in pheromone concentration between the two antennae
- Towards the edges, the antennae closer to the edge detects a lower pheromone concentration
- This keeps them in the center of their given trail
- 2) Ants have an innate “preferred” direction: away from the colony
when searching, toward the colony when carrying prey - 3) Individuals turn to avoid collisions; outbound ants are more
maneuverable than those carrying prey and have a higher rate of avoiding collisions
Local rules in fish schools
- Pink = zone of repulsion
- Move away from others
w/in this zone to avoid a
collision - Blue = zone of orientation
- Align with the movement of others within this zone
- Purple = zone of attraction
- Move towards others in this
zone
Voting in African Buffalo
- Females and calves live in large, stable herds of several hundred individuals
- During the day, they lay down in the shade
- Towards dusk, they all get up and move off
together - How do they decide where to go?
- Before the herd sets off, individuals stand up and then lie down again…but they’re not stretching their legs!
- They’re voting! When they stand up, they adopt a special posture and gaze steadily in one direction
- The herd’s eventual direction of motion can be predicted by the vector of individuals’ gazes
Describe the social lives of wild dogs
- Highly social canids
- Non-aggressive, dominant-directed group living
- Dominant breeding pairs affect the behavior of the pack as a whole
- Facilitate feeding by pups at kill sites, suppresses pregnancies in sub-dominant individuals, ensures collective care for denning females and pups
What are ‘social rallies’ (African Wild Dogs)
High energy, socially intricate, pre-departure greeting ceremonies that help the pack to decide if it should go hunting
What preliminary observations motivated the study on African Wild Dogs?
- Preliminary observations during rallies indicated that audible ‘sneezes’ appeared to be frequent
- Hypothesized these sneezes acted as a pre-departure signal
More on results of African Wild Dogs sneezing study
- Rally success was influenced by who initiated: rallies initiated by dominant individuals were more likely to result in group movement than rallies initiated by subordinate individuals
- Rally success was influenced by who initiated: rallies initiated by dominant individuals were more likely to result in group movement than rallies initiated by subordinate individuals
- The number of sneezes required to initiate movement was lower for dominant individuals than for subordinates
- Lower-ranking individuals had to sneeze more times to get everyone to move than did dominant individuals
- A rally never failed if (1) a dominant initiated it and (2) there were at least 3 sneezes
