Behavioral Ecology, ecology lecture 3
Behavioral Ecology Notes
Introduction
Behavioral Ecology, a subfield within introductory ecology (BIOL 2060), focuses on the study of social relations and the interactions between organisms and their environment that are mediated by behavior. The scientific study of behavior is commonly referred to as ethology.
Behavior: Defined as the observable response of organisms to external or internal stimuli. These responses can be driven by:
Proximate Causes: The immediate genetic and physiological mechanisms underlying a behavior.
Ultimate Causes: The evolutionary effects of a behavior on an organism's reproductive success.
Sociobiology: A branch of biology specifically concerned with the study of social relations.
Fitness: Quantified as the number of offspring (or genes) an individual contributes to future generations.
Evolution and Behavior
Natural selection acts on individuals, driving evolutionary changes within populations. Behavior, much like physical traits such as beak length, is subject to the pressures of natural selection.
Example: Honeybees: A single gene can determine whether a bee will work within the hive or become a forager. The adaptive value of switch-over behaviors depends heavily on local conditions; for instance, bees foraging actively before the hive is constructed or before plants produce nectar are likely to be less fit than those that delay foraging.
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Proximate immediate causes explain how a behavior happens
ultimate causes explain why a behaviour happens
Evolution and behaviour
Natural selection acts on individuals, causing evolution within population
Inclusive Fitness
Introduced by Hamilton in 1964, the concept of inclusive fitness extends the traditional view of fitness.
Inclusive Fitness: An individual's overall fitness is determined not only by its own survival and reproduction (direct fitness) but also by the survival and reproduction of its relatives (indirect fitness), with whom the individual shares genes.
Survival and reproduction plus the survival and reproduction of its relatives
Direct Fitness Effect: The actor's own reproductive success.
Indirect Fitness Effect: The impact on the reproductive success of social partners, weighted by the relatedness of the actor to the recipient.
r is the coefficient of relatedness of the donor (the altruist) to the recipient.
Kin Selection
Selection for helping your relatives is called kin selection
key: ensuring the survival of gene
Kin selection is a type of natural selection rooted in the concept of inclusive fitness. It is an evolutionary strategy that favors the reproductive success of an organism's relatives, even if it comes at a cost to the organism's own immediate survival and reproduction. The key principle is ensuring the survival of shared genes.
Social Interactions and Fitness Consequences
Hamilton (1964) classified social interactions based on their potential fitness consequences for both the donor (the individual performing the action) and the recipient (the individual who recognizes the behavior). Interactions can have positive (benefit) or negative (cost) impacts.
donor performs a given action, such as singing a song… etc
the recipients us the individual who recognizes the given behaviour
Cooperation: Donor benefits, Recipient benefits (e.g., cooperative hunting).
Selfishness: Donor benefits, Recipient is harmed (e.g., infanticide).
Altruism: Donor is harmed, Recipient benefits (e.g., alarm calls).
Spite: Donor is harmed, Recipient is harmed (e.g., certain parasitoid wasp behaviors).
Notably, cooperation and selfishness, which provide positive fitness consequences for the donor, are generally favored by natural selection, while altruism and spitefulness, which incur negative fitness consequences for the donor, might appear to be selected against.
Examples of Social Interactions
Spite: Parasitoid Wasps: In polyembryonic parasitoid wasps, sterile soldier larvae (mostly females from fertilized eggs, thus genetically identical to sisters) attack less-related brother larvae (from unfertilized eggs) to ensure greater access to food for their more closely related sisters. This results in negative fitness consequences for both the attacking soldier and the attacked larvae, but a net inclusive fitness gain for the soldier's genes.
Altruism: An act that benefits the recipient but imposes a cost on the donor.
Squirrels: In populations like those in Kluane, red squirrels occasionally adopt offspring of other squirrels, often closely related individuals, demonstrating kin selection.
Belding's Ground Squirrels: A sentry squirrel emits alarm calls warning kin of predators, drawing attention to itself and becoming an easier target while benefiting others.
Parental Care: When parents care for their young, they are promoting the survival of their own genes. Genes for altruism toward offspring are therefore favored by natural selection, increasing their frequency in subsequent generations.
Group Selection: A controversial concept where an individual acts counter to its own interest for the betterment of the group. While strongly criticized as inconsistent with evolutionary understanding, it is now known to occur under some specific circumstances.
Selfishness: Hanuman Langurs: Male Hanuman langurs ( Semnopithecus entellus ) aggress toward, and even kill, the young of other males. This behavior hastens the day females come into estrus, allowing the new male to father his own offspring sooner, thus increasing his direct fitness at the cost of the infant's life.
Hamilton's Rule for Altruism and Kin Selection
Hamilton proposed that an altruistic gene will be favored by natural selection when the benefit to the recipient, weighted by their relatedness, outweighs the cost to the donor.
Where: rB > C
In this inequality, r represents the genetic relatedness between the altruist and the recipient, B denotes the benefit received by the recipient, and C refers to the cost borne by the altruist, providing a mathematical framework for understanding kin selection.
(coefficient of relatedness): The probability that a gene present in the donor is also present in the recipient and derived from the same source.
(benefit): The fitness benefit received by the recipient of the altruistic act.
(cost): The fitness cost incurred by the donor.
Example: Male Turkeys
Male turkeys form coalitions of to birds to court and defend females. Only the dominant male mates, but subordinate individuals are often brothers or half-brothers.
For a specific scenario:
r = 0.42 (coefficient of relatedness between brothers/half-brothers)
B = 6.1extra offspring for the dominant male
C= 0.9offspring lost to the subordinate male
Applying Hamilton's Rule:
\mbox{rB} = 0.42 \times 6.1 \approx 2.56
Since 2.56 > 0.9, the altruistic gene for helping relatives is favored by natural selection in this context.
Sociality
The evolution of group living brings fundamental changes to relationships among individuals.
Characteristics of Evolved Sociality: Generally accompanied by:
Cooperative feeding.
Defense of the social group.
Restricted reproductive opportunities for some individuals.
Cooperation: Often involves exchanges of resources or other forms of assistance.
Eusociality
More complex level of sociality
three major characterictics
individuals of more than one generation living together
cooperatuve care of young
division of individuals into non-reproductive and reproductive castes
Cooperative Breeders
Groups of adults cooperate in the process of producing and/or rearing offspring, even those that are not their own.
Benefits for Helpers: Why do individuals help?
Inclusive Fitness: Assisting family members (relatives) improves their survival and reproductive rates, thereby increasing the helper's inclusive fitness.
Inherited Territory: Helping may increase the helper’s probability of future reproduction and recruiting helpers, especially if they stand to inherit the territory.
This altruistic behavior in cooperative breeding is often justified by Kin Selection.
Example: African Lions
African lions are highly social predators, exhibiting complex cooperative behaviors.
Females: Live in prides, typically to adults (ranging from to ), along with dependent offspring and a coalition of adult males.
Female cooperation is crucial for defending young against male infanticide, which frequently occurs when new male coalitions displace existing ones. While an individual female has little chance against a larger male, cooperating females are often successful in repelling attacks. Since females in a pride are almost always close relatives, their cooperation fits within the framework of kin selection.
Males: Also cooperate to defend territories against invading males and predators. Male coalitions can sometimes be relatives, but not always, suggesting that kin selection alone is not always a sufficient explanation.
An individual male cannot claim and defend a pride of female lions alone; forming coalitions is necessary. Even in coalitions of unrelated males, individuals may produce offspring to increase their fitness.
Relatedness and Coalition Size: Studies show that males in coalitions of two or three are often unrelated, whereas larger coalitions ( to males) consist mostly of related males.
Reproductive Success: The probability of siring young depends on an individual's rank within the coalition and the overall coalition size. Reproductive success is more variable for males in coalitions of three to four than for those in coalitions of two.
Eusociality
Eusociality represents the most complex level of sociality, characterized by specific organizational traits.
Three Major Characteristics:
Individuals of more than one generation living together.
Cooperative care of young.
Division of individuals into non-reproductive (worker) and reproductive (queen/king) castes.
Example: Leafcutter Ants
Leafcutter ants, particularly Atta species, are among the most socially complex ant groups and exemplars of eusociality.
Castes: Individuals in leafcutter ant colonies are divided into physically distinctive castes, each engaging in specialized behaviors within the colony. For example, smaller ants may ride on leaf fragments carried by larger workers to offer protection against aerial attacks by parasitoid flies.
Comparison of Castes and Behaviors: Different ant species exhibit varying degrees of task specialization and caste numbers, with leafcutter ants like Atta sexdens displaying a high number of castes and behaviors per colony compared to other ant species.
Evolution of Eusociality
Two main factors are thought to contribute to the evolution of eusociality:
Kin Selection: This plays a significant role, especially in Hymenoptera (ants, bees, wasps), due to their haplodiploid genetic system.
Haplodiploidy: Males develop from unfertilized eggs and are haploid (possessing a single set of chromosomes), while females develop from fertilized eggs and are diploid (possessing two sets of chromosomes).
Consequence: Due to this system, full sisters share on average about of their genes (all genes from their haploid father and from their diploid mother), which is a higher coefficient of relatedness () than that between a mother and her daughter (). This genetic asymmetry can make it more advantageous for female workers to help raise their sisters than to produce their own offspring, thereby favoring the evolution of sterility and complex social structures.
Ecological Constraints: The demands of the environment may necessitate complex social behavior. For example, the extensive burrow systems required by many social insects or the specialized tasks involved in cultivating fungi (as in leafcutter ants) may demand a division of labor that encourages a more complex social structure.
Summary
Natural selection consistently favors those behaviors that enhance the inclusive fitness of individuals. The evolution of sociality, particularly eusociality, is generally marked by cooperative feeding, the collective defense of the social group, and the restriction of reproductive opportunities to a specialized caste or few individuals.