Goals: learn various hypotheses that may explain spatial distribution patterns of individuals within species, including whether or not they live in groups and what mating systems they have.

The Lecture:

Animals differ greatly in their spatial distribution pattern and social behavior.  Different levels of group living include:

1. Solitary species -- may interact with members of the same species to mate and to raise offspring, but other than that are primarily alone (examples: many territorial bird species, mammals such as weasles)
2. Species that form groups that travel together while feeding, but within which there are few complex behavioral interactions, and within which all individuals have similar roles (examples; fish that form schools, mammals that form herds, birds that form flocks)
3. Highly social species that form groups within which there is division of labor such that some individuals are reproductive and others are not.

• cooperative breeding: indiviuals in addition to genetic parents help raise young.  Some division of labor among helpers (ex: some bring food to young, some watch for predators).  Rare behavior but occurs in several bird and mammal species.
• eusociality: large social groups in which few individuals breed and many are non-breeders who help the group and offspring of the breeders.  Extreme division of labor.  Ex: social insects (ants, termites, many bees, many wasps), naked mole-rats

Advantages related to predation:

• more "eyes" to see predator -- less need for vigilance, so more time to spend feeding.  Evidence: studies of starlings of different group size; individuals spend less time in vigilance (looking up) in large groups
• earlier predator detection.  Evidence: flocks of pigeons of different size had a trained hawk flown toward them.  Larger groups detected the hawk sooner.
• the "selfish herd" phenomenon -- if a predator encounters a single individual, it will attack that individual, but if that individual is in a group and the predator encounters the group there is a good chance that it will attack someone else

Advantages related to finding food:

• if food has a patchy distribution, individuals may use each other to find food patches

Some costs to group living:

• increased competition for food
• more chance of attracting the attention of predators
• risk of disease

• delayed dispersal of young from territory; young from previous brood help pair of parents raise subsequent brood(s)
• plural breeding: on a territory, several breeding pairs are helped by many non-breeders
• colonial cooperative breeders: non-territorial species -- individuals live in large colonies and each pair may have non-breeding helpers.

• increased offspring production by pair during a breeding effort
• more breeding efforts per season
• higher survivorship of breeders

• general hypothesis: something constrains helpers from breeding
• specific hypotheses within this general hypothesis:
• limited habitat available for breeding so all territories typically full AND little/no marginal habitat
• possible causes:
• habitat is rare, occurs in isolated patches (ex: Florida scrub jays)
• major modification of habitat by animals (ex: granaries of acorn woodpeckers, mounds of banner-tailed kanagroo rats)
• Experimental evidence supporting hypothesis that helpers don't breed because territories limited comes from experiments that increase the number of territories.  Two approaches:
• remove breeders.  In superb fairy wrens, removal of male breeders resulted in almost all males that were previously helpers becoming breeders
• increase territories.  In red-cockaded woodpeckers, adding artifical nest holes (real ones are extremely time/energy consuming for the woodpeckers to excavate) led to increase in new areas in which breeding occurred compared to control areas with similar habitat

• evidence: strongly male-biased sex ratio in several cooperative breeders.  Helpers in these species typically male.

• hypothesis: improved future reproduction.  Possible reasons (more specific hypotheses):
• inherit territories from breeders (observed in many species)
• form coalitions: groups of individuals that together have better chance of obtaining a territory than would a single individuals (lions)
• when breed, better success because of experience gained as helper (mixed results: support in some species, not others)
• Another general hypothesis: production of non-descendent kin:
• helping can evolve (genetic basis for tendency to help if no breeding opportunities available) if:
• helping directed toward kin, so helpers and breeders genetically similar
• so breeders likely to carry genes for helping
• so when breeders reproduce more because of help, result is more genes for helping passed on
• predicts helping increases reproduction of breeders AND helpers typically close relatives of breeders
• evidence: white-fronted bee-eaters -- probability of helping strongly correlated with predicted increase in non-descendent kin
• Another general hypothesis: helping is side effect of instinct to feed begging young.

• polygyny:  one male mates with many females (probably the most common system in animals)
• monogamy: one male and one female mate with each other, and not with other individuals, for the duration of a reproductive attempt (common in birds, but uncommon in other animals)
• polyandry: one female mates with many males (rare, but occurs in some birds, fish, frogs, a few mammals)
• promiscuity: many males mate with many females.  Ex: species that broadcast sperm and eggs into the environment (many marine invertebrates)

Social versus genetic mating systems: the social mating system, that is, what we observe by looking at interactions between individuals, often differs from the genetic mating system (based on genetic determination of parentage of offspring).  A species may appear monogamous in that a male and female will spend time together, mate, raise a brood of offspring together, but it may be that they actually mate with other individuals and have other offspring.

Some hypotheses to explain variation in social mating systems:

• since females of most species put more time and energy into raising each offspring than males do, males will have most surviving offspring by attracting most mates, while females will have most offspring by choosing a high quality mate so that they have high quality offspring.  This leads to polygyny.
• species will be monogamous if it takes more than one parent to successfully raise young. Otherwise, they will be polygynous (for the reason stated above)
• species will be monogamous if resources require females to have large, separate home ranges and males can not defend more than one female ("spaced out female" effect.)  In contrast, anything in the environment that leads to being clumped together so that females group together results in males being able to defend more than one female and leads to polygyny; this is called having a higher "environmental potential for polygyny."  The factors that lead to female clumping are like those that lead to non-social grouping in general; protection from nest predators is likely a major factor.
• polyandry expected when males put more time, energy into raising each offspring than do females