RESEARCH INTERESTS AND ACCOMPLISHMENTS


 

Stephen T. Emlen has studied a wide range of both mechanistic and adaptive questions in animal behavior. He has focused primarily on birds as model systems to understand general issues, and his study species have ranged from Jacanas to Bee-eaters and from Indigo buntings to Bank swallows and Social weaverbirds. His doctoral students (26 to date) have extended the range of study organisms to include red-backed spiders, Sierra-dome spiders, bluegill sunfish, clown anemonefish, barking tree-frogs, and additional avian species such as great tinamous, brown boobies, semipalmated sandpipers, Australian brush-turkeys, groove-billed anis, Guianan cock-of-the-rock, turquoise-browed motmots, bay wrens, barn swallows, Taiwan yuhinas, superb starlings, Montezuma's oropendolas, red-winged blackbirds, brown-headed cowbirds, cardinals, red-billed quelias and white-throated sparrows.

The paragraphs below give a brief synopsis of Emlen's research accomplishments in each of the areas of interest enumerated in the Biographical Profile.

1) Animal orientation and navigation: Emlen's studies of migratory orientation are considered classics. Through both laboratory and field experiments, he documented the use of multiple cues by nocturnally migrating birds. His studies of the use of celestial cues by Indigo buntings, completed some 35 years ago, still provide the most complete information available for any orientation system known to date. By choosing to do much of this work under the artificial skies of a planetarium, he was able to experimentally manipulate the patterns of stars and their rotation through the night skies. Emlen documented the use of configurational information provided by the stars and discovered a form of programmed learning analogous to imprinting in which constellations of stars acquire directional meaning through a learned association with the north-south axis of celestial rotation. His studies also demonstrated that the reversal of direction between North American spring and autumn migrants was caused by hormonally-mediated changes in the responses of birds to the same stellar patterns. The 'Emlen funnel' remains the preferred cage design used by current avian orientation researchers today.

Emlen also studied the orientation of free-flying migrants under a variety of natural and experimental conditions. In a unique collaboration with the U. S. National Aeronautics and Space Administration, he released individual migrants several thousand feet above the ground, at night, and then followed them (using NASA tracking radars) for 25-40 km as they selected the direction, speed, and altitude of their migratory departure. By experimentally altering the information available to the released birds, he was able to demonstrate the importance of geomagnetic cues, the position of sunset, and the patterns of winds aloft as additional important orientation cues. Emlen was one of the first scientists to recognize that animals have redundant means of determining direction. Many of the research questions he pioneered in asking (e.g. how are different cues systems integrated and calibrated against each other?) still dominate orientation research today.

2) Animal communication: Emlen was one of the first field experimentalists to study how information is encoded in bird song. He experimentally altered various components of Indigo bunting song, and then played back the modified songs to male territory holders. By quantifying the intensity of responses to altered songs, he documented the importance of specific temporal, structural, and syntactical information for both individual, and for species, recognition.

3) Animal mating systems: Together with Lewis Oring, Emlen pioneered the integration of ideas of economic defensibility with sexual selection theory to develop one of the first predictive models of animal mating systems. The logic behind the model argued that the reproductive success of males is limited by access to potential female mates, and that ecological factors often determine the distribution, and hence the defensibility, of females. Polygyny occurs when individual males can monopolize access to multiple females. Emlen and Oring listed a number of social and ecological variables that have proven to be strong predictors of whether a given species exhibits monogamy, polygyny, or polyandry as a mating system. This work is one of the most cited in the area of behavioral ecology. (In 1988 it became a 'Citation Classic'.) The basic predictions of the Emlen-Oring model have since been validated for species in a large variety of taxa including arachnids, insects, fishes, amphibians, birds and mammals. The model has served as the stimulus for a myriad of additional theoretical and empirical studies covering virtually every animal group, including humans.

4) The costs and benefits of group living: In studies of the adaptive significance of colonial living in Bank swallows, Emlen provided some of the first documentation that the fitness of individuals increased both as a function of colony size and colony breeding synchrony. He speculated that this was due to enhanced capabilities of locating difficult-to-find, unpredictable and ephemeral food resources. In essence, unsuccessful foragers were able to follow successful ones as they left the colony to find food. He expanded this idea into a new hypothesis for synchronized breeding.

5) The adaptive bases of sex differences in behavior: Emlen recently completed a long-term study of Wattled jacanas in the Republic of Panama. Jacanas are neo-tropical shorebirds that represent a vertebrate extreme in the reversal of traditional sex roles: males are the sole care-takers of the eggs and young while females compete to obtain harems of male mates. Sexual selection is thus intense but reversed, making jacanas an ideal test case for the theory that inequalities in parental investment underlie differences in the behaviors we typically associate with maleness and femaleness. Emlen confirmed that female jacanas show many behaviors considered male-like in other species. These include high levels of aggression, accumulation of harems of mates, multiple mating with co-mates, competitive behaviors to obtain and retain mates, and forcible take-over of neighboring territories (including infanticidal killing of eggs and chicks of newly acquired mates).

Polyandrous mating among jacanas (where a female pairs simultaneously with several male mates) creates a conflict of interest between the sexes because it increases the reproductive success of females but decreases that of males. Emlen discovered that a male resolves this conflict by engaging in behaviors that enhance its own success at the expense of its female partner. These include active egg tossing, nest abandonment, and inciting a neighboring female to take-over ownership of his territory from his current female mate. Each behavior occurred predictably when one or more of the following conditions were met: the male was a member of a large harem, the probability of his having been cuckolded was high, and/or when neighboring females with smaller harems were available as alternative mates. Jacanas are thus an exception that proves the evolutionary rule --- that a reversal of parental investment and sexual selection can lead to reversals in many behaviors of the two sexes.

6) The evolution of cooperative and altruistic behavior in animal societies:

A) The evolution of altruism: Emlen has made major contributions to our understanding of the paradox of altruism. He has modeled the costs and benefits of helping-at-the-nest and has clarified the roles of ecological, social, and genetic relatedness factors in the evolutionary maintenance of such behaviors. His long-term study of the African White-fronted bee-eater provides one of the best documented cases of the importance of kin selection in the expression of an altruistic behavior. His economic models go a long way in predicting both when specific individuals will become helpers, and to whom they will provide their help. He demonstrated the sensitivity of the helping response to each variable of Hamilton's equation: r, the closeness of kinship; b, the benefit to the recipient of receiving such helping aid; and c, the cost to the participant of providing it. By performing cross-fostering experiments, Emlen further confirmed that the developmental mechanism by which different categories of kin come to be recognized in bee-eaters is one of differential early association. This is the same mechanism that underlies kin recognition in human societies.

B) The evolution of despotic versus egalitarian societies: Recently Emlen has begun modeling and field-testing various explanations for the distribution of despotic versus egalitarian societies in nature. A despotic (or high skew) society is one in which reproduction is monopolized by one or a few individuals. An egalitarian (low skew) society is one in which reproduction is shared more equally among most or all group members. Nearly 30 years ago, Emlen and his former student, Sandra Vehrencamp, independently proposed what is now termed the 'concession' model of reproductive skew to explain this difference. They each argued that dominant individuals in social groups might concede reproduction to subordinates as an incentive to retain the subordinates in the group (leading to increasingly shared breeding). These initial models attempted to predict the degree of shared reproduction on the basis of ecological factors (the difficulty of breeding independently and the benefits of group association), genetic factors (the degree of relatedness) and social factors (differences in competitive ability). In recent years there has been a surge of interest in reproductive skew theory and new models and model extensions have proliferated at a rapid pace. Emlen is currently developing models specifically tailored to vertebrate species that exhibit bi-parental care, and is testing these and alternative models with an African weaverbird, the Grey-capped social weaver.

7) An evolutionary theory of the family:

A) The formation of family groups: Emlen has modeled the conditions that favor delayed dispersal and family formation in animal societies. His 'ecological constraints' hypothesis argues that families develop when there is a shortage of acceptable breeding opportunities, and young can improve their competitive chances of obtaining a future, high quality, breeding position by temporarily staying at home. The model leads to many predictions about family stability/instability and when animal family 'dynasties' (the continued occupation of a high quality area over several generations by one genetic lineage) are likely to occur. Emlen has further suggested that extended (as opposed to nuclear) families form when environmental conditions are unpredictable. This favors a fission/fusion family structure in which individual pairs can reproduce successfully when conditions are favorable, but helping coalitions of relatives can easily form when conditions are harsh.

B) The social dynamics of family groups: The social and genetic structure of a family group strongly influences the types of interactions expected among its members. Synthesizing information on the variety of conditions that lead to expected cooperation and conflict within families, Emlen published a list of 12 evolutionary-based predictions relating to family dynamics and conflict resolution. The predictions address such topics as genetic relatedness-sensitive altruism, incest avoidance, behavior of unrelated 'in-laws', and changes in behavior following the replacement of a mate (i.e., stepfamily dynamics). Although the predictions were only published in 1995, they have stimulated a great deal of research in a wide diversity of taxa, and many of the predictions have found robust support from insect, bird, and mammal species.

8) Applications of Emlen's research to Human Issues:

A) Behavioral ecology and the Agency for International Development: Much of Emlen's early work in behavioral ecology focused on the effects of different spatial and temporal resource distributions on resulting social organizations of animal societies. Distributions of resources are major predictors of observed group size, sedentariness, mating systems, and family structure (see 3, 4, 7 above). Emlen realized early on that when wealthy nations export industrial or agricultural aid overseas, the result is often the introduction of new resource bases, or changes in the distribution of existing resource bases. As a result, these aid programs frequently have the undesired side effect of causing disruptive changes in the social structure of the recipient peoples.

Emlen believed that our knowledge of how changes in resource distributions lead to changes in the social organizations of animal societies was sufficiently robust to predict that similar effects were likely to be occurring in human groups. He argued that incorporating the findings from behavioral ecology could enable aid workers to predict many of these changes in advance. In the late 1970s, Emlen campaigned to require 'social impact statements' for development projects that would evaluate the probable shifts in social structure that would result from the implementation of particular programs. Evolutionary anthropologists and economists joined in making similar arguments, and today most foreign aid projects are required to undergo a social soundness assessment before their implementation.

B) The evolutionary study of human family dynamics: Emlen is currently applying data and predictions derived from evolutionary studies of avian families to better understand the social dynamics of human families 'in transition'. He argues that humans are themselves 'cooperative breeders' who, in past times, could count on a built-in work force of genetic relatives to assist with child rearing. With the weakening of extended family ties to grandparents and other collateral relatives (by virtue of persons settling far from their parents) humans are losing this kin assistance, and societies are struggling to develop alternative forms of child care to replace it.

The recent rapid increase in divorce and remarriage rates has created an increase in the number of stepfamilies in current society. Both Darwinian logic and empirical data (from birds and from humans) indicate that the potential for conflict is greatly increased in stepfamilies. Emlen believes that incorporating an evolutionary perspective into family counseling can help us to dramatically decrease the statistically higher incidence of sexual abuse and physical violence that currently occurs within reconstituted stepfamilies. His 12 predictions, derived for animal families, specify the precise contexts, as well as the likely participants, of different forms of social conflict. They tell us when, where, and between whom, conflict is most likely to occur. Such information can allow counselors and family members to identify and anticipate these flash-points of potential violence before they occur. This can lead to an increase in awareness that can further empower persons to more effectively substitute positive behaviors for behavioral feelings that would otherwise escalate into more serious conflict. Emlen has written articles for, and been interviewed by, various social science journals. A small, but growing, number of family counselors and therapists are incorporating these ideas into their thinking, and are currently 'field testing' the value of this evolutionary approach to human family studies.