Research

Our research interests are generally focused on ecology and the evolution of adaptive behavior in birds. We utilize a variety of approaches, usually including close behavioral observations of adults and offspring in their natural habitat. We also employ concepts from ecophysiology, molecular biology, and capture-mark-recapture methods. Some ongoing research projects are described below. Feel free to email Keith (ekbowers  ‘at’  memphis  ‘dot’  edu) for PDF copies of any papers. You can also read about some of our work and ideas discussed in more popular venues by clicking their respective logos below.

 

 

 

 

PhysOrg

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Parental care & investment

Research - Hands

 Nearly a century ago, Sir Ronald Fisher observed that, “In organisms of all kinds the young are launched upon their careers endowed with a certain amount of biological capital derived from their parents.”

Research - Parental Care 1

Much of Fisher’s work opened the door for modern evolutionary and behavioral ecology, and an appreciation of the fact that parents often vary widely in the degree to which they invest in offspring. Work to explain this variation over the past half-century has yielded a robust framework involving a network of trade-offs thought to govern the lives of all species. For example, investment in one’s own survival should necessarily reduce one’s ability to invest in the survival of others, namely kin. Indeed, there is evidence that fighting pathogens – i.e. investing in immune function and one’s own survival – compromises an individual’s ability to invest simultaneously in reproduction.

Research - Parental Care 2 - Variation

Paradoxically, however, there is also recent evidence that increased immune activity does not reduce reproductive success. These findings are in line with the concept of terminal investment, which predicts that, if immune activation signals to an individual that it has been parasitized and may therefore not survive to reproduce again, this individual should invest all of its capital into current offspring as these offspring will likely be its last. We recently found that maternal house wrens facing an immune challenge alter their reproductive effort by producing sons of above-average size and body condition and daughters with enhanced immune responsiveness. The increase in offspring condition following immunostimulation is predicted by the terminal-investment hypothesis, where individuals with reduced residual reproductive value (i.e., probability of survival and future reproduction) should invest heavily into current reproduction as they are unlikely to survive to reproduce in the future.Research - Parental Care 3

We also followed up on these findings in a series of experiments that have revealed that females increase allocation in the egg, which positively affects both offspring growth and immune responsiveness. Moreover, females also increase their provisioning of food to nestlings after hatching, and the increased effort is mediated, in part, by increased circulating corticosterone, the major avian glucocorticoid. Corticosterone production typically increases with immune activation, as expected if fighting pathogens is energetically costly, and the increase in corticosterone was passed to offspring during egg formation and positively predicted the maternal provisioning of food to offspring after hatching. These results challenge the commonly held paradigm that corticosterone always negatively affects reproduction.

Representative Publications

Bowers EK, Bowden RM, Thompson CF, Sakaluk SK. 2016. Elevated corticosterone during egg production elicits increased maternal investment and promotes nestling growth in a wild songbird. Hormones and Behavior 83:6−13. Link to paper.

Hodges CJ, Bowers EK, Thompson CF, Sakaluk SK. 2015. Cascading costs of reproduction in female house wrens induced to lay larger clutches. Journal of Evolutionary Biology 28:1383−1393. Link to paper.

Bowers EK, Bowden RM, Sakaluk SK, Thompson CF. 2015. Immune activation generates corticosterone-mediated terminal reproductive investment in a wild bird. American Naturalist 185:769−783. Link to paper.

Bowers EK, Smith RA, Hodges CJ, Zimmerman LM, Thompson CF, Sakaluk SK. 2012. Sex-biased terminal investment in offspring induced by maternal immune challenge in the house wren (Troglodytes aedon). Proceedings of the Royal Society B 279:2891−2898. Link to paperMedia coverage by The Scientist and Phys.Org

Bowers EK, Sakaluk SK, Thompson CF. 2012. Experimentally increased egg production constrains future reproduction of female house wrens. Animal Behaviour 83:495−500. Link to paper.

 

Sex allocation

Research - Sex Allocation

A major component of sex-allocation theory, the generalized Trivers-Willard Model, posits that sons and daughters are differentially affected by variation in the rearing environment, with changes in the amount of parental care received having differing effects on the fitness of males and females. When this occurs, selection should favor adjustment of the offspring sex ratio in relation to the expected fitness return from offspring. We have demonstrated that the growth and size of sons is more strongly influenced by variation in the neonatal environment than that of daughters and that the reproductive success of offspring as adults varies with the body mass of sons, but not daughters, prior to independence from parental care. Thus, selection should favor changes to the progeny sex ratio in relation to the quality of offspring that parents are able to produce. As predicted, females with high investment ability over-produce sons relative to those with lower abilities. However, broods with 1:1 sex ratios are more productive, and produce more recruits to the breeding population on a per-nestling basis, than broods biased more strongly toward sons or daughters, suggesting that selection favors the production of mixed-sex broods, constraining sex-ratio variation.

Representative Publications

Bowers EK, Thompson CF, Sakaluk SK. 2016. Within-female plasticity in sex allocation is associated with a behavioral polyphenism in house wrens. Journal of Evolutionary Biology 29:602−616. Link to paper.

Bowers EK, Thompson CF, Sakaluk SK. 2015. Persistent sex-by-environment effects on offspring fitness and sex-ratio adjustment in a wild bird population. Journal of Animal Ecology 84:473-486. Link to paper

Bowers EK, Thompson CF, Sakaluk SK. 2014. Offspring sex ratio varies with clutch size for female house wrens induced to lay supernumerary eggs. Behavioral Ecology 25:165−171. Link to paper

Bowers EK, Munclinger P, Bureš S, Nádvorník P, Uvírová L, Krist M. 2013. Cross-fostering eggs reveals that female collared flycatchers adjust clutch sex ratios according to parental ability to invest in offspring. Molecular Ecology 22:215−228. Link to paper

Bowers EK, Sakaluk SK, Thompson CF. 2011. Adaptive sex allocation in relation to hatching synchrony and offspring quality in house wrens. American Naturalist 177:617−629. Link to paper.

 

Sibling rivalry

Research - Sibling Rivalry

Age-differences among siblings have long been known to influence the pecking order among them, and for many avian families, things can get even worse for the youngest members as their older siblings leave the nest. The older siblings are traditionally expected to leave the nest as soon as they can, reducing their susceptibility to nest predators. This introduces a dilemma for the younger, less mature siblings because their chances of survival outside the nest plummet if they leave at too young of age, yet they get abandoned by their parents and starve if they do not fledge with their older siblings. We have found that, when age-spans among siblings are broad, the oldest ones actually postpone leaving the nest for several days beyond the age at which they are capable, providing their younger siblings time to mature prior to leaving the nest with them, and this significantly improves the survival of the younger, but not older, siblings. Thus, competition still dominates the landscape of sibling interactions, but older siblings may value their younger ones to a greater extent than has traditionally been thought.

Representative Publications

Bowers EK, Thompson CF, Sakaluk SK. 2016. Within-female plasticity in sex allocation is associated with a behavioral polyphenism in house wrens. Journal of Evolutionary Biology 29:602−616. Link to paper.

Bowers EK, Sakaluk SK, Thompson CF. 2013. Sibling cooperation influences the age of nest-leaving in an altricial bird. American Naturalist 181:775−786. Link to paper. Media coverage by dugdug.com 

Bowers EK, Sakaluk SK, Thompson CF. 2011. Adaptive sex allocation in relation to hatching synchrony and offspring quality in house wrens. American Naturalist 177:617−629. Link to paper.

 

2013 05 04 - Bluebells in Zone 6 G