Date of Award

Spring 2015

Document Type


Degree Name

Master of Science (MS)


Biological Sciences



Committee Director

Eric L. Walters

Committee Member

Walter D. Koenig

Committee Member

Lisa A. Horth

Call Number for Print

Special Collections LD4331.B46 B765 2015


Reproductive skew, the degree to which reproduction is shared among same-sex individuals in a social group, is a pattern affected by ecological conditions, sociality, cooperation, and the inter- and intrasexual behavior of individuals in complex animal societies. Transactional and compromise skew models assume that high skew is the product of dominance hierarchies among cobreeders, yet this has rarely been tested. Both model types fail to incorporate the decisions of more than two individuals, generally overlooking the effect of female behavior on male reproductive success in multi-male groups, and are ineffective at predicting skew in larger groups characterized by more than two same-sex cobreeders. To test these assumptions, I examined the potential behavioral causes of reproductive skew in acorn woodpeckers (Melanerpes formicivorus), a cooperative polygynandrous species that lacks dominance hierarchies among closely related cobreeding males despite highly skewed paternity.

I quantified male mate-guarding behavior by measuring overall attendance time and successful follows of breeding females prior to egg-laying to determine if these behaviors resulted in a breeding hierarchy reflected by realized paternity. Mate-guarding behavior of small (two-male) and large (three or more male) cohort groups was compared to examine the effect of larger group sizes on reproductive skew. Moreover, the potential effect of female behavior on male reproductive skew was examined by investigating female mating behavior in polyandrous social groups.

Reproductive competition among cobreeding males was high, as evidenced by consistently high attendance behavior throughout the fertile period of breeding females. Groups with three or more cobreeding males mate guarded for more days than groups with two cobreeding males. Attendance and following behavior were good predictors of reproductive skew in two-male groups, but did not predict paternity in groups with three or more cobreeding males. I hypothesized that this species may use tree cavities in which to copulate, and tested the prediction that the frequency and duration of tree cavity visits by breeding females with breeding males determined the realized paternity of the offspring of polyandrous groups. The timing and duration of cavity use behavior of males and females in this study closely resembled the timing and duration of copulation behavior in other polyandrous species. The cavity use behavior of females and males, however, was not a good predictor of realized paternity. The ways in which acorn woodpeckers agree with, and diverge from, the predictions of reproductive skew theory highlight both the potential insight derived by, and the limitation in scope of, this theory as a whole.


In Copyright. URI: This Item is protected by copyright and/or related rights. You are free to use this Item in any way that is permitted by the copyright and related rights legislation that applies to your use. For other uses you need to obtain permission from the rights-holder(s).