Individual variation in testosterone and parental care in a female songbird; the dark-eyed junco (Junco hyemalis)

Abstract

When competition for sex-specific resources overlaps in time with offspring production and care, trade-offs can occur. Steroid hormones, particularly testosterone (T), play a crucial role in mediating such trade-offs in males, often increasing competitive behaviors while decreasing paternal behavior. Recent research has shown that females also face such trade-offs; however, we know little about the role of T in mediating female phenotypes in general, and the role of T in mediating trade-offs in females in particular. Here we examine the relationship between individual variation in maternal effort and endogenous T in the dark-eyed junco, a common songbird. Specifically, we measure circulating T before and after a physiological challenge (injection of gonadotropin releasing hormone, GnRH), and determine whether either measure is related to provisioning, brooding, or the amount of T sequestered in egg yolk. We found that females producing more T in response to a challenge spent less time brooding nestlings, but provisioned nestlings more frequently, and deposited more T in their eggs. These findings suggest that, while T is likely important in mediating maternal phenotypes and female life history tradeoffs, the direction of the relationships between T and phenotype may differ from what is generally observed in males, and that high levels of endogenous T are not necessarily as costly as previous work might suggest.

Keywords: Dark-eyed junco (Junco hyemalis); Endogenous steroids; Gonadotropin releasing hormone (GnRH); Life history tradeoffs; Maternal care; Testosterone; Yolk hormones.

Figure 1

Scatter plots relating parental behavior (left: brooding behavior; right: provisioning behavior) to ability to produce T in response to GnRH challenge. Points in the scatter plots for T are leverage pairs, akin to partial correlation (see statistical methods), P values are from single effects in multiple regression.

Figure 2

Scatterplot illustrating the correlation between female ability to produce T in response to a GnRH challenge (post challenge T) and the concentration of yolk T (ng/g) deposited in eggs. Lines and symbols differ according to year; 2008, cross and dashed line; 2009, x and dotted line, 2010 square and solid line.

Figure 3

Network model illustrating the relationships between measures of hormonal phenotype and maternal behaviors. Each node represents a variable of interest, colored according to type of trait (hormone measures are blue, maternal effort is orange, egg metrics are yellow). Lines connecting nodes denote the nature of the relationship between the variables (solid lines are positive; dashed line is negative, faint lines indicate a trend that is not statistically significant). The length of the line is inversely related to the strength of the relationship; short lines are stronger. Notice tight clustering around post-challenge T as compared to initial T.