Females as the Gatekeepers to Seasonal Breeding: What We Can Learn by Studying Reproductive Mechanisms in Both Sexes

Abstract

<jats:title>Abstract</jats:title> <jats:p>Seasonal reproduction is a widespread adaptation in vertebrates, such that individuals time their reproductive efforts to match peak resource abundance. Individuals rely on environmental cues to regulate hormonal mechanisms governing timing of breeding. Historically, studies on physiological mechanisms of seasonal reproduction, specifically in birds, have disproportionately focused on males compared to females. For this review, I conducted a literature search of the last decade of avian research and found a persistent sex bias in the field of physiological mechanisms of seasonal reproduction. Using work conducted with the dark-eyed junco (Junco hyemalis) as a case study, I present a possible solution to combat the sex bias: natural comparisons of populations that differ in reproductive timing to investigate mechanisms of reproduction in both sexes. Populations of dark-eyed juncos that differ in migratory behavior (i.e., migrant and resident) exhibit overlapping ranges during winter and early spring; residents begin breeding in early spring prior to the departure of migrants. This system, and others like it, provides an opportunity to compare mechanisms of reproduction in populations that differ in reproductive timing despite experiencing the same environmental conditions in early spring. In juncos, migrant and resident females and males exhibit similar patterns of hypothalamic regulation of reproduction in early spring, but sex differences in gonadal sensitivity between the populations could be an important distinction that partially explains sex differences in reproductive development. Comparing mechanisms of reproduction in free-living populations and in captivity can reveal important mechanisms that determine the onset of reproductive development, as well as potential sex differences in these mechanisms. Understanding the mechanisms of reproductive phenology has important implications for understanding how species will survive and reproduce in a changing climate.</jats:p>

Beyond a biased binary: A perspective on the misconceptions, challenges, and implications of studying females in avian behavioral endocrinology

“…Lastly, we offer solutions to address and overcome male bias in future research and increase female inclusivity. Our perspective shares important insights with many recent publications on the inclusion of female animals in biology ( Shansky, 2019 ; Orr et al, 2020 ; Rosvall et al, 2020 ) including birds ( Ball and Ketterson, 2008 ; Caro, 2012 ; Kimmitt, 2020 ). This is by no means an exhaustive list of topics for which significant improvements can be made.…”

“…This sex bias in the literature was demonstrated in a systematic review, which found that 84% of avian physiology, ecology, and reproduction studies between 2003 and 2011 involved male birds, whereas only 58% involved female birds (Caro, 2012). A 10-years follow up study found that this discrepancy persists at similar rates in the current literature (Kimmitt, 2020). In addition to being understudied, female variation in key life history traits is typically examined in the context of how such variation affects males, rather than an interesting and important subject itself (Alonzo and Warner, 2000;Rosenthal and Ryan, 2022).…”

“…One commonly used explanation for persistent sex bias in avian endocrinology is that males are more conspicuous or aggressive than females, and therefore easier to capture and study in the field ( Caro, 2012 ; Kimmitt, 2020 ). However, as evidenced by recent research efforts studying free-living females (e.g., Cain and Ketterson, 2012 ; DeVries et al, 2015 ; Covino et al, 2018 ; George and Rosvall, 2018 ; Kimmitt et al, 2019 ; Lipshutz et al, 2021 ), a better understanding of the natural history of female birds can make sampling less difficult.…”

Plasticity in female timing may explain earlier breeding in a North American songbird

“…Accurate predictions of future responses to climate change will require further consideration of mechanisms of female reproductive timing (Chmura et al, 2020; Kimmitt, 2020; Williams, 2012). Past and ongoing work in the junco are elucidating the physiological mechanisms driving reproductive timing in females based on life history, including endocrine systems and costs of early breeding (Graham et al, 2019; Greives et al, 2016; Kimmitt et al, 2019; Kimmitt et al, 2020).…”

Plasticity in female timing may explain current shifts in breeding phenology of a North American songbird

“…Accurate predictions of future responses to climate change will require further consideration of mechanisms of female reproductive timing (Williams 2012;Chmura, Wingfield & Hahn 2020;Kimmitt 2020). Past and ongoing work in the junco is elucidating the physiological mechanisms driving reproductive timing in females based on life history, including endocrine systems and costs of early breeding (Greives et al 2016;Graham et al 2019;Kimmitt et al 2019;Kimmitt, Sinkiewicz & Ketterson 2020).…”