One in the Hand Worth Two in the Bush? Reproductive Effort of Young Males Is Not Affected by the Presence of Adult Males

Kevin L Monteith,Kyle B Monteith,Rhiannon P Jakopak,Jonathan A Jenks

doi:10.3389/fevo.2020.00274

Kevin L Monteith, Kyle B Monteith + Show 2 more

Open Access

https://doi.org/10.3389/fevo.2020.00274

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Publication Date: Aug 18, 2020
Citations: 1	License type: CC BY 4.0

Affiliation: University of Wyoming, South Dakota State University

Abstract

Reproduction is a costly endeavor, and most large, long-lived, and iteroparous mammals exhibit conservative life-history tactics wherein an individual may forego or abandon a reproduction event for the sake of survival. Nevertheless, risks and benefits associated with reproduction are not equal across males and females, nor across their life. Whereas expenditure for females is associated with rearing young (e.g., lactation), expenditure for males occurs with securing mating opportunities. Young males may be more successful when dominant males are lacking, but it is less clear whether—and at what cost—young males will expend effort when those opportunities arise. We designed an experiment to quantify reproductive effort (e.g., food intake, somatic loss [body mass and fat]) of male white-tailed deer (Odocoileus virginianus) to better understand the reproductive ecology of male ungulates, with an emphasis on determining how adult males (≥4.5-years old) affect timing and extent of reproductive effort expended by yearling males (1.5-years old). Food intake, hormone levels, body mass, and somatic loss during rut were similar between yearling males that interacted with adult males and those that did not. Somatic loss by all males was greatest during peak estrus of females, but forage intake relative to metabolic body mass for yearling males was nearly twice that of adult males. Testosterone levels were lower for yearling than adult males early in rut and were related negatively to forage intake. Whereas adult males lost 20% (23.5kg) of body mass and 31% (5.3 percentage points [ppt]) of body fat during the rut, yearling males lost 12% (9.3kg) of body mass and 22% (4.7ppt) of body fat. Reproductive effort by young males was not influenced by the presence of adult males, though young males expended less reproductive effort than adults. Instead, reproductive allocation occurred in a state-dependent manner, where pre-season levels of somatic reserves dictated reproductive effort, regardless of age. Like female ungulates, male deer displayed risk-sensitive reproductive allocation wherein current reproductive allocation occurred as a function of resources garnered during the prior season and were expended in a way that should have avoided over-investing and creating a tradeoff between reproduction and survival.

Highlights

Reproduction is a costly endeavor, and animals must balance the potential fitness benefits of investing energy into reproduction with other functions critical to life, such as avoiding predators, somatic growth, and maintenance (Kozłowski, 1992)
If resources are inadequate to sustain offspring growth and survival, female ungulates will withhold resources necessary to preserve their survival at the cost of their offspring (Martin and Festa-Bianchet, 2010; Shallow et al, 2015)—a concept known as risk-sensitive reproductive allocation (Bårdsen et al, 2008)
In contrast to the hypothesis that adult males suppress reproductive effort in young males during rut, food intake (Figure 2), hormone levels (Figure 6), and loss in somatic reserves (Figure 3) of yearling males was not affected by the presence of large, dominant males

Summary

INTRODUCTION

Reproduction is a costly endeavor, and animals must balance the potential fitness benefits of investing energy into reproduction with other functions critical to life, such as avoiding predators, somatic growth, and maintenance (Kozłowski, 1992). To better understand how young males navigate the relative risks and benefits of breeding at a young age, we quantified extent and timing of reproductive effort (i.e., weekly forage intake, change in body mass and body fat, and hormone levels; Charnov, 2002) for young and adult male white-tailed deer using a manipulative experiment. We hypothesized that yearlings would vary forage intake and subsequent somatic (i.e., body mass and fat) loss based on the presence of adult males. Because we repeatedly monitored the same individuals through time within each year, we used repeated-measures analysis of variance with a heterogeneous autoregressive error structure and a lag of one to evaluate differences across weeks, treatment groups, and their interaction for feed intake, digestibility, and body mass.

RESULTS

DISCUSSION

ETHICS STATEMENT