Skewed sex ratios at birth are widely reported in wild populations, however the extent to which parents are able to modulate the sex ratio of offspring to maximize their own fitness remains unclear. This is particularly true for highly polytocous species as maximizing fitness may include trade-offs between sex ratio and the size and number of offspring in litters. In such cases, it may be adaptive for mothers to adjust both the number of offspring per litter and offspring sex to maximize individual fitness. Investigating maternal sex allocation in wild pigs (Sus scrofa) under stochastic environmental conditions, we predicted that, under favorable conditions, high quality mothers (larger, older) would produce male-biased litters and invest more in producing larger litters with more males. We also predicted sex ratio would vary relative to litter size, with a male-bias among smaller litters. We found evidence that increasing wild boar ancestry, maternal age and condition, and resource availability may weakly contribute to male-biased sex ratio, however, unknown factors not measured in this study are assumed to be more influential. High quality mothers allocated more resources in litter production, but this relationship was driven by adjustment of litter size, not sex ratio. There was no relationship between sex ratio and litter size. Collectively, our results emphasized that adjustment of litter size appeared to be the primary reproductive characteristic manipulated in wild pigs to increase fitness rather than adjustment of offspring sex ratio.

This study was conducted at the Savannah River Site (SRS), a 78,000 ha U.S. Department of Energy facility located in Aiken, Barnwell, and Allendale counties, South Carolina, USA. We sampled from live-trapped (and consequently released for other studies, e.g., neonate survival [Chinn et al. 2021]) and humanely-euthanized according to established protocols (A2015 12-017) wild pigs throughout the year between March 2017 and July 2019. We collected fetal data, tissue for genetic analysis, and morphological measurements from females >27 kg to quantify litter and maternal attributes (n = 160). We weighed and measured each female dorsally from snout to base of tail. We calculated a standardized body condition index (subsequently referred to as maternal condition) for each individual as mass/length (LaBocha et al., 2014). We measured extraneous fat reserves, a quantitative measurement of maternal nutritional condition, from culled females and used it as a proxy for local resource availability. We assessed age by tooth eruption and replacement patterns (Matschke, 1967; Mayer, 2002), and individuals were classified into 3 age classes: juvenile (between 6 months and 1 year), yearling (1-1.5 years), and adults (≥ 1.5 years old). If present, fetuses were removed, weighed and measured in a straight-line from crown to rump (CRL). Fetuses ≥36 days old were sufficiently developed to visually determine sex. We quantified the ancestry of wild pigs following methods described in Smyser et al. (2020). 

We centered and z-transformed continuous variables to a mean of 0 and a standard deviation of 1 to allow for standardized comparison for all analyses. We censored individuals from analyses if they did not have all variables measured (i.e., removed a female from analysis if she was missing rump fat data). 

All analyses were performed in R 4.0.5 (R Core Team, 2021). We used package lme4 (Bates et al. 2015) for some analyses.