Phenotypic quality and senescence affect different components of reproductive output in roe deer

Abstract

Summary Females should adjust their reproductive effort prior to substantial investment. For roe deer, due to delayed implantation, this adjustment may occur at ovulation/fertilization during the summer rut, or at implantation in mid‐winter. We investigated the effects of climate and maternal phenotype (mass, condition, age) on potential litter size (number of fertilized ovulations) and implantation failure (number of fertilized ovulations less number of fetuses) of 818 individual roe does from nine populations across Britain. Potential litter size varied from 1 to 4, with 0–100% of does per population polyovulating. Among prime‐aged does, 16.7–54.5% subsequently failed to implant at least one blastocyst. Individual fecundity was determined by the combined effects of maternal phenotypic quality and age, but acting at different stages of the reproductive process: potential litter size increased with increasing maternal body mass; implantation failure was independent of phenotypic quality, but varied in relation to maternal age, being lowest for prime‐aged does, somewhat higher for yearlings, but higher still for senescent females. Implantation failure increased with increasing initial potential litter size, perhaps related to physiological malfunction. Implantation was often an all‐or‐nothing process, with females either implanting all or failing to implant any of their fertilized blastocysts. Climatic factors were not consistently correlated with individual female fecundity within populations, but between populations implantation failure increased with climatic severity. For species such as roe deer, where females rely on food intake rather than fat reserves for reproduction, we suggest that a two‐step process shapes patterns of reproductive output: body mass first sets an upper limit to potential litter size at conception, then reproductive output is limited mainly by senescence and climatic severity through implantation failure.