Animals possess many unique ways to devote effort (time, energy, and/or nutrients) towards reproduction in seasonal environments. An extreme response to seasonality is seen in the presence of reproductive delays (i.e. temporal pauses in reproduction) that occur between copulation and fertilization, between conception and implantation, or after an embryo has implanted. Here I ask how delays might allow mammals to allocate limited resources to reproduction (i.e., are delays adaptive?) and explore the causes and consequences of reproductive delays in mammals. Sperm storage also termed ‘delayed fertilization’ occurs when females store sperm prior to ovulating. By increasing the time over which fertilization occurs, delayed fertilization may result in increased sperm competition or cryptic female choice and glans elaborations (penile spines). I discuss data indicating that taxa with delays are more likely to possess genital elaborations (spines) which may function for locking, sperm removal or female stimulation. I also summarize what is known about similar elaborations in other mammalian taxa. Next, to understand why delays may have evolved I explore the ecological and phylogenetic predictors of delayed implantation in the order Carnivora. In this group there is evidence that the presence of delays is positively related to body size, seasonality, diet as well as evolutionary relationships (Phylogeny). I will conclude this discussion of reproductive delays by examining the hypothesis that delays allow females to time the most expensive stage of reproduction, lactation, with periods of greatest food abundance in a species of bat that has pregnancies both with and without delays.