Reproductive synchronization is considered a life history strategy that maximizes reproductive success. There are numerous theoretical predictions as to how reproductive synchrony, or asynchrony, contributes to fitness. For example, synchronous reproduction may increase reproductive success by coinciding with critical resource availability, coinciding with a narrow optimal climate window, or predator swamping. At higher population densities, synchrony may increase reproductive success by reducing intraspecific competition, or aiding in predator avoidance. Here, we analyze the relationship between reproductive synchrony and reproductive output in the context of local density of two subalpine, cavity nesting bee species, Megachile relativa and M. pugnata (Megachilidae). From 2012 – 2014, individual bee nesting activity was monitored in artificial nesting blocks in the Colorado Rocky Mountains, USA, and the reproductive success of each individual (i.e., nest) was tracked. We found that despite fluctuations in local variation in density across the three-year study period, nests that were more synchronous produced more bees in the following year, for both M. relativa and M. pugnata. This general pattern indicates that there could be selection for greater reproductive synchrony in this environment for cavity nesting bee species, and it does not appear to be density dependent. Although further research is needed to understand the underlying mechanisms for this positive effect of synchrony on reproduction, we hypothesize that it might be attributed to a narrow optimal climate window in the subalpine environment, parasite activity, or synchronization with critical resources.
