Variation in maternal solitary bee nest defence related to nest state

Parental protection of offspring is found in numerous animal species. Protection provides offspring with a greater chance of surviving to be able to reproduce, while at the same time, often posing a cost to the parent. Therefore, the net value of defence for the parent can vary depending on the developmental stage of the offspring and their ability to defend themselves. For example, in commonly studied organisms (e.g. birds), defence level increases over time until offspring are able to leave the nest and parental defence wanes. We examine these nest defence decisions in the solitary bee system that poses an interesting variation to traditionally studied organisms. Nest value does not increase in a simple additive manner as offspring are added to the nest. Here, individual investments are allocated to each offspring, then as a final act, the nest entrance is sealed. This sealing action provides increased protection for all developing offspring in the nest, therefore strongly increasing their value. Our observational experiment using Megachile rotundata found that mothers slightly increased nest defence as nest size increased. However, unlike traditionally studied organisms, this increase in defence continued until the nest was completed.