Red mason bees analysed in the present study showed similar cocoon size and nest structure (number of cells/nest) in both years. Most of the bees in our study were correctly oriented. Nevertheless, the proportion of misoriented individuals was not negligible, reaching 29 and 10 % for males and females, respectively. Overrepresentation of the smaller sex (males) in both years, together with the generally smaller cocoon size of misoriented individuals, confirms the earlier findings of Torchio (1980), suggesting that misorientation depends on available space in the nest. Analyses of available space and cocoon size showed that as expected, cocoon size together with available space were the traits that affected most dramatically cocoon orientation. Nest identity explained some of the observed differences, but this is no surprise since nest identity determined nest width and the maximum space an individual can use. Additionally, nest building females in the present study clearly adjusted provision size of female progeny to nest diameter, but on the contrary provisioning male offspring equally. Contrary to the earlier findings of Tepedino and Torchio (1989), females in our study did not adjust their progeny’s sex ratio to nest diameter, but used all available sizes regardless of their progeny’s sex. However, Tepedino and Torchio (1989) used a different species than the one examined here, namely the North American O. lignaria propinqua. Both species share similar nesting biology, but differences in their behaviour or decision making cannot be excluded. Discrepancies in provisioning rate or shifts in sex ratio due to nest-building female’s body size in various bee and wasp species were shown already by Bosch and Vicens (2006).
The correlation between nest diameter and the ratio of misoriented cocoons per nest also confirms the findings of Torchio (1980). Unlike in our study, Torchio used only two different nests (with diameters of 6 and 7 mm) to compare the effect of nest size, while we have examined a whole spectrum of sufficient nest sizes. Both Torchio’s and our results show that the ratio of misoriented cocoons is lower in thinner nests.
Space-dependent cocoon orientation may be explained by cocoon construction. Already Torchio (1980) had observed that relatively small larvae compared to their cell size (thus with more available space) spin cocoons in a different way than larger larvae, more snugly adjusted (having less space) to their cells. Small larvae attach individual strands to all or almost all wall surfaces forming a cushion made of strands before spinning the first layer of the cocoon. Larger individuals or those that better fit their cell size use only a few strands and do not form a cushion to support the larvae, but attach the outer layer of the cocoon directly to the ventral and basal surface of the cell. This observation suggests that perception of restricted space in the cell activates different mechanisms/procedures of cocoon spinning (Torchio 1980).
Other studies suggest possible environmental cues determining the orientation of the larvae inside the cell. Matthew and Kislow in 1973 using reversely placed provision masses concluded that O. lignaria cocoons determine their orientation based on the placement of provision within the cell. However, few years later, Johnson (1980) conducting a similar, though not identical experiment found that O. lignaria did not rely on provision placement. Similarly, the same year, Torchio (1980) using O. lingnaria propinqua as a model concluded that in horizontal linear nests (in which the effect of gravity on cocoon orientation can be excluded), differences in the structure of the partition walls serve most probably as a clue for proper cocoon orientation. In most species, including the red mason bee, the posterior wall is usually smooth and concave, while the anterior is rough and convex (Krombein 1967). This difference in wall structure is caused by the wall building process itself. The egg-laying female brings mud particles in its mandibles to the nest, gluing them first to the cell’s cylindrical inner surface. After creating an initial mud-ring, she narrows the ring by adding new layers of mud until the cell is closed (personal observation), and also pushes and smoothens the anterior surface using the front of her head (Krombein 1967). The larvae can probably recognize both the structural difference of walls and the space available in the cell. According to these factors, larvae either adjust their orientation towards the entrance during spinning the first (outer layer) of their cocoons, or if sufficient space is available, simply disregard their orientation inside the cell. In the second case, they construct the above-mentioned cushion supporting the larvae and in consequence orient the spinning of the cocoon more randomly.
Construction of the cocoon either according to or disregarding the direction of the nest entrance suggests possible selection on proper cocoon orientation since bees usually orient their cocoons properly even when enough space is available. Unfortunately, no detailed data are available showing the effect of misorientation on the success of emergence from such cocoons. The only documented effect was given by Torchio (1980) who noted that large, misoriented females failed to emerge; however, this effect was not observed in smaller females. We observed that small males can turn around and in some cases even push their way through other small siblings and emerge successfully; however, this behaviour was not noted in the case of large individuals (personal observation).
In summary, misorientation of cocoons in linear bee nests is clearly dependent on individual cocoon size and available space in the cell. Additionally, due to significant sex-related size differences and possible adjustments made by the nest building female, it can be more pronounced in the smaller sex, i.e. in males, but less in larger, better adjusted to nest size sex, i.e. females. The significantly higher ratio of correctly oriented cocoons indicates that selection on proper orientation during spinning may be relevant, and further studies are required in this direction.