Have you ever had a roommate that you stuck with simply because they made the cost of housing cheaper? As it turns out you are not alone: some animals find themselves in similar situations. Researchers at Arizona State University have found that carpenter bees (Xylocopa sonorina) will live socially to avoid the high energy costs of independently building a nest in which to raise their young.
Carpenter bees build extensive nests by drilling tunnels into wood in which to live and rear young. It has been observed that females will either search for an abandoned nest or join a small social group of other females in an established nest. However, these bees do not share the burdens of foraging and excavating. These tasks are the responsibility of the one reproductively dominant female in the social group. Therefore, instead of living socially because of the benefits of division of labor, it could be that sociality is due to low-cost nesting available in a nest that is already built. This leads Otswald et al hypothesize in their article “Social consequences of energetically costly nest construction in a facultatively social bee” that sociality in these carpenter bees is driven by the energy costs of nest building.
To test this hypothesis, they had to quantify the cost of the nest building. First, they measured the metabolic rate, the amount of energy expended during an activity, of excavating bees and compared it to that of resting and flying bees. To do this, they placed a bee in a syringe filled with CO2-free air. Bees that were being tested for the metabolic rate of excavation were provided with a small block of either hard or softwood, as carpenter bees have been known to nest in both. The content of CO2 that was produced by the insect while performing the behavior it was being studied for was measured.
Next, the researchers measured the volume of the bee’s nests at three different points in the breeding season. Scans of preexisting nests were taken with a CT machine before the breeding season, at the peak of egg laying, and at the end of the breeding season when excavation had stopped. The change in the volume of the nests over the season was used to determine the renovations needed for a female to raise her young in an existing nest. Furthermore, the volume of tunnel necessary to raise a single young was calculated. These two data points, metabolic rate and volume, were used to calculate the energy expenditure related to different nest volumes and with the rearing of a single offspring.
It was found that the metabolic cost for excavating was much more than that of resting but less than flying, and that excavating hardwood was more costly than excavating softwood. However, per offspring produced, the excavation cost was close to that of seven hours of flight. This is a large cost, as it is widely accepted that insect flight is one of the most metabolically costly behaviors in the animal kingdom. Additionally, it was found that the building of a nest with a volume to accommodate the average amount of offspring takes the energy of roughly eighty hours of flight. While on average, renovations of existing nests were less costly, there was a wide variation in renovation cost.
The large variation in renovation cost led Otswald and colleagues to believe that the initial condition of the inherited nest was a factor in the benefits of social living. Considering the higher metabolic costs of excavating hardwood, they believe that building in hardwood could have benefits for the condition of the nest that offset the initial energy investment. Softwood breaks down easier so might require extensive renovations meanwhile, hardwood could increase the longevity of the nest. Therefore, hardwood nests would be more desirable to inherit due to the low renovation cost but high cost to the founder. This lower cost associated with good quality inherited nests could lead to reproductive benefits for the insect, as more energy could be put into producing offspring than excavating the nest.
These findings provide an important link between behavioral decisions and ecological conditions. Additionally, it shows how energetic tradeoffs are important as social living originates.
References
Oswald M, Fox T, Harrison J, Fewell J. 2021. Social consequences of energetically costly nest construction in a facultatively social bee. Proc. R. Soc. B. 288, 20210033. (doi.org/10.1098/rspb.2021.0033)
