Effects of Age and Prior Reproduction on the Cost of Reproduction in Two Species of Burying Beetles
Damron, Ethan; Jo, Dane
Mark Belk, Biology
Understanding how much energy an organism allocates to breeding to maximize reproduction over their
lifetime, or the cost of reproduction, is a central idea to understanding the variations in patterns of
reproduction among living organisms (Creighton et al., 2009). In most studies of cost of reproduction, age is confounded with prior reproductive experience. As individuals age, they tend to reproduce making it difficult to disentangle these two factors when evaluating costs of reproduction. It is not clear whether reproduction early in life costs the same as reproduction later in life and how much of that difference might be due to prior reproduction or aging. Analyses of changes in reproductive effort are impeded by fundamental difficulties in measuring the cost of reproduction (Clutton-Brock, 1984). Unlike many other species, such as mammals and birds, burying beetles breed in discrete cycles in conditions easily replicated in a laboratory. Evaluation of brood size and the amount of mass gained by the parents gives us a very clear indication of how much energy a male and female burying beetle put into a reproductive bout. There are very few organisms that have breeding conditions and practices that allow for such clear measurement of the cost of reproduction as in burying beetles. These beetles provide the perfect experimental organism that will allow us to independently look at the effects of age and previous reproductive experience on the cost of reproduction. We will explore this question in both sexes of two different species to test for generality of patterns.
Understanding the cost of reproductions helps us understand how organisms allocate their resources over
their lifetimes. Therefore, by testing for generality we will provide results on the cost of reproduction that can be used and repeated by researchers all over the world.
The goal of this project is to determine if the energy allocated to reproduction is constant throughout life or if reproductive cost varies with age and prior reproductive experience. The terminal investment
hypothesis states that organisms tend to favor increased reproductive effort with age and that organisms put more energy into reproduction at the end of life because older organisms have limited reproductive opportunities remaining (Charlesworth, 1976). In line with this theory, we hypothesize that as burying beetle’s age and reproductive experience increases, so does the cost and allocation of energy to reproduction. To test this hypothesis we will have three treatment groups: young virgin, old experienced, and old virgin. The young virgins will only be bred once during their lifetime and will start their reproductive bout at twenty-one days of age. In the second treatment we will breed beetles at twenty-one days of age and then again at fifty days of age. After their second reproductive bout they will not breed again. For our third treatment we will breed old virgins at fifty days of age. They will only breed once in their lifetime. We will use the same methods that have been used to study cost of reproduction in previous experiments (Billman et al., 2014; Creighton et al., 2009). We will raise the beetles in captivity and allow the parents to process and raise a brood on a mouse carcass. When reproduction is finished we will count the offspring and maintain the parents that will breed once for the rest of their lives on ad libitum food. For the beetles in the second treatment that will breed a second time we will set up a new reproductive bout three days after their first reproductive cycle ends. After the second bout is completed we will maintain the parents on ad libitum food for the rest of their lives. Treatment three will be treated the same as treatment one, but their only reproductive bout will begin at fifty days of age.
Both of these species of burying beetles exhibit bi-parental care and therefore incur costs of reproduction in similar ways. When larvae first appear on the carcass both parents will participate in filial cannibalism to adjust the size of the brood to the size of the available food source. The resulting brood size is a reliable measure of energy allocation to the reproductive bout. Feeding and consequent mass gain by the parents is a reliable indicator of energy reserved to future reproductive bouts (Creighton et al., 2009). We will record the number of larvae both before and after parental reduction of brood size. We will also weigh the parent beetles before and after the reproductive bout to determine mass change. Using these data we will be able to determine if the beetles attempted to save energy during the reproductive bout. After the larvae and parents have completely consumed the carcass, the larvae will disperse into the dirt to pupate. At this point we will remove the parents. We will record the date in which they die and calculate their total lifespan. After all of our treatments are complete, we will compare brood size, mass change, and the length of life of the parent beetles in the three treatments to evaluate whether or not the independent effect of age at which a beetle breeds and the independent effect of prior reproductive experience causes variance in the cost of reproduction.
We predicted that as age and reproductive experience increased, so would the cost of reproduction. Our
data, however, was inconclusive and did not support our hypothesis. The average lifespan of beetles who
did not reproduce was very similar to the lifespan of beetles who did reproduce at the three different ages. Brood size decreased with age for many beetles. The data we collected for the weight gain of parent beetles was also inconclusive and did not have any sort of pattern.
The findings of our experiment suggests that there is no cost of reproduction for these burying beetles and that the cost is constant throughout life and independent of reproductive experience. This finding
contradicts the terminal investment theory. Our research team is quite skeptical that this data is reliable since experiments we have conducted prior to this one have fully supported the idea of terminal investment and have shown that reproduction has associated costs. We have discussed a few sources of error that could have affected our experiment. First, the male N. Marginatus population seemed to be affected by some sort of genetic mutation that caused them to die at very young ages. This mutation could have caused offspring to die prematurely and therefore skew our lifespan data but it also reduced the overall population and caused problems with inbreeding and genetic diversity. Second, incorrect sexing of offspring beetles caused dozens of reproductive bouts to fail which caused us to have a small set of data. We have decided to make adjustments to this experimental design and then conduct the project again to determine if this data is reliable or not. In the new project we will only focus on the cost of reproduction of females in both species since males and females experience slightly different costs. We will also only focus on the effect age has on the cost of reproduction.
This experiment did not pan out the way we had hoped it to but we were able to gain valuable insight on
how to improve our experimental design to help us get more accurate and reliable data.
Creighton JC, Heflin ND, Belk MC (2009) Cost of Reproduction, Resource Quality, and Terminal
Investment in a Burying Beetle. Am Nat 174:673-684
Cluttonbrock TH (1984) Reproductive effort and terminal investment in iteroparous animals. Am Nat
Billman EJ, Creighton JC, Belk MC (2014) Prior experience affects allocation to current reproduction in a burying Charlesworth B, Leon JA (1976) Relation of reproductive effort to age. Am Nat 110:449-459