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Original Paper: Engel, K. C. et al. 2016. A hormone-related female anti-aphrodisiac signals temporary infertility and causes sexual abstinence to synchronize parental care. Nature Communications 7: 1–10.

Burying beetles are called nature’s undertakers because they bury the remains of small dead animals. These animal bodies serve as food for the beetles’ young. A mating pair of beetles typically meets over a fresh animal carcass, copulates, and buries the carcass. The female lays fertilized eggs in the soil above the carcass. Some 50–60 hours later, the eggs hatch and both parents spend the next 3 days eating from the carcass and regurgitating the partially digested material to feed their young. After 3 days, the larvae feed themselves.

Researchers used these beetles to study trade-offs in time and energy investments between brood care and production of new broods. Both activities are demanding, so how do the beetles control their apportionment of energy? The researchers provided female beetles with mating partners and carcasses to stimulate egg laying. There were three treatment groups: (1) no larvae group: just as the eggs hatched, the new hatchlings were removed and the parents were presented with a new carcass; (2) new larvae group: just after their eggs hatched, the parents were left with 10 larvae and given a new carcass; and (3) old larvae group: 4 days after their eggs hatched and they had cared for their hatchlings, the parents were presented with a new carcass. The researchers measured the number of females in each group that produced eggs. The results are shown in Figure A.

The researchers measured juvenile hormone (JH) levels present at various times in the breeding cycles of females with and without larvae. Juvenile hormone is known to suppress the fertility of the females. The results are shown in Figure B.

The researchers also measured the effects of a possible pheromone, methyl geranate, on the beetles’ sexual behavior. Methyl geranate is a volatile compound that shares the same biosynthetic pathway with juvenile hormone. The researchers recorded whether or not males copulated with females and the amount of methyl geranate emitted by those females. Figure C summarizes these results. The data points represent meetings resulting in copulation or not.

Questions

Question 1

When females have no larvae or old larvae that no longer need parental care, they invest their energy in producing new broods. However, during the short period when larvae are dependent on parental investment (“new larvae”), the female fertility rate is low. Thus females seem to direct their investment of time and energy into the activities most likely to result in reproductive success

Question 2

Question 3 of Tinbergen applies. It asks about how a behavior influences survival and reproductive success, and therefore, it deals with ultimate causes of behavior. This study looked into the egg-laying behavior of female beetles under different circumstances. The results can be used to support the hypothesis that female beetles shift their energy expenditures to maximize their reproductive success. This is an ultimate cause of behavior since it deals with evolutionary pressures to maximize reproductive success.

Question 3

The data indicate a correlation between increases in juvenile hormone and suppression of egg-laying behavior. The data also indicate a correlation between increases in methyl geranate and suppression of copulation behavior. These correlations suggest that hormone and pheromone changes may be underlying physiological mechanisms that influence the shift in egg-laying behavior and sexual behavior to parental care behavior in burying beetles.

Question 4

Tinbergen’s question about immediate causation 1 applies, and it deals with proximate causes of behavior. The studies were aimed at elucidating the underlying biological mechanisms that produce the observed changes in behaviors. The results can be used to support the hypothesis that expression of different behaviors comes about as the result of changing levels of hormones and pheromones produced by the female. This is a proximate cause of behavior since it deals with an immediate physiological mechanism that determines how an individual is behaving at a particular time.

Question 5

A cost of the parental care behavior is the opportunity lost for mating and egg laying that could be done instead of caring for young larvae. This means that there are times when the beetles do not actively reproduce, and so fewer offspring are produced over time. A benefit is the protection and nurturing of young larvae, which improves the likelihood that they will survive to carry on the next generation.