Reproductive Compensation Hypothesis

The reproductive compensation hypothesis assumes that when an individual is faced with future reproductive losses they should increase current reproductive investment[i]. This is based on the assumption that all individuals should attempt to maximize overall lifetime reproductive success. Therefore, we expect to find a negative correlation between the probability of future reproductive success and current reproductive effort.

Parasitized crickets never live for more than a few hours following the emergence of O.ochracea. Therefore, the lifespan of a parasitized cricket is significantly reduced (depending on age). This leads to a loss in future offspring and therefore, we can assume that the reproductive compensation hypothesis applies.

Reproductive compensation has yet to be observed in the G. texenxis/O. ochracea system. parasitized males do not increase calling effort as predicted by the reproductive compensation hypothesis[ii]. Additionally,  in Hawaii, where O. ochracea uses the cricket Teleogryllus  oceanicus as a host, parasitized male T. oceanicus did not differ in calling activity (proportion of males calling; number of bouts; length of longest bout; calling duration) from unparasitized males[iii].

Figure 11: Mating crickets. The female mounts the male and a sperm package is transferred.

One problem with questions driven by the reproductive compensation hypothesis is that the assumptions made by this hypothesis have yet to be confirmed in this system. For example, the reproductive compensation hypothesis requires that an organism ‘knows’ that its lifespan is shortened, but how is the cricket expected to know that its future reproductive success is being compromised? If we are speaking of a strictly physiological response, this implies that the presence of the larva in the cricket causes some sort of hormonal, neuronal, or otherwise physiological change. This change then should result in an increase in reproductive effort, whether by means of increasing activity (and thereby the possibility of finding mates) or influencing hormones relating to mating and reproduction. It is possible that the encapsulation response the cricket’s body has to the larva[iv]may serve as a catalyst for such a process, but there has not been any work published to this effect. In the absence of a mechanism by which the cricket can detect its shortened lifespan, it is unrealistic to expect this system to serve as a model for the reproductive compensation hypothesis.  If G. texensis females are incapable of distinguishing between healthy and parasitized males, we would expect that an evolutionary arms race would evolve only if there is some cost associated with mating with a parasitized individual. If there is no cost associated with mating with a parasitized individual, there is no pressure for a parasitized individual to increase its reproductive effort. Furthermore, the possibility of increasing reproductive effort in both male and female G. texensis may not exist.

Studies on the impact of O.  ochracea on female hosts have looked mainly at egg laying behaviour and very little at mate choice of parasitized versus healthy females. Two week old parasitized Acheta domesticus lay the same number of eggs per day (18) as healthy females for the first five days once removed from the colony[v]. This rate is significantly decreased in parasitized females the day before parasite emergence. Therefore, female A.  domesticus also fail to support the reproductive compensation hypothesis as we would expect parasitized females to increase the number of eggs that they lay. It is important to note however that A.  domesticus is not parasitized by O.  ochracea in the wild and therefore the pressure to evolve a response to parasitism by O.  ochracea is absent. If A.  domesticus were to exhibit a change in egg laying behaviour, we could assume that this was a general response to the presence of a foreign body (which does not always mean reduced longevity). Another possibility to consider, is that female G.texensis are laying the optimal number of eggs and are physiologically incapable of the up regulation demanded by the reproductive compensation hypothesis. It is therefore necessary to investigate this in G.  texensis females.

Additionally, females may be compensating by re-mating quickly or by becoming less ‘choosy’ when face with parasitism. Since most orthopterans store sperm, it seems unlikely that males would be reducing their sperm load to such an extent that a female would not receive enough sperm to fertilize as many eggs as she wanted to immediately, therefore making a decreased refractory period unnecessary. A more logical explanation is that females re-mate quickly because the size of the spermatophore serves as an indication of the male’s quality, but they could also be re-mating because they can identify a parasitized male by some other means.

Figure 12: Female G. texensis  ovipositing.

Photo taken by Crystal Vincent

Home