What is Male Senescence?
Looking at the effects of male senescence on female fitness, it is clear that this type of sexual conflict is driving the evolutionary and
behavioral mating changes seen in the Darkling Beetle. Senescence, the act of aging, is divided into two main categories— Organismal and
Cellular. Cellular senescence corresponds to the failure of diploid cells to continue cell division; this is characterized by DNA damage from
telomere shortening and apoptosis, a type of programmed cell death. Organismal senescence is simply associated with natural aging and
corresponds to a loss of homeostatic regulation. These two types of senescence, found in old (83-85 days post eclosion) Darkling Beetles, are
driving sexual conflict as the age of the male beetle plays a major role in its sperm quality. The effects of senescence on sperm can be further
divided into two distinct types—premeiotic sperm senescence and postmeiotic sperm senescence. The first is a type of organismal senescence
as the overall quality and production of sperm decreases as the genome ages (Carazo et al 2011). The latter, postmeiotic sperm senescence, is
a result of cellular senescence as it is the individual sperm cells that deteriorate due to prolonged sperm storage.
How Does Male Senescence Influence Mating Behavior?
The overall reduction in sperm quality and count can be costly to the females who are coerced into mating with older males. Sperm
competition and the mating skills of the older male beetles work in tandem against the female as she is subjected to more aggressive, yet less
effective, mating attempts as male Darkling Beetles age. However, studies show that older males are shifting their mating behavior in order
to optimize their mating success and overall fitness. These behavior shifts are most easily noted in the counter- adaptive measures being taken
by female beetles. Females have been noted to increase their escape attempts, although severely limited by the spines on the male aedeagus
(penis) that makes ending copulation difficult. In addition, females had lower rates of postcopulatory associations (PCA) or aggressive
spermatophore guarding after copulating with older males. Research also suggests that females may manipulate their ova position in order to
distort the affects of mating with older males (Carazo et al 2011). Older beetles have understood the female preference for younger beetles
and sperm competition has driven these men to increase copulation time, increase courtship length and style, increase their intensity of mate
guarding as well as increase probing duration(Carazo et al 2011).
How Does Male Senescence Influence Female/Offspring Fitness?
The intense battle between females to avoid mating with older males and the older male’s more aggressive and elaborate attempts at mating
causes the offspring of this antagonist pair to pay a heavy fitness cost. The female Darkling Beetle also pays a high evolutionary cost as she
experiences increased levels of stress and aspermic matings (copulations without sperm delivery) when mating with old beetles. Simply, the
female experiences a drastic reduction in biological fitness as her children are smaller, have a lower survival rate, lower body weight, and are
less attractive when sired by older male Darklings. These children also show increased malformation rates and lower levels of mating success.
Overall, the conflict lies in the tug of war between older males and their aggressive attempts at mating and increased terminal investment and
the female’s attempts to avoid copulations and the subsequent brood of lower quality children. Taken together, it is seen that sperm
competition is driving the beetle species towards allelic changes, behavioral shifts, and co-evolution. Simply, males that continue to mate past
their genetic prime greatly reduces female and offspring fitness and cause sexual conflict to be exacerbated in Darkling Beetles.
Number of offspring
Behavioral Effects
Larvae Survival
Fitness Effects
O
Y
O
Offspring With
Malformations
Reference
Y
Courtship
Probing
Mean Offspring
Weight
Copula
Carazo, Pau, Pau Molina-Vila, and Enrique Font. "Male Reproductive Senescence As A Potential Source Of Sexual Conflict In A Beetle." Behavioral Ecology
22.1 (2011): 192-198. PsycINFO. Web. 28 May 2013.