Este tipo de estudios contribuyen al entendimiento de aspectos relacionados con la teoría de la selección sexual, en particular respecto a los costos y beneficios del apareamiento y la elección de pareja en ambos sexos.
They like them with experience
Male age and sexual experience seem to be important factors in mate choice matters. Male mating success might depend on their size, diet, their previous mating events and age. However, females’ choices are not simple: there might be advantages in choosing young suitors but also in choosing more aged ones.
According to some studies females should prefer aged suitors due to the fact that, by being alive they are already showing their survival abilities and consequently, are likely to be the carriers of “good genes”. They might also have better quality ejaculates and better parental abilities.
Choosing the young ones might also have some advantages. For instance, females could avoid the negative consequences of deteriorating sperm that comes with age and/or repeated mating and could also be at a minor risk of contracting a sexually transmitted disease.
Some empirical evidence on birds, lizards and sheep suggest that females prefer older males.
In biological terms we can say, however, that a certain preference (of any other attribute or behaviour for that matter) is either better or worst when we take into account its effects on individuals’ biological fitness. Fitness is measured by basically counting the number of descendants that –ideally- survive until reproductive age. With this in mind, one might actually wonder, what about female Mexican fruit flies (Anastrepha ludens)? Do they prefer young or more senior males? Do they like them with experience or without it? What are the consequences of their choices on their fitness?
Questions of the sort motivated Maria Martinez’ honours project, which was supervised by Diana Pérez-Staples at the Universidad Veracruzana and Martín Aluja at the Instituto de Ecología, both Institutions located in Veracruz, México. Their results were published in the journal Ethology. For their study, Diana and their team captured wild fruit flies in several places in Veracruz, they kept them in captivity and waited until the flies reproduced and formed a new generation of semi-wild flies. Afterwards, those flies were separated by sex.
Individuals were individually marked with different colours of paint. Males were separated in four groups according to the following: young males (13 days older), old males (18 days older), experienced males (that mated once between days 13 and 15) and sexually naïve males (with no mating on their records). Females were all virgins between 13 and 15 days.
Mating experiments started when young males (with and without experience) were 18 days old and when older males (also with and without experience) were 36 days old. In other words, males that have had previous contact with females had enough time to replenish their sperm reservoirs before the experimental observations started.
Diana and collaborators recorded mating occurrence and the duration of all copulations in several experimental boxes with three males of each treatment (12 males in total) and six females. In that way, each experimental box contained one female for every two males, ensuring competition between males and female choice. Males had to compete and females had four different kinds of males to choose from. On the next day, females that had copulated laid their eggs on agar spheres (artificial ovipositing devices) specially designed for that purpose. All eggs were counted and hatching recorded. Researchers also measured females’ longevity.
What the Mexican team found was that older males were more likely to mate and that aged and experienced males were more likely to obtain the first mating. These results might suggest that males become more competitive with age and experience; time might help them to improve their mating strategies.
It is important to highlight that Anastrepha ludens males exhibit an elaborated courtship that involves wing fanning during dust and emitting all available sexiness in the form of pheromones. There is no evidence of forced mating in this species and females could potentially resist males’ approaches as they can actually kick males off them and hide their ovipositor to avoid mating, or simply fly away from wing fanning and flirtatious males. Consequently, males’ experience in courtship displays seems to be essential.
On the other hand, Diana and her team also found that male condition did not affect females’ fecundity, fertility or longevity. In that case, if females preferred aged and experienced males with no apparent benefit, what is going on?
One explanation is that females cannot discern between males with different attributes and that males may be more efficient in overcoming female resistance. It is also possible that for females there are some other benefits –in fitness terms- that were not measured by the study of Diana, Maria and Martin.
Another explanation is that ejaculate capacities are not diminished at the age of 36 days. In that case, there might be disadvantages for females choosing even older males. However, in order to elucidate this, more experiments are needed.
It is possible then that –as it has been suggested in other studies- sexual experience might be more relevant for males in long lived species where there is intrasexual competition, where courtship and copulation are highly elaborated and where males have several mating chances throughout their lives; exactly as it happens in Mexican fruit flies. In these tiny flies, mating with older males does not seem to provide any advantage to females, but males seem to have the right amount of experience to persuade them.
This kind of studies contribute to widen our understanding of those aspects related with the theory of sexual selection, particularly those related to male competitiveness and condition and how this in turn affects female fitness. 
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