Male nutritional history affects female fecundity in a male-dimorphic mite: Evidence for a nuptial gift?

Abstract

In male-dimorphic species, males are often either armoured 'majors' that can monopolise access to females, or unarmoured and defenceless 'minors' that cannot. However, majors, unlike minors, have to spend energy to maintain their weaponry. Like-for-like, this could mean that minors have relatively more energy available to increase their reproductive output through e.g. sperm competition, or the transference of nutrients by means of a nuptial gift. Such a fitness advantage to minors could therefore contribute to explaining the coexistence of both morphs in single populations. We tested if food-deprived females of the male-dimorphic bulb mite Rhizoglyphus robini produced more eggs when mated to a minor or to a major male, and whether egg production depended on whether their mates were starved or fed prior to mating. We found no effect of male morph on female fecundity, but females did produce more eggs when mated to previously fed males. We also found that females increased in mass, but males decreased in mass over the course of the experiment. From these observations we infer that fed males are able to transfer nutrients, a nuptial gift, to their mate. This is the first observation to suggest nuptial gift transfer in mites. Though males of both morphs appeared able to produce nuptial gifts; other factors, like habitat complexity, should be considered to identify fitness benefits of minor over major males to understand why the two morphs coexist.

Costs of weaponry: Unarmed males sire more offspring than armed males in a male‐dimorphic mite

“…We did not find any effect of nutritional state on the reproductive output of males of different morphs, neither did we find that scramblers performed better than fighters under starved conditions. This does not support our hypothesis or the results obtained by Van den Beuken and Smallegange (), who, in a similar experiment, found that starved females mated to “fed” males produced more offspring than starved females mated to “starved” males. Importantly, after reaching maturity, “fed” males were fed for a single day and “starved” males were always starved in the study by Van den Beuken and Smallegange ().…”

“…Females were measured only once, after mating. The mating period was set at 24 hr because (a) this should have been more than enough time to complete mating (and mate searching) (Radwan & Siva‐Jothy, ), (b) a previous study found that fighters often kill females if left without food for an extended period of time (Van den Beuken & Smallegange, ), and (c) we wanted to assess whether males, particularly starved males, died more often after mating, perhaps as a result of a terminal reproductive investment; however, no male died of such causes.…”

“…Scramblers also live longer than fighters (Radwan & Bogacz, ); however, the reproductive output of both morphs decreases with age and the number of previous mates (Radwan & Bogacz, ); hence, the reproductive benefit of an increased longevity is questionable. There is no evidence that either male morph differentially invests in post‐copulatory attributes such as sperm competition (Radwan, ); however, Van den Beuken and Smallegange () found that, after allowing 1 day of feeding, males of both morphs sired more offspring than starved males. These results suggest that males transferred resources to their mate which increased offspring production, for example a nuptial gift.…”

Life-history consequences of bidirectional selection for male morph in a male-dimorphic bulb mite

“…Possibly, this was a demographic response if juvenile or adult females were killed more frequently by the larger number of fighters in fighter lines (cf. Van den Beuken and Smallegange 2018). The higher number of females likely yields a high clutch growth rate in growing populations (Lee et al 2011).…”

“…These four stock cultures have been intermixed approximately every 6 months to reduce the level of inbreeding. In order to obtain virgin females to start the first generation, we collected 240 quiescent tritonymphs (moulting stage immediately preceding adulthood) from our four stock populations (60 from each) and stored each in 7-mL ‘individual tubes’ (50 × 16 mm) (the maintenance of the stock cultures is described in Van den Beuken and Smallegange 2018). Individual tubes were filled for about two-thirds with plaster of Paris, with charcoal for contrast.…”