Male reproductive investment changes in relation to female condition such as age, body size and mating status (i.e. virgin or mated) because of male mating strategy and/or female control. In this seminar, I review the relationship between male investment and female condition from the perspective of male strategies and female control.
Generally, males mate multiply during their lifetime, but their resources are limited. Males are predicted to allocate their reproductive investment strategically in response to female condition (1).
In Drosophila melanogaster, males delivered significantly more sperm to mated, large or young females compared with virgins, small or old females, respectively (2). In cotton bollworm Helicoverpa armigera, ejaculate size was correlated with female body size. Moreover, males delivered significantly more sperm to mated females than to virgin ones (3). It might be adaptive for males to invest more to more fecund females, and males might also be adaptive to sperm competition risk if more fecund females (e.g. young and large) mate more frequently. The allocation of more sperm to mated females might be related to sperm displacement of previously-mated males or sperm competition advantage. Similarly, in Mediterranean flour moth, Ephestia kuehniella, males preferred younger and larger females for copulation when given a choice and allocated more apyrene (anucleate) and eupyrene (nucleate) sperm to them. However, allocation of more sperm by males did not increase female reproductive output because high-quality females are more likely to re-mate sooner. In this species, the allocation of more sperm may function as sperm competition against rivals (4).
The strategic male reproductive investment may also change depending on age of males because they face a trade-off between investments in present versus future mating. In scropionfly, males offered high-quality females significantly larger salivary mass as nuptial gift than low-quality females, but relative mating effort in matings with low-quality females increased when males became old. Old males, with a relative small number of expected future matings, did not invest differentially in copulations with high-versus low-quality females (5).
Not only males but females may regulate the male investment. Sperm transfer rate can also be facilitated by the physical size of females and/or females in good condition may be more cooperative during sperm transfer. In scropionfly, Panorpa cognate, significantly more sperm were transferred in mating with females in good condition than in mating with females in poor condition. Moreover, male sperm transfer per time unit is different depending on female condition: males transfer sperm at a faster rate in mating with females in good condition than in mating with females in poor condition (6). To explore whether females counteract male sperm transfer rates, females were treated with the insecticide Propoxur which leads to total paralysis caused by tremors. It was found that males had significantly lower sperm transfer rates when mating with treated females (7).
In conclusion, female traits (age, size/weight and mating status) are likely to be important in determining male reproductive investment. We need to interpret the changes in male investment in terms of both sexes.
References
1. Galvani AP & Johnstone R. 1998. Sperm allocation in an uncertain world. Behav Ecol Sociobiol. 44: 161-168.
2. Lüpold S, Manier MK, Ala-Honkola O, Belote JM & Pitnick S. 2010. Male Drosophila melanogaster adjust ejaculate size based on female mating status, fecundity, and age. Behav Ecol. 22:184-191.
3. Teng ZQ & Zhang QW. 2009. Determinants of male ejaculate investment in the cotton bollworm Helicoverpa armigera: mating history, female body size and male age. Physiol Entomol. 34:338-344.
4. Xu J & Wang Q. 2009. Male moths undertake both pre- and in- copulation mate choice based on female age and weight. Behav Ecol Sociobiol. 63:801-808.
5. Engqvist L & Sauer KP. 2001. A life-history perspective on strategic mating effort in male scorpionflies. Behav Ecol. 13:632-636.
6. Engqvist L & Sauer KP. 2003. Determinants of sperm transfer in the scropionfly Panorpa cognate: male variation, female condition and copulation duration. J Evol Biol.16:1196-1204.
7. Vermeulen A, Engels S & Sauer KP. 2008. Mating effort and cryptic sperm choice in scorpionflies: Male investment strategy vs. female control. Ethology 14: 1166-1172.