Sperm competition within a dominance hierarchy: investment in social status vs. investment in ejaculates.

There is increasing recognition that male-male competition can take many forms, but as yet the form is not predictable a priori. Many recent studies have focused attention on how males in disadvantaged mating roles compensate through sperm competition. However, mating systems in which subordinate males are reproductively suppressed, particularly through the stress of social interactions, may limit the ability of males to respond by increasing investment in sperm quality. We examined the interaction between social status and ejaculate tactics in Nauphoeta cinerea, a cockroach that has a mating system with well-characterized dominance hierarchies. Both social experience with other males and social status influenced aspects of ejaculates. The stress of social interactions reduced the size of the ejaculate and number of sperm inseminated. In ejaculates formed prior to social experience, however, males that go on to become dominant inseminated more sperm than males that go on to become subordinate, suggesting innate differences among males. Our results show that though selection for increased success in sperm competition for subordinate males in a hierarchy can occur, both the traits and the way in which the balance between pre- and post-copulatory strategies is negotiated will depend on specific details of the mating system. These details will include how the physiological effects of social interactions may limit selection through male-male competition.

Tamara Montrose V ，Edwin Harris W ，Moore AJ ，Moore PJ ... -  《-》

Sperm competition roles and ejaculate investment in a promiscuous mammal.

Theoretical models of sperm competition predict how males should allocate sperm and seminal fluid components to ejaculates according to their mating role (dominant vs. subordinate). Here, we present a detailed analysis of ejaculate expenditure according to male roles in the bank vole (Myodes glareolus). Sperm competition occurs regularly in this species, and dominant males typically achieve higher fertilization success than subordinates. Contrary to theoretical predictions, we found that dominant male bank voles invest more sperm per ejaculate than subordinates, both absolutely and relative to body and testes mass. The testes of dominant males were also absolutely (although not relatively) larger than those of subordinates. However, we found no evidence that subordinate males compensate for lower sperm numbers per ejaculate by increasing ejaculation frequency or sperm velocity. Similarly, we found no evidence for differential investment in copulatory plug size according to male roles in sperm competition, although dominant males had significantly larger seminal vesicles (both absolutely and relative to body mass) compared with subordinates. We conclude that sperm competition roles can have significant but unexpected influences on ejaculate investment in mammals with clearly defined differences in male social status.

Lemaître JF ，Ramm SA ，Hurst JL ，Stockley P ... -  《-》

Changes in sperm quality and numbers in response to experimental manipulation of male social status and female attractiveness.

Cornwallis CK ，Birkhead TR 《-》

Sperm competition and male ejaculate investment in Nauphoeta cinerea: effects of social environment during development.

Harris WE ，Moore PJ 《JOURNAL OF EVOLUTIONARY BIOLOGY》

Sexual selection and sperm quantity: meta-analyses of strategic ejaculation.

Multiple mating or group spawning leads to post-copulatory sexual selection, which generally favours ejaculates that are more competitive under sperm competition. In four meta-analyses we quantify the evidence that sperm competition (SC) favours greater sperm number using data from studies of strategic ejaculation. Differential investment into each ejaculate emerges at the individual level if males exhibit phenotypic plasticity in ejaculate properties in response to the likely risk and/or intensity of sperm competition after a given mating. Over the last twenty years, a series of theoretical models have been developed that predict how ejaculate size will be strategically adjusted in relation to: (a) the number of immediate rival males, with a distinction made between 0 versus 1 rival ('risk' of SC) and 1 versus several rivals ('intensity' of SC); (b) female mating status (virgin or previously mated); and (c) female phenotypic quality (e.g. female size or condition). Some well-known studies have reported large adjustments in ejaculate size depending on the relevant social context and this has led to widespread acceptance of the claim that strategic sperm allocation occurs in response to each of these factors. It is necessary, however, to test each claim separately because it is easy to overlook studies with weak or negative findings. Compiling information on the variation in outcomes among species is potentially informative about the relevance of these assumptions in different taxa or mating systems. We found strong evidence that, on average, males transfer larger ejaculates to higher quality females. The effect of female mating status was less straightforward and depended on how ejaculate size was measured (i.e. use of proxy or direct measure). There is strong evidence that ejaculate size increased when males were exposed to a single rival, which is often described as a response to the risk of SC. There is, however, no evidence for the general prediction that ejaculate size decreases as the number of rivals increases from one to several males (i.e. in response to a higher intensity of SC which lowers the rate of return per sperm released). Our results highlight how meta-analysis can reveal unintentional biases in narrative literature reviews. We note that several assumptions of theoretical models can alter an outcome's predicted direction in a given species (e.g. the effect of female mating status depends on whether there is first- or last-male sperm priority). Many studies do not provide this background information and fail to make strong a priori predictions about the expected response of ejaculate size to manipulation of the mating context. Researchers should be explicit about which model they are testing to ensure that future meta-analyses can better partition studies into different categories, or control for continuous moderator variables.

Kelly CD ，Jennions MD 《-》