Experimental evidence for testis size evolution via sperm competition

Sperm competition theory predicts increased spermatogenic investment with increased sperm competition risk when competition is numerical. There is ample correlational evidence for this relationship in a wide range of taxa. However, as with all correlations, this does not establish cause and effect. Nevertheless, there are no published experimental studies of the evolutionary influence of sperm competition on testis size. We report here on evolutionary responses of testis size to variation in sperm competition intensity in the yellow dung fly. Experimental flies were divided across two treatments, polyandrous or monogamous, with four replicates of each. There was a rapid evolutionary response in testis size resulting from selection via sperm competition, with larger testes found when sperm competition intensity was greatest. These results provide direct experimental evidence of evolutionary change consistent with macro‐evolutionary patterns found across a wide range of taxa.     

Sperm size of African cichlids in relation to sperm competition

We compared pairs of closely related taxa of cichlid fishesfrom Lake Tanganyika to examine the relationship between spermsize and the presumed intensity of sperm competition. In contrastto previous reports of relatively short sperm in polygamousfishes across a variety of taxa, we found that polygamous cichlidshad significantly longer sperm than their closest monogamousrelatives. In addition, sperm length was significantly relatedto relative testis size (controlling for body size and phylogeny).The site of fertilization may also be correlated with spermlength, as species that fertilize in the female's buccal cavityhad significantly shorter sperm than those that fertilizedeggs on the substrate. Assuming that relatively large testesand polygamous mating are indicative of more intense sperm competition, these results indicate that sperm length is related to the intensityof sperm competition in this clade of cichlids, as has beenfound previously in insects, birds, and mammals.     

Contrasting effects of large density changes on relative testes size in fluctuating populations of sympatric vole species

Across species, there is usually a positive relationship between sperm competition level and male reproductive effort on ejaculates, typically measured using relative testes size (RTS). Within populations, demographic and ecological processes may drastically alter the level of sperm competition and thus, potentially affect the evolution of testes size. Here, we use longitudinal records (across 38 years) from wild sympatric Fennoscandian populations of five species of voles to investigate whether RTS responds to natural fluctuations in population density, i.e. variation in sperm competition risk. We show that for some species RTS increases with density. However, our results also show that this relationship can be reversed in populations with large-scale between-year differences in density. Multiple mechanisms are suggested to explain the negative RTS–density relationship, including testes size response to density-dependent species interactions, an evolutionary response to sperm competition levels that is lagged when density fluctuations are over a certain threshold, or differing investment in pre- and post-copulatory competition at different densities. The results emphasize that our understanding of sperm competition in fluctuating environments is still very limited.     

Larger testes are associated with a higher level of polyandry, but a smaller ejaculate volume, across bushcricket species (Tettigoniidae)

While early models of ejaculate allocation predicted that both relative testes and ejaculate size should increase with sperm competition intensity across species, recent models predict that ejaculate size may actually decrease as testes size and sperm competition intensity increase, owing to the confounding effect of potential male mating rate. A recent study demonstrated that ejaculate volume decreased in relation to increased polyandry across bushcricket species, but testes mass was not measured. Here, we recorded testis mass for 21 bushcricket species, while ejaculate (ampulla) mass, nuptial gift mass, sperm number and polyandry data were largely obtained from the literature. Using phylogenetic-comparative analyses, we found that testis mass increased with the degree of polyandry, but decreased with increasing ejaculate mass. We found no significant relationship between testis mass and either sperm number or nuptial gift mass. While these results are consistent with recent models of ejaculate allocation, they could alternatively be driven by substances in the ejaculate that affect the degree of polyandry and/or by a trade-off between resources spent on testes mass versus non-sperm components of the ejaculate.     

Testicular Size,Mating System,and Maturation Schedules in Wild Anubis and Hamadryas Baboons

We report body mass and testicular size in 258 anubis (Papio anubis or P. hamadryas anubis) and 59 hamadryas (P. hamadryas or P. h. hamadryas) baboons, live-trapped in Ethiopia. As predicted by theories of sexual selection by sperm competition, among hamadryas baboons, which are monandrous, fully adult males have absolutely and relatively smaller testes than those of comparable males among anubis baboons, which are polyandrous. Male hamadryas are also ca. 10% smaller in bodily mass as adults. The intertaxonal difference in adults is due entirely to the fact that in male anubis baboons, testicular and bodily mass continue to grow up to full adulthood–the age at which most males emigrate from their natal troop and initiate a confrontational breeding strategy among unrelated animals. By contrast, male hamadryas baboons, which are usually philopatric, attain adult body mass and testicular size as subadults. In both species, juveniles experience rapid testicular growth peaking in rate at ca. 12kg body mass, but testicular descent and growth starts earlier in hamadryas than in anubis baboons. Juvenile hamadryas baboons have relatively larger testes than their anubis equivalents, perhaps because male philopatry allows the mating strategy of male hamadryas baboons to be initiated during juvenile life and therefore permits some sperm competition between juveniles and adults.     

Sexual selection and sperm competition in primates: What are male genitalia good for?

One hundred twenty-five years ago, in The Descent of Man and Selection in Relation to Sex,¹ Charles Darwin proposed the theory of sexual selection, as distinct from natural selection, to explain why, in some species, males have such magnificent ornaments and, in other species, such impressive weapons. He suggested two processes, which we now term female choice and male-male competition: either females choose particularly ornate males or, alternatively, relatively passive females accept the winner of fights among males. By now, knowledge of species in which the females are more brightly colored or aggressive than males has led to a more general formulation of the principle of sexual selection, in which, instead of “females”, we write “the sex with the lower potential reproductive rate”, and, instead of “male”, “the sex with the higher potential reproductive rate”.²     

Mammalian sperm and oviducts are sexually selected: evidence for co-evolution

Oviduct length was measured in 48 species representing 33 genera of mammals in order to examine possible relationships between female morphology and the occurrence of sperm competition due to matings with multiple males. Multiple regression analyses revealed that residuals of oviduct length were positively correlated both with residuals of testes weight and with sperm midpiece volume in the genera and species studied. These correlations remained significant after application of comparative analysis of independent contrasts to control for possible phylogenetic biases in the data set. These results indicate that sexual selection (relating to sperm competition and cryptic female choice) has influenced co-evolution of oviduct length, testes size and sperm morphology in mammals.     

Long lasting stickleback sperm; is ovarian fluid a key to success in fresh water?

Although the sperm of externally fertilizing fishes usually has a brief life span of up to a few minutes, this study showed that the sperm of the three-spined stickleback Gasterosteus aculeatus moved for several hours in brackish water and up to at least 10 h in the presence of ovarian fluid. Three-spined sticklebacks were able to spawn in waters ranging from full-strength sea water to fresh water, an ability unusual among fishes. The influence of salinity on sperm motility was examined, using three-spined sticklebacks from sea (salinity 30), brackish (5·5) and freshwater (0) populations. All three populations were found to have sperm with long motility periods in brackish water, lasting 165–270 min. Seawater three-spined sticklebacks had sperm motile for up to 65 min in sea water, whereas sperm from fresh- and brackish-water fish were quiescent in this medium. In fresh water, sperm from all three populations showed a very brief motility period, lasting <60 s. The presence of ovarian fluid, however, prolonged the motility period of sperm from both fresh- and brackish-water three-spined sticklebacks, for up to 7 and 10 h in fresh and brackish water, respectively, with some sperm found to be motile for up to 24 h. The results indicated that ovarian fluid created a favourable environment for the sperm and might have facilitated the three-spined sticklebacks' successful penetration of fresh water.     

Male reproductive effect of arsenic in mice

Arsenic, a known human carcinogen, was given to mice via drinking water as sodium arsenite at a dose 53.39, 133.47, 266.95 and 533.90 mol l for 35 days. A decrease in the activity of 17 HSD along with increase in LDH, GT activity were observed at 533.90 mol l. The observed sperm count, motility and morphological abnormalities in sperm were similar to control at lower dose levels. However at 533.90 mol l a significant decrease in sperm count and motility along with increase in abnormal sperm were noticed. Significant accumulation of arsenic in testes and accessory sex organs may be attributed to the arsenic binding to the tissues or greater cellular uptake. No effects were observed on indices studied for reproductive effects at 53.39 mol l arsenic close to which human being are exposed through drinking water under the present set of experimental conditions.     

Sperm competition mechanisms in birds: models and data

This study presents three models to explain the mechanism oflast male sperm precedence in birds. Because passive loss ofsperm from the female reproductive tract occurs, all modelsincorporate this process. The three models are passive spermloss alone, stratification with passive sperm loss, and displacementwith passive sperm loss. With two inseminations containing thesame number of sperm, the models make the following predictions.For passive sperm loss alone, (1) differential paternity ispositively and linearly related to the time interval betweeninseminations, (2) with a slope that is equal to rate of lossof sperm from the female reproductive tract, (3) with an interceptthat is the same as the differential fertilizing capacity betweenthe semen of the two inseminations, and (4) the ratio of offspringfrom two inseminations remains constant over time. For stratification,(1) the relationship between differential paternity and theinterval between inseminations is nonlinear and exhibits a "brokenstick" pattern, with a substantial first-insemination precedencefor short intervals, and (2) the proportion of offspring fatheredby the first insemination increases over time. For displacement,the relationship between differential paternity and the intervalbetween inseminations is nonlinear and also exhibits a "brokenstick" pattern, but in contrast to the stratification model,sperm from the last insemination have precedence. Data fromthree experimental studies of the domestic fowl and one forthe turkey provide the opportunity to test these models, albeitto different extents. The data from all studies are consistentwith the passive sperm-loss model, except that one aspect ofone data set provided ambiguous support for stratification.None of the data provided any support for the displacement model.