International Journal of

Introduction
 
Sperm is a highly specialized cell for delivering DNA from male to the ovum. Sperm cells are produced in testis by a dynamic process known as “spermatogenesis”, this process show great similarity among organisms (1). However, there are extensive variations in sperm morphology across species. Incredibly, wide degree of diversity in sperm morphology in their basic structures i.e. head, middle piece and tail is found across species. Differences in terms of overall size of the sperm, shape and number of sperm produced are also incredible.
One of the keys to this variations or diversity in sperm may be associated with female reproductive tract, sperm competition, testicular size and sperm size and number. The evolution of sperm in sexual method of reproduction had shown complex developmental processes, leading to extreme species variation in sperm morphology (2-4). Establishing a correlation between sperm morphology and factors influencing them is a phenomenal task. A general adaptive explanation for this diversity is lacking. In this mini-review these associations and the anatomical and functional adaptations among different species that have evolved to optimize fertilization success are discussed.
 
Influence of female reproductive tract
The expectation is justified that sperm morphology should be influenced by female reproductive anatomy and/or the fertilization environment (5, 6). The sperm phenotype comes from enigmatic female choice (7-9). The sperm must operate inside the females' reproductive tract hence; the female reproductive tract can have important effects on sperm performance. The potential importance of female-derived effects in mammals, birds, and insects showing that sperm length is positively associated with the length of the sperm storage organs of females (spermatheca) (3).
Thus, sperm length appears to respond positively or negatively to evolutionary increases in female reproductive tract dimensions (10, 11). Reports also suggest that smaller the sperm more may be the advantage in terms of numbers in competing for fertilization. Across the species the sperm of internally fertilizing species are generally longer than those of externally fertilizing species, where the influence of female reproductive tract is minimal or nil (3, 12).
The variation in survival of sperm in female tract is tremendous across species. Sperm of rat survives for few hours in female tract and in human and monkey sperm survives for few days (13-15). Sperm survival in female reproductive tracts could extent to months in bats or even years in some fishes and reptiles (16-18). This shows the influence of female reproductive tract on sperm survival and their successes; here the role of sperm or middle piece length (mitochondria volume) seems to be negligible. It appears that rationalize understanding of sperm-female co-evolution is challenging.
 
Sperm competition
Sperm competition (sperm from different males compete for fertilization) in polyandry (females mate with more than one male) is recognized as a potent evolutionary force that has led to sexually selected adaptations (9, 19-21). These adaptations either enhance a male’s own sperm competitiveness, or eliminate rival sperm and the whole organism (9). Such adaptations can cause sexually selected sperm with winning architectures over rival sperm (22). Basically, long tail sperm were seen in lower animals than highly evolved species.
There are several theories regarding the evolution of sperm size and sperm competition favoring males with longer sperm, due to their enhanced swimming velocity and therefore competitiveness (23). Studies have reported that sperm cells are on average longer in polyandrous species compared to monandrous species (female mating with single male in a season) (10, 24). The evolutionary explanation for finding adaptive reason that how longer sperm might be more competitive than shorter ones requires inter-disciplinary re-evaluation of accumulated knowledge and input from studies involving fields like physics, biomechanics, hydrodynamics etc. which seems to be mandatory to understand the link between sperm form (length) and function (velocity) and how the selection acts (25).
 
Testicular size
Literature indicates relationship between sperm competition and the relative size of the testes. The adaptation of sperm competition and the relative size of the testes of an animal were identified across many taxa such as primates, mammals, birds, frogs, bats and fishes (16, 26-31). There is strong evidence that taxa whose mating pattern generates higher levels of sperm competition have evolved relatively larger testes. Thus the risk of sperm competition in which a male is likely to emerge as an adult develop larger testes and produce bigger ejaculates. However, from the literatures it seems that there is no causal relationship between sperm size or length and testicular size (32).
 
Sperm size and number
There is an enormous diversity in shape of the sperm in animal kingdom; such as giant sperm, even longer than the animal itself, as seen in fruit fly (Drosophila); where the fly is just half a centimeter long and its sperm length is up to 6centimetres.This indicates that sperm competition and enigmatic female choice lead to decreased sperm number by favoring longer sperm (6). In this case, as the sperm become less abundant, ova become relatively abundant, and competition between males for fertilization success is predicted to weaken, therefore, is expected to be self-limiting (33).
The presence of multi-flagellate sperm and sperm that lack flagella entirely are another form of diversity (34, 35). It appears that potentially a flagellate sperm can be produced in more number and are less costly to produce, both in terms of energy and time when compare to multi-flagellated sperm. Moreover, selection may therefore favour sperm that lost flagellum in monandrous taxa, where a single male available for a group of females, hence, the competition for sperm was absent and where sexual selection is not a factor.
Coming to the mammalian sperm which are characteristically tiny, but vary in length, starting from 28 μm in the porcupine Hystrixafricaeaustralis to 349 μm in the honey possum Tarsipesrostratus and the average length of sperm in human, monkey and rat are 60 μm, 70 μm and 160 μm, respectively (36). Wide spectrum of variation in size and number do show mild relation to the presence or absence of sexual selection for given species.
 
Conclusion
 
Concluding remarks
Nevertheless, explaining these morphological diversities in sperm cells is a challenging question and it seems that evolutionary biologists have only recently engaged in exploring its links and patterns. From the literatures it seems that there is no causal relationship between sperm size and testicular size, however, the accumulated knowledge do indicates evolution of sperm morphology across species has some associations with female reproductive tract, sperm competition and sperm size and number, however interpreting these results for phylogentic correlations should be approached with caution. Further studies with multi-disciplinary approach would through more light on this science. Collective evolutionary adaptability of the male gametes in an organism determines the survival of species and then they go for a fight among themselves for successful coitus.