In the closing paragraph of on the origin of species Darwin famously said that nature was a war in which individuals struggle against each other and the environment for survival. However, while survival may be important from an individuals point of view, from an evolutionary perspective mere survival is not enough. Reproduction is what matters and success or failure at producing offspring is what determines an individual’s evolutionary success. Of course, survival is important too, but only when it leads to reproduction.
In most species the reproductive success of females is limited by the rate at which they can produce offspring. When a female is pregnant or carrying eggs she has no choice but to wait until she has given birth or laid her eggs before she can reproduce again, and this can take a long time. Males have no such constraints to their reproductive success and can potentially mate with hundreds of females over their lifetime and raise an enormous number of offspring. The only thing stopping them is that there just aren’t enough females to go around. This shortage of females coupled with the need to reproduce leads to intense, and often aggressive, competition among males for limited mating opportunities.
Male red deer (Cervus elephus) fight for their chance to mate by using their huge antlers to batter their rivals into submission, while male northern elephant seals (Mirounga angustirostris) grow to enormous sizes allowing them to dominate harems of many females and guard them against the advances of smaller, weaker males. Not all species are so aggressive in their tactics. Males of many bird species such as peacocks (Pavo cristatus) and birds of paradise produce fantastic and colourful displays with which they attempt to attract females, as do a large number of insects and fish. In these species, rather than fighting with each other, males try to out-perform and out-class each other in the hope that females will choose them while their rivals are left unwanted on the sidelines. This may seem a more peaceful strategy but make no mistake, although these males don’t actively fight each other the competition between them is every bit as intense as among more aggressive species.
So fighting or displaying are two ways in which males can improve their reproductive chances, but what happens in species in which each female mates with lots of different males in quick succession? How is a male to improve his odds of being the true genetic father of the offspring? Well, as is often the case evolution has found a way and that way is called sperm competition (yes, really).
In species in which females mate promiscuously males compete not just for mating opportunities but also for direct access to eggs. In these cases competition between males happens after mating has occurred as the sperm of multiple males compete with each other within the females reproductive tract as they race towards the eggs. In species in which sperm competition is known to exist an incredible variety of different sperm adaptations have been found, all of which serve to improve the sperms chances of reaching the eggs first.
For individuals of many species adaptation to sperm competition simply means producing more sperm so as to swamp the sperm of their rivals and increase the odds that some of their sperm will make it to the eggs before anyone elses. For other species adaptation to sperm competition is more complex. For example, the wood mouse, Apodemus sylvaticus, has evolved sperm that have a hook-like structure on the head which allows them to intertwine with one another to form long sperm ‘trains’ which are much faster at swimming than individual sperm.
In a similar and recently discovered case, a team led by Morgan Pearcy of the Université libre de Bruxelles looked for evidence of sperm competition in the desert ant, Cataglyphis savignyi. The queen ants of this species mate with up to 14 males in rapid succession and store their sperm jointly in a special storage organ called the spermatheca. Only those sperm which make it to this storage organ have any chance of fertilising an egg and so competition for access to the spermatheca is intense. In response to this pressure C. savignyi males have evolved highly cooperative sperm that team up into bundles of 50-100 cells which can swim much faster than they could alone and so are better able to outcompete their rivals.
It is not just the way sperm behave that can change due to sperm competition, the shape and function of sperm can change too. For example, Philip Byrne and his colleagues from the University of Western Australia found that in a group of Australian frogs those species under the most intense sperm competition produced sperm with the longest tails, possibly to improve their swimming speed. Other species known to have oddly shaped sperm include the water beetle Dytiscus marginalis which has sperm that fuse at the head into pairs with two tails, and the tiny fruit fly Drosophila bifurca which at 6cm long produces the longest sperm on earth.
Some species have taken a more sinister approach to sperm competition and have evolved infertile “parasperm” which contain enzymes capable of breaking down the sperm of rivals. A similar and fantastically named kamikaze sperm hypothesis has even been proposed for humans in which some sperm are adapted to kill the sperm of rivals rather than fertilise eggs. The evidence for this hypothesis is equivocal at best but given the adaptations that have been discovered in other species it is not entirely unbelievable. In fact, given the adaptations that have been discovered so far, almost nothing is completely unbelievable.
Sperm competition by producing large quantities of sperm
Moller, A. (1989). Ejaculate Quality, Testes Size and Sperm Production in Mammals Functional Ecology, 3 (1), 91-96 DOI: 10.2307/2389679
Sperm trains in the wood mouse
Moore H, Dvoráková K, Jenkins N, & Breed W (2002). Exceptional sperm cooperation in the wood mouse. Nature, 418 (6894), 174-7 PMID: 12110888
Cooperative sperm in the desert ant
Pearcy M, Delescaille N, Lybaert P, & Aron S (2014). Team swimming in ant spermatozoa. Biology letters, 10 (6) PMID: 24919705
Sperm competition in Australian frogs
Byrne PG, Simmons LW, & Roberts JD (2003). Sperm competition and the evolution of gamete morphology in frogs. Proceedings of the Royal Society B: Biological Sciences, 270 (1528), 2079-86 PMID: 14561298
The two tailed sperm of the water beetle
Mackie JB, & Walker MH (1974). A study of the conjugate sperm of the dytiscid water beetles Dytiscus marginalis and Colymbetes fuscus. Cell and tissue research, 148 (4), 505-19 PMID: 4836644
The world’s largest sperm in drosophila
Bjork A, Dallai R, & Pitnick S (2007). Adaptive modulation of sperm production rate in Drosophila bifurca, a species with giant sperm. Biology letters, 3 (5), 517-9 PMID: 17594959
Buckland-Nicks, J. (1998). Prosobranch parasperm: Sterile germ cells that promote paternity? Micron, 29 (4), 267-280 DOI: 10.1016/S0968-4328(97)00064-4
Baker, R., & Bellis, M. (1989). Elaboration of the Kamikaze Sperm Hypothesis: a reply to Harcourt Animal Behaviour, 37, 865-867 DOI: 10.1016/0003-3472(89)90074-2
Criticism of the kamikaze sperm hypothesis
Moore, H., Martin, M., & Birkhead, T. (1999). No evidence for killer sperm or other selective interactions between human spermatozoa in ejaculates of different males in vitro. Proceedings of the Royal Society B: Biological Sciences, 266 (1436), 2343-2350 DOI: 10.1098/rspb.1999.0929