ABOVE: Some researchers have hypothesized that in species such as Peromyscus maniculatus, hooks may help sperm aggregate and swim faster en masse than they could alone. Heidi Fisher

If you look at a mouse sperm cell through a microscope, as postdoctoral researcher Kristin Hook did hundreds of times while working in Heidi Fisher’s lab at the University of Maryland, College Park, you’ll likely see a striking, talon-like hook curving from its head. These sperm hooks are found not only in animals in the Peromyscus genus, which Hook studies, but across almost all mice—which is why it’s so surprising that scientists don’t know what they’re for.

Despite their singular purpose—fertilizing an egg—sperm are among the most diverse cell types on Earth. “Some have multiple hooks [while] some have multiple heads. . . . They have this huge amount of diversity of sizes and shapes,” Fisher, an evolutionary biologist, tells The Scientist. “The question is, why?” She says that Hook, now a biologist with the US Government Accountability Office, was an excellent person to have on the case—and not just because of her last name.

A clue about sperm hook function seemed to emerge about two decades ago, when a study in wood mice reported that hooks help sperm lock together—either at their heads or head to tail—to form clusters or trains that swim faster than individual cells do. This speediness might be advantageous in more-promiscuous mouse species, in which females mate with multiple males in quick succession, with particularly hooky sperm gaining a boost over sperm from other males.

But there was counterevidence, too. The sperm from some mouse species don’t aggregate. In fact, sperm aggregation is rare. Evolutionary biologist Renée Firman of the University of Western Australia, for example, has surveyed the sperm morphology of various Australian mouse species, including the sandy inland mouse (Pseudomys hermannsburgensis). In 2013, she published a study detailing that these mice have “elaborate sperm hook complexes,” Firman says, that contain three individual hooks—but the sperm don’t clump together. 

If you look at a mouse sperm cell through a microscope, you’ll likely see a striking, talon-like hook curving from its head.

Peromyscus is a great system to test hypotheses about sperm hooks, says Fisher. Closely related species in this genus vary in how promiscuous they are—some species are monogamous, while others have multiple mates—and only some species’ sperm group together, even though all of them have hooks. In one study, published online last fall, she, Hook, and PhD student David Webber found that sperm aggregations in polygamous Peromyscus mice swim faster than those in monogamous species, supporting the idea that aggregation has something to do with sperm competition.

To measure how sperm hooks influence aggregation, Hook and Fisher recorded videos of sperm from six species of Peromyscus, analyzing how the hooks’ length, width, and curvature affected the number of sperm in aggregates. The evidence, published in Cells, suggested that there was an optimum hook size for sperm clumping: while the structures were necessary for fast-moving sperm bundles to form, bigger hooks prevented sperm from grouping effectively. But the researchers also found that hook size showed no correlation with species’ promiscuity, leaving the link between hook size and sperm competition murky. 

One possibility, Hook and Fisher realized, was that hooks might influence not only aggregation, but an individual sperm’s swimming. And, thanks to a lucky break, the pair was recently able to test this idea. A few months ago, as Hook was taking videos of a sample of Peromyscus leucopus sperm in a petri dish to document their size and speed, she noticed that the sperm’s hooks were missing. She immediately ran next door to tell Fisher. Fisher was initially skeptical—it’s hard to see hooks on sperm as they’re darting around in a petri dish, she tells The Scientist. But Fisher took a look and saw, to her surprise, that the sperm were indeed hookless.

Fisher told Hook to move quickly and preserve the sperm, as well as every tissue in the mouse they came from, thinking they might have stumbled on something very rare. But just a few weeks later, Hook found another male with hookless sperm. Then another, and another. After they’d analyzed dozens of animals, the researchers realized that about 1 in 10 animals in their Peromyscus mouse colony don’t have hooks. 

Image of a sperm hook (<em>Peromyscus leucopus</em>)
HELP OR HINDRANCE? A study of sperm hooks in Peromyscus leucopus finds that larger hooks hinder the sperm’s swimming.
Kristin Hook

It was the perfect opportunity to test the hypothesis that hooks increase the speed of individual sperm, Fisher says. But after analyzing more than 30 animals, both with hooks and without, Hook found the opposite. “[The hook] seems to slow them down as single cells, which is interesting,” Fisher says. “Why is it in most sperm in most species if it inhibits sperm velocity?” The hooks also didn’t seem necessary for a male to reproduce—mice with hookless sperm were fertile, although they seemed to have lower sperm counts. Fisher says she suspects that the hookless sperm may have arisen spontaneously as a result of the mice being in captivity, and that hooklessness is not representative of P. leucopus as a whole.

Firman says that the results are consistent with her findings in Australian wild mice. “There’s no evidence [in the Cells study] to suggest that . . . the mouse hook has something to do with sperm competition,” she says. But Eduardo Roldan, an evolutionary biologist at the Spanish National Research Council in Madrid, says he still thinks there is a role for sperm hooks in sperm competition in some species. “My general . . . conclusion is that there may be differences between species or between groups of species as to how the hook serves either to aggregate sperm or influence swimming patterns, not only velocity but also the trajectory of the sperm,” he says. Even though bigger hooks can prevent sperm from forming aggregates, for example, some cross-species studies show that, compared to males in less-promiscuous species, males in more-promiscuous species produce sperm with larger hooks that curve more tightly, perhaps influencing swimming patterns. 

Fisher says she thinks that, instead of influencing aggregation or swimming speed, sperm hooks might have “something to do with the female reproductive tract or interactions with the egg.” The sperm, Fisher says, could be using their hook to snag onto the sides of the female reproductive tract, staying there longer so as not to expend too much energy as they swim up the tract and wait for fertilization. “Most of us in the field assume that [sperm] diversity has come from . . . diverse mechanisms or modes of fertilization . . . or the coevolution of sperm shape with different features of the female reproductive tract,” she says. She adds that Hook’s studies help bring the field one step closer to linking the “form and function” of sperm, giving insight into why sperm have the diversity that they do.