Sperm head morphology is associated with sperm swimming speed: A comparative study of songbirds using electron microscopy

Abstract

Sperm exhibit extraordinary levels of morphological diversification across the animal kingdom. In songbirds, sperm have a helically shaped head incorporating a distinct acrosomal membrane or "helical keel," the form and extent of which varies across species. The functional significance of this helical shape, however, remains unknown. Using scanning electron microscopy, we quantified inter- and intraspecific variation in sperm head morphology across 36 songbird species (Passeriformes: Passerida). Using phylogenetic comparative methods, we investigated the relationship between sperm head morphology and both sperm swimming speed and the frequency of extra-pair young (EPY). We found that species whose sperm had a relatively more pronounced helical form (i.e., long acrosome, short nucleus, wide helical membrane, and a more pronounced waveform along the sperm head "core") had faster-swimming sperm. We found no evidence of a relationship between interspecific variation in sperm head morphology and EPY, although we did find that among- and within-male variation in sperm head traits were negatively correlated with EPY. Applying principles of fluid mechanics, we discuss how the helical form of the sperm head may influence swimming speed, and suggest that further studies considering aspects of sperm morphology beyond sperm length are needed to improve our understanding of sperm structure-function relationships.

Sperm head abnormalities are associated with excessive omega‐6 fatty acids in two finch species feeding on sunflower seeds

“…Both species are frequent visitors to sunflower feeders, and they tend to include sunflowers as a substantial part of their diet (Brittingham and Temple 1992). Moreover, greenfinch populations are declining in the UK (Harris et al ), in Norway (Falkenberg et al ), and in the Czech Republic (Reif et al ), and the decline in the UK has been linked to disease spreading through their use of feeders (in particular trichomonosis, Robinson et al ). As such, investigating further adverse effects from supplementary feeding on this species is particularly important, as it could help inform potential conservation actions and ensure that provided foods improve the situation of urban greenfinches rather than contributing to further decline.…”

“…Additionally, the nutritional composition of provided foods can negatively influence body condition and reproductive success of wild birds (e.g. smaller offspring and reduced fledgling success, Plummer et al ). Households often provide large quantities of foods that would not be found abundantly in the local environment, for example bread, peanuts or sunflower seeds (Jones and Reynolds , Støstad et al ), though the amount and type of food provided can differ between urban and rural areas (Tryjanowski et al ).…”

“…Providing sunflower seeds for wild birds has become increasingly popular during the last few decades; Plummer et al () estimate that the amount of sunflower seeds fed in British gardens (by participants of the British Trust for Ornithology's ‘Garden Bird Watch’) has more than doubled between and . In Norway, there has been a gradual increase in the annual import of sunflower seeds intended for animal feeding from zero in 1993 to nearly 8 million kg in , with an annual import value of 6 million USD in (Statistics Norway ). Although it is difficult to know the exact historical extent of supplementary feeding of sunflower seeds in the Western world, it appears to have grown from almost nothing to a major food source for urban birds in a relatively short period of time.…”

“…In captive bird populations, the effect of dietary PUFA content on sperm performance appears to be variable, and depends on age and levels of other nutrients such as vitamin E (Cerolini et al , Khatibjoo et al ). Diets enriched with ω‐3 PUFAs have been associated with increased fertility (Blesbois et al , Feng et al ) and larger semen volume (Surai et al 2000) compared to ω‐6 rich diets; although one study showed that an ω‐3‐rich diet led to a slight decrease in sperm motility yet an increase in fertility (Kelso et al ), and other studies have found no difference in sperm motility (Cerolini et al , Khatibjoo et al ).…”

“…In captive bird populations, the effect of dietary PUFA content on sperm performance appears to be variable, and depends on age and levels of other nutrients such as vitamin E (Cerolini et al , Khatibjoo et al ). Diets enriched with ω‐3 PUFAs have been associated with increased fertility (Blesbois et al , Feng et al ) and larger semen volume (Surai et al 2000) compared to ω‐6 rich diets; although one study showed that an ω‐3‐rich diet led to a slight decrease in sperm motility yet an increase in fertility (Kelso et al ), and other studies have found no difference in sperm motility (Cerolini et al , Khatibjoo et al ). However, these are captive birds on strictly controlled diets, and to our knowledge there are no studies on the impact of the nutritional content of wild bird diets on sperm quality; though sperm motility and swimming speed are negatively associated with oxidative stress in the ejaculate in wild great tits (Helfenstein et al ) and with circulating levels of oxidative stress in wild barn swallows Hirundo rustica (Bonisoli‐Alquati et al ).…”