Certain females consume male ejaculate and sperm as if they were food, using the nutrients to fuel their own bodies as well as their eggs, according to new research.
The study, published in the journal Biology Letters, adds yet another dimension to the battle between the sexes.
"If males have their sperm consumed, rather than used for egg fertilization, they will lose that reproductive opportunity. Therefore, it is in the male's best interests to try to ensure at least some of his sperm reaches the female’s eggs," lead author Benjamin Wegener, a researcher at Monash University’s School of Biological Sciences, explained to Discovery News.
Wegener said that ejaculate consumption is well-documented among numerous species. Humans are included in that group, but the behavior is not a standard part of our reproductive process.
According to Columbia Health, human male ejaculate contains fructose sugar, water, ascorbic acid (a.k.a. Vitamin C), citric acid, enzymes, protein, zinc and more. It reads like the list of ingredients for a protein-infused sports drink.
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Sperm consumption — as opposed to just ejaculate swallowing — in the animal kingdom "is far less common," according to Wegener. Species that exhibit this include carrion flies, picture wing flies, a strange marine invertebrate known as Spadella cephaloptera, a type of leech, a marine nudibranch and the southern bottletail squid (Sepiadarium austrinum).
Humans again may swallow sperm, but it's not standard behavior during reproduction.
It appears to happen a lot among squid, the focus of the new study. Wegener and his team discovered the behavior and tracked how the nutrients were utilized after consumption. It is the first time that the phenomenon has been observed in a female with external fertilization.
"This is an important distinction, as even if the female consumes some of the ejaculate in those internal fertilizers, at least some of the sperm remains inside in the reproductive tract," he said. "For an external fertilizer with short-term sperm storage, if the female doesn't lay eggs in time, the male loses his chance to fertilize the eggs."
To help combat this problem, squid sperm and the sperm of many other animals may contain manipulative compounds that stimulate female reproduction. So far, more than 80 proteins have been identified in other types of sperm that could do the following: decrease female receptivity to further mating, encourage her to lay eggs sooner, stimulate ovulation and egg production, and affect how long females store sperm and affect egg fertility.
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Females, on the other hand, can control whether or not they will consume the sperm or ejaculate.
This calls to mind numerous questions, such as whether females sometimes use males as a food source, whether females sample sperm to determine its quality, and whether they eat it to allow other sperm to fertilize eggs.
Tom Tregenza, a professor of evolutionary ecology at the University of Exeter and director of research for CLES Cornwall, told Discovery News that it’s been known for some time that insects get nutritional benefits from eating male sperm packets (squid, certain insects and other species encase sperm in a membrane-sealed, spring-loaded package). He said that "this finding of exactly the same sort of thing having evolved completely independently in such a distantly related group is really fascinating."
He agrees that the behavior can put pressure on the males, which have to balance providing enough sperm for fertilization, but perhaps not so much that females start to rely upon it as a regular "tempting meal."
"As the authors point out," Tregenza added, "she might even choose to eat the sperm packets from less attractive males and use the sperm from more attractive ones for fertilizing her eggs."
Copyright 2013 Discovery Communications LLC.