Scientists with the Natural History Museum of Los Angeles County and the University of Southern California have found evidence that provides support for a long-held but largely unseen evolutionary theory of mating strategies.
The idea is that males may face a trade-off between evolutionary investment in traits that would make them better able to secure mates physically and ones that increase their sperm production, thus making it more likely their genetic material will fertilize eggs.
The scientists were able to find this tradeoff in certain marine mammals, including species of sperm whales, narwhals, beaked whales and humpback whales. Those animals each have physical traits, such as increased size or natural combat weaponry, that improve their ability to compete for females prior to mating. Each also has relatively small testes for their body size. Testes size correlates strongly to the production of sperm.
"I think it's a very clear trend, at least in this group," said Jim Dines, who co-led the study and manages the mammalogy collection at the Natural History Museum.
Dines said he began his study by examining the collection housed at the museum. He and his collaborators also scoured scientific literature for information about the mating strategies of cetaceans, a classification that includes whales and dolphins. Dines said the information gleaned was sparse because of the difficulty of studying animals in the ocean.
"Although we're getting better at it, it's really hard to observe them. So many of these species have never been observed mating, let alone engaging in these combat situations," Dines said.
The team was able to identify that the tradeoff of physical armament for reduced testes size occurred in cetaceans that engage in physical competition for access to mates, but only when the species typically groups multiple females in protected harems.
"To get that payoff, you really need to be in a situation where you are monopolizing access to females," Dines said. "When a male is able to monopolize access to females, then it's the combat situation that we usually see — the investment in precopulatory traits, like things that help in combat."
The study's authors note that much is still unknown about the mating habits of cetaceans and that other factors could play a role in shaping reproductive strategies. However, they conclude in their study that the correlation they found could aid researchers in predicting the behaviors of other cetaceans.
Dines said it's also exciting to find instances of tradeoffs between precopulatory and postcopulatory development in nature.
"There are very few examples of this theory being borne out, and just by having this unique data set of cetaceans, we were able to show that," Dines said.