Mating is costly for both males and females, but in this example we will consider male mating costs because of the competition for females. With intense competition males can benefit by becoming more choosy over whom they mate and how much effort they invest. In examining the male effort, scientists are often interested in sperm transfer, but quantifying ejaculate is difficult, so copulation duration is commonly used as a proxy to measure sperm transfer. Longer duration equals more sperm transferred. While this has been found to be true in some systems in others it remains untested and therefore may not be universal.
Photo credit: Atsushi Ono
In the paper by R. Lange et al., 2012 from Animal Behavior, the authors test this assumption for the simultaneous hermaphroditic sea slug Chelidonura sandrana. These sea slugs often mate in aggregates and can only assume one role at a time being either male or female during the mating. After completing copulation in one role they often switch to the other role. Previous research established from Anthes et al., in 2006 suggested that individuals mate longer in the male sex role with partners that were on the large side or had not recently mated which is suggestive of a positive relationship between copulation duration and sperm transfer.
To test whether or not copulation duration is in fact a reliable proxy for sperm transfer the authors used two types of assays. The first incorporated specific time intervals and the second allowed uninterrupted copulation. The sea slugs have a sperm duct that stop the sperm from entering the water and the authors cauterized this duct. In this manner, the sperm would be released into the water and could be subsequently measured.
In the first assays, sea slugs were allowed to mate for 2, 4, 6, 8 or 10 minutes. Sea water and ejaculate were collected and counted upon termination. In the second type of assays, they were allowed to mate without interruption and upon their termination the time was recorded and water and ejaculate collected.
Analysis of results suggest there is no effect of sperm donor or receiver body size on sperm contribution. A clear increase of sperm number was detected between 2 and 4 minutes, but after 4 minutes no further increase was noted. The average uninterrupted mating lasted about 9 minutes with the range from around 4-15 minutes.
In short, sperm transfer is actually only taking place in the first 4 minutes of copulation, so why do some of the matings last well beyond that time? This time may allow for the transfer of other non-sperm ejaculate components like seminal fluid to aid in sperm competition or nuptial gifts to increase the receivers reproductive physiology. This time period after sperm transfer may also be some sort of mate guarding so that others in the male role do not have access. Exploration of these possibilities may yet find support for a positive relationship existing between male mating effort and the copulation duration.
In the future, such studies could take this last point a step further by measuring fitness and paternity.
