As Monty Python memorably taught us, sperm should never be squandered, abused or otherwise mismanaged. This is particularly sage advice for certain members of the animal kingdom for whom inefficient mating practices can lead to reproductive failure. However, in spite of this, it seems the bottletail squid has not got the message, as new research reveals that males of the species are surprisingly “imprudent” with their sperm investments.
Male bottletail squid (Sepiadarium austrinum) transfer their sex cells to females in prepackaged bundles called spermatophores, which then become depleted after mating. Since these can only be regenerated at a rate of about 4.5 per day, it makes sense for males to be picky about which females they mate with, only selecting those that are the most fertile.
As such, the researchers behind the study, which appeared in the journal Animal Behaviour, had expected male bottletail squid to show a preference for larger females, since these carry higher numbers of eggs. This attribute is advantageous for a number of reasons. For instance, as females carry spermatophores in the buccal cavity (mouth), they are vulnerable to being removed by rival males. However, the larger this cavity, the more spermatophores can be stored, reducing this vulnerability.
To determine how male bottletail squid invest their sperm, the researchers devised an experiment whereby each male was presented with two females, one after the other. In some cases, a small female was presented first, followed by a large female, while in other cases this order was reversed.
Although the study authors predicted that the males would be more willing to mate with the larger females and would transfer a higher proportion of the sperm to them than the smaller females, they were surprised to find “no evidence of strategic sperm allocation to larger females.”
Rather, the males were seen to mate with whichever female was presented to them, even when they were already in a sperm-depleted state. Furthermore, female size did not appear to have any effect on the proportion of spermatophores transferred by males during each copulation, with up to 60 percent being allocated to the first mate, regardless of size.
As a possible explanation for this liberal sperm expenditure, the authors propose that “low mating rates in the wild may overcome benefits of prudent male choice in this species.” Indeed, mating opportunities are few and far between for many male bottletail squid, due to intense competition for females. As a result, large numbers of males experience “zero reproductive success” in the wild, which is why it may be more beneficial for them to simply transfer as much of their sperm as possible when they get the chance, rather than discriminate in favor of “superior” females. In other words, beggars can’t be choosers.