Regular readers of this blog will know that I’m very interested in animal behaviour. In recent weeks, I’ve written about a novel mode of visual communication in razorback suckers, a novel mechanism by which aquatic mammals can smell underwater, co-operative hunting between groupers and moray eels, and more. Animal behaviour is truly extraordinary, and shows that many of our distant evolutionary cousins have far greater cognitive capabilities than we normally give them credit for. The study of animal behaviour can also shed light on various aspects of human behaviour.
The February issue of The American Naturalist contains another fascinating story from the animal kingdom. Biologists the University of Wisconsin and Simon Fraser in British Columbia report that males of a particular fish species cannibalize their offspring if there is any possibility that the young fish were fathered by another male.
Telmatherina sarasinorum is a small and very colourful fish native to Indonesia’s Lake Matano. Like other fish, T. sarasinorum reproduce externally, with males and females simultaneously releasing sperm and eggs into the water. T. sarasinorum spawns throughout the year in shallow beach sites or in overhanging vegetation at flat offshore sites. During fertilization, so-called “sneaker” males swim into a position parallel to the spawning pair in an attempt to fertilize some of the female’s eggs. The mating behaviour of a male can change very quickly – an individual male may switch from spawning to sneaking within a matter of seconds.
The act of eating one’s own offspring is known as filial cannibalism. Cannibalistic parents can either consume their entire brood (total filial cannibalism), or just some of the eggs (partial filial cannibalism). There are a number of hypotheses explaining why filial cannibalism occurs in nature; it is generally assumed to be a trade-off between rearing the current brood and future reproductive success. Parents will expend much energy rearing their offspring, but the chances of all offspring surviving in a resource-poor habitat are probably small. Partial filial cannibalism therefore reduces the energy expended in rearing offspring, provides an additional food source for the cannibalizing parent, and increases the probability that the rest of the brood will survive.
Suzanne Gray and her colleagues observed the mating behaviour of T. sarasinorum, and found that the greater the number of sneaker males present during a spawning event, the more likely a male was to cannibalize his offspring. The authors speculate that males use the number of sneakers present as an indication of their relatedness to the brood. However, the assessment of the number of sneakers present during spawning is not always accurate, as males were occasionally observed to cannibalise their offspring in the absence of sneakers. Males were also seen to consume the eggs they had just fertilized if other males were doing the same.
An examination of the stomach contents of a number of specimens confirmed that males, but not females, consume their own offspring – conspecific eggs (that is, eggs of fish of the same species) were found in the stomachs of 11 out of 13 males, but none were found in the stomachs of females. T. sarasinorum males do not invest in offspring after fertilization, so, at least in this case, the cannibalism is not a trade-off between present and future reproductive success. So it seems that the only benefit of filial cannibalism to males of this species is the energy obtained from the eggs, which are more likely to be eaten when the male is less likely to have fertilized them.
Gray, S. M. et al. (2007). Cuckoldry incites cannibalism: Male fish turn to cannibalism when perceived certainty of paternity decreases. Am. Nat. 169: 258-263.
Lissåker, M. et al. (2003). Effects of a low oxygen environment on parental effort and filial cannibalism in the male sand goby, Pomatoschistus minutus. Behav. Ecol. 14: 374-381.