Cannibalism as a mechanism to offset reproductive costs in three-spined sticklebacks

Why I highlight this preprint:

Cannibalism is prevalent in many different species, and I found it interesting that this paper delved deep into understanding the complexity of this phenomenon and why it might be advantageous. This study was meticulous in investigating different factors that might contribute to cannibalism, and I think it provides a lot of new insights.

Background:

Parents invest a significant amount of effort in caring for their offspring by protecting and feeding them, often at the cost of their own survival and ability to reproduce in the future. Parents may offset these costs by cannibalizing some of their brood, which gives them more energy to care for future offspring while improving the success rate of the survivors. As these offspring are eaten rather than abandoned, scientists predict that the parents recover energy with this strategy.
However, previous studies have indicated that this cannibalism may occur regardless of food availability or health. This inspired the authors of the preprint highlighted here to investigate other variables that may influence cannibalism. This study examined two different stickleback populations from Alaska and looked at factors that may increase cannibalism rates such as parentage, brood size, life stage of the offspring, and the physical health of the parents. The three-spined stickleback was chosen because in this case only the males take care of the offspring and the cost of rearing offspring is high. As a result, they eat a large percentage of early-life stage offspring. It is unclear, however, whether they eat their own embryos, ones that may belong to other males (that sneak fertilize eggs) or steal from other nests.

Approach:

The research team devised four different tests to investigate cannibalism among male three-spined sticklebacks. The first test was to compare males with and without offspring to determine if the parenting males cannibalized more due to having more embryos to eat. Next, they checked to see if this behavior helped recoup energy to care for the rest of their nest, with larger males and those with lower health cannibalizing more due to higher energetic demand. The third test was to confirm if males prioritized consuming lower-quality offspring, specifically younger ones with lower reproductive value, and ate more if they had larger broods. Finally, they tested whether the parents were eating individuals that had been fertilized by other males, with the assumption that parents offset the costs of cannibalism through these strategies.

Key findings:

1 Males cannibalized more when they were caring for embryos in their nest compared to males with no offspring

Cannibalism rates were high in parenting males of both stickleback populations, with 72% observed in the Spirit Lake population and 85% in Watson Lake (Fig. 1). When compared to the males without any offspring, these rates were much lower, with 13% in Spirit Lake and 17% in Watson Lake (Fig. 1).

Figure 1. Cannibalism rates were higher in parenting compared to non-parenting males and similar in both populations. The proportion of males that had cannibalized at least one embryo for a) parenting males (n = 18 Spirit Lake, n = 20 Watson Lake) and b) non-parenting males (n = 16 Spirit Lake, n = 6 Watson Lake). Significance of comparisons between groups was determined using Fisher’s exact tests. Figure was made available under a CC-BY-NC-ND 4.0 International license.

2 Cannibalism rates were higher in males with larger broods, but were not related to the size or health of the parents

The number of offspring eaten was not related to the size or physical condition of the parents. Their life stage also made no difference in whether they were eaten or not, suggesting that parents do not selectively eat lower quality embryos. Brood size did impact how many embryos the parents ate, with larger brood sizes leading to higher cannibalism rates. Although this does support the hypothesis that parents may indeed prioritize consuming lower quality embryos, the higher rates may also be due to there being more embryos to consume.

3 Brood size was related to the size of the males, but not their health or population

Although the males with larger nests ate more embryos compared to those with smaller nests, the amount was proportional to the size of the nest, giving an advantage to embryos in larger nests due to a lower chance of being eaten.

4 Males often ate the embryos of other parents, with different rates depending on the population

Males in both populations ate a high percentage of embryos that were not related to them, with only 16% matching in the Spirit Lake group and 46.5% in Watson lake.

Questions for the authors:

1. Cannibalism rates were high when using the sampling method that measures embryos eaten in a single day Is it possible that the males consumed embryos at different rates depending on the day? It would be interesting to see how they determine how many embryos to eat day to day, and if they tend to eat more (or less) over time as the individuals develop.
2. You observed that some males with empty nests had embryos in their stomach, indicating that either they had cannibalized their entire nest or had eaten embryos from another nest. If they had eaten their entire brood, are there any ideas on why they might do so based on your other results?

Tags: fish

doi: https://doi.org/10.1242/prelights.42905

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