Thought Experiments: Picky Eaters

Observation: The cannibal male travelled >40 m to grab a pup.

To the two members of our team who focused their PhD research on male grey seal behaviour, this observation stuck out in the pattern of the cannibalism events as somewhat unusual.

Male grey seals, the original couch potatoes, are known to rest for up to 80% of the day and tend to remain within a focused area during the breeding season. While seals’ fusiform (spindle shaped) morphology allows them to glide easily through the water—undulating 250-300kg across rocky terrain is not exactly efficient. Resting for a large proportion of the day therefore helps them minimize energy usage while they are fasting. Also, neighboring males enforce loose ‘territories’ through aggressive behaviours and threats; so a male travelling over 40m to pick a pup would be risking costly interactions with rivals, energy expenditure, and possibly losing his place (and his breeding opportunities!) altogether.

Two males exchanging open mouth threats.

Two males exchanging open mouth threats.

So why then, we wondered, would a male bother to travel that distance, passing plenty of weaned pups along the way, to pick a target and then drag it all the way back?

From previous studies observing and mapping individuals across time, we know that some male grey seals wander around the outskirts of the colony trying to mate with females as they leave for sea, or engaging in ‘sneaky copulations’ at night. Other males establish their location among breeding females and stay there for 5-40 days! Some of these ‘Tenured males’ are known to return to the approximately same spot in subsequent years. So perhaps, if the cannibal male had been on the island in 2013 and mated, the pups around his location in 2014 might be his own offspring. Could he be moving away to avoid killing his own progeny?

Two males holding their positions amongst a group of females, while another male sleeps behind them; perhaps waiting for the opportune moment.

A male sleeps on his side while holding a position among a group of females, while another male sleeps behind them; perhaps waiting for the opportune moment.

There is already some evidence suggesting pinnipeds can differentiate between relatives and non-relatives. We know that females have mechanisms for distinguishing their pups from those of other seals’. Female sea lions and fur seals go to sea for days to forage, and then are able to find their pup in the mess of the colony upon return. Some studies have suggested this is an olfactory (scent) signal, while others suggest recognition of the pup’s vocalization. There is even evidence that female Antarctic fur seals select mates that are unrelated. Do male seals have a similar mechanism for recognizing relatives?

A female grey seal with her newly born pup.

A female grey seal with her newly born pup.

Because males aren’t routinely monitored, we have no records to say for certain if the cannibal male was on the Isle of May or sired any pups the year before our observation. However,  the evidence from these other studies suggests our hypothesis at least makes biological sense.

So, now that we have a hypothesis: that the male travelled that distance to avoid his offspring, how would you test that?  If we had a DNA sample from the cannibal male we could run a paternity test on all the cannibalized pups to see if the pups eaten were offspring, but it would also be important to know the paternity of the live pups that he left alone nearby. Since we didn’t do this at the time it would be hard to retrospectively test this hypothesis. Can you think of any other experiments or tests that might be used to explore this hypothesis in the future?

We’ve discussed the hypothesis that the male might be travelling to avoid his offspring, but this isn’t the only plausible explanation for the observation. Perhaps the male travelled away from his pool to look for females to mate with, and on the way back just picked a pup at random. Can you think of any other explanations for the male’s behaviour?



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