Australasian Science: Australia's authority on science since 1938

The Rise of Zombie Tits

By Magdeline Lum

Scientists have discovered bat-eating birds and sea slugs with disposable penises.

Finnish wildlife photographer Lassi Kujala recently found a number of dead birds, including more than ten common redpolls (Carduelis flammea) and a yellowhammer (Emberiza citrinella). They had all been killed by two or three great tits (Parus major). This is not the first incident where this 14 cm-long bird weighing up to 21 grams has been reported to kill other birds.

For the most part, great tits feed on insects and seeds. For their size they are quite powerful, as their bill can be used to break hazelnuts and acorns. They can also use tools. In 1993, great tits were reported to use conifer needles to reach insect larvae hidden in bark.

In 2009, Peter Estock of the Max Planck Institute for Ornithology spent two winters observing great tits interacting with hibernating pipistrelle bats in Hungary. Estock and his team were following up on anecdotes of dead and injured bats found with wounds that were possibly inflicted by the great tit.

Estock observed several kills, and the bodies of the bats were picked clean enough to rule out the possibility that great tits were killing them for roosting space.

The great tits would fly slowly and systematically across the cave face listening out for bat noises. When a bat was located, it would be killed and feasted upon.

To further test whether the great tits were killing for food, Estock hung a feeder containing seeds and bacon outside the entrance of the cave. During 10 days when the feeder was present there was one kill , while 17 kills were observed when the feeder was absent.

The great tits would start eating the bats from the head, back or abdomen. The researchers noted: “Considerable parts of muscle mass had been pecked off these carcasses; on one, only skin and bones remained”.

This has earned the small songbird a nickname: the great zombie tit.

A Disposable Penis and a Couple of Spares

We live in a throwaway society where many items are taken for granted and are used once before being discarded. Penises are not disposable – unless you’re the nudibranch (or sea slug) Chromodoris reticulata, which discards its penis about 20 minutes after mating.

C. reticulata is a hermaphroditic shell-less mollusc found in waters around South-East Asia. During copulation, both partners simultaneously exchange sperm. After mating, the partners crawl away from each other.

Dr Ayami Sekizawa of Osaka City University noticed that after 20 minutes the still-swollen penis would fall off. “No other animal is known to repeatedly copulate using such ‘disposable penes’,” Japanese biologists wrote in the journal Biology Letters.

Researchers examined the male organs under a microscope to understand their function. They observed internal spiral structures that are believed to grow into a replacement penis. There were up to two internal spiral structures found.

“We propose that the tissue at the spiral part of the penis is compressed and undifferentiated, gradually differentiating into the ‘next penis’,” the team wrote.

It was also determined that a recently mated nudibranch would have a new penis and be ready for mating after a 24-hour period.

On closer inspection, the penis of C. reticulata was covered in backward-pointing spines that had entangled many sperm, suggesting that the spines are useful in removing the sperm of previous partners. The scientists surmised that while the spines were useful for reducing competition for their sperm, they would also make it difficult to retract the penis and hence it is easier to discard it.

“Chromodoris reticulata may compensate for the short-term cost of decreased reproductive opportunities caused by the loss of a penis with the reproductive advantage gained by sperm displacement,” wrote the study authors.