Cuckoo–hawk mimicry? An experimental test

We found clear differences in tit responses depending on the mounted species to which they were exposed. During the presentations of the two harmless controls, a familiar collared dove and a novel teal, the tits often continued to visit the feeders, and afterward their attendance returned to pre-exposure levels. By contrast, they avoided the feeders during and after both sparrowhawk and cuckoo presentations. The most striking result from experiment 1 was that the response was similar to sparrowhawks and cuckoos, even though cuckoos are of no threat to adult tits. 

The plumage manipulations in experiment 2 suggested that the strong alarm response to cuckoos depended on their resemblance to hawks because when their hawk-like underpart barring was obscured, the tits treated them as no more of a threat than doves. This supports the idea that the evolution of barring in parasitic cuckoos, revealed by the comparative analysis (Payne 1967; Kru¨ger et al. 2007), enhances their resemblance to hawks. 

However, underpart barring cannot be the only feature inducing an alarm response because the tits showed equally strong alarm to barred and unbarred hawks. Furthermore, little alarm was shown to barred doves. Therefore, the underpart barring must combine with other cuckoo features, for example, their grey upperparts and elongated wings and tail, to cause hawk resemblance. We found no significant effect of the specimen, which suggests that these results cannot be attributed to any peculiarities of the particular mounts we used. 

We also found no effect of the study site, so the tits on Wicken Fen, which would have experienced daily encounters with cuckoos during the previous summer, had equally strong responses to cuckoos as the tits in Cambridge and Madingley Wood, which were unlikely to have encountered cuckoos. This suggests that the strong effect of the cuckoo at both sites was not simply one of alarm to a novel stimulus. Tits attending rich food sources are especially vulnerable to attack because sparrowhawks learn that these are good locations for finding prey (Hinsley et al. 1995). 

Because sparrowhawks make surprise attacks (Newton 1986; Cresswell 1996), alarm to any hawk-like stimuli is likely to be adaptive, despite the loss of feeding time from frequent false alarms. Nevertheless, the 5 min exposure of the specimens gave the tits ample opportunity for close inspection, so it is remarkable that a cuckoo caused a strong alarm response, given that it lacks a hawk’s lethal weapons, namely talons and a hooked beak. If the inspection of a potentially dangerous predator is costly, then even a slight resemblance through shape, grey upperparts, and underpart barring may be sufficient to deter approach. 

Other studies have shown that mimics do not have to resemble the model perfectly to gain protection, especially when signal receivers regard the model as highly noxious or dangerous, or if the model is relatively common (Ruxton et al. 2004). Perhaps the tits’ response depends not only on the stimulus but also on the context; cuckoos are absent in winter so hawk-like stimuli at this time are more likely to be hawks. 

The motivation of the signal receiver (value of the resource it is exploiting) may also influence responses to models and potential mimics (Barnett et al. 2007; Cheney & Coˆ te´ 2007). For example, it may pay a more hungry tit to risk a closer inspection of hawk-like stimuli when there is the potential for the stimulus to be a harmless mimic. Previous work on egg discrimination has shown that both great and blue tits, like other species with no history of cuckoo parasitism, will accept eggs unlike their own. This suggests that the egg rejection exhibited by cuckoo hosts has evolved specifically in response to cuckoo parasitism (Davies & Brooke 1989; Moksnes et al. 1991). 

Our results here show that, at least in one context, great and blue tits respond to adult cuckoos as if they were hawks. This raises the possibility that the discrimination by cuckoo hosts of the adult cuckoo as an enemy distinct from hawks, which can be attacked (Moksnes et al. 1991; Duckworth 1991;Welbergen & Davies in press), is also an evolved response to cuckoo parasitism. 

Experiments have revealed that in response to brood parasitism, hosts pay closer attention to the features of their own eggs so they are better able to discriminate against foreign eggs (Rothstein 1982; Lotem et al. 1995). Similarly, hosts may pay closer attention to multiple features of hawks so they can better discriminate cuckoos. Further studies are now needed to test the features used by cuckoo hosts to distinguish cuckoos from hawks and to test whether, despite some host discrimination, parasitic cuckoos still gain from hawk resemblance. Just as host improvements in egg discrimination have been selected for better cuckoo egg mimicry (Brooke & Davies 1988), so perhaps have improvements in their plumage discrimination selected for better cuckoo–hawk mimicry. 

The study followed the guidelines for the treatment of animals in behavioral research and teaching (Association for the Study of Animal Behaviour). The mounted specimens were obtained from licensed taxidermists. We thank Chris Thorne and the Wicken Fen Group, Nancy Harrison, Julia Mackenzie, and Camilla Hinde for color-ringing tits; Jan Davies for making the barred/unbarred underparts for the mounts; John Parker, the director, for permission to work in the Cambridge University Botanic Garden; two anonymous referees for their helpful comments and the Natural Environment Research Council for funding.

N. B. Davies* and J. A. Welbergen

GIANT GROUND SLOTH

GIANT GROUND SLOTH Giant Ground Sloth—The human silhouette in this picture gives an idea of how huge these extinct sloths were. They could even rear up on their hind legs to reach lofty food. (Natural History Museum at Tring) Giant Ground Sloth—The ground sloths were perhaps the most impressive of all the extinct South American mammals. 

Scientific name: Megatherium americanum

Scientific classification:

Phylum: Chordata

Class: Mammalia

Order: Pilosa

Family: Megatheriidae

When did it become extinct? The last giant ground sloths are thought to have died out

around 8,000 to 10,000 years ago.

Where did it live? The giant ground sloths were found throughout South America.

The largest species (Megatherium americanum), the one depicted here, was about the same size as a fully grown elephant. South America is probably the most biodiverse landmass on earth, yet, many thousands of years ago, the fauna of this continent was even more remarkable. A perfect example of this long-gone South American fauna is a ground-dwelling sloth that was the same size as an elephant. This was the giant ground sloth, and it was an immense and unusual animal. 

Fully grown, the giant ground sloth was about 6 m long and estimated of its weight range between 4 and 5 tonnes. Several skeletons (real and copies) of this animal are to be found in museum collections around the world, and one of the most astonishing things about these remains is the size of the bones. The limb bones and their supporting structures are massive and give an impression of a heavy, powerful animal. In life, the digits of the animal were tipped with long claws, which may have been used to grab plant food or even as weapons. 

We know from the skeletons of this animal that the bones of the hind feet were arranged in a very peculiar way, making it impossible for the living animal to place its feet flat on the ground. The animal could certainly rear up onto its hind legs, and perhaps even manage to amble around in this posture, using its thick tail as a strong prop, but it had to shuffle around on the outside of its feet with the long claws pointing inward. The giant ground sloth may have been able to make better progress on all fours, possibly reserving its two-legged stance for feeding or defense. 

As the giant ground sloth is related to the living sloths, it was always assumed that they were gentle plant-eating animals, but some recent, controversial scientific research has shed some light on how this massive beast used its forelimbs. These studies suggest the forelimbs of a giant ground sloth were adapted for fast movement. Such an ability was of little use to a plant-nibbling animal that needed a strong, sustained pull to bring tasty leaf-bearing branches within reach of its mouth. The research suggests that the muscles of the forelimbs were used to power the large claws into other animals, and maybe not only in defense. 

The animal’s teeth also give intriguing insights into the way it fed. They are not the normal grinding blocks that are found in the mouths of plant-feeding mammals. Th ey and the jaws they sit in appear to be adapted for slicing, much like the jaws and teeth of meat-eating animals. The claws and teeth of this giant mammal have led some people to suggest that the giant ground sloth was not a plant feeder at all, but a scavenging animal that used its size to drive predatory animals from their kill before digging into the carcass. 

The image of a 5-tonne brute ambling over to a group of dire wolves, scaring them off, and then devouring their kill is quite fantastic. Regardless of this research, it is decidedly unlikely that this giant lived in this way, and like its living relatives, the giant ground sloth was probably a herbivore, but it may have been able to use its forelimbs and teeth to defend itself. As with almost all of the long-dead animals that once roamed South America, we cannot be certain what brought about the demise of the giant ground sloth. It has been speculated that the arrival of modern humans, with spears and arrows, led to their extinction, but it is reasonable to assume that there was something much more far-reaching happening at the time that wiped these animals out. 

Climate change is one of the usual suspects, and we know that the earth’s habitats were going through some massive changes at the time these animals went extinct. Global temperatures were changing, and land-dwelling animals everywhere were being affected. Hunting may have had an effect, but it may have been minor compared to the ravages of climate change. Today, there are still vast areas of South America where people rarely venture, and some people believe that a species of giant ground sloth may have somehow survived the events that wiped out its relatives and is alive and well in these remote areas. 

Local inhabitants call the beast the mapinguary, and it is said to rear up on its back legs and emit a foul-smelling odor from a gland in its abdomen—not only that but the creature is said to be impervious to bullets and arrows, thanks to some very tough skin on its belly and back. Without a specimen or an excellent photograph, it is difficult to take these stories seriously, but it is worth remembering that previously unknown species of mammal are discovered fairly regularly, and some of them are surprisingly large. If a live giant ground sloth was found today, it would be the zoological story of all time. 

• It is thought that there were around four species of giant ground sloth. The species mentioned here (Megatherium americanum) was by far the biggest. The closest living relatives of these extinct animals are the anteaters, armadillos, and tree sloths. The biggest of these, the giant anteater, would be dwarfed by even the smallest giant ground sloth.

• In 1895, a rancher by the name of Eberhardt found some hide in a cave in Patagonia that turned out to be giant ground sloth skin. The skin was in very good condition, and some people believed that it was from an animal that died relatively recently. When techniques became available to age the skin, it was found to be several thousand years old—it was just that the very dry conditions in the cave had prevented it from rotting. Interestingly, the mummified skin was studded with bony nodules, which probably gave the animal excellent protection from the teeth and claws of predators, and perhaps even the spears and arrows of early humans. 

• It would be fantastic if a species of giant ground sloth had somehow survived into the modern-day, but accounts of the mapinguary may be due to confusion with other animals or derived from folk memories of when humans encountered these animals thousands of years ago.

CATNIP (Nepeta cataria)

Labiatae (Lamiaceae; mint family)

Also known as Catmint, catnep, catrup, catwort, English catnip, field balm, nep, nip

DESCRIPTION

• Catnip, a hardy perennial that grows 0.6 to 1 m (2 to 3 feet) tall, is native from the eastern Mediterranean region to the western Himalayas, central Asia, southern Siberia, and China. Most cats love catnip and purr contentedly, tear delightedly, and roll in ecstasy on its crushed leaves. The generic name Nepeta comes from the Italian town Nepete, where catnip was once cultivated.

• Light green, scalloped, opposite leaves have heart-shaped bases, pointed tips, and velvety, grayish-white undersides. The edible leaves, which have a strong mint-like, warm, pungent, bitterish fragrance and flavor, grow in massed profusion before the plant flowers. After the blossoms appear, the leaves become more sparse.

• Catnip has erect, square, branching stems that are covered in soft hairs. The root becomes quite woody and branched with age. Each spring the root sends up an increasing number of new items, many of which are rather close together.

• Produces spikes of small whitish or pinkish, purple- or red-dotted flowers in midsummer.

• May be grown indoors for winter use.

• Catnip is an excellent honey plant.

CULTIVATION NOTES

• Catnip grows bushiest in well-drained, moderately rich soil, although it also grows well in dry, sandy soil. Add a light layer of compost to the top of the soil before planting. Recommended pH range is 4.9 to 7.5.

Culinary Herbs for Short-Season Gardeners

Thrives in partial shade, but can be grown in full sun.

• Grows easily from seed, which should be started indoors about 6 to 8 weeks before your last spring frost date. Sow seeds no more than 6 mm (¼ inch) deep. Seedlings usually emerge in 8 to 10 days.

• Space transplanted seedlings 30 cm (12 inches) apart.

• Can also be propagated by dividing the roots in the spring or fall, or from softwood or stem tip cuttings. Cuttings from young plants tend to root more quickly, often in just a week. Stem cuttings should be about 10 cm (4 inches) long. Grow rooted cuttings to about 15 cm (6 inches) in a moist medium before you transplant them to the garden.

• Catnip self-sows easily, so be prepared to remove unwanted plants. Weed as Required.

• For bushier plants, pinch flower buds as they appear.

• Usually pest-free, but susceptible to rust and root rot.

• Cats are the biggest problem confronting catnip gardeners. Give young plants a chance to get established by enclosing them in a sturdy chicken wire cage, which will protect them from enthusiastic felines. Cats are drawn to catnip only when the branches are broken or the leaves are bruised, thereby releasing the attractant chemicals, so if the plants aren’t damaged, cats will probably leave them alone.

• Overwinters outdoors up to zone 3.

• Indoor plants should be potted in moist, but not soggy soil that is supplemented with lime. Plants need at least 5 hours of direct sunlight daily. Prune as required, as plants are inclined to become scraggly.

HARVESTING NOTES

• Pick leaves for fresh use at any time throughout the summer, although the taste is milder if you pick the leaves before the plant flowers. Collect the leaves in the morning, after the dew has evaporated.

• To dry catnip, harvest complete stems, including the flowering head and the tender leaves. Cut stems about 5 cm (2 inches) from the ground, and hang upside down in a shady location. When dry, strip off the leaves, crumble them, and store in airtight jars out of the light.

CULINARY USES

• Catnip was a familiar herb in English kitchen gardens as far back as the 13th century. Catnip leaves were once used for rubbing meats before they were cooked, and were chopped and sprinkled into green salads. Snip a few leaves into your salads and see how you like it.

• Add fresh or dried leaves to soups, stews, and hearty sauces.

 

HARVESTING NOTES

• Pick leaves for fresh use at any time throughout the summer, although the taste is milder if you pick the leaves before the plant flowers. Collect the leaves in the morning, after the dew has evaporated.

• To dry catnip, harvest complete stems, including the flowering head and the tender leaves. Cut stems about 5 cm (2 inches) from the ground, and hang upside down in a shady location. When dry, strip off the leaves, crumble them, and store in airtight jars out of the light.

CULINARY USES

• Catnip was a familiar herb in English kitchen gardens as far back as the 13th century. Catnip leaves were once used for rubbing meats before they were cooked, and were chopped and sprinkled into green salads. Snip a few leaves into your salads and see how you like it.

• Add fresh or dried leaves to soups, stews, and hearty sauces. Make a refreshing, soothing cup of tea by pouring 250 mL (1 cup) of boiling water over

15 mL (3 teaspoons) of fresh leaves or 5 mL (1 teaspoon) of dried leaves. Alternatively, add dried catnip leaves, along with dried mint or dried lemon balm, to your favorite black tea.

CRAFT USES

• Sew cat toys and stuff them with uncrushed dry leaves for all your favorite felines.

MEDICINAL USES

• In traditional folk medicine, catnip was used to treat everything from cancer, insanity, nervousness, nightmares, scurvy, and tuberculosis, to colic, diarrhea, flatulence, hiccups, whooping cough, the common cold, measles, asthma, yellow fever, scarlet fever, smallpox, and jaundice. Catnip poultices were applied to hives, and to the sore breasts of nursing mothers.

• Catnip does have sedative qualities and is occasionally used in herbal medicine as a calmative and to treat insomnia.

• Catnip is not used in modern Western medicine.

CAUTIONS

• Catnip has some capacity to cause uterine contractions and stimulate menstruation, so you should avoid it if you are pregnant or suffering from menstrual disorders.

• While a cup of catnip tea is helpful if you don’t sleep well at night, the herb’s diuretic properties mean that your peaceful sleep may be disturbed by an urgent need to go to the bathroom.

• It has been said that catnip may be smoked like marijuana; however, there is no proof that this herb has the intoxicating effects of marijuana. Nevertheless, your suspicions may be justified if young people of your acquaintance seem unusually interested in your catnip plants.

• Bees like catnip, so make sure there are none in the flowers that you pick.

CULTIVARS AND RELATIVES

Here’s a cultivar you won’t have to fight over with your cats quite so much.

• Lemon catnip (N. cataria var. citriodora). Has an appealingly mild lemon aroma, which you may prefer over regular catnip Makes a delicious tea. When candied with egg white and sugar, the leaves make a refreshing after-dinner mint. In the belief that catnip roots made even the kindest person mean, early American hangmen used to eat the roots before executions to harden themselves for their work. While most cats are affected by catnip, not all felines are “nipaholics.” Apparently, a dominant gene is responsible for inheriting the euphoric response. It also appears that cats do not react in their customary delighted way to catnip until they are 3 months old.

Horned Turtle

Scientific name: Meiolania sp.

Scientific classification:

Phylum: Chordata

Class: Sauropsida

Order: Testudines

Family: Meiolaniidae

When did it become extinct? The last of these turtles are thought to have become extinct about 2,000 years ago.

Where did it live? The bones of these extinct turtles have been found on Lord Howe Island, 600 km from mainland Australia and the islands of New Caledonia.

Horned Turtle with their spiked heads and tails, the horned turtles are among the largest and most bizarre turtles ever to have lived. There would be very few people who would fail to recognize a turtle; such is the familiarity of these unusual reptiles. Although the fossil record is full of peculiar beasts, it has been said that the turtles are among the oldest vertebrates to have ever lived.

 Although their skeleton has the same bones like any other vertebrate, they are put together in a very different way. Their body is protected by a bony shell, which is, essentially, a hugely modified rib cage. The strength of this external carapace depends on the species, but it ranges from the leathery dome of the soft-shelled turtles to the almost impregnable shell of the giant tortoises. Also unique is the position of the hip and shoulder girdles, as they are found inside the rib cage. 

These animals are most familiar with being able to withdraw their heads and legs into the safe confines of their shells. The way they withdraw their head allows scientists to identify two groups of turtles: the cryptodires and the pleurodires. The latter is often called side-necked turtles because they bend their long necks into an S shape to keep their heads out of harm’s way. The turtles that people often keep as pets fall in the first group, the cryptodires, and these can pull their heads right into their shells by bending their necks below the spine. 

There’s no doubt that some turtles, especially the land-dwelling species, are very slow, lumbering creatures, characteristics that are often linked to evolutionary failure and poor adaptability. However, nothing could be farther from the truth for the turtles. These shelled reptiles are a successful group of animals that have been around since the Triassic—at least 215 million years (and probably considerably longer)—which makes them much older than the lizards and snakes. Not only are they ancient, but they are among the very few living reptiles that have become almost completely amphibious, only leaving the water to lay eggs (some species of snake also only leave the water to lay eggs). 

Today, there are around 300 turtle species, ranging from tiny, 8-cm tortoises all the way up to the oceangoing giant, the leatherback turtle (Dermochelys coriacea), which can be 3 m long and weigh 900 kg. Even though some truly bizarre turtles are still with us today, they pale in insignificance compared to an immense, land-living turtle that only became extinct in the last couple of thousand years. 

This was the horned turtle, and in life it must have been an astonishing animal. The horned turtle was around 2.5 m long, and it must have weighed in the region of 500 to 700 kg. By comparison, the largest living land-dwelling turtle is the Galápagos tortoise (Geochelonenigra) at about 300 kg and 1.2 m long. Imagine a horned turtle alongside a Galápagos tortoise and you get an idea of the size of this extinct beast. Not only was the horned turtle big, but it also had a very bizarre appearance. Sprouting from its skull were large horns and spikes, the longest of which grew from toward the back of the head and could reach a span of 60 cm. 

This formidable forward armory was combined with the typical tortoise carapace and a heavily protected tail that also sported spines. The horns of this extinct turtle made it impossible for the head to be pulled into the shell during times of danger. It is possible that these horns were used by the turtle to defend itself, but we don’t know what predators lurked on the islands where these extinct reptiles lived. Male giant turtles can be quite aggressive to one another during the breeding season, and maybe the extinct giant used its horns and tail spikes to fight other males for the right to mate. 

As with other island animals, the horned turtles may have grown to great size because there was very little in the way of threats in their isolated home terrain. Alternatively, great size is a simple yet effective defense against many predators. The truth is that we’ll never know the evolutionary force behind the incredible size and appearance of these turtles. What we can be surer of is their diet. Large land-dwelling turtles are slow, heavy animals, so fast-moving animal prey is out of the question. 

We know that the Galápagos tortoise and other terrestrial giant turtles are herbivores that eat a wide range of plant matter. The horned turtle was obviously unsuited to climbing trees or rearing up on its back legs to reach lofty vegetation, so it must have been dependent on the unique, low-growing plants that grow on New Caledonia and the surrounding islands. 

All living turtles lay eggs, and we can assume that the horned turtle was no different, but how it laid them and where will never be known for certain. Perhaps it excavated a pit before laying its eggs and forgetting about them. It is amazing to think that these giant, bizarre turtles roamed some of the isolated islands of the western Pacific into very recent geological times, but exactly why they died out is another mystery. 

We do know that island animals have suffered badly at the hands of humans, and we can be almost certain that the first thing to spring to the mind of the first human who saw these shelled giants was, “Can I eat it?” A slow-moving turtle, regardless of its size, is no match for humans and their various weapons. Lord Howe Island and New Caledonia are small areas of land, and they could never have supported large populations of such big animals; therefore it is very likely that when humans did discover the horned turtle, they wiped them out in a matter of centuries, or possibly even decades. 

• Apart from the way that living turtles bend their necks to hide their heads, we can divide them another way into three groups: there are marine forms, with legs modified into flippers, for example, the leatherback turtle; terrestrial forms, with thick, pillar-like legs, for example, the Galápagos tortoises; and semi-aquatic forms, for example, terrapins and snapping turtles. • 

Many of the living species of turtle may soon follow the horned giant to extinction as they are incredibly endangered. Some of the very rare species only survive in small populations on isolated islands, while the oceangoing species are at risk from fishing hooks, drift nets, and direct hunting. Without complete and active protection, it is very likely that some of the most amazing turtles could be extinct within 30 years. 

• As turtles lead such slow lives, they are among the most long-lived of all the vertebrates. The Galápagos tortoise can live to be at least 150 years old. One famous, long-lived radiated tortoise (Geochelone radiate) was presented to the Tongan royal family in 1777 by none other than Captain Cook. Known as Tu’iMalila, this tortoise died in 1965, at age 188. The longevity of an immense turtle like the horned giant can only be guessed. 

• Further back in the fossil record, in the age of the dinosaurs, there were other extinct turtles that were truly enormous. One of these, Archelon, is only known from 70-million year- old fossils. It was about 4 m long, and the span of its flippers was around 4.5 m. fully grown, Archelon probably weighed in the region of 2 to 3 tonnes. Its large head and powerful bite appear to be suited to eating shelled mollusks such as the extinct ammonites.