Insectivorous Vertebrates (Insects)

Insects are a part of the diet of a vast array of animals. Indeed, there are almost no animal groups that do not include some representatives that consume insects, incidentally or intentionally, actively or accidentally. Even obligate herbivores consume insects,although never in any large number. Several groups of animals are insectivorous as juveniles and shift dietary focus as adults, whereas other groups become insectivorous only as adults.

DIVERSITY OF INSECTIVORY

A simple list of the insectivorous terrestrial animals would be lengthy and include many freshwater fishes, most frogs and salamanders, lizards and snakes, and birds and mammals. Because marine ecosystems do not include many insects, the incidence of insectivory among marine invertebrates and vertebrates is much less common.
In freshwater fishes, insects are ubiquitous and widely consumed. In streams, for example, 70-90% of the macroinvertebrates are insects, comprising as much as 99% of the numbers of individual organisms and 99% of the biomass. Fishes take advantage of these abundant resources and consume insects from all of the 13 orders of insects with aquatic life stages.
Terrestrial salamanders and almost all frogs, which are often filter feeders or herbivores as larvae, become predominantly insectivorous as adults. Frogs and salamanders that remain aquatic, or become so secondarily, possess a wide array of dietary choices that can include other aquatic life, including (but not limited to) insect adults or larvae.
Lizards, more so than snakes, include insects in the diet. Consumption of insects also occurs in other reptiles such as turtles and juvenile crocodilians. Insect prey selected by lizards is somewhat size dependent: smaller lizards consume more small insects, whereas larger lizards can also consume larger insects. Some lizards that are insectivores as juveniles become more herbivorous as adults. In snakes, smaller insectivores become more carnivorous as they get larger, focusing on other vertebrate prey, especially mammals, frogs, and other snakes. Among some groups of lizards and snakes, specialization for insectivory is a familiar pattern; in these instances, ants and termites are most frequently consumed.
There are many birds that consume insects as a dominant part of the diet. Some of these insectivorous lineages include pipits and wagtails (Motacillidae), bulbuls and allies (Campephagidae, Pycnonotidae, Chloropseidae), waxwings and allies (Ptilogonatidae, Bombycillidae, Dulidae), dippers (Cinclidae), warblers and gnatcatchers (Sylviidae, Parulidae), flycatchers (Muscicapidae), and titmice, nuthatches, and treecreepers (Paridae, Sittidae, Certhiidae). Many birds capture insect prey in flight, whereas other birds forage in shrubs and trees or on the ground. Some birds specialize by obtaining insect food, grooming large mammals or following behind large mammalian herbivores and foraging on the insects disturbed by large mammal movements.
Mammals include many insectivorous groups, some generalists and others obligate specialists. Most of those that specialize in eating insects eat either ants or termites. Generalized insectivores will eat insects along with other arthropods such as centipedes, millipedes, spiders, and scorpions. Marsupials, bats, primates, rodents, carnivores, and other groups of mammals include insectivorous lineages. Specialized feeding on particular insects, especially ants and termites, occurs among a few frogs, many lizards, and some snakes and has occurred in several different lineages of mammals (see Table I).


ANATOMICAL SPECIALIZATIONS FOR

INSECTIVORY

Although eating insects is a dominant part of many diets, anatomical and behavioral specializations for insectivory are not as widespread. Many lizards, for example, feed on whatever suitable prey item might be available. Nevertheless, there are several anatomical specializations that seem to assist in the capture of insect prey. Perhaps the most obvious and remarkable trait is a highly projectile tongue. This sort of tongue evolved many times, in many different lineages, and in many different ways. Most frogs have a projectile tongue whose intrinsic muscles attach to the lingual edge of the symphysis of the jaw. The tongue is flipped out of the mouth, in the same way that a catapult works, so that the back of the tongue after it is extruded contacts the prey first. Salamanders have evolved several different types of projectile tongues. One group of lungless salamanders, the plethodontids, uses the hyobranchial skeleton and associated muscles, once used to ventilate lungs, in a tongue protrusion mechanism that is quite spectacular. Contraction of these muscles results in protrusion of the tongue as well as large parts of hyobranchial skeleton, resulting in an extruded tongue that can reach 80% of body length.
Chameleon lizards have an unmatched ability to accurately aim, project, and hit arboreal insect prey. The muscles that chameleons use to accomplish this ballistic feat contract faster than any other vertebrate muscle. Furthermore, the tongue can be accurately projected up to 200% of body length. The tongue has a large tip covered with viscous mucus. A muscle in the large fleshy tip contracts just after the tongue tip strikes a prey item, creating a slight vacuum that assists in prey capture.
Mammals eat insects and the most distant ancestors of mammals may have been insectivores. Generalized features derived from the primitive amniote condition are associated with mammalian feeding,

TABLE I

Examples of Frog, Lizard, and Mammal Lineages in Which

Termites and/or Ants Are a Significant Part of the Diet
Frogs Mammals
Dendrobatidae—poison frogs Canidae—dogs and foxes
Microhylidae—narrowmouth Cercopithecidae—Old World
frogs and toads monkeys
Pelobatidae—spadefoot toads Cricetidae—New World mice,
hamsters, etc.
Rhinophrynidae— Dasypodidae—armadillos
Mesoamerican burrowing toads
Lizards Didelphidae—opposums
Agamidae—angelheads, calotes, Herpestidae—mongooses
dragon lizards, and allies
Amphisbaenia—wormlizards Lorisidae—lorises
Gekkonidae—geckos and Manidae—pangolins
pygopods
Iguaninae Muridae—Old World rats
Lacertidae—wall lizards, rock Myrmecobiidae—marsupial
lizards, and allies anteaters
Phrynosomatinae Myrmecophagidae—anteaters
Scincidae—skinks Orycteropidae—aardvarks
Teiidae—whiptail lizards, tegus, Sciuridae—squirrels
and allies
Tropidurinae Tachyglossidae—echidnas
Snakes Talpidae—moles
Anomalepididae—early Thylacomyidae—bilbies
blindsnakes
Leptotyphlopidae—thread- Vespertilionidae—bats
snakes and wormsnakes
Typhlopidae—blindsnakes

Note: Not all the taxa that are a part of these lineages eat only ants and/or termites. There are many other lineages not listed that eat insects.
including a longer secondary pallate, heterodont dentition, higher metabolic rates, and more active foraging behaviors. Several of these features are reversed in obligate ant- and termite-eating mammals. For example, anteaters, pangolins, and the echidna, numbat, and aardvark possess highly simplified teeth few in number or lack teeth entirely. Some ant and termite specialists have lower metabolic rates, but it is not clear if these rates are retained from a primitive mammalian ancestor or if they are a response to prey that may present chemical challenges to typical mammalian digestive systems. A long, sticky, and protrusible tongue is a common feature among ant- and termite-feeding mammals. Details of tongue anatomy confirm that many of these lineages evolved these specializations independently.

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